Although the original designs for the AAT were based on the expectation of extensive wide-field photography at f/8, improvements in both instruments and photographic materials since then have conspired to ensure that the Cassegrain focus is rarely used for direct photography. As a consequence, the f/8 camera and its accessories are pretty much in their basic, as-received state, unrefined by user pressure or experience. However it is possible to take plates at Cass. and the following notes outline the system and working procedures.
The camera is attached directly to the Cassegrain acquisition and guide unit and carries a manually operated roller-blind shutter, electric vacuum pump and reservoir and the filter and plate holder guides.
The vacuum system, essential to suck the plate so that it is concave to the incoming beam, is activated automatically when the plate holder is fitted. After a minute or so the pump stops and restarts at (long) intervals to maintain the vacuum. A switch is provided to cut out the pump; the intention in this refinement was to prevent vibration spoiling image quality. In our experience there is no vibration and the switch is not used. However, a vacuum gauge indicates the system pressure and a red LED light is illuminated when the system pressure falls to a stage when further pumpdown is necessary. This light only operates when the automatic system is switched off.
The camera was designed to use 14 x 14 inch plates, 1mm thick, which are sucked concave in the plate holder as described above. There is only one 14 x 14 inch plate holder, though parts exist for the fabrication of another. Loading the plate holder is simple and trouble free but the dark-slide is a flimsy affair which leaks light slightly. It must be fully removed during exposure and on removal a ~ 1cm gap is left between plate holder and filter which is completely open to any stray light. Fortunately, the dark-slide is easily re-inserted in the dark but care must be taken to avoid fogging plates at this stage. The plate holder has been equipped with a nitrogen inlet to flush the space between plate and filter during long exposures. To avoid both excessive nitrogen loss and fogging it is suggested that during exposure a strip of opaque tape is used to block the gap left by the dark-slide. The nitrogen flow pattern has not yet been tested for uniformity.
The camera also accepts an adapter which enables it to accommodate the 10 x 10 inch plate holders normally used at prime focus. Of course, these plates are exposed flat, so image quality deteriorates away from the plate centre. More serious is the fact that the emulsion surface is 25.6mm farther from the primary than that of the 14-inch plates and this focus adjustment places the telescope focus outside the range of focus available on the guide probe. Thus exposures on 10-inch plates cannot be guided.
While this itself is merely an inconvenience, it considerably complicates focusing since the only useable knife-edge is the device which fits the 10 x 10 inch camera at prime focus. To use this, the Cass-to-prime adapter plate is fitted to the open 14-inch plate holder. The adapter plate has an effective thickness of 13.45mm, away from the f/8 secondary mirror. The knife edge adds an additional 11.15mm, and since the adapter sits on the surface where the back of the plate normally rests, an additional 1.00mm should be added, to give a total focus offset of 25.60mm from nominal. This figure is divided by 6.76 to give an effective shift of focus of 3.78mm from the actual emulsion surface of a plate in the 14-inch holder. This number is to be added to the knife-edge focus value found with the 10 x 10inch knife edge.
During one of the infrequent f/8 photographic sessions, in January 1984, the 10inch knife-edge, fitted by its adapter to the 14inch camera gave a focus reading without filter of 46.25mm. Correct focus, derived from this is 46.25 + 3.78 = 50.03mm. An additonal 0.1mm must be added for a 4mm thick filter thickness, resulting in a final focus of 50.13mm for this run.
Only one filter holder is available but changing filters in this holder takes only a few minutes and can be undertaken by - for example - the night assistant during plate changing.
The following 358.8mm square filters are available, 4mm thick
We do not keep stocks of 14 x 14inch plates at the AAT. Instead, we have an arrangement with the UK Schmidt that they will provide us with hypered 14 x 14inch plates for use at the AAT f/8 focus. This arrangement may have to be changed if we were to use many plates but at present this seems unlikely.
Our processing tray rocker is compatible with UK Schmidt developing trays so the development and stop bath stages of processing can be carried out in the 5th floor darkroom. Fixation and washing can also be completed there but only in open trays, since our tanks were not designed to take 14inch plates. The UK Schmidt have offered us the use of their processing facilities if necessary but again this is subject to usage. To date all plates have been processed at the AAT.
Prime focus photography is sometimes specified as a back-up either as insurance against instrument failure or to be used if the seeing is particularly good. Until recently, the availability of hypersensitised plates was a constraint, mainly because the stainless steel sensitizing boxes in which they were stored required three or four hours to warm to room temperature. Hypered plates stored at room temperature for lengthy periods fog badly so they are always kept at deep-freeze temperatures when not required. This warm-up delay has now been overcome and an observer can be ready to step into the prime focus cage, complete with a selection of hypersensitised plates, in less time than it takes to change top ends, typically 45 minutes.
If such a change is a possibility, the prudent observer will go through check-list A (below) on arrival on the mountain. If direct photography becomes inevitable, check-list B will ensure that you are ready to start before the top end change is completed. List B assumes that everything in List A is satisfactory and that supplies are where they should be. Some items in List A, for example choice of corrector, may require day staff to rectify and must be identified well in advance.
We often have some hypersensitised plates either in the 5th floor darkroom refrigerator or in the first floor deep freeze. The emulsion types available will vary but as a minimum there should be sufficient IIIaJ, IIaO and 098-04 for at least one full night's work. These emulsions will be foggier and slower than is normal but should be quite usable. Details of speed and fog and the date of the last sensitometric test are recorded on the box label.
If PF photography is a possibility, the following should be checked:
If PF Photography becomes a certainty you have 45 minutes to:
A more comprehensive checklist appears in Chapter 2.13