Top and side-on views of the WFI dewar showing the eight CCDs in place
The focal plane will be housed in an evacuated vessel (the "dewar") and will be cooled to about 170 K by the use of liquid nitrogen refrigeration.
The dewar fits onto exposure controllers located on the AAT Prime Focus and the 40" Cassegrain Focus, each comprising a filterwheel with at least four positions, shutter and their control electronics frames. The AAT exposure controller will also accommodate their existing acquisition and guide cameras.
Autoguiding at the 40" will be accomplished by the use of one of eight small guide CCDs located on the focal plane next to the scientific detectors. They are connected to (preferably) a commercial CCD controller (SBIG ST4 or similar/better) and a PC which interfaces to the Telescope Control System. Alternatively, the guiding facility which will have to be incorporated in the exposure controller for the tiptilt system can be used. This can probe inside the camera field, shadowing it. If such a system is compatible with the existing AAT autoguider, it may be used there as well.
Two San Diego State University second generation CCD controllers will be mounted on the dewar to read out the CCDs and transfer the data over fibre optic links to the data acquisition computer system.
The control and data acquisition computer system will consist of two Sun workstations with enough capacity to efficiently handle the data stream from the CCD controller for display, storage and archiving, plus data reduction. The use of two computers will allow the time-critical control of CCD readout to be separated from the user interface and data reduction computer. It will also allow user interface and data reduction computers to be set up permanently at both telescopes, optimally configured for each organisation's data pipelining procedures. The control and data handling software will be the MSSSO CICADA package, suitably adapted to deal with reading out CCD mosaics through the SDSU controller.