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The LBL f/1 wide-field imaging system

The f/1 focal reducer for wide-field imaging was custom-built for the AAT by Applied Physics Specialities, Toronto, Canada, in collaboration with Carl Pennypacker of Lawrence Berkeley Laboratories, and was commissioned in 1989. The focal reducer converts the f/3.3 prime focus beam of the AAT to f/1, and comprises a hyperbolic mirror and three BK7 glass lenses optimised for imaging in the 5000-9000Å region. Large off-axis chromatic aberrations are seen at redder and bluer wavelengths (e.g. 2" chromatic aberrations at 8' 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"/pixel, giving a total field of view of 16.7' x 16.7'. 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 about 40% over the wavelength range 5000-8000Å (see § 6.2.4).

With the fastest readout speed available (NONASTRO), the entire Thomson CCD (including an overscan region of 5 rows) can be read out in 28 seconds. At this speed, the gain is 5 e-/ADU and the readout noise 10 electrons. For all practical purposes, therefore, Poisson noise from the sky dominates the readout noise and images are sky-limited within a few seconds.

Photometric standard stars must be chosen carefully, as many are too bright to observe with the f/1 system. The minimum recommended exposure time is 5 seconds, set by the shutter timing because the large shutter of the f/1 system takes a significant time to open and close. Stars brighter than about V=14 may saturate the CCD in less time than this, and are therefore unsuitable.

The AAO CCD Imaging Manual contains further information on the f/1 system. Most of the contents of an earlier draft document, (AAO UM 27: Notes for users of the AAT f/1 imaging system ) are now contained there.



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This Page Last updated: Mar 5, 1996, by Chris Tinney.