This Chapter provides more details on the hardware specific to the Prime Focus imaging systems. Details of how to run the OBSERVER control system are given in §4 - The Imaging Cookbook.
Introduction The Telescope & Optics The Detectors
The Imaging Cameras An Imaging Cookbook The Data you Take Away
Exposure times OFFSET_RUN files CCD Windows Data Catalogs
On-line Reduction Filters Flat-fields Blank Fields Orientation Shutters
Currently, Prime Focus f/3.3 imaging is usually carried out using the Doublet Corrector (see §1.2 The Prime Focus Optics, and §1.2.2 The Doublet for more details on the optical train) and the TEK 1024x1024 thinned CCD (see §2.1 The TEKTRONIX CCD for more details) mounted on a CCD adaptor plate attached to the photographic camera head.
In this sense the current imaging system is somewhat of a hybrid : guiding is carried out in exactly the same manner as for photographic imaging; a Uniblitz 63(?)mm shutter attached to the CCD adaptor plate is used to expose the CCD; and, a range of mannually operated filter wheels have been constructed to fit into the space normally occupied by a single photographic filter. Currently, both the guide probe head, and the filter wheel must be operated manually, requiring the presence of an observer in the Prime Focus cage for all but the simplest operations.
Moreover, the CCD is controlled from terminals in the Console Room, so TWO OBSEVERS must normally be present for all Prime Focus imaging runs.
The `cage' is in fact a solid-walled, slightly oval-sectioned cylinder, approximately 0.72m in diameter and 2m deep, mounted on its own interchangeable top end. The cage is open ended so the observer can enjoy the sky. The most obvious feature within the cage is the prime focus (PF) camera head and pedestal, (see Fig. 3.1) mounted centrally, and a chair mounted on the wall. Less apparent are several important switches and controls which are also mounted on the wall, which are identified in this section.
The Prime Focus Cage is accessed by slewing the telescope to `Prime Focus Access' , which is at a zenith distance of about 77° in the north and on the meridian. In this position the cage can be accessed from a landing which is reached by climbing the two flights of stairs directly opposite the lift on the 6th (console) floor.
Figure 3.1: The Prime Focus (PF) Cage at the PF Access Position. Note the TEK CCD in its cylindrical orange dewar, which is mounted on the Camera Head in the centre of the cage. A larger version (232K) of this image, with annotations, is available here, or by clicking on the thumbnail. A detail of the camera head is shown in Fig. 3.3.
This aspect of observing at prime focus can cause problems for the unprepared.
Because the telescope can be pointed at almost an orientation, the cage
is designed to rotate, so that an observer can always be at least moderately
comfortable sitting (or in extremes cases lying) with her back in the direction
of gravity. The camera head itself, of course, does not rotate - only the
walls and floor and everything attached to it. It is remarkably easy to
become disoriented in semi-darkness as the telescope slews, the dome rotates,
and the cage can be in motion, while the only other obvious reference point,
the camera head, can be in any one of four positions.
When the telescope is slewed to the PF access platform where the observer can climb aboard it has a ZD of about 77° degrees and is pointed towards the north. The chair is normally on the north side of the cage with its backrest at the lowest point on the cage wall. This is the `park' position. Slip inside the cage and ask for the telescope to be driven to zenith. On the right-hand wall of the cage is a handset with three buttons marked +, PARK and -. The + button drives the cage clockwise from the northern PARK position around the optical axis through east and south into the west, 270° from the park position, BUT NOT BEYOND. The - button drives the cage a little more than 90° into the west from park, but again, not beyond. The PARK button returns the cage to its northern parked location from any position.
NOTE : The telescope will normally slew from pointing in the south west to PF access directly; the cage however must move through S and E on its way to the northern park position, an operation which takes much longer than the telescope slew. It is normal and safe practice to ask the night assistant to slew to prime focus access from the south west or west via the zenith.
The cage can be rotated during an exposure without seriously upsetting telescope guiding or tracking, but as a precaution the shutter should be closed during rotation. The height of the chair above the cage floor can be adjusted by a short lever on the left of the squab. The angle of the squab itself is adjustable by a wheel on its right; however be careful to avoid rapid movements in the cage as these can be detected on the autoguider readout and may affect image quality.
