The f/1 focal reducer for wide-field imaging was custom-built for the AAT by Applied Physics Specialities, Toronto, Canada, in collaboration with Dr. Carl Pennypacker of Lawrence Berkeley Laboratories. The 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 redder and bluer wavelengths (e.g. 2 arcsec chromatic aberrations at 8 arcmin off-axis for images taken in the B passband).
The mirror-based design means that the CCD dewar, shutter and filter holder sit inverted below the AAT prime focus cage. Autoguiding is not possible with this setup, and exposures should be limited to less than 5-10 minutes to avoid trailing. A filter holder accommodates up to three filters (generally V, R and I), which can be changed remotely from the AAT control room, so there is no need for an observer to ride in the prime focus cage.
The focal reducer is used with a 1024x1024 Thomson CSF THX 31156 CCD. The scale on the CCD is 0.98 arcsec/pixel, giving a total field of view of 16.7x 16.7 arcmin. The overall efficiency of the f/1 system (telescope plus focal reducer plus filter) is roughly 50%, while the Thomson CCD has a detective quantum efficiency of around 40% over the wavelength range 5000-8000Å (see section 2.9).
With the fastest readout speed available (NONASTRO), the entire Thomson CCD (including an overscan region of 5 rows) can be read out in 38 seconds. At this speed, the gain is 5 e-/ADU and the readout noise 10 electrons (see section 2.8). For all practical purposes, therefore, Poisson noise from the background sky dominates the readout noise and f/1 images are sky-limited within a few seconds.