To reduce the dark current and to allow the CCD amplifiers to operate with lower thermal noise, the CCD is generally cooled with either a Cryotiger closed-cycle cooler or liquid nitrogen (inside a vacuum housing).
The temperature of each CCD is monitored with a platinum RTD (1PT100KN815). Heater resistors are used to maintain the temperature of the CCD at a predetermined set-point.
[insert Cross-section of CCD cooling scheme]
[ insert diagram of CCD dark current ]
At room temperature, the typical ccd dark current in
multi-pinned phase
(MPP) mode may be 250 e-/pix/s.
After cooling to -100 °C, the dark current will typically be a
few
counts per hour. The reduction is about a factor of xxx.[ insert diagram of CCD dark current ]
BASE temperature sensing circuit
The temperature sensing circuit works by passing a small current from the current source (500 uA) through the platinum RTD, then sensing the voltage across the RTD using an instrumentation amp.
Adjustable current source for heater resistors
The BASE system provides eight temperature sensors and eight adjustable current sources which share a 1 amp power supply. The CCD temperature is controlled with a digital PID feedback system implemented in the dsp (see notes below).
 Temperature sensor design notes (1) |
 Temperature sensor design notes (2) |
 PID controller notes (1) |
 PID controller notes (2) |
Greg Burley (burley@obs.carnegiescience.edu)
Ian Thompson (ian@obs.carnegiescience.edu)