Interested readers are referred to the recent article by Lewis et al. 2002: ``The Anglo-Australian Observatory 2dF Facility" ADS or gzipped postscript (MNRAS, 333, 279), which gives the historical background to 2dF, the design philosophy, a full technical description, 2dF performance, and some of the science that has been done with 2dF. Many of the figures below are taken from this article.
2dF users should also see the 2dF WWW page for the most recent 2dF information, and to obtain 2dF software.
The 2dF system consists of the following main components:
The 2dF corrector is a 4 component system designed by Damien Jones based on an original concept by C.G. Wynne; see Figure 1.3. 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. The most serious image degradation is 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 1.4.
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 zenith distances (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 arcsec/mm in the centre to about 14.2 arcsec/mm at the edge. The corresponding change in focal ratio is from f/3.4 to f/3.7.
The ADC consists of two prismatic doublets (which are also the first two elements of the 2dF corrector), each made of a cemented pair BK7 and F2 lenses. These two lenses may be driven to any angle using stepper motors controlled by a SSX stepper motor controller or more usually from higher level software (a DRAMA ADC task running as part of the 2dF control system).
When observing away from the zenith the atmosphere acts to disperse the light from target objects into small spectra. The two elements are arranged so that their resultant dispersion vector is equal and opposite to the to the atmospheric dispersion.
By having two prisms of fixed dispersion able to rotate we can vary the resultant dispersion from zero (prisms opposed) to double the dispersion of the individual prisms (prisms aligned).
The atmosphere acts to disperse the blue light towards the horizon along the parallactic angle so the resultant dispersion vector of the ADC must point towards the zenith along the parallactic angle.
This means that in general the thick part of the prisms must point towards the zenith. So for a target field in the north west, the parallactic angle is shown on the mimic display pointing towards the zenith (in the south east relative to the field) and the dispersion vectors of the two ADC elements will be symmetrical about the parallactic angle. In real life the reference marks will point towards the northwest as they represent the thin part of the prism.
Similarly for a field in the southeast, the parallactic angle will be drawn in the northwest with the dispersion vectors drawn symmetrically either side of the parallactic angle. In real life the reference marks will be seen pointing to the southeast.
Figure 1.5 shows FPI star images with the ADC nulled, tracking, and anti-tracking.
The design of the 2dF positioner was driven by the requirement that fields may need to be reconfigured as frequently as once every hour 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 one hour. Also, in order to avoid unacceptable dead time, a double buffered arrangement has been adopted to allow the next field to be configured while observing the current one. Figure 1.6 shows a picture of the 2dF robot positioner.
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. Figure 1.7 shows a schematic of the tumbler unit, fieldplates, and fibre retractors.
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. Figure 1.8 shows a picture of one of the field plates. 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 1.9.
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; see Figure 1.10. Figure 1.11 shows a picture of the fibres on a fieldplate, and Figure 1.12 is a closeup of a few fibres on the fieldplate.
Each guide fibre bundle contains a central fibre surrounded by six more fibres in a hexagonal arrangement. The guide bundles feed an autoguiding TV camera which is used to acquire the field, and then to guide on the field. From 2002 onwards, we now have a true autoguider, which works very well. 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). See Figure 1.13 for a schematic of the guide bundle layout.
In May 2002, we began to implement plate rotation with 2dF. Here, the rotation of the plates can be controlled by the Night Assistant or Support Astronomer, by up to ± 0.5 deg. This allows any residual rotation to be taken out during field acquisition, and will help to some extent to counter the effects of differential refraction during long exposures.
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. Figure 1.14 shows the main components and optical layout of the 2dF spectrograph(s), while Figure 1.15 is a picture of one of the 2dF spectrographs.
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 Chapter 9, and here is a link to the AAO Gratings WWW page.
The detectors are 1024 × 1024 thinned Tektronix CCDs. With 200 spectra on each detector the spectra are positioned roughly 5 pixels apart. Both CCDs are now science-grade devices, with good cosmetics. Spectrograph #2 suffers from halation from an unknown source, and thus slightly higher levels of scattered light. The 2dF WWW page gives up-to-date information about 2dF and all its components.
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.
2dF is controlled by a distributed system involving a number of different types of computer systems; see Figure 2.1 for a `flowchart'.
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 DRAMA 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 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.
with thanks to Mike Irwin
This section describes what is involved in preparing an observing project for
2dF. The basic steps involved can be summarised as follows:
Step 3 is performed using the Configure program which is described in Section 5.
Successful use of 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. We believe that all of the major measuring machines (SuperCosmos, APM, USNO, the STScI PDS, and others) have more than adequate accuracy for 2dF purposes, i.e. relative positions can be measured to 0.3 arcsec or better across the two-degree field. However, special care still needs to be taken when doing astrometry, and the following considerations need to borne in mind.
