The 2dF system consists of the following main components:
The 2dF uses a new telescope top-end, with the spectrographs, and much of the control electronics being mounted on the top-end ring.
The 2dF corrector is a 4 component system designed by Damien Jones based on an original concept by C.G. Wynne. Its very large field of view is achieved at some cost to its general broad band imaging performance; however for multi-object fibre spectroscopy utilising ~ 2 arc second input fibres, this compromise is well worth taking in order to maximise the system's field of view. The most serious image degradation is that of a strong Chromatic Variation in Distortion term which has the effect of dispersing images in the radial direction by up to 2 arc seconds over the full 350-1050nm wavelength range. This dispersion reaches a maximum at about half the full field radius as shown in figure 1.1.
The corrector incorporates an atmospheric dispersion compensator (ADC). The ADC is formed by making each of the first two elements of the corrector prismatic doublets which can independently rotate to compensate for the dispersion effects of the atmosphere for all ZDs less than 65 degrees. These prismatic doublets are close to a metre in diameter and hence represent some of the largest lenses ever made for astronomy. The glass blanks for the corrector were cast by Ohara (Japan) and the optical figuring and mechanical mounting was done by Contraves (Pittsburgh, USA).
The large radial distortion introduced by the corrector results in an image scale which varies from about 15.5 arc sec per mm in the centre to about 14.2 arc sec per mm at the edge. The corresponding change in focal ratio is from f/3.4 to f/3.7.
The design of the 2dF positioner is driven by the requirement that fields may need to be reconfigured as frequently as once every 30 minutes to cope with the effects of differential atmospheric refraction over the 2 degree field. Thus it must be possible to set up a 400 fibre field in 30 minutes, but also in order to avoid unacceptable dead time, a double buffered arrangement has to be adopted to allow the next field to be configured while observing the current one.
This is achieved with a Tumbler arrangement on which two field plates are mounted. A robotic gripper head mounted on an X-Y gantry moves over the upper plate and places magnetic buttons attached to the fibre ends at the required positions on the plate. A TV system in the gripper head is used to measure and refine the position of the fibre. Once the field set up is complete the tumbler can rotate to place the field plate in the lower position where the fibres receive light from the telescope, while the second plate is now positioned for fibre set up. A second X-Y gantry at the base of the positioner carries the focal plane imager (FPI), a CCD camera which can be used for viewing objects in the field. It can be used to assist with field acquisition and for calibrating field distortion and related effects.
The X-Y gantrys are driven by linear motors rather than the lead-screw technology used in the previous Autofib system. The X axis uses two linear motors to drive both ends of the Y beam simultaneously, while a single motor drives the gripper along the Y-beam.
Each field plate has 400 object fibres and an additional 4 guide fibre bundles, making a total of 808. The fibres are 140mm in diameter corresponding to about 2.16 arc sec at the field centre and 2 arc sec at the edge of the field with a variation with field radius as shown in figure 1.2.
The fibres are terminated by a magnetic button which can be picked up by the gripper and placed on the field plate. A prism at the head of the button reflects the light from the object into the fibre.
In addition to the 400 object fibres, their are four guide fibre bundles on each field plate. These each contain a central fibre surrounded by six more fibres in a hexagonal arrangement. The guide bundles feed an autoguiding TV camera which is used to determine guiding corrections by comparing the fraction of starlight falling into each of the fibres.
The fibres in the guide bundles have a diameter of 95mm (about 1.4 arc sec) and are spaced by 120mm (1.8 arc sec).
The two identical fibre spectrographs each receive 200 fibres. The spectrographs consist of an off-axis Maksutov collimator feeding a 150mm collimated beam to the gratings and thence, at a collimator/camera angle of 40 degrees to an f/1.2 camera. The camera is a modified Schmidt design using a single, severely aspheric corrector plate.
The spectrographs use the same gratings as the RGO spectrograph. A number of duplicate gratings have been bought to allow simultaneous use of identical gratings in the two spectrographs. The gratings are listed in appendix 9.
The detectors are 1024 × 1024 thinned Tektronix CCDs. With 200 spectra on each detector the spectra are positioned roughly 5 pixels apart.
The spectrographs contain a slit assembly which allows for switching between the fibre bundles coming from the two field plates. This also provides for the back illumination of the fibres needed by the positioner.
The 2dF is controlled by a distributed system involving a number of different types of computer system.
A fourth VxWorks system (2dFAg) in the control room is used to run the Autoguider. It receives images from the Quantex TV camera via a frame grabber, and sends offset demands to the telescope via a Camac interface.
Most mechanisms in the 2dF positioner and spectrographs are controlled via Delta Tau PMAC cards in the VME system. The PMAC is a versatile programmable servo controller capable of providing the fast and precise control needed for the positioner's XY gantrys.
Other interfaces include a number of frame grabbers used to read images from the TV cameras in the positioner and autoguider. The controller for the focal- plane imaging CCD is controlled via an IEEE bus interface from the VME system.
To meet the needs of the 2dF project for a distributed software system running over a variety of different processors and operating systems, the AAO software group have developed the software environment. DRAMA provides for the development of modular software systems made up of a number of tasks which communicate via a message system. DRAMA tasks can run on the VxWorks, UNIX and VMS systems and efficient communication is possible between all the systems on the network as well as locally between tasks on the same machine. The messages are encoded using a self-defining data system (SDS) which automatically handles differences in data representation over the different machine architectures.
DRAMA is used for most of the software in the 2dF system, except for the OBSERVER system used for the CCD data taking which uses the older ADAM environment.
The main software components are shown in the figure 2.2 which presents a simplified view of the system. Many of the boxes on this diagram actually represent a number of tasks. For example the positioner system consists of a main task, and subtasks to control the gripper gantry, FPI gantry and tumbler.
All 2dF software is controlled through graphical user interfaces most of which have been developed using the Tcl/Tk system developed by John Ousterhout at Berkeley. All 2dF user interfaces follow the Motif style guide and should be easy to follow for anyone used to Motif or a similar system such as Microsoft Windows or the Macintosh interface. The main conventions are described in Appendix A.
This section describes what is involved in preparing an observing project for the 2dF. The basic steps involved can be summarised as follows:
Step 3 is performed using the configure program which is described later.
Successful use of the 2dF depends on accurate source positions. With fibre diameters of 2 arc seconds, positions accurate to better than 0.5 arc seconds are needed to avoid significant light loss. Most 2dF projects are likely to be based on astrometry from Schmidt plates measured with the APM or SuperCosmos machines. With care it should be possible to achieve the necessary accuracy but the following considerations need to be borne in mind.
The best currently available reference star catalogue with sufficient star density for Schmidt plate reduction is the PPM (Positions and Proper Motions) catalogue of Roeser and Bastian. The PPM catalogue is particularly accurate south of the equator where the mean error is 0.11 arc sec in each coordinate. The northern PPM catalogue is somewhat less accurate with a mean error of 0.27 arc sec in each coordinate. All APM and SuperCosmos astrometry is now reduced using the PPM catalogue, but some older data, such as the Cosmos database, uses the SAO catalogue which is much less accurate (1.2 arc sec). Make sure that you use the final version of the PPM South catalogue. The NASA ADC "Selected Astronomical catalogues" CD-ROM contains a preliminary version of PPM South which is much less accurate than the final version.
The accuracy of astrometry from Schmidt plates is dependent on the magnitudes of the objects being measured. Over most of the range of interest the accuracy appears to be of the order of 0.2 to 0.3 arc seconds as determined by comparing measurements of different plates of the same field. The accuracy falls off somewhat for the faintest objects on the plates, but is also poor for bright objects where the images are large and saturated. Unfortunately the reference stars used to calibrate the astrometry fall into this range. Provided these errors are random this is not a problem, as there are a large number of reference stars available to determine a small number of plate constants. However, systematic magnitude dependent effects could cause problems, and there is some indication that such effects are present, particularly if the brightest available reference stars are used.
It is therefore advisable to use only the fainter reference stars. This is another advantage of the PPM catalogue over the older SAO catalogue, since the PPM includes more fainter stars.
Analysis of HST Guide Star catalogue data has shown the existence of a pattern of systematic distortions in Schmidt plate astrometry at a level of about 1 arc second. This implies that better astrometry can be obtained using local fits to a small area of the plate, than using a standard global solution for the entire plate. However, Irwin (Working Group on Wide Field Imaging, newsletter 5) has shown that APM data from Palomar plates shows this distortion pattern, but UKST plates measured with the APM show very little evidence for such distortion.
UKST plates are therefore to be preferred for 2dF astrometry. If Palomar plates are used some correction for the systematic distortions should be included in the astrometric reduction.
The effect of proper motions of stars over the time interval between the taking of the plate and the 2dF observation can give rise to errors in the positions. Even if the target objects are extragalactic, proper motions in the guide stars can give rise to errors in field acquisition. There are a number of steps that can be taken to minimise this problem:
There are four fibre bundles which are used for guiding whose pivots come located at the North, East, South and West cardinal points of the two degree field. Due to the fibre pivot angle constraints each of them can only access about 0.25 deg2 of sky, so a density of about 20-30 stars are required per 2dF to ensure all 4 bundles are allocatable.
The TV used to guide with 2dF can see stars down to V = 15 in typical seeing. This limit has been established empirically using Landolt standards. Be careful that your magnitudes are on this scale - in particular APM `galaxy' magnitudes for stars can be way off.
Further the positions should be checked by eye on Sky Survey plates to ensure they haven't had their positions corrupted by faint halos, diffraction spikes or companions. Halo effects are especially a problem for stars with V<12 when the positions have been derived from survey plate scans.