A handset with a 4-position selector switch and 4 buttons is attached
to the cage wall, though it can be hand-held if required. The selector
switch can be set to SET, GUIDE, OFFSET, FOCUS.
Appropriate rates for SET, and GUIDE and angles for OFFSET are loaded into the telescope control computer from the control console by the night assistant, and by pressing the buttons the observer can move the telescope in the selected mode. The relative orientation of the N, S, E and W buttons is appropriate for a direct view of the sky. When viewing the field through the prime focus guide eyepiece, there is an odd number of reflections, so the view is reversed. If manual guiding is required, it may be helpful to hold the handset with its face away from you so the sense of the buttons is consistent with the movement they actuate.
In the FOCUS position TWO buttons must be pushed simultaneously to move telescope focus in or out, fast or slow. To focus, the observer must request that the night assistant sets the console buttons to FOCUS MANUAL.
When the PF top end has been fitted to the telescope the following must be checked either by staff members or by the observer. The items to be checked by the observer are marked with an asterisk. Some of the items in this list are described in detail in subsequent sections.
It is also prudent to know where the following are stored:
The camera head carries all the controls necessary for setting up on a field, including the autoguider control panel and guide probe. A schematic diagram of the camera head controls is shown in Figure 3.2. Figure 3.3 is a colour photograph of the camera head with the TEK CCD attached. It has been annotated to point out the major features shown in Figure 3.2.
Fig. 3.2 The controls on the prime focus camera head. The small carriage with the autoguider eyepiece and optical adjustments is attached to the probe and moves in the X direction as the X position is shifted. You will observe most comfortably when this carriage is in front of you, or to your right or left.
Fig. 3.3 A colour photograph of the camera head with the TEK CCD mounted. The small carriage with the autoguider eyepiece and optical adjustments is attached to the probe and moves in the X direction as the X position is shifted. You will observe most comfortably when this carriage is in front of you, or to your right or left. Note that this image has been rotated 90° clockwise relative to Fig 3.1, in order to put it in the same orientation as Figure 3.2. An image twice as large (128K) is available here.
The CCD sits inside its cylindrical orange-red dewar, which is mounted on top of the black CCD adaptor plate. The Uniblitz shutter is mounted in the circular orange- and blue-anodised plates which mount between the black adaptor plate and the orange-red dewar. Dry nitrogen is continually flushed across the the dewar window during the night to stop the window misting up.
The camera head can be rotated around the optical axis to any of the four cardinal points where it is retained by a spring-loaded latch. The rotation is limited to a single 270 degree range because of cable wrap constraints. The camera head is moved by swinging the clamp lever to UNCLAMP and then pressing the INDEX LATCH RELEASE (marked PRESS) while rotating the camera head. The latch drops every 90 degrees. Whenever the camera head is unclamped a red warning UNCLAMP light will show on the camera head - the autoguider will not work with the camera unclamped. A four-quadrant `Camera Head Orientation' indicator shows the real (i.e. sky) orientation of the guide probe with respect to the optical axis. For example, the camera head might be rotated so that the probe and its eypiece are near the parked chair, which parks in the north. The quadrant indicator will show S (south) because it refers to the image of the sky reflected by the primary mirror. A sifficiently large CCD image, taken in this configuration, would have the shadow of the guide probe along its southern edge.
The ideal observing position is one with the autoguider eyepiece in front of you. It is also perfectly possible to work with the eyepiece to the right or left (depending on your handedness). If the eyepiece is across the CCD dewar from the observer it can be difficult to reach especially at large zenith distances, though a specially extended (and seldom used) eyepiece tube is available for this contingency. This position is further complicated by the photographic calibrator lamp housing which sticks out of the camera at crotch level.
So the most comfortable position in which to have the camera head is usually determined by where in the sky most of your objects will be. If they will be mostly in the North, the camera head will be most comfortable in the `SOUTH' orientation, and visa versa. Similarly for objects in the East, the camera head will be most comfortable in the `WEST' orientation.