The best currently available reference star catalogue with sufficient star density for Schmidt plate reduction is the Tycho-2 catalogue of Hog et al. (2000; A&A, 355, 27). This contains positions and proper motions of around 2.5 million astrometric reference stars on the TYCHO-HIPPARCOS system. All online, and any recent APM and SuperCosmos catalogues, are reduced using this TYCHO astrometric grid. This generally provides several hundred astrometric stars per Schmidt field down to V = 11 and greatly alleviates many of the earlier problems found with doing accurate astrometry on sky survey Schmidt plates. The Tycho-2 catalogue supersedes the Tycho-1 and ACT catalogues, as well as earlier catalogues such as the PPM (Positions and Proper Motions) catalogue of Roeser and Bastian. Systematic errors in the TYCHO+ACT catalogue are negligible at the few mas level, while the random errors in individual star positions are 30 mas at the mean epoch of the catalogue ( ~ 1990) degrading by 3 mas/yr due to proper motion errors.
Although all the recent data is reduced using the Tycho-2 catalogue, some older data, such as the Cosmos database, uses the SAO catalogue which is much less accurate (1.2 arc sec). If you want to reduce your own data on the new system make sure you use the Tycho-2 Catalogue.
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 0.1 to 0.3 arcsec as determined by comparing measurements of different plates of the same field. The accuracy degrades somewhat for the faintest objects on the plates, but can also be poor for bright objects where the images are large and saturated and show both strong diffraction spikes and large reflection halos. Unfortunately the astrometric 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 can cause problems, and there is some indication that such effects are present, particularly if the brightest available reference stars are used. To a large extent this problem has been reduced to acceptable levels with the latest Tycho-2 astometric catalogue, which reaches 1 magnitude fainter than the PPM catalogue. However, the relative positions of bright and faint targets can still be systematically different at the few 10ths of an arcsec level, due to the differences in intrinsic image profile with magnitude. This effect is likely to be machine-dependent, since it depends on both the effective scanning beam profile of the machine, the algorithms used to determine image centroids and perform background followong; and the astrometric reduction technique adopted. It also varies across Schmidt plates and between different plates, since it is partially dependent on such things as `seeing' and tracking. It is therefore advisable to use only the fainter reference stars.
Both APM and SuperCosmos have adopted a similar method for dealing with these problems using essentially the same algorithms for the whole measurement and reduction process.
Analysis of HST guide star catalogue data by Taft and colleagues (1990) revealed the existence of a pattern of systematic distortions in Schmidt plate astrometry, from measurements made using the STSci PDS machines, at a level of about 1 arcsec. This implies that better astrometry can be obtained using local fits to a small area of the plate, rather than using a standard global solution for the entire plate. Irwin (Working Group on Wide Field Imaging, newsletter 5) demonstrated that APM data from the first epoch Palomar sky survey plates (POSSI) shows a similar very stable distortion pattern, as do UKST plates measured with the APM, albeit at a much lower level. The distortion pattern is very stable and is automatically mapped out of the final astrometric product for the on-line catalogue data provided by both APM and SuperCosmos. However, it is worth noting that residual magnitude-dependent distortions do still remain after this operation at the 0.25 arcsec level for UKST plates and around 0.5 arcsec in the corners for POSSI plates, since the distortion pattern itself is also a function of magnitude.
Much of the magnitude-dependent systematic error is radial and appears to strongly follow the vignetting pattern. That is, up to a radius of 2.7 degrees from the plate centre there is little effect, beyond 2.7 degrees radius there is a strong, roughly linear increase in astometric distortion between the bright images and faint images, to as much as 1 arcsec at the corners. The onset of the effect is progressive and appears to be at around V = 14, where the diffraction spikes start to become visible. Fainter than this the effect is negligible, but by about V = 9 is can be as much as 1 arcsec in the corners.
This problem is what currently limits the delivered external accuracy of Schmidt plate data.
Second epoch Palomar sky survey data (POSSII) show a similar low level overall distortion pattern to the UKST plates and therefore if possible either UKST or POSSII plates should be used for 2dF astrometry. In any case some correction for the systematic distortions should be included in the astrometric reductions or judicious use of a local astrometric fit can alleviate the problem.
Another reason for using recent epoch material is to mitigate the effect of proper motion of the guide stars selected for 2dF use, relative to the target objects. 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 are 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 field to ensure all 4 bundles are allocatable. Now that plate rotation has been implemented with 2dF, it is recommended that each configuration have at least 2 guide stars near the edge of the field (to give a better handle on rotation corrections), which requires even higher numbers of candidate guide stars (40-50/field).
Many of the potential astrometric problems discussed above are minimized by choosing guide stars as faint as possible. 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 stellar magnitudes diverge from an accuracy of ~ 0.25 magnitudes at the faint limit due to plate variations in the internally derived magnitude calibration system.