A standard format for the input file used to describe a field has been adopted for the 2dF on thr AAT and the Autofib-2 instrument on the William Herschel Telescope in La Palma (Lewis, 1993). The file is an ASCII text file listing the field details, and the details of each target. This section describes the required format.
The file consists of character lines. Comment lines can be indicated by an asterisk character in the first column. The first four data lines in the file must contain information on the target field, each item beginning with one of the following keywords (which may appear in any order).
Subsequent lines describe target objects, one per line. Each line consists of a number of items separated by spaces.
* This is a comment line LABEL target field number 1 xyz cluster UTDATE 1994 05 12 EQUINOX J2000.0 CENTRE 12 43 23.30 +10 34 10.0 * end of required header info * F1 12 40 20.55 +10 30 11.4 F 9 12.0 1 brightest star F2 12 38 10.31 +09 59 58.9 F 9 13.5 1 fiducial star * NGC1002 12 41 30.55 +10 31 56.9 P 2 15.0 1 small fuzzy galaxy ic3082 12 40 18.40 +10 32 21.5 P 2 17.0 1 candidate satellite * sky-1 12 40 10.00 +10 32 21.5 S 5 99.9 1 blank sky (checked)
The 2dF configuration program is used to perform the following main functions:
The program is started with the command configure typed at the UNIX shell prompt. This will bring up two windows. The control window contains a menu bar, status display and message region. The other window will be used to display a graphical representation of the 2dF field configuration being generated.
Note: when run at Coona configure configure will automatically pick up the current fibre and astrometry information from ~2dF/positioner/ and ~2dF/config/. The latter can be overridden by setting the environment variable CONFIG_FILES which can be useful when setting up fields for different Declinations (see section 8.7).
The configuration program can read data from two types of files:
To open an SDS configuration file select Open SDS... from the File menu and select your file using the resulting file selection dialogue. By default a file extension of .sds is expected for SDS configuration files.
On opening your file the status display will show a summary of information on the field, and the objects in the field will be drawn on the graphical display. In the case of a text file no fibre allocations will be present so the fibres will all appear on the graphical display at their park positions. An SDS file may already include fibre allocations and these will be shown on the display. To remove the existing allocations in order to start from scratch use Remove Allocations from the Commands menu.
The configuration program can be used as a way of converting configuration files between text and SDS formats in both directions.
To convert a text configuration file to an SDS configuration file use Open... from the File menu to open the file, then use Save or Save As... to save the file in SDS format. This sequence can be performed non-interactively using the -d switch on the command line when configure is invoked (see section 6.14 for more details).
To convert an SDS configuration file to a text file use Open SDS... from the File menu to open the file, then use List... to output the file in text format. The output file produced by List... is a valid configuration text file containing the unallocated objects from the configuration with EQUINOX set to J2000. It may also include a listing of the fibre allocations in the form of comments in the listing.
Before allocating fibres it is important to set the desired observing wavelength. Because of the large wavelength dependent distortion in the 2dF it is possible for an allocation that is valid at one wavelength to be invalid at another wavelength. Therefore use the Set Wavelength... option in the Commands menu to set the desired central wavelength of observation.
Sometimes you will want to configure for a particular plate, for example when running online during a telescope run, or checking a field for the telescope. This is necessary because both plates always have different fibre and astrometry information.
To set the desired field plate use the Set Field Plate... option in the Options menu. The default is to use Plate 0, but this can be changed using the -p option.
Alternatively one may wish to configure a field for both plates, for example to observe it continously for 4 or more hours and hence reconfigure to allow for the Hour Angle effects. In this case one would configure for one plate, set the other plate, do a Check Allocation (see section 6.9) and adjust the allocations of any fibres which cause clashes.
An automatic allocation of fibres can be done by selecting Allocate... from the Commands menu. This will bring up a dialogue box with a number of parameters controlling the allocation process. The default parameters should normally be suitable. During the allocation process, which will typically take a few minutes to complete, a progress window will report on the progress of the allocation, and the graphical display and status display will be updated as new fibres are allocated.
The default is to leave ~ 20 fibres for sky. Once the object allocation is complete you can assign these to sky positions. These can be either generated on a standard grid (using Allocate Sky Grid... from the Commands menu) or they can be supplied in the input file. Alternatively you can add sky positions manually (see section 6.10).
Once the allocation is complete it will be checked for validity at the current position. As an extra step you should select Check Over HA Range from the Commands menu to check the validity of the field over a range of hour angles (the default is to check for ±4 hours from the meridian on the date set by the UTDATE field).
Saving the configuration as an SDS file will now give you an input file for the 2dF positioner. You should ensure all the available guide fibres were allocated.
The above recipe will suffice for the majority of 2dF fields. The hardware constrains the fibres to an angular limit of about 14 degrees from the radial direction (or, more rigourously, from the direction in which they exit the retractor block!), but fibres are allowed to cross multiple times so in most cases all of them can be allocated to targets.
The algorithm currently used (developed by Gavin Dalton at the University of Oxford) is very optimised and has proved to give the best results for `typical' fields. After an initial allocation pass it searches down a tree of multiple fibre swaps, looking for swaps which give increased allocations. The algorithm is not unlike a chess program. The tree search terminates at a depth of ten swaps where it is has been found, empirically, that the expenditure of CPU time required to deepen the search is not rewarded by increased allocations.
The algorithm handles objects of different priorities by trying to allocate the highest priority objects first. During the swapping process it will continue to search until it becomes possible to allocate a fibre which was previously parked, or it becomes possible to promote an allocated fibre to a higher priority object.
After the swapping phase there is a final `uncrossing' pass which looks at all pairs of fibres which cross to see if they can be reversed. This is important, as reducing the number of fibre crossings in the final configuration provides a significant reduction in the field-field setup time by reducing the numbers of fibres that must be parked in transit, but this reduction is provided without constraining the allocation itself.
The targets are allocated in order of priority, with a numerical value of 9 being the highest priority objects and 1 being the lowest. To ensure all guide fibres are allocated it is best to give them the very highest priority values.
The parameters here control the various steps of the allocation. The default is to allocate the maximum number of targets and leave 20 fibres for subsequent sky allocation.
The recommended procedure is to first allocate the objects, leaving a certain number for sky, and then assign the leftovers to sky positions.
The observer can supply sky positions in the input catalog - for example positions which have been carefully checked on images to ensure absence of bright objects). To assign to these simply select `only allocate sky' in the Allocation dialog.
Alternatively a grid of sky positions may be generated and allocated automatically by selecting Allocate Sky Grid... from the Commands menu.
Finally arbitrary sky positions can be assigned interactively (see below).
It may be desirable to check the positioning of sky fibres that have been automatically allocated or added by hand, to ensure that these are not contaminated by stray objects, particularly brighter stars. This can be done by selecting List... from the Commands menu, and checking the Allocated Sky as DSS input button. This will list all allocated sky positions as J2000 coordinates to a file (the default is the same as the input file, but with the extension .dss) which is in the correct format to be read by the commonly available StScI getimage program. The sky positions are named S??? in the .dss file, where ??? is the fibre number. The content of the sky fibres can then be conveniently be checked using a FITS aware visual browser (e.g. the visual schnauzer in xv)
The allocation process results in a fibre configuration which is valid at a single time. However, small changes in the relative positions of objects as a result of refraction and other effects could make this configuration invalid at other times and different telescope positions. It is possible to change the Hour Angle using the Set HA... option in the Commands menu, and then check the validity of the allocation using Check Allocation.
The Check over Range of HA... option in the Commands menu performs a check that a fibre configuration remains valid over a range of dates and telescope hour angles. The check should run through with Allocation OK reported in the message region for each position tested.
Occassionally one or two fibres or buttons which are OK at the zenith will cause collisions at large hour angles. The simplest procedure is to manually reassign these as extra sky fibres, although it is usually possible to manually adjust the configuration to preserve the target allocation whilst removing the collisions. An automated procedure for this task is under construction.
Manual allocation is performed by interacting with the graphical display as follows:
There is a short cut to the manual allocation procedure which avoids the use of the menu as follows:
It is also possible to deallocate fibres manually as follows:
The display can be zoomed to magnifications of 2, 4 or 8 times its normal scale using the Zoom menu. The zoomed display can be scrolled using scroll bars.
It is also possible to zoom the display by a factor of two about any selected point by clicking the right mouse button with the cursor positioned at the desired centre.
In the display fiducial stars are shown as large red circles, objects as small black circles and sky fibres as blue squares. The fibres buttons are coloured blue for normal fibres, green for guide fibres and grey for disabled fibres. Selected objects and fibre buttons are shown in red. Information about individual objects or fibres in the display can be examined by double clicking on the object or fibre button.
To locate an arbitrary fibre button enter the the pivot number in the fibre info popup and press RETURN - the fibre button selected in red changes to the requested one.
The display can be printed by selecting the Print... command from the File menu. A postscript version of the display will be generated which can either be sent directly to a printer, or saved as a file according to selections in the resulting dialogue box.
After completing the allocation process and checking its validity the resulting configuration can be saved as a SDS file using Save or Save As... from the File menu. The resulting file is in a form suitable for use by the 2dF observing system.
A text file listing the fibre allocations and/or the unallocated objects can be obtained by selecting List... from the File menu.