There is an added complication, however. The TEK CCD has an offset filler tube (the white frosted 'blob' on the top of the dewar in Fig.3.3). This means that there is a preferred orientation in which to fill the CCD at PF access - the preferred orientation being that in which the CCD's electronics box (the grey slotted box on the left of the orange-red dewar in Fig.3.3) is on the North side of the dewar (ie. pointing down towards the floor at PF access). If you will be observing objects predominantly in the North, you should ask the AAT technical staff to mount the CCD with the electronics box on the NORTH side of the dewar, when the Camera Head is in position `SOUTH'. (The usual default is to mount the CCD with the electronics box on the NORTH side of the dewar, when the Camera Head is in position `NORTH'.) It is desirable to decide on an orientation before your run starts, and then leave the Camera Head untouched throughout your observations, otherwise you may not be able to flatten your data!
Lastly, the observer in the cage must be beware of the TEK CCD's offset filler tube - it is possible for the telescope to be slewed to a location in which liquid nitrogen will pour onto the observer's crotch region. It is generally a good idea to rotate the cage so as to put yourself `under' the CCD, after large slews are finished.
The most important functions on the camera head are the guide-probe
controls and the filter wheel/slide system, both of which are discussed
below. A few of the remaining controls should be mentioned however.
The PF observer has available three ways of preventing light reaching the detector.
Obviously, the first two must be open before normal CCD observations can be carried out. If you find you aren't getting any light through, this is an obvious thing to check first.
A probe ranges over an area approximately 60 x 120mm (Y x X) (16 x 32 arc min) of the focal plane to one side of the field. Two interchangeable probes (``short'' and ``long'') double the area of field available, in the Y direction. The short probe is designed for use with the triplet and the long with the doublet. At the cost of some vignetting of the guide star image the short probe can be used with the doublet with no intrusion of the probe shadow into the unvignetted field and in practice the short probe is used for both doublet and triplet. If no suitable guide stars are found in the area available to the probe then the camera may be rotated, possibly at the cost of some inconvenience in reaching the eyepiece. Alternatively, if your astronomical target is sufficently small (compared to the CCD field of view), the telescope can be moved to bring your guide star into the guide probe's field of view.
The range of focus in the guiding optics is not sufficient to cover all possible applications of the prime focus camera - observers with very thick filters (~10 mm or greater) should therefore consult with AAT staff about inserting a weak lens into the guide-probe relay to allow the probe to be focussed. These lenses are kept in the prime focus optics box which is normally at PF access.
There are two ways of finding stars which are suitable for
i.e. brighter than about 14th mag.
Because of the excellent performance of the AAT, short exposures (5-10 minutes) are possible without using the autoguider. Therefore, for programs which are sky limited in this length of time (in fact most broadband observations), observers can seriously consider not guiding at all. Unfortunately, this can impose a heavy data reduction load later on, if many images have to be stacked - on the other hand the acquistion of guide stars will waste some fraction of the available time. The decision as to whether to guide or not is one which has to be made on a program-by-program basis.
Once a selected guide star is visible in the autoguider eyepiece it can be centered by the X and Y guide probe knobs. The star image, which might at this stage be out of focus, is seen together with an illuminated graticule consisting of two pairs of crossed lines. The central square that these form is 2 arc sec on a side when the probe is used without an auxiliary lens. This square can be used to estimate the seeing.
To set up the optical system, first select one of three available eyepieces and focus it on to the graticule by rotating the upper smaller knurled ring. Then focus the guide star on to the graticule by rotating the LENS FOCUS knob on the guide panel. The autoguider optics are confocal with the graticule, which is illuminated by a green light whose brightness is adjustable. Do not move the LENS FOCUS knob once guiding has begun - there may be some shift of position.
A labelled three-position lever-switch operates a prism assembly to direct the guide star light to the guide probe eyepiece (labelled EYEPIECE), to the autoguider (labelled PM TUBE) or to a beamsplitter (labelled BOTH) which splits the light roughly 90% to the eyepiece, 10% to autoguider. The BOTH position is preferred if the guide star is bright enough, though this is not often the case. The 3 prism positions do not exactly coincide in positioning a star image on the graticule or autoguiding CCD, therefore the prism position must not be changed during an exposure.
NOTE: The following discussion of the autoguider system is somewhat out-of-date. The guide head now carries a commercial ST4 CCD camera, which is used to derive guiding corrections for the telescope. Acquisition of guide stars is carried out as before, however now the night assistant controls guiding from a PC. Searching for guide stars is only necessary if the guide star falls outside the ??' x ??' field of view of the ST4 camera, in which case the observer will have to move the guide star into the cross hairs manually. There is no equivalent of the `STAR-TO-PROBE' mode, with the ST4. A more detailed description of the use of the ST4 control software is in preperation.