Fiducial stars should not cover more than a 1 mag range, due to limited dynamic range on the guiding TV. A further safety measure is to use guide stars which are themselves part of the target set wherever possible; this works particularly well in the case of star clusters or Magellanic Cloud samples, since then all targets and fiducial stars have the same proper motion.
It is vital that the positions of fiducials should be checked by eye on Sky Survey plates or by downloading digitised maps of the region and checking to ensure that their positions have not been grossly corrupted by: faint halos, diffraction spikes, blending with companions etc... Halo effects are especially a problem for stars with V<12 when the positions have been derived from survey plate scans. Fiducials measured from CCD data may well be okay but can also suffer from charge bleeding and so on. Fiducials can also be checked using the Digital Sky Survey (DSS) CDRoms with the getimage program or by WWW access.
A standard format for the input (.fld) file used to describe a field has been adopted for 2dF on the 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 .fld 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 Configure program is used to perform the following main functions:
You may obtain Configure from the 2dF Software WWW page. Here you will find a link to the 2dF ftp site where you can download Configure; the compressed tar file contains instructions for installing Configure, and versions are available for Solaris and Linux. From this page you can get the latest 2dF fibre and astrometry files. Remember to get the astrometric files for the declinations that you'll be using; see the README file there for more information. The 2dfdr data reduction software may also be obtained from this site.
Before running Configure, it needs to know where the current fibre and astrometry information files are. This is done by typing: setenv CONFIG_FILES directory_where_files_are; e.g ``setenv CONFIG_FILES . '', if the files are in the current directory. At Epping, the files are stored in: /pub/2df/latest_config_files/mxx, where xx= declination (e.g. /pub/2df/latest_config_files/m30). At Coona, they are in: ~ 2dF/config/poscheck_xxxyy/mzz, where xxx=month, yy=year, and zz= declination (e.g. ~ 2dF/config/poscheck_may02/m30). At other sites, grab the latest files via ftp as described above. Realize that fields usually have to be reconfigured at the telescope, closer to the actual time of observing, since fibre and astrometry information can change on short notice (i.e. fibres break!).
The program is started with the command configure typed at the UNIX shell prompt. You will first be asked if you want 2dF or 6dF, answer `2dF'. This will bring up two windows. The control window contains a menu bar, status display and message region: see Figure 5.1. The other window will be used to display a graphical representation of the 2dF field configuration being generated: see Figure 5.3.
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 5.14 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...Allocations to output the file in text format. The output file produced by List...Unallocated Objects 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 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 continuously 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 5.10) and adjust the allocations of any fibres which cause clashes. However, this may not work very well (may find lots of collisions). An alternative is to change the fibre and button clearances, and the maximum pivot angle from their default values: in the Options menu, select `Expert' mode; then when you Allocate, you will find more options on the popup window; change `Fibre clearance' and `Button clearance' from 400 to 600 microns, and `Max non-radial pivot angle' to 12 degrees; then set the other Allocate parameters in the usual way. Now, when you check the allocation on the other plate, you will have many fewer collisions; the penalty is a slightly decreased allocation of objects (but only by 2-3%, even in very crowded fields). You should also find less collisions when you do the Check over HA Range (see next Section).
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 5.11). One should aim for at least 10-15 sky fibres for each spectrograph, i.e. a minimum of ~ 30 total.
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 a high percentage 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 (preceded by the mysterious `Generating Cone Tree' message), 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 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.
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; see Figure 5.2.
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 the 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 Section 5.11 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. To enable the full functionality of Check over Range of HA... `Expert' mode (found under the Commands menu) has to be chosen.
Occasionally 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 or to deallocate (park) them (see Section 5.11 below), 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. To unzoom, select `Normal' in the 'Zoom" menu item in the Configure control panel.
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.
configure supports a number of command line options:
There are also options that can be used to perform specific actions when invoked with the -f option:
One may wish to configure a field for both plates, for example to observe it continuously for 4 or more hours and hence reconfigure to allow for the Hour Angle effects. Usually, however, if you allocate on one plate using the default parameters, the allocation will fail dramatically on the other plate, with many collisions. A better way is to change the fibre and button clearances, and the maximum pivot angle from their default values: in the Options menu, select `Expert' mode; then when you Allocate, you will find more options on the popup window; change `Fibre clearance' and `Button clearance' from 400 to 600 microns, and `Max non-radial pivot angle' to 12 degrees; then set the other Allocate parameters in the usual way. Now, when you check the allocation on the other plate, you will have many fewer collisions; the penalty is a slightly decreased allocation of objects (but only by 2-3%, even in very crowded fields). You should also find less collisions when you do the Check over HA Range
When allocating in crowded fields (e.g. Galactic globular clusters), one can usually obtain significant increases in the number of allocated objects via the following procedure:
The 2dF system should only be started up and shut down by AAO technical staff and support astronomers, who should refer to the `2dF Survival Guide' for details of how to do this. Any `serious-looking' problems should only be dealt with by these people as well.