On typical workstations (e.g. a SPARC 5) the configure run generally only takes a few minutes. It is recommended that the configure program be run interactively. However there is a limited feature for running it in batch mode. This can be done as follows from the command line.
configure -a -f sample_field.fld(Note: The command to start configure may be different on your system.) This will produce two new files, filename.sds and filename.lis. The first can be fed back into the program in interactive mode to show how the fibres have been allocated. The second is a text file that contains details of the pivot allocation and could be processed by some other program to analyse the way the field has been configured. Sky fibres will be left for manual allocation.
configure supports a number of command line options:
There are also a number of options that can be used to perform specific actions when invoked with the -f option:
This section of the manual is intended for AAO support astronomers ONLY.
Currently, at the end of the startup you will be asked `Do you wish to null the ADC'. Generally `NO' as it takes a few minutes - when observing you will set it tracking anyway. You will also be prompted for the names of the observers.
This is currently subject to rapid change. Please consult the local 2dF site manual for the latest instructions.
Follow the instructions in the local site guide for the exact hardware power-down sequence.
This topic is dealt with in a separate document `The 2dF Error Recovery Guide' because it is subject to rapid change and requires utter caution. The first version of this document is still being written. Until this is available please report errors to technical support personnel and under no circumstances attempt error recovery yourself. This risks irreversible decommissioning of the instrument.
The 2dF software system is controlled through a number of user interface tasks which appear on the Sparcstation and the adjacent X-terminal. These are as follows:
The main window of the control task is shown in figure 7.1. This window contains status displays for the six subsystems managed by the control task (Telescope, ADC, Autoguider, CCDs, Positioner, Spectrographs). Any of these sections can be removed from the display by clicking the button in the top right hand corner. It can be restored to the display using the Display menu.
A number of options are controlled through the Commands menu which contains the following commands:
The Positioner control window is shown in figure 7.2.
The top section of the display shows the plate in the configuring position (Config Plate) is currently field plate 0. This means that plate 1 is in the observing position. The last configuration file which each plate was set up with is shown below. You can double click on the file name to get further details on the configuration file.
The Fibres Moved? button indicates if any fibres have been moved since the field was set up. The All Parked button will be on if all the fibres on the plate are parked.
The lower section of the display is a `notebook' widget with most of the main positioner control functions in the Setup Field page.
The Survey... button starts a survey of the grid of fiducial marks in the field plate. This calibrates the relationship between the encoder units of the gripper or FPI gantry with the actual XY of the field plate. A field plate survey should be done before any position critical operations are done with a gantry, such as setting up a field, or doing an astrometric calibration with the FPI. Because of flexure between the gantries and field plates, the survey should be done at the same telescope position at which the setup or calibration will be done. The Survey... button brings up a dialogue which allows selection of the FPI or Gripper gantry and provides options for a number of different types of survey. The All option is the one normally used for calibration.
The Tumble button `tumbles' the positioner interchanging the positions of the two field plates. The plate that was at the configuring position now becomes the observing plate. After tumbling a check of a single fiducial mark on each plate is done to adjust the calibration of the gantry coordinate systems.
The Setup button initiates the setting up of a fibre configuration. The configuration file to be set up should be entered in the Config file entry section. Clicking on the button at the right hand end of this will bring up a file section dialogue to enable the file to be selected.
By default configuration files are `tweaked' before being set up. This involves recalculating the XY positions of the fibres for a specified observing time and for the current astrometric model and refraction parameters. If you are going to tweak your file make sure that the information in the Weather and Wavelengths pages are correctly set up as these are used for the refraction calculation. The Obs Time value is used to set the time in minutes from now which the field will be set up for. If you turn off tweaking the XY values in the configuration file will be used (which may have been calculated using an old model).
Once the field setup starts, the Configuration Progress section displays a progress bar, and the number of the fibre currently being positioned.
The Park All button parks all the fibres on the plate.
The Fibre Moves page contains options to move or park individual fibres.
The telescope control window is shown in figure 7.3. The upper section shows the current telescope status. The Slew page can be used to slew the telescope to a source. A position can be entered directly into the Position section, or the telescope can be slewed to the field centre position of the configuration file associated with either the observing or configuring field plate using the buttons in the Config File Positions section. The ADC can also be set to track the same source by setting the ADC Track button.
Other pages in this display allow the telescope to be parked at zenith or prime-focus access, offset control of the telescope, and focus control.
The CCD control window is shown in figure 7.4. The CCD control works by sending commands to the standard CCD Observer system running on the VAX. However, it is advisable to always control the CCD via the 2dF system rather than by directly typing commands on the Observer terminal. This ensures that control is properly integrated with other 2dF subsystems and (eventually) will ensure that the correct header information is written to the data files to allow them to be reduced by the 2dF data reduction system.
The upper section of the window specifies the exposure time, the type of run, and whether it is to be recorded. The options here are as follows:
The Window button allows the CCD readout window to be specified. The TEK1K_2DF window should be used for all standard 2dF observations.
The Readout Speed This sets the readout speed for the CCD which determines the readout time, readout noise and gain.
The ADC control window contains a display of the ADC prism angles and buttons to allow the ADC to be nulled, stopped or set to track a specified telescope position. In normal operation this window is not needed as the ADC can be controlled from the telescope control window.
There is an autoguider window in the control task which can be used to provide the basic control functions for the autoguider. However, it is normal to control the autoguider from the FGT user interface which normally appears on the X terminal. The use of the autoguider is described in section 8.11.
The spectrograph control window is shown in figure 7.5. The window provides a display of the status of the two spectrographs. It also provides control of the following functions:
This is used to control the Focal Plane Imaging gantry which carries a small CCD which can be positioned anywhere in 2dF's focal plane to take images. It is used for astrometric calibration and setups as described in Chapter 8.
The calibration lamps are not yet under computer control. An interim control system has been provided via a box labeled 2dF COMPARISON LAMP SWITCHBOX in the control room.
The calibration lamps are turned on via switches on the switchbox. There is a rotary switch to set the power level for the quartz lamp, and individual switches for a number of wavelength calibration lamps (Copper Argon, Copper Helium, Mercury, Helium). Remember that to take a calibration lamp exposure you also need to close both the flaps using the 2dF FLAP CONTROL box. Note this is a workaround system: always open or close both flaps simultaneously and ON NO ACCOUNT OPEN THE FLAPS SEPARATELY.
A typical observing session with the 2dF is likely to involve the following steps:
A fundamental operation, for acquiring objects on to 2dF, is `centering a star on the FPI camera'. This is so common it warrants a special section, it is described below.
Although the ADC has its own control window, the basic control of the ADC is most easily done from the Telescope control window of the control task via its SLEW page. If you slew the telescope from here the ADC can be automatically slewed with it. From here you can select one of the following modes.
The first setup to do when 2dF goes on is to focus the telescope. We find it is very stable (it is temperature compensated) throughout a 2dF run though it is advisable to check every few nights during twilight if there is time, especially if the seeing becomes exceptionally good. This is done by the following procedure:
Pointing calibration involves setting up the values of the parameters ID, CH which determine the position of the pointing axis. Although this is standard procedure for the AAT it is particularly important that it is done correctly for the 2dF. This is because the CH value not only determines the telescope pointing, but also feeds into the field rotation offset which is used in configuring the fibres. Therefore an incorrect CH value can lead to errors in fibre positioning as well as errors in telescope pointing. Since we do not yet have the plate rotators operating these errors cannot be corrected.
The ID and CH values are different for the two field plates (as they refer to the somewhat arbitrary (0,0) position on the plate which is the origin of the fibre positioning). There is also a difference between the values for the field plate and the focal plate imager, due to a small offset between the camera viewing the field plate and the FPI camera viewing the sky. This difference is sometimes referred to as the magic offset.
These parameters can change slightly each time the 2dF top end is put on so it is necessary to re-calibrate on the first night of each 2dF run. Further the positioner does not incorporate temperature corrections to allow for thermal expansion of the field plate and encoder, so if the mean night-time temperature changes by more than 5 degrees the poscheck should be redone. Also the plate rotators are not available and due to flexure and pointing model effects there is a residual Declination dependent field rotation. For this reason we always use POSCHECK calibrations taken at a similar declination (within ~ 10°).
We need to measure 3 offsets, (all expressed as differences in CH and ID):
First we need to measure the FPI-Fibre offsets for both plates:
The new MAGIC OFFSET is DCH WEST and DID SOUTH (and are both usually positive, with values about 15W 2S).
This is the physical offset between what is nominally (0,0) on plate 0 and plate 1.
From the above formulae:
where the RHS values are as calculated in section 8.5.1.
The above derived values of DCH and DID are all relative, because they were only done for one star, they now need to be given a fixed reference by deriving a pointing model for 2dF averaging over many stars.
By convention we do this referenced to a guide fibre positioned at (0,0) on plate 1. Do a gripper survey then position this fibre. Set the ADC to Null.
A SNAFU on about 12 stars ranging over ±3 hours in HA and -10° to -70° in Declination will give good values for ID, CH and IH. This will normally take about 30 minutes.
Each star should be centred on the fibre. If the star is not visible and cannot be found by moving the telescope a few arcseconds (quite likely at the start of the run!) then move the FPI to (0,0), centre in the FPI, park the FPI and move the telescope by the `magic offset'. The final model should be saved for permanent use during the 2dF run.
This gives an ID and CH value for the fibres on plate 1 (CH1,ID1) The offsets determined in the previous section can then be used to derive final ID and CH values for the fibres and FPI on the two plates (four sets of values altogether). For convenience (and avoidance of sign confusions) the following formulae can be used:
|FPI||1||CH1 + DCH1||ID1 + DID1|
|Fibres||0||CH1 + DCHP||ID1 + DIDP|
|FPI||0||CH1 + DCHP + DCH0||ID1 + DIDP +DID0|
In order to perform an accurate transformation from RA,Dec to x,y on the field plate the 2dF system requires two calibration files which specify the linear transformation between the predicted and actual x,y coordinate system, and the distortion model. There are two versions of these files, one for each field plate.
