Two types of detectors are currently available: CCDs and an IPCS.
CCDs tend to change every few years. The most up--to--date
information will be found in AAO Newsletters or from support astronomers.
At the time of writing, the most common choice is a Tek 1024 square thinned CCD
with 24 
m square pixels. The usual alternative is a coated
1024 square Thomson CCD with smaller, 19 
m square pixels
and a lower cosmic ray strike rate than the Tek chip, but lower
quantum efficiency.
The IPCS is an image photon counting system, described in the AAO User Manual
10. The size of the scanned area is adjustable, but the normal configuration
uses a data window of 32.6 mm 
17.8 mm and a pixel size of 16 
m
along the spectral direction and 74 
m in the cross--dispersion direction.
Because of the large format normally used
with the IPCS (2048 
256), non-linearity effects become evident at
counting rates as low as 0.2--0.4 Hz. When observing bright objects it is
important that neutral density filters be inserted to reduce the count rates
below this level.
The IPCS now sees infrequent use, as CCDs have grown in size and quantum
efficiency. It offers advantages only at very low count rates, typically
for quasars fainter than 16.5 magnitude for which the readout noise of CCDs
would swamp the meagre signal. For almost all other applications, CCDs have
superseded the IPCS.
Detector data are summarised in Table 
, and
quantum efficiencies are compared in Figure .
Table: Detector characteristics
Figure: Relative quantum efficiency of AAT detectors