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 6.1. 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 6.2.
The top section of the display in Figure 6.2 shows that the plate in the configuring position (Config Plate) is currently field plate 1. This means that plate 0 is in the observing position. The last configuration file which each plate was set up with is shown below this. 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 (a survey is now done automatically when a field is setup), 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, or for a given local time. If you turn off tweaking (though why you would want to do this, we can't imagine), the XY values in the configuration file will be used (which may have been calculated using an old model). It's hard to imagine a situation where you would want to disable tweaking.
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 6.3. 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 (useful for doing standard stars), 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: choose Observation Plate File or Config Plate File, then Load position from file to input fields, and finally Commence Slew and Track. 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.
Check with Night Assistant before moving telescope!
The CCD control window is shown in Figure 6.4. The CCD control works by sending commands to the standard CCD Observer system running on the VAX. 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 will ensure that the correct header information is written to the data files, allowing 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. Since the 2dF data reduction software relies on correct header information, it's very important to enter the correct run type; see the sections below for more information. The options for recording data are as follows:
Section 7.8 explains in more detail about actually taking data and calibration frames.
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 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. Figure 6.5 shows the ADC control window.
Autoguiding is done by the Night Assistant on another console. Hence, the autoguider control on the 2dF Control Task is currently not implemented.
The spectrograph control window is shown in Figure 6.6. The window provides a display of the status of the two spectrographs. It also provides control of the following functions:
This section still under construction
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 field acquisition as described in Chapter 7. The FPI control window is shown in Figure 6.7. The FPI has the following functions:
Menu Items: File, Commands, Imager, Options
To be done
Camera Control (left-hand side):
to be done
A typical observing session with 2dF is likely to involve the following steps:
`Centering a star on the FPI camera' is a fundamental operation for acquiring objects with 2dF, and warrants its own section here:
Is it cloudy?
If not, the usual cause of target field acquisition problems are obscuration in the optical path, namely:
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. The telescope focus is not fully temperature-compensated (the metal structure of 2dF in particular), so it's adviseable to check the focus each night during twilight if there is time, especially if the seeing becomes exceptionally good. It is especially important to check the focus if there is a large temperature change. This is done by the following procedure:
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 fully commissioned (hopefully by the end of 2002), 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. In practice, we usually do poschecks for DEC= -5, -30, -50, and -70 degrees.
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 ...
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:
Usually the field is acquired straightaway by the Night Assistant on the TV, who will then tweak it in. However, if this doesn't happen, here's what to do:
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 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. Runs can be done in single or count modes. Finally, exposures can be done on both CCDs (the usual) or on either of the CCDs singly.
The following types of 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. Bias frames are used by the 2dfdr data reduction software if available, but are not required to reduce the data since 2dfdr can also do bias-subtraction using the chip overscan region (this latter option is the default, and is usually adequate).
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. Again, these are used by 2dfdr if available, but are not required for data reduction. Note that the dome must be dark when taking DARK frames, as the spectrographs are not absolutely light-tight.
It is intended that eventually 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:
You can also click to whether to leave the flaps closed after the exposure; this is useful if you are doing another calibration frame (e.g. ARC) immediately after, as no time is wasted opening and closing the flaps.
Wavelength calibration (or ARC) frames are taken using the lamps in the calibration unit. These illuminate the flaps below the corrector. There are four Copper-Argon, two Copper-Helium, and two Iron-Argon 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. The default is 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.
There are other options however:
Here we discuss observing standard stars through individual fibres, for instance to observe radial velocity and spectrophotometric standard stars. You should allow ~ 15 minutes to observe one standard in both spectrographs. The observer has to supply the 2dF SA with star positions in J2000 coordinates.
The basic idea is to centre a standard star in a guide fibre, then use a blind offset to put the standard star down a spectroscopic fibre. For this reason the spectroscopic fibre should be as close as possible to the guide fibre (within 20mm (5 arcmin) if possible, for best results; the default is 50mm (12.5 arcmin), but this is a little large).
This process is a bit cumbersome but works well. An offsets calculator is available and is now built into the 2dF control system. You will require the position (RA,Dec in J2000.0) of the standard star or stars that you wish to observe.
Two basic procedures are possible:
The former procedure is advised if you only want to observe one standard star at a time, the latter procedure is much faster if you want to observe several standard stars say at the end of the night.
(1) Using an existing field configuration
The basic procedure is to centre a standard star in a guide fibre then use a blind offset to put the standard star down a spectroscopic fibre. For this reason the spectroscopic fibre should be as close as possible to the guide fibre.