The files are called tdFlinear0.sds and tdFdistortion0.sds for field plate 0 and tdFlinear1.sds and tdFdistortion1.sds for field plate 1. The files are normally kept in the directory ~2dF/config. They are used by the FPI control software, the configuration software, and the tweak process run by the 2dF control task.
The files are set up by a calibration procedure which involves running the FPI around a number of stars in a field and recording the x,y positions of the FPI gantry at which the stars are found. The file produced by this process can be analysed using a program which will fit a model to the data and determine the linear and distortion files.
It is non-trivial to find calibration stars which have a high density and cover a large part of the sky. Therefore we have provided the following sets of standard astrometric calibration fields:
The wavelength should be 0.7 microns - this corresponds to the filter that is in the FPI camera.
The CH value (or ``Collimation Offset on the Hour Axis'') should be set to that determined for the FPI on this plate as described in section 8.5. The astrometry files (tdFlinear0.sds etc.) default to those in the system location (~2dF/config) and this default should not normally be changed.
The Star should then be visible on the FPI camera, and can be centred up by moving the telescope.
This is advised for normal operations. This is on by default (but can be turned from the Options menu in FPICTRL). The automatic calibration procedure causes the FPI gantry to be driven to each star in turn, and a centroid taken. Before doing this the centroid box size should be setup on the IMG widget using Shift-left-mouse-drag to define the box. A reasonable size in typical seeing (1-2 arcsec) is 20-30 pixels. Note the approximate pixel scale of the FPI camera is 0.3 arcsec. If you have a poorer calibration a larger size will be needed (e.g. 50 pixels for the first POSCHECK of a run).
An integration time of 0.2 seconds usually works well with PPM stars. If the FPI camera is running in movie mode turn it off before starting the calibration.
The POSCHECK button in the Select Object popup window starts the automatic calibration. You will be prompted for the names of files to save the fit results in - best not to overwrite the system files at this point!
Error messages may come up if a centroid fails, usually because the star image is saturated. This is not a problem - the star will simply be marked as unusable and the calibration will continue.
The final RMS reported should be in the range 20-40 microns (remember 1.0 arcsec is 70 microns). Don't forget to do the POSCHECK for both plates!
The files containing the final results (tdFlinear0.sds tdFlinear1.sds tdFdistortion0.sds tdFdistortion1.sds) should be copied to either the system location or a Declination dependent sub-directory (see section 8.7).
This is not advised except when doing tests. If the calibration is a long way out, possibly because the plates were rotated during maintenance, an automatic calibration may not be feasible. In this case a manual calibration can be performed on a smaller number of stars. The new linear and distortion files derived from this will then provide an approximate calibration which will enable the automatic calibration to run.
To do a manual calibration use the Record On button. Then select a star from the list and drive the gantry to it using the GO TO RA/Dec button. Run the FPI camera in continuous mode and centre the star on the cross using the Gantry Jogger. When the star is centred select Record from the gantry commands menu. Repeat for as many stars as required.
By default the calibration run is automatically reduced and a new linear and distortion file created. In most cases this is all that is needed.
However, it may in some cases be useful to record the calibration data in a file and reduce it later. To do this turn off the AutoCalibrate option in the Options menu of the FPI task before running your calibration.. An output file will be written which can be analysed using the tdffit Unix program or the 2dFModel Macintosh program described below.
If you used the default AutoCalibrate option when you did your calibration ignore this section. The calibration will already have been reduced automatically.
The program tdffit available from the Unix observer account can be used to fit a model to the calibration run data, obtained with the AutoCalibrate option turned off, and derive new tdFlinear.sds and tdFdistortion.sds files. To use this program to reduce the run test1.sds, login as observer, cd to the ~2dF/config directory and type the following command line.
tdffit ~observer/test1.sds 280 890 0.5 0.7where the four numbers following the file name are the temperature, pressure, humidity and wavelength as entered previously in the Init Transform... dialog. This program will fit a model to the data and output the values on the terminal. It writes new tdFlinear.sds and tdFdistortion.sds files. Again the files will need to be copied to their final destination.
To make the FPI software pick up the new files go into Init Transform... and select the files again.
The configuration software picks up the new files when you start it up or when you select a field plate.
2dF has a Declination dependent field plate rotation due to a combination of the intrinsic geometric rotation of the sky and plate flexure.
Until the field plate mechanical rotation is available the only way to compensate for this is to use setup files (tdFlinear0.sds tdFlinear1.sds tdFdistortion0.sds tdFdistortion1.sds) for the correct Declination (i.e. within about 10°). This means a POSCHECK must be performed for each Declination which fields will be configured for.
Currently we organise this by creating sub-directories in ~2dF/config/ for each month (e.g. ~2dF/config/poscheck_jul98/) and then below that for each declination that has been measured (e.g. ~2dF/config/poscheck_jul98/m5/, ~2dF/config/poscheck_jul98/m30/ for d = -5°, -30°, etc.) The files have the same name as in the top level directory (i.e. tdFlinear0.sds etc.) and can easily be copied back to the top level before setting up a field. For example:
% cp ~2dF/config/poschecks_jul98/m30/* ~2dF/configIt is also advisable to check field validity using configure with the right setup files. You can change the default location configure picks up its files by setting the environment variable CONFIG_FILES before starting configure, e.g.:
% setenv CONFIG_FILES ~2dF/config/poscheck_jul98/m30 % configure & Opening distortion file /instsoft/2dF/config/poscheck_jul98/m30/tdFdistortion0.sds Opening linear file /instsoft/2dF/config/poscheck_jul98/m30/tdFlinear0.sds Reading /instsoft/2dF/positioner/tdFconstants.sds ...
The following steps must be carried out before setting up a field:
Now in the Setup Field section of the Positioner Control window select the configuration file you want to set up, and set the required time of observation.
The field setup can now be started using the SETUP button. The XY positions of the fibres will be recalculated for the specified observing time before starting to set up the field.
If you have problems (for example fibre/button collisions) go back in to configure and edit the configuration (e.g. deallocate these fibres). Currently there are some constraints that tdfct knows about (e.g. the location of the plate screw holes!) which configure does not.
Also you may have made a mistake and not checked the field for the correct Hour Angle.
This requires the following steps:
If you still can not find the stars, repeat the above procedure, this time doing an FPI survey at the field position first to take out any local flexure.
Data taking should be controlled via the CCD window of the control task rather than directly from the observer control terminal. This ensures correct headers are fed through to the data reduction system.
The data reduction system also requires that 2dF data should be taken with a window that covers the full chip without binning and includes a number of overscan columns on the right hand side. The window TEK1K_2DF is recommended. This is the default and is controlled using the Window button. The Readout Speed button can be used to set the CCD readout speed. Data frames are taken by setting the type of run and then clicking on the Start CCD Run button.
The following types of calibration frames can be taken:
A bias frame is taken by setting the run type to Bias Run, record option to Record Run and then clicking on the Start CCD Run button.
It is generally a good idea to take a number of bias frames which can be combined to minimise the effect of readout noise. To take a set of 10 bias frames select the Count mode and set the count value before starting the observation.
To take a dark frame set the exposure time and specify Dark Run and Record Run and then click on the Start CCD Run button. As with biases it is advisable to take a number of darks using the Count option so that they can be combined to remove cosmic rays.
It is intended that long slit flat fields can be taken by moving the slit unit backwards and forwards to blur the fibres into the appearance of a long slit. Currently the hardware and software to do this is not complete and there is no way to take such a frame. Flat fielding therefore has to be done with Multi-Fibre flat fields as described below.
Multi-Fibre Flat Fields are taken using the quartz lamp in the calibration unit. This illuminates the flaps below the corrector. Set it up as follows:
Wavelength calibration (or Arc) frames are taken using the lamps in the calibration unit. These illuminates the flaps below the corrector. There are four Copper Argon and four Copper Helium lamps which can be turned on separately or in combination.
To accurately subtract sky is is necessary to calibrate the throughput of the fibres. These vary at the 10% level between configurations, though they are stable to about 0.5% while tracking on a given configuration.
This calibration ideally requires a bright, flat source, unfortunately the `flatfield' lamps are neither flat or stable enough. Also because of the sheer size of the field (2 degrees) the twilight sky will vary by a few percent across it. Thus we have to use the dark sky. Because it is dark the data reduction system bins up along the wavelength axis so as to avoid the need for excessively long exposures.
The standard procedure is to take 3 exposures with the telescope offset 10 arcseconds in different random directions from the observing position. We find 3×3 minutes gives good results. Select `SKY' for these observations.
The Autoguider is currently still under development.
The autoguider hardware consists of a Quantex camera mounted on the top end, the Acquisition TV Memory system next to the Telescope control desk and a VME rack in the control room (at the bottom of the rack containing the XMEM VME system). To start the 2dF autoguider system, you need only ensure the VME rack is turned on and has booted. To autoguide, you will need the Quantex camera and TV Memory system turned on.
The autoguider software is started automatically during the 2dF control task startup. The user interface will be put on the NCD XTerminal next to aatssf and is known as ``FGT'' (Fibre Guider Task).
We assume you have calibrated the autoguider as described in ``2dF Autoguider Calibration'', 14-Aug-1996.
You must configure a field to put the guide fibres on objects of better than about 12th Magnitude. There are two basic approaches to doing this.
Slew to the object field and acquire it roughly using the TV memory display of the guide fibre output. You may need the help of your local TV memory expert (probably your night assistant).
