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Despite advances
in solid state imaging devices over the last twenty years, the choice of a
camera to replace the aging vacuum tube devices on the WHT telescope was not
obvious. All ING telescopes originally had Westinghouse (Intensified Secondary
Electron Conduction) ISEC TV cameras, which could be used at the standard
25 frame s–1 TV rate, had an advantage of photon multiplication
within an intensifier stage, and could be made to integrate charge at the
photocathode. Being able to integrate for longer than a single TV frame made
these cameras attractive in astronomical applications. The integrating facility
works by inhibiting the electron beam from a thermionic cathode for an integer
number of TV frames before reading off the two dimensional charge pattern
with a single raster scan, the image is then held in a digital frame store
until refreshed by the next scan but is presented to the observer on a TV
monitor as a stationary image during the interscan period. The problem with
ISEC cameras is that they can be damaged by over illumination and have poor
geometric stability because image distortion is dependent among other factors
on the linearity of the camera time-base oscillators. Furthermore these cameras
are no longer supported by the manufacturer and obtaining or engineering around
obsolete spares was starting to become a problem. Solid-state detectors offer
excellent geometric stability are not damaged by over illumination and have
a large dynamic range.
An important decision when retrofitting an acquisition camera to a telescope
intended to operate with a camera such as the Westinghouse ISEC is whether
to replace the reimaging optics and design a system utilising a smaller format
imaging device. Small format devices are of course cheaper and several cameras
are commercially available, many work at TV frame rates and some have thermoelectric
cooling and can integrate for a few tens of seconds. A problem with many of
these products is their poor quantum efficiency of the CCDs used and relatively
high dark currents, these shortcomings combine to make it difficult to achieve
the limiting magnitude of the ISEC cameras ~mV=20. A solution adopted
at some observatories is to use an intensified CCD camera, generally a MCP
ahead of a cooled CCD, but we wanted to avoid this solution partly because
of the nuisance of managing the over illumination risk, but also to offer
better photometric performance than the ISEC cameras. The performance requirements
for future upgrades to ING acquisition cameras had previously been specified
in a memo (Rutten and Barker, 1996) and these included features such as on-line
bias subtraction, generation of FITS files and a minimum of 8 bit precision
over the dynamic range of the detector. These requirements were easier to
meet using commercially available cameras aimed at the university observatory
market. Several suppliers offer cameras aimed at the university telescope
niche but the Micro Luminetics Cryocam offered the possibility of remote
PC operation over a single 50 ohm coaxial cable, whilst other contenders typically
needed multicore cables of restricted length between the camera and PC. This
cable limitation was an important factor in choosing the Cryocam although
other observatories like the AAT have chosen to accept the cable restriction
and have mounted the controlling PC on the telescope.
The Cryocam model presently operating at the Cassegrain focus of the WHT
has a thinned back illuminated 1k square SITe chip with 24m pixels. Replacing
the original camera with a CCD having a similar format meant the acquisition
FOV at Cassegrain has been retained and slightly increased in the y-direction,
the possibility of interposing a focal reducer to change between a 1.5 arcmin
and a 4 arcmin FOV remains along with the original camera filter wheel. The
chip is operated at a temperature of 220K and has a dark current better than
0.2 e– pixel–1 s–1 with a QE>80% between
600 to 750nm. The Cryocam will operate in either fast readout 8 bit ‘focus
mode’ when acquiring bright ~mV=5–7 calibrate stars or monitoring
the position of a bright source on the ISIS slit or the ‘normal mode’ with
16 bit precision. Any shutter speed ≥200ms is available in either mode giving
the possibility of confirming the position of faint or extended objects on
the ISIS slit without the need to trust a ‘blind offset’. The camera is operated
by the observer from the control room and the control and display application
run on a PC under the Windows 98 operating system. The camera CCD does not
operate in the frame transfer mode and requires a mechanical shutter to close
during readout and the long term reliability of this component is an area
of concern, with bright stars there is a temptation to use the minimum 200ms
integration time and a readout time of less than a second in focus mode implies
several thousand shutter operations per hour. The recommendation is that in
these circumstances it is preferable to integrate for longer and use a red
filter, we have however operated a Cryocam at the Cassegrain focus for both
direct and slit viewing since May 2001 and have not experienced any serious
problems. The spare camera was recently tested with AF2 to manually guide
utilising the recently upgraded coherent guide fibres and we are considering
the purchase of enhanced Cryocam software so a Cryocam can be used for autoguiding
AF2.