PMT specific notes : The autoguider, when so commanded by the night assistant, searches an area 12 arc sec square in a raster scan. It senses the position of the brightest star and proceeds to guide on this star by scanning in a cross pattern 6 arc secs by 6 arc secs over the star image. The autoguider gives a display to the console operator of the seeing profile of the guide star.
The astronomer must be aware of the two ways in which the guide star can be acquired by the autoguider's image dissector. In the PROBE-TO-STAR mode the telescope position at the time guiding began is taken to be correct, and the guider then maintains the guide star at its initial, arbitrary position. Once guiding has occurred, as the guide star drifts with respect to the telescope, the telescope will be moved so the guide star will be brought back to the initial position until OFF is selected by the operator, at which time the arbitrary position is lost. Any subsequent guiding will then, of course, begin by defining a new position corresponding to the telescope orientation during the first few seconds after re-acquisition. Temporary loss of the guide star due to cloud will not cause the guiding position to be lost unless OFF is selected.
In some cases - where the telescope has been accurately set to some particular position ( e.g. with the program star centred in an aperture) - the PROBE-TO-STAR mode must be used to maintain the telescope position as initially set.
The second method of acquiring a guide star, the STAR-TO-PROBE mode will cause the autoguider to shift the telescope a few arc secs as it acquires the guide star and brings it to a standard position on the photocathode. This nominally corresponds to the centre of the guiding eyepiece graticule but may in practice not do so precisely. Where a choice is possible, e.g. during direct photography, the STAR-TO-PROBE mode is preferred as there is no risk of double exposure if the guiding position is lost due to malfunction or misoperation. If the autoguider should fail during an exposure it is possible to continue by guiding manually without risk of losing position if the operation is started in the STAR-TO-PROBE mode.
There is also a `hold' facility called FREEZE/THAW. The night assistant selects FREEZE on the control panel which stops the autoguider from inserting corrections but does not exit from the guiding loop. Selecting THAW will resume guiding instantly, without the need to acquire the star again. This is a very useful mode when there is cloud around, or for speeding up re-acquisition when doing remote exposures. (See section 3.7). The autoguider is described in more detail in the AAO Technical Manual (TM 7) covering the telescope control system. XXXX
To use autoguider:
|Position||Colour Filter (Knob C)||ND Filter (Knob D)|
|1||Clear 350-750nm (2mm WG280)||Clear|
|2||Blue ~400nm (1mm BG25 + 1mm BG39)||ND=0.7|
|3||Green ~ 550nm (1mm GG495 + 1mm BG39)||ND=1.4|
|4||Red ~ 630nm (2mm RG630)||ND=2.0|
It is easy for the filters (either ND or colour) to get stuck `half-way' between their positions. If you do not plan on using the filters, ensure that both are set to position `1'.
The observer can confirm that the autoguider works by offsetting the telescope about 5 arc sec with the guiding handset and with light splitting prism set to BOTH. Watch the autoguider return the star to its original position.
The CCD filter wheels are rotated manually, and fit into a modified photographic filter slide. These slide into the camera head, underneath the CCD adaptor plate. The handle for the filter slide can be seen projecting from the top of the Camera Head in Fig.3.3. The filter slide locks into place in the head once it is inserted - it can be released by pulling on the `Filter Slide Release' lever, which is hidden in Fig.3.3, but shown in Fig.3.2. The filter slides themselves can be seen in Figure 3.4 and Figure 3.5 below.
|Figure 3.4 - The Prime Focus Filter Wheels and Slides|
|Figure 3.5 - The Six Position (57mm) Filter Wheel installed in its slide. The wheel can be replaced in this slide one of two Three position wheels to suit 83 or 75mm diameter filters.|
Insertion of the filter slide can be tricky. The mechanism has aluminium sliding on aluminium, which tends to stick. It is also easy to inserted the filter slide upside down, or to put it in not quite straight. Be patient, and DON'T attemp to force the slide in. The CCD adaptor plate and the filter wheel slide are also very light-leaky. Once the filter wheel is inserted it should be taped up with black tape.