Now we need to confirm the autoguider image grabber is working. Select ``Imager'' from the FGT commands menu. Then select ``Simple Exposure'' from the commands menu. The exposure should appear on the same imager used by the Fpi. Confirm that the display image reflects what is seen on the TV memory display. If you get errors at this point, or you don't get the same image as on the TV memory display, there is probably a wiring error. Talk to your technical support staff.
This is done for each new field. To correctly guide, the autoguider requires flat field and sky background information. It also requires a 2dF configuration file which describes where the fibres are, their rotation and the x/y to RA/DEC conversion.
If you configured your field using the 2dF control task then you need only tumble the positioner (using the control task).
If you configured the positioner using the engineering or pos interfaces, or using the control task but not immediately prior to the run, then you must do the following
If all four guide fibres are not configured, you will see error messages but these can be dismissed - the system will attempt to guide on the fibres it has.
You need to take a flat field. This calibrates the transmission levels of each fibre. Put some flat field data down the fibres (say turn on the dome lights). Get your TV memory expert to try to get as even as possible illumination of the fibres.
Select the Flat Field button to do the flat field
The item Transmission in the Extra Commands menu will display the current transmission values. The item Clear Flat in that menu allows you to reset the transmissions to 1.
You can save the flat field data and it will automatically be loaded next time the autoguider is started. Use the Save Flat item from the Extra Commands menu to do this. You may need only take one flat for each plate on a run, but may need to take another one if fibres are changed at all. Also note that for a flat field to be valid for other configurations, all valid the guide fibres that are to be used must be allocated when the flat field is done.
Note that flat fields and sky information are preserved across tumbles of the autoguider, and flat fields are preserved in general, but they are sensitive to fibre usage when they are taken. For example, if a plate has only three fibres enabled when you take a flat field or sky, then only those three are used for that field. If you enable a fibre late, you will have to do these again.
If you don't wish to use all fibres which have been configured, then you can use the following procedure
Fgt can guide on any number of fibres, including only one, but if only one is available, then there is no rotation information available. In addition, since guiding is done using a weighted average of available fibres, the more you have the better guiding.
Now put the telescope on the objects and line them up. Have your TV memory expert set up the quantex to get a good image of the objects. (Objects should be clearly separable from the background)
Offset the telescope such that only sky is going down the guide fibres. Do not adjust the TV Memory at this point. Grab you sky frame using the Take a Bias/Sky button on the Fgt user interface. Return the telescope to your objects.
You can display the Sky frame details using Show Bias/Sky from the Extra Commands menu and you can clear it using Clear Bias/Sky from the same menu.
Note that you will need a new sky frame if there is any significant variation in the sky. For example, the cloud and moon conditions are changing.
You will need to de-select the Imager (Commands menu) as displaying every image while guiding could cause problems (and the TV Memory video display shows you the image.
You should now start up the CCS side of the autoguider software. Type G2DF on the control terminal. The offsets being sent to the telescope will be displayed here. The values being sent by the autoguider software are displayed on the VxWorks console terminal (window 2dFAg on the aatssf console).
Assuming the telescope is tracking, start guiding by pressing the Guide button on the FGT user interface. This will then change to a Stop button which can be used to stop guiding.
Various options are available.
A graph of the measured offsets is available using the Graph Offsets button in the Options menu. Graphing starts from when you enable the graph.
This is a ``check button'' on the Fgt main window. When enabled (box is yellow), offsets will not be sent to the telescope. Calculation of errors continues and if the graph is enabled it will be updated. When you disable the freeze mode, the autoguider will attempt to re-acquire.
This mode can be used to allow you to offset the telescope manually for whatever reason and then to resume guiding.
Two modes of guiding are available. The default mode attempts to pull in the guide fibres, which assumes that centering of the guide fibres indicates centering of the field.
The alternative mode attempts to keep the guide fibres at the position at which they were when guiding was started.
The mode is changed using the Pull In Stars check box in the Options menu. The former mode is enabled when the box is coloured yellow.
Extra logging of what is going on is available by selecting a Log Level option in the Extra Commands menu.
Various items can be set using the Settings dialog. Select Settings in the Options menu.
The weight is that proportion of a calculated offset which is actually used. The default is 1/3.
The Guide Delay is the delay between taking exposures.
The Repeat Delay is the delay used when repeating exposures (activated using the Repeat Exposures item in the Commands menu.)
When you are finished with guiding, the you will need to enter the command DELE G2DF in the main console 02 terminal of the CCS (at the other end of the room, not the one next to the console).
Also ensure the Quantex camera is shutdown.
For each unbroken fibre of a configured and enabled bundle, the software sums all the pixels in that fibre. It subtracts the equivalent value from the sky image and scales the result according to the transmission value determined from the flat field.
Then for each bundle, we calculate the centre of the image profile in the bundle. From that we calculate the offset of the new image centre from the zero position (which may be the center of a bundle when ``Pull In Stars'' mode is enabled or will be the position found in the first image of a guide sequence).
We then rotate this offset to take account of the button rotation which gives us the offset of the profile centre in terms of the fibre-fibre centre distance. We convert this offset to RA/DEC on the sky.
We average all valid bundles to obtain the average offset and we calculate the rotation if there is more then one bundle. We do not consider at this point bundles the offset of which is excessive or for which none of the fibres has a value significantly above the sky.
If all bundles are invalid (not significantly above sky or offset is too high) then we ignore this image. We will try again a predetermined number of times before giving up.
If we have a valid offset value, we multiply by the weighting factor (1/3) and add the result to the current telescope offset if enabled (guiding not frozen).
Currently, the autoguider is not able to sync itself to the integration rate of the quantex, when the quantex is in integration mode. It just grabs an image at a specified rate (see the ``6.5 Settings'') from the 50Hz output of the TV memory video display.
When you are integrating for longer then the update time, then you will get multiple identical frames, which negates some or all of the weighting factor. This will often result in overshooting, and potentially leads to losing the image. Try setting the update time so something appropriate for the integration time being used on the quantex.
This document describes version 1.5 of the 2dF data reduction system released in Nov 1998. The system is designed to provide fully automatic on-line and off-line reduction of data taken with the 2dF.
The data reduction system currently does bias and dark subtraction, flat fielding, tram-line mapping to the fibre locations on the CCD, fibre extraction, arc identification, wavelength calibration, fibre throughput calibration and sky subtraction.
The data reduction system normally runs on Sun Solaris systems. A version for DEC Alpha systems is also available. On the AAO systems the 2dF data reduction system will currently be found in the following directories:
To run the system you should set up the environment variable DRCONTROL_DIR to point to the directory containing the reduction system with a command such as:
> setenv DRCONTROL_DIR /prog/ssz/1/jab/2dfdrand then source the following file to define commands:
> source $DRCONTROL_DIR/2dfdr_setupYou can include these commands in your .cshrc or equivalent so they are run when you log in.
Having done this the following commands will be defined:
|drcontrol||The command to run the data reduction system|
|2dfinfo||List header information from 2df files|
|fits2ndf||Convert 2dF files from FITS to NDF|
|ndf2fits||Convert 2dF files from NDF to FITS|
|objlist||List object and sky fibres for old 2dF data|
|cleanup||Message system cleanup|
To reduce a 2dF data set you should have the following set of files:
If you don't have the full set of files listed above it is still possible to reduce the data, but some stages of the calibration may be skipped.
Data to be reduced using the 2dF Data Reduction system should consist of raw NDF files (which have a .sdf extension) as written by the OBSERVER CCD system. Data from the archive may be provided in FITS format in which case it needs to be converted back to NDF. This is done using the Starlink Convert utility which has been modified for us by Malcolm Currie at Starlink to handle all the extensions in the 2dF data.
To convert a single file use the command:
> fits2ndf run0001.fts run0001 \\
Multiple files can be converted using the form:
> fits2ndf *.fts * \\Other FITS reading programs such as FIGARO WDFITS should not be used, as they don't handle the special extensions used to hold the fibre header information, though they will read the data itself correctly.
There is another command ndf2fits to convert an NDF file to a FITS file. For 2dF data you must specify the keyword proexts on the command line to ensure that the 2dF fibre header is handled correctly (it becomes a FITS binary table in the FITS file). So the command would be:
> ndf2fits proexts run0001 run0001.fts \\
2dF data taken before October 1997 does not include the header information present in current data files and may have other problems. In this case you will need to read section8.20 and prepare the data files accordingly.
Data taken between Oct 1997 and Jan 1998 may have a number of errors in the headers. There is a program tdffix which can be used to fix these problems. Make sure these files have been fixed, before trying to run 2dfdr on them.
You need to ensure that the X windows display device is set up appropriately, for your workstation by setting the DISPLAY environment variable or using a command such as the starlink xdisplay command.
Before running the system it is a good idea to close down any X-windows applications that use a lot of colour table entries as this may prevent the system creating its display windows. Netscape is one aplication that can cause this problem.
First cd to the directory containing your data. Then start the data reduction system using the following command:
> drcontrol &The Data reduction system is closed down by the EXIT command in the File menu.
If you have problems starting the system, or get timeouts on the initialization type the following command:
> cleanupand try again.
The user interface main window is shown in figure 8.1. It is divided into three main sections. On the left is a ``Notebook'' widget which has a number of pages which can be selected by means of tabs. One of these pages is the ``Data'' page which can be used to select data files by run number or file name and perform operations on them. The other pages are used to set parameters which control data reduction or display.
At the top right section of the screen is the automatic reduction section, and below this is a window which displays the progress of current operations and any messages output during the reduction progress. There are two execution tasks and the windows for the two tasks are organized as another ``notebook'' widget.
The standard way of automatically reducing a set of data is as follows:
The system will now locate all the raw data files in your directory and check their classes. It will also check if they have already been reduced. Once this is done you can use the DATA notebook page in the left hand part of the display to step through your files by run number and select a file to work on.
Data files can also be reduced individually. To do this, select the file in the DATA notebook page and click on the REDUCE button.
Whether you use automatic or manual reduction the file will be reduced in one of the two DREXEC tasks, and messages on the status of the reduction will be displayed in the appropriate window. You have to select the right page of the notebook widget for whichever DREXEC task is being used to see the messages and progress bar.
The PLOT button in the DATA notebook page can be used to plot the currently selected run. You can choose to plot either the raw data or the reduced data. The data will appear in the plot window with the title `DRPLOT2 - General Plots'.
You can plot the data for one run while reduction of the next is in progress.
If you use the hardcopy option a file with the name gks74.ps, (or gks74.ps.2, gks74.ps.3 etc for subsequent plots) will be generated. The ``Hard'' parameters page allows you to select different kinds of hardcopy plots. You can also produce hardcopy plots by using the Print... entry in the File menu of the plot window.
Having reduced a set of data there are two important checks you should make to ensure that the reduction has gone OK?
To check the arc reduction select the arc file in the `DATA' notebook page and plot the reduced file using the Plot button. The plot should show lines running straight up the image (see figure 8.3) as all the fibres have been scrunched onto the same wavelength scale. If the reduced arc doesn't look like this see section 8.17.6
All information about the progress of the data reduction is contained in the reduced files themselves. Thus if a data reduction session is interrupted (or the system crashes) drcontrol can be restarted, and the SETUP button used to pick up things exactly as they were. It will know which files have been reduced and be able to reduce any files which have not.
If you really want to start a data reduction session from scratch, redoing everything. Then delete all the *red.sdf and *tlm.sdf files and restart drcontrol.
The automatic data reduction depends on the use of a file naming convention in which the name consists of a root name which is the same for all files followed by a four digit integer run number. Raw data from the AAT conforms to this convention with names of the the form 13apr0001.sdf, 13apr0002.sdf etc. Data from the archive also conforms to the convention though the names are changed to run0001.fts etc.
It is possible to reduce indivdual files which do not conform to the naming convention. They can be loaded into the system using the Open... entry in the File menu or the Reduce entry in the Commands menu. However such files cannot form part of the automatic reduction of a sequence of files, unless they are renamed to conform with the other files in the sequence.
The data reduction system adds suffixes to the file names for the results of each stage in the reduction process. Thus the file run0001.sdf would result in the following files:
On start up the system creates a number of calibration groups. Each of these contain reduced calibration files of a certain type (e.g. BIAS, DARK, FLAT, ARC etc.). Whenever a calibration exposure is reduced it is inserted into the appropriate group.
At each stage in reduction when a calibration is required, the appropriate group is searched for a matching file (i.e. one with the same CCD and Spectrograph settings). If there is more than one matching file the closest in time is chosen. This works fine provided calibration files are reduced before those files that will need the calibration data. In automatic reduction the correct sequence is chosen based on the class of the file, to ensure that calibration data are available when needed.
You can use the Show History button in the DATA notebook page to find out which calibration files were used to reduce a selected file.
It is no longer necessary to precombine the offset sky files before running the data reduction system. When reducing an offset sky file the system automatically checks if it matches any previous files. If so all the matching files are combined and a throughput map derived from the combined file. This throughput map is stored with the reduced file. When calibrating an object frame the system will choose the matching throughput map combined from the largest number of frames.
The reduced file for the offset sky frames contain the combined throughput map, but the data array is only that for the single frame (this allows it to be combined with subsequent frames if necessary). The combined sky is contained in a temporary file with the name _COMBINED.sdf.
A crucial part of the reduction is the generation of a tram-line map which tracks the positions of the fibre spectra on the CCD. This is generated using an optical model for the spectrograph and a file listing the positions of the fibres on the slit. There is one of these files for each slit block (fibposa1.dat for spectrograph A, Field plate 1, and fibposa0.dat, fibposb1.dat fibposb0.dat for the others).
A new tram-line map is created from the first file to be reduced in each session. This is then used for all subsequent files with the same spectrograph setting. Although a tram-line map can be created from any file the best results will normally be obtained from a fibre flat field because of its high S/N. A twilight sky exposure, if available, would also be a good choice. If you are reducing a new set of data for the first time, the automatic reduction will always reduce flat-fields first.
To test whether the tram-line map is a good match to the data use the Plot Tram Map... entry in the Commands menu. This will plot a display of the tram-line map overlaid on the data it was generated from. The plot can be zoomed in or out with the Z or O keys on the keyboard (you need to zoom in several times to see anything useful). The P key pans to centre on the cursor position. The Q key quits the display and allows data reduction to continue.
It is also possible to plot the tram-line map overlaid on the data during the reduction by switching on the Plot Tram Map check box in the Extract notebook page. The plot gets put up twice during the course of reduction. Once with the initial tram line map, and a second time after adjusting the map in shift and rotation to match the data.
After zooming several times the plot should look like that in figure 8.2. If the tram-lines don't correctly overlay the data then there are a number of possibilities.
If it is not possible to get a good tram-line match to the data, it is likely that the fibre position file (which lists the positions of the fibres on the slit) is not valid, either because the slit assembly has been modified since the file was created, or perhaps just because a grating change has changed the positions of the fibres on the CCD sufficiently to alter their apparent spacings. There are four of these files named as follows:
|fibposa0.dat||Spectrograph A (1) - Field Plate 0|
|fibposa1.dat||Spectrograph A (1) - Field Plate 1|
|fibposb0.dat||Spectrograph B (2) - Field Plate 0|
|fibposb1.dat||Spectrograph B (2) - Field Plate 1|
A new fibre position file can be created from the data. Use a well exposed flat field covering all the fibres. Use the Find Fibres... command in the file menu. Select the raw data file for the flat field. The positions of the peaks will be located and used to generate a new fibre position file. A tram line map will then be generated from this file and a display of this overlaid on the data will be presented in the Diagnostic Plots window. Note that at this point the tram-line map will not have been corrected for rotation so may not overlay the data perfectly. This plot should be carefully examined to check that the fibre numbering looks correct (use the Z key to zoom the plot, and the Next and Previous buttons to step up and down the chip).
If any problems in fibre numbering are visible on the display fibres can be deleted or added interactively. To delete a fibre place the cursor on the fibre and press the D key on the keyboard. To add a fibre place the cursor on a point through which the tram line should pass and press the A key (it is advisable to have the plot highly zoomed for accuracy here).
If more than 200 fibres are found only the first 200 are plotted. Fibres beyond 200 can however still be deleted by placing the cursor at the expected position of the next fibre beyond 200 and using the D key. Alternatively deleting a fibre number below 200 will cause them to be plotted.
As fibres are added or deleted, the current number of fibres will be shown in the DREXEC message window. When you are happy with the fibre numbering (at this point there should be 200) click on the QUIT button in the plot window. If there are 200 entries at this point the fibre positions will be written to the appropriate file (fibposa0.dat etc.). If there are not 200 entries it will be written to a temporary file with the name fibpos_temp.dat.
Problems may arise if the data is incorrectly positioned on the CCD such that some fibres are off the chip area. In this case the number of fibres identified will be less than 200. In this event you should edit the fibpos_temp.dat file (it is just a text file listing the fibre positions) to add additional entries at one or other end to bring the total number to 200 and then rename it appropriately (e.g. to fibposa0.dat). It does not matter if these positions are off the chip. Do not attempt to run the system with a fibpos file which contains less than 200 entries.
The fibre extraction process uses the tram-line map and the image to extract the spectrum for each of the 200 fibres. The default method is a simple extraction (the TRAM option in the Extract parameters) which sums the pixels around the tram-line over a width slightly less than the spacing of the tram-lines.
An alternative method is the FIT option which does an optimal extraction based on fitting profiles determined from a flat field frame. This should give somewhat better S/N than the tram extraction, and should better handle the overlap between adjacent fibres, since the fit is done simultaneously to the two fibres on either side of the one being extracted. FIT extraction is much slower than TRAM extraction. A flat-field must be reduced using the FIT method before any other frame, to provide the profiles needed for the extraction.
If the data were taken with the spectrograph in the blaze-to-camera configuration the spectrum id reversed at this point to get wavelength in the conventional direction.
The software can perform a subtraction of background scattered light before doing the extraction. This is turned on using the Subtract Scattered Light option in the Extract notebook page. This option is recommended when FIT extraction is used, and is optional with TRAM extraction. The background is determined by fitting a function through the 'dead' fibres in the image. When a flat (class MFFFF) is reduced the list of dead fibres is taken from the header of the image, and each is checked to see if it has signal in it. The dead fibres actually found are written to a file (deadfibresa0.dat, deadfibresb0.dat etc. - one for each of the four slit assemblies). You can edit this text file if you want to add or remove and fibres. Once such a file is present in the directory it will be used for all subsequemt reductions.
The Plot Fits option turns on a plot of the fits to the data for selected columns for the background subtraction, and the fit extraction.
Cosmic rays are rejected during the extraction process on the basis of the spatial profile across the fibre. The spatial profile for a single wavelength channel is compared with the median profile over a block of pixels on either side of the current pixel. If it differs by more than a threshold value (the default is 20 sigma) the pixel is rejected and flagged as bad in the resulting spectrum.
In principle such a procedure based on only the spatial profile should be insensitive to the spectral structure of the data and there should be no danger of it mistaking a strong emission line for a cosmaic ray. However, in practice this is not true with 2dF data for two reasons.