The filter actually used for observing, once the wheel/slide arrangment
is inserted, is selected by a notched thumbwheel which is accessed inside
the slide's handle. Each position on the wheel has a detent, and each position
has a unique number of large notches on the thumbwheel so that the correct
position can be located in the dark. Three wheels are available:
A blank slide is also available for holding a single filter, which can be up to 160mm in diameter. It is possible to change quickly between the blank slide, and one of the filter wheels - but not between two wheels. Smaller filters will need specific adaptors. The minimum filter diameter is ??mm to avoid vignetting the TEK chip
The filter wheels are stored in a cupboard in the prime focus darkroom on the sixth floor of the AAT dome. Filters and other useful bits and pieces are also kept there.
Appendix 6 lists the filters available at AAO for CCD imaging, and also gives transmission curves for the most commonly-used filters. because of their high throughput, and rectangular band profiles, the KPNO interference filters are generally used for broad-band photometry - though their response is not well matched to the photoelectric BVRI bands. Appendix 1 gives some colour term data for these filters.
The telescope is usually focussed by obtaining a series of exposures at different focus values. These are then inspected and the best focus value chosen. The usual way to do this is via a MULTIPLE exposure (ie one where several exposures are taken, without reading out the CCD, the telescope and focus being stepped in between each exposure). This procedure is described in §4 - The Imaging Cookbook.
If the filters used are of different optical thickness, the focus must be adjusted for each filter. To a first approximation, the focus setting must be increased by the increase in effective optical thickness of the filter (bearing in mind that the focus readout value increases as the top end is raised). However, because the corrector and plateholder are moved by the focus drive, and the correctors have some power as lenses, the adjustment for true focus differs slightly, but significantly, from that implied by the optical thickness.
It should be unnecessary to focus through each filter individually, since optical thicknesses can be measured beforehand. It is quite adequate to assume:
FP(1) = FP(0) + 0.33 * d * k,
where FP(1) is the telescope focus position for a filter, FP(0) is the position for no filter, d is the physical thickness of a glass filter, k is 1.04 for the doublet corrector, and 0.90 for the triplet.
In practise, the focus offsets between the various AAO filters are well known, and it is only necessary to determine the focus through one filter. Observers bringing their own filters will usually want to check focus using a focus sequence - however, it is helpful to estimate the likely focus, so that it can be used as a starting point for a sequence
The telescope focus is continually adjusted for changes in the temperature and attitude of the telescope's steel structure, by comparing the distance between the primary mirror cell and the fixed top ring against a tensioned invar wire. In respose to changes in this distance, computer-derived corrections are made to the focus drive which moves the whole top end structure. The corrections applied are up to about 0.5mm with changing attitude and about 0.1mm/°C.
When the CCD dewar is topped up with nitrogen at the prime focus access position, it can only be filled to about 60% capacity and so should be re-filled every eight hours (or every twelve hours if the power is off.)
to be written
The f/1 focal reducer converts the f/3.3 prime focus beam of the AAT to f/1, and consists of a hyperbolic mirror and three BK-7 glass lenses optimised for imaging in the 5000-9000Å region. Large off-axis chromatic aberrations are seen at redder and bluer wavelengths. More detailed optical information on the f/1 system can be found in §1.4 Sample flat fields (which show the vignetting of the system) can be found in Appendix 7.
The f/1 system has a filter holder which accepts up to three 50.6mm diameter circular filters, and V, R, I and V+R filters are currently provided. Filter specifications, along with transmission curves, are given in Appendix B. There is also provision for a single filter to be placed below the shutter.
Filter selection is via a small metal box located in the AAT control room. The box has a switch with four positions (generally V, R, I and CLEAR). There are also three meters which provide a check on the position of the three filter arms, and two sets of shutter lights which indicate whether the shutter is open or closed (one set for each half of the two-part shutter).
At present, the filters are not encoded, so you should be very careful that you have selected the filter you want, and that it is recorded correctly in the data log. It is a good idea to incorporate the filter name into the header of each of your observations (by typing e.g. OBJ NGC 123 R as the object name in OBSERVER).
Auto-guiding is not available with the f/1 imaging system. This means exposures are limited to 5-10 minutes, over which time tracking errors are only noticeable on the AAT in exceptional seeing. However, even at new moon in the VRI passbands in which the LBL camera operates, a 600s expsure will get 4000-20000 adu/pixel from the sky - so longer exposures are not possible in any case.