In very high S/N data it may be necessary to increase the threshold further or turn off cosmic ray rejection entirely. For this reason cosmic ray rejection is automatically turned off when flat fields and arcs are extracted.
The fibre spectra are packed very close together on the detector. The design specification was that they would overlap at about the 1% level on the profile. However, the actual situation is worse than this, and in some early data it is much worse due to poor focus. Some data is also affected by halation giving extended wings on the profile. This means that there is often significant contamination of a fibre spectrum by light from the adjacent fibre. This is particularly bad when there is a bright object in the fibre adjacent to a fainter one. TRAM extraction can be seriously affected by contamination from adajacent fibres. The problem is much less serious with FIT extraction.
The data reduction system performs an approximate wavelength calibration using the information from the spectrograph optical model. It then refines this using data from an arc lamp exposure. The lines found in the arc lamp exposure are matched against a line list. Then a cubic fit is performed to the predicted and measured wavelengths of all the `good' lines for each fibre. A good line is one that is not a blend (i.e. there is no nearby line in the line list), is not too wide (which usually indicates saturation), and is not too weak. This fit is then used to refine the wavelengths and the arc spectrum is scrunched onto the new wavelength scale. If you plot a reduced arc the lines should be straight across all 200 fibres, and the wavelengths should be correct.
The details of the arc fit to each fibre are output to a text file with a name of the form arclistnnn.dat for run number nnn (e.g. arclist021.dat for the fit to arc spectra in run 21).
If you don't get a good wavelength calibration, the most likely reason is that the central wavelength specified in the header is not close enough to the true value for the software to correctly match the lines. You can check this by comparing the raw arc data with an arc map. Other things that could cause problems are incorrect grating information in the header.
When other files are reduced the calibration from the best matching arc exposure will be used to set the wavelength scale.
The 2dF includes CuAr, Helium and FeAr lamps which can be used in any combination. Wavelengths and approximate intensities of lines are contained in a number of files for the various combinations (e.g. cuar.arc for the CuAr lamp, hecuar.arc for the Helium, CuAr combination). If you have problems getting a good arc fit, it may be possible to improve things by editing the line list, removing lines which are causing problems. An arc file in your working directory will be used in preference to one of the same name in the standard 2dfdr directory.
The sequence of data reduction for an object file is as follows:
The fibre extraction stage also adds an approximate wavelength calibration to the data based on the spectrograph optical model. The end result of this stage is the extracted file which is indicated by an ex suffix. This file contains the data in the form of a 200 by 1024 array giving the data for the 200 fibres.
2dF data files contain two types of header information. A standard FITS header contains information on the telescope, CCD, spectrograph etc. However the information on the fibre configuration is contained in a special FIBRES extension in the NDF files. This contains information on the name, position, type etc. of the object allocated to each fibre. In addition reduced files contain an NDF History extension which describes the reduction history of the data. For example, which data reduction steps were applied to it, and which calibration files were used.
This header information can be accessed from the data reduction system using the Show Header, Show Fibres and Show History buttons. However, it is sometimes more convenient to access this information without having to run up the data reduction system. The command 2dfinfo has been provided for this purpose. It can be used in a number of ways as follows:
> 2dfinfo run0001.sdf fitslists the FITS header of run0001.sdf.
> 2dfinfo run0001 fibreslists the fibre configuration information for the same file (note that the .sdf extension is optional).
> 2dfinfo run0001red historylists the reduction history of the reduced file run0001red.sdf.
It is also possible to request information for an individual fibre or an indivdual fits keyword as follows:
> 2dfinfo run0001 fibre 37lists the information on fibre number 37.
> 2dfinfo run0001 fits_item GRATIDlists the value of the fits header item GRATID (grating ID).
The File menu contains only two active commands at present. The Open... command is used to open a data file and add it to the list of files accessible through the ``Data'' notebook page. Files opened in this way can be reduced or plotted manually, but do not form part of the list which will reduced automatically (These are loaded using the Setup button in the automatic reduction section).
The Exit command is used to exit from the data reduction system.
This is unlikely to be needed in normal use. The Tasks... option displays the status of the subtasks which the data reduction system uses to do its work. The Tcl Command... option allows a Tcl command to be entered directly and exists mostly for debugging purposes.
The commands menu contains the following commands:
This menu controls the enabling and disabling of balloon help information which is provided by default for most aspects of the user interface.
This section is used to control automatic data reduction. In the 2dF reduction system automatic reduction is taken to mean reduction of a sequence of files in one go, as opposed to reducing files indivdually.
The Setup button is used to load files into the list on which automatic reduction will be performed. As mentioned in section 8.17.1 these files must obey a naming convention. Once loaded the files are accessible through the `Data' notebook page. The number of files loaded, and the number which have been already reduced are indicated on the display.
The Start button starts automatic reduction. This will cause all files in the automatic reduction list which are not already reduced, to be reduced. The sequence of reduction is chosen according to the priority of the various data file classes to ensure that calibration files are reduced before the data that needs calibrating.
The Stop button stops automatic reduction after reduction of the current file has completed. If you want to stop immediately then use the stop button, followed by the abort button in the execution task window to abort the reduction currently in progress.
The `Data' page is selected by means of the Data tab on the notebook widget on the left of the screen. It can be used to select any file which is known to the system. Known files are all those loaded onto the automatic reduction list by means of the Setup button, as well as other files loaded into the system by means of the Open... or Reduce... menu entries.
Files can be selected by run number. Either step through the run numbers using the up and down arrow keys, or type a run number into the entry field. Alternatively select by file name using the File: section. If a file is opened which does not fit the standard naming convention then it may only be possible to select it by file name.
The class, status (whether the file is reduced or not), and the name of the reduced file (if any) are displayed for the selected file.
Buttons in the `Data' page provide the follwing operations on the selected file.
The other pages in the notebook are used to set parameters which control data reduction or display.
This contains a number of check buttons which can be used to turn off some stages of the data reduction. Subtract Bias Frame, Subtract Dark Frame, and Divide Image by Flat Field are all on by default, though they won't happen if no suitable calibration file is available. Divide by Fibre Flat Field is off by default. This is a possible way of flat-fielding the data using the normalized fibre flat field. However, it tends not to be very succesful as there is usually residual structure in the fibre flat field after the extraction process.
This section also contains the Verbose button. If this is turned off the number of messsages output during reduction is much reduced.
This section controls the method used when multiple reduced frames are combined. It applies to combination of reduced runs using the Combine Reduced Runs... menu command, or to the automatic combination of offset sky frames which occurs during normal reduction.
Adjust Continuum Level causes the continuum level to be adjusted for each fibre by subtracting the difference between a smoothed continuum and the median for the data being combined. This is on by default when combining object frames to allow for variations due to seeing, telescope tracking etc., but can optionally be turned off. The setting is ignored when combining offset sky frames. Adjustment is always turned off when combining offset skys to ensure that accidental alignements with stars get rejected in the combination.
The Rejection Threshold is the number of sigma deviation from the median of the combined frames at which a point is rejected.
The Smoothing Scale is the length of the median smoothing applied to get the smoothed continuum during the continuum adjsutment phase.
This section contains parameters controlling the fibre extraction and tram-line map generation process.
The Method menu selects the fibre extraction method. The TRAM method which performs a simple sum of the pixels is the default method. The FIT method performs optimal extraction and handles fibre overlap by simultaneously fitting profiles to overlapping fibres.
The Plot Tram Map option causes the tram-line map to be plotted overlayed on the data, during the reduction process, The plot is put up twice, both before and after a shift and rotation correction is applied to match the data. (See section 8.17.4).
The Rotate/Shift to Match option causes the tram-line map to be adjusted by means of a rotation and shift to match the data. It normally needs to be on since flexure in the spectrograph means that at least a shift correction is needed for each data frame. The matching operation can fail in a frame with very few fibres illuminated. In this case the Rotate/Shift to Match option can be turned off, but it will be necessary to use a tram-line map derived from a frame taken at the telescope position to avoid problems with flexure.
Use Default Correction causes the software to add a correction to the tram-line map derived from the ray tracing model of the spectrograph, based on the empirically derived difference between the model and typical actual data. This option should normally be on. The only reason for turning it off is when a new correction map is being derived.
Fit Tram Map to Data controls the final step of the derivation of a new tram-line map, in which a surface fit to the difference between the data and the tram-line map is applied as a final correction. Normally this option should be on, but sometimes low signal or noise in some part of the frame may mean that a poor fit is obtained in some regions. In this case better results may be obtained by turning this option off.
Reject Cosmic Rays controls whether cosmic rays are rejected during the extraction process. Cosmic ray rejection with the default threshold should work OK in most cases, but with very high S/N data it may result in rejection of real features. In this case it may be better to turn it off.
Subtract Scattered Light controls subtraction of background light from the data before extraction using a fit to the 'dead' fibre light levels. However, scattered light subtraction is not performed for sky frames (class MFSKY) unless the next option is also selected.
Subtract Scattered Light light from Offset Sky Frames turns on scattered light subtraction from sky frames as well as other frames. This is separately controllable, since offset sky frames usually have verly low scattered light levels and low signal levels, and there is a possibility that the scattered light subtraction process could introduce some excess noise.
Plot Fits turns on plotting of the selected fits to the data during the scattered light subtraction and fit extraction.
NSigma (for CR rejection) controls the number of sigma which a point has to deviate from the profile to be rejected as a cosmic ray. The default value of 20 is about the lowest value that is found to be reasonably safe, i.e. unlikely to reject anything which is real in typical data.