Because the two-part shutter of the f/1 system is large and takes a finite time to open and close, very short exposures are not recommended. Shutter timing tests using dome flat fields show the timing over the whole frame is accurate to better than 0.5% over the whole frame for a 5s exposure. For exposures of 3 seconds or less, flat fields show a noticeable brightening in the centre relative to the outer part of the frame, due to the finite opening/closing time of the shutter.
The minimum recommended exposure time is 5 seconds, but observers who plan to do accurate photometry of extended objects at low light levels might prefer to use minimum exposures of at least 10 seconds and should make sure that their flat field exposures (if done on twilight or dark sky) are also at least this long.
The usual way to do this is via a MULTIPLE exposure (ie one where several exposures are taken, without reading out the CCD, the telescope and focus being stepped in between each exposure). This procedure is described in §5 - The Imaging Cookbook. In the standard V,R,I filter set provided for the f/1 imager, all three filters have the same thickness and should have the same focus. Thus it is only necessary to do a full focus sequence for one of the filters.
The Thomson CCD dewar has only a short hold time, and will need refilling every six hours. Normally this will be done by the night assistant or daytime technical staff, but support astronomers and observers may be required to lend a hand at weekends, when the afternoon staff do not start work until 2pm.
To fill the dewar, move the telescope to prime focus access and connect the liquid line of the grey liquid nitrogen dewar to the thick black hose which emerges through the floor of the prime focus cage. The clear plastic nitrogen line should also be connected to the dewar (via a snap-on connector). Once the liquid nitrogen dewar is pressurized to about 10psi, open both valves to allow nitrogen to flow to the CCD cryostat. This will take about 10 minutes to fill - when liquid nitrogen starts to spill noisily from the CCD end it is time to stop. Close both valves and disconnect the grey dewar (using the hairdryer to unfreeze the frosted-up connector if necessary).
A simple `no frills' adaptor has been built to mount one of the AAO CCD cameras with a manually-operated filter wheel at the Cassegrain auxiliary focus. Since it is possible to swap between the main and auxiliary foci at the push of a button (in ~20 seconds), this is useful for programs requiring both spectroscopy and `quick look' imaging, and also enables the best use to be made of periods of photometric weather. Since the filter wheel is not remotely controlled, the present system is best suited to programs requiring photometry through only one filter (which can be any of the standard UBVRI 2 inch square filters).
For imaging requiring more than one filter at the Cassegrain focus, it is necessary to have an observer riding in the Cassegrain cage.
In practise, there is no advantage to imaging for its own sake at Cassegrain over Prime. Cassegrain focus is now only used for CCD imaging in conjunction with another Cassegrain instrument - usually the RGO spectrograph.
The image scale is quite large, so binning of the Thomson CCD (the usual device used when the TEK is otherwise occupied at the RGO focus) is worthwhile.
In recent years, the progressive decline in the sensitivity of our TV acquisition system, together with the demands of observers to put fainter and fainter targets down the RGO slit, has forced us to push our science CCDs into use for acquisition.
In practise this means a CCD (currently the Blue Thomson) is mounted at the Auxiliary focus for all RGO spectrograph runs. A push of a button on the control panel can then allow imaging of faint (22-24th magnitude) targets for acquisition.
To use the Auxiliary CCD in this mode you need to know
Now to start doing some science. Slew to your target and switch to Auxiliary imaging. Take a picture, and find your target. Now what we want to do is work out the RA,DEC offset to move the telescope which will put that object at our slit X,Y position.
Luckily there is a tool to help you here ... QIKLOOK. QIKLOOK is a simple GUI tool to do quick look reduction. Start it up, and then click on the 'OFFSET' action button. You'll get a pop up like so
|Focus Offset : RGO Slit - Aux+CCD+KPNO I = -6.7 mm|
Introduction The Telescope & Optics The Detectors
The Imaging Cameras An Imaging Cookbook The Data you Take Away
Exposure times OFFSET_RUN files CCD Windows Data Catalogs
On-line Reduction Filters Flat-fields Blank Fields Orientation Shutters
This Page maintained by : Chris Tinney (email@example.com)
This Page last updated: 28 Feb 1996, by Chris Tinney