These parameters control sky subtraction. Either throughput calibration (based on the offset sky frames) or sky subtraction may be turned off. The Sky Fibre Combination Operation menu sets the operation used to combine the sky fibres in the data before subtracting.
The Plot Combined Sky option causes the sky spectrum from the combined sky fibres, to be plotted during each reduction. The Plot Throughput Map similarly causes the throughput map to be plotted during each reduction.
The Throughput Calibration Method provides two options for calibrating the fibre-to-fibre throughput method. The OFFSKY option is the default and determines the throughput based on the signal in the combined offset sky frames for the fibre configuration. The SKYLINE method is an alternative which can be used if no offset sky frames are available, which determines the throughput from the strength of sky lines in the actual data frame being calibrated.
These parameters control plots on the screen as a result of the Plot button or the Plot... menu entry. The 95% Scaling? option scales plots between a high and a low scaling level which exclude the top and bottom 2.5% of the data values. This is the default option, if it is turned off data is scaled between the minimum and maximum values.
The Plot Type options controls how image data is displayed. The options are COLOUR for a false colour plot, GREY for a greyscale plot and CONTOUR for a contour plot. Pixels per bin controls the binning of data displayed as spectra. The Remove Residual Sky option causes the strongest sky line at 5577Å to be removed from plots by interpolating across it.
These parameters control hardcopy plots. Some of them are the same as parameters in the screen plots section. The additional option is to plot data as multiple spectra with a number of spectra per page. For example to plot all 200 fibre spectra, 20 to a page, set the parameters as follows:
This section is also organized as a notebook widget, with two pages for the two execution tasks, DREXEC1 and DREXEC2. Every operation which involves accessing the data files is dispatched to one of these tasks for execution (except plotting operations which go to special plot tasks). Having two tasks means that more than one operation can be carried out at the same time. In principle it is possible to reduce two files at the same time, though this is not recommended as there may be conflicts with simultaneous access to the same files. However, the two tasks make it possible to do simple operations such as setting the class of a file, or viewing a FITS header while reduction of another file is preceding. If necessary more execution tasks will be loaded as they are needed.
Each execution task contains a message region in which messages from the task are displayed. There is also a progress bar in which the progress of data reduction is indicated and a description of the current step in the data reduction process.
The Abort button is used to abort reduction of the current file. It may not take effect immediately as the status of the button is only checked at intervals during the reduction process.
On startup the system creates two plot windows, one labelled `General Plots' and one labelled `Diagnostic Plots'. The diagnostic plots window is used for graphical output generated during the data reduction process. At present the only plot of the this type is that generated if the Plot Tram Map option is turned on. The `General Plots' window is used for graphical output resulting from the Plot button or the Plot... menu entry.
Some features of the plots can be controlled using the buttons to the left of the plot window. Other options are obtainable by placing the cursor over the plot and typing keys on the keyboard. The main options available in this way are as follows:
|X||Plot a cut through the image in X direction.|
|Y||Plot a cut through the image in Y direction.|
|Z||Zoom in by a factor of 2.|
|O||Zoom out by a factor of 2.|
|P||Center plot on cursor indicated position.|
|[ ]||Select a region which will be expanded to fill the display.|
|H||Set the high scaling level to the value of the point under the cursor.|
|L||Set the low scaling level to the value of the point under the cursor.|
In addition clicking the mouse button on a point causes the position and value of the point to be output in the message window at the bottom of the plot window.
If the file being displayed is a reduced multi-fibre image, then an X cut through the data (obtained with the X key) will be a plot of the fibre spectrum through the cursor position. It is then possible to step through the fibres using the Next and Prev buttons.
The size of the plot window can be changed to a number of different settings using the Size menu. A change in size will lose the current plot. The file has to be replotted in the new size plot window.
By default when a new file is plotted it will overwrite the one already in the plot window. To prevent a plot being overwritten click on the Lock check box at the lower left of the window. If a plot window is locked and a new file is plotted, then a new plot window will be created to receive it. There can be up to three `General Plot' windows at any one time.
The Print... command in the File menu of the plot window can be used to output the current screen display as a postscript file or send it directly to a printer. Note that the result will be output at the resolution it is displayed on the screen. You will get better quality hard copy from a larger plot window. Alternatively you can produce hardcopy output directly from a file using the Hardcopy Plot... command in the Commands menu of the main window.
Data taken before October 1997 will not contain all the header information needed for reduction. The necessary header items must be added as described below. If your data was taken from October 1997 onwards you should be able to ignore this section.
To be usable by the reduction system the following header items must be present.
|LAMBDAC||Central Wavelength (Angstroms)|
|GRATID||Grating ID (e.g. 300B, 1200V etc.)|
|GRATLPMM||Grating lines per mm|
|SPECTID||Spectrograph ID (A or B)|
|SOURCE||Spectrograph Source (``Plate 1'' or ``Plate 0'')|
In addition wavelength calibration lamp exposures need the name of the lamp in the FITS item LAMPNAME. This should be CuAr for the Copper/Argon lamp or Helium for the helium lamp.
If spectrograph information is not in the headers the missing items can be added using the Figaro FITSET command. A shell script to add these items could be set up as follows:
#!/bin/csh figaro fitset $1 LAMBDAC 5880.0 '"Central Wavelength"' fitset $1 GRATID 300B '"Grating ID"' fitset $1 GRATLPMM 300 '"Grating Lines per mm"' fitset $1 ORDER 1 '"Grating Order"' fitset $1 SPECTID A '"Spectrograph ID"' fitset $1 SOURCE '"Plate 1"' '"Spectrograph Source"'Then for the arc exposures you will have to set the additional LAMPNAME item which can also be done with the fitset command.
The other thing required in the files is the NDF_CLASS item which identifies to the data reduction system how to process the file. This is set up with the data reduction system itself.
For data taken from October 1997 onwards fibre information is included in the headers. The data reduction system will automatically know which fibres are sky fibres for example.
Data taken before October 1997 does not contain headers describing the information on each fibres. For these runs, a utility program called objlist is provided with the data reduction system to list this information in a useful form. When you run this program it generates two files:
Use the command:
objlist configuration.sds plate0.pivs
where configuration.sds should be replaced by your actual configuration file. Use plate0.pivs for field plate 0 and plate1.pivs for plate 1.
You can ignore this section if the correct run command setting (i.e. NORMAL, FLAT, SKY, ARC etc) in the 2df control task was used when you took your data. The 2dF data reduction system will use the RUNCMD item written into the header to determine the class of your files. If not you have to set the class as described below.
In order to reduce a data file the system has to know what type of data file it is. It does this using an NDF_CLASS extension stored within the file. Eventually these will be written into the files by the observing system, but currently they have to be added by hand. To set this up you need to be running the data reduction system. Use the SET_CLASS entry in the COMMANDS menu. This will bring up a file selction dialog to allow you to select a file, and then another dialog to allow you to set the class. The class should be set as follows:
|LFLAT||Long Slit Flat Fields|
|MFFFF||Multi-Fibre Flat Fields|
|MFOBJECT||Multi-Fibre Object Data|
|MFSKY||Offset sky or twilight sky|
|MFARC||Multi-Fibre Arc Frames|
Some early data taken with the 2dF shows some contamination of the spectra by artificial lights on the 2dF electonics and power supplies. Usually this just affects a few fibres on each image. The contamination takes the form of emission spectra from neon lamps (see figure 8.6) which show a series of emission lines mostly in the range from 6000 to 7000Å as well as broad emission features from light emitting diodes. Common wavelengths for the LED emissions are 6600Å (see figure 8.7) and 7100Å though others have been seen occasionally.
|(90% eff.)||(75% eff.)||(Å/mm)||(Å)|
|1200I||1||10000||2||4700-6000..||4350-6000..||21.8||0.8 (but see below)|
|1200J||1||12000||2||4700-6000..||4350-6000..||21.8||0.8 (but see below)|
The detector size is 24mm, so the wavelength coverage of a grating is 24 times the dispersion given in the above table (the dispersions allow for the order given in column 4). Note that for the low resolution gratings the wavelength coverage can be more than a factor of two in wavelength, and in this case it is not possible to use the full length of the CCD without some order overlap.
The above resolutions assume a FWHM of ~ 1.7 pixels, a quality of focus which is not yet achieved for both cameras over the whole CCD at the time of writing. A more realistic estimate at the time of writing is a FWHM of 2.5 pixels.
The dispersions above refer to the blaze to collimator configuration. We have limited experience of the blaze to camera configuration but that which we have (1200B, first order) suggests a gain in resolution of ~ 20%.
At the time of writing we have found considerable problems in use of 2nd order due to a slight mis-collimation of the spectrographs. Because of this 2nd order currently offers no resolution gain over first and we do not currently advise it's use.
The following tables give the S/N that should be achieved for a point source, or for an extended source (the latter is the figure given in parentheses). The figures are calculated for 10000s exposure time and are based on observations of faint Landolt standards in January 1997. For other settings a S/N calculator is available on the web ( Figures are given for high resolution (1200V grating) and low resolution (300B grating)
|V magnitude||1.0 arc sec seeing||2.0 arc sec seeing|
|18||54.8 (34.6)||38.0 (27.6)|
|20||17.0 (9.2)||10.4 (6.8)|
|22||3.6 (1.7)||2.0 (1.2)|
|V magnitude||1.0 arc sec seeing||2.0 arc sec seeing|
|19||55.8 (33.2)||36.9 (25.5)|
|21||14.6 (7.4)||8.5 (5.3)|
|23||2.7 (1.3)||1.5 (0.9)|