To reduce the dark current and to allow the CCD amplifiers to operate with lower thermal noise, the CCD is thermoelectrically (TE) cooled within a sealed, dry gas filled housing. The cooling system uses an off-the-shelf two-stage Melcor/Laird thermo-electric cooler (2-CP-055-065-31-17L). The CCD temperature is monitored with an off-chip sensor (AD590) mounted inside the housing.
[Note: TE coolers are solid state semiconductor devices, made from doped bismuth telluride. A typical TE cooler consists of alternating blocks of n-type and p-type material sandwiched between two opposing thin alumina plates. Driving a current through the TE material carries heat across the material, creating a temperature difference (Peltier effect).]
The CCD47 can be bought from E2V in a special hermetically-sealed housing, but we preferred to build a housing around a standard device package. The housing is constructed from FR-4 fiberglass, a poor thermal conductor, to minimize heat transfer to the gas inside.
At room temperature, the typical ccd dark current in multi-pinned phase mode (MPP) of 250 e-/pix/s will be noticeable even in short (50 ms) exposures. At a target ΔT of 40 °C, the dark current is reduced by a factor of 125 to about 2 e-/pix/s, calculated using n = 122 n0T3 exp[-6400/T], for temperature T (K), and n0 dark counts at room temperature.
Adjustable precision current source for TE cooler
With the temperature sensor, it would be possible to implement closed loop feedback. In practice, running a fixed current through the cooler works well for guider applications.
A liquid-cooled heat sink removes the heat from the TE cooler hot side. Even at low flow rates (a few litres/min, the entire CCD housing can be rapidly chilled to the fluid temperature (nominally 10 °C at the telescope). The TEC drive transistor is mounted on the heat sink as well, to prevent it overheating.
Greg Burley (firstname.lastname@example.org)
Ian Thompson (email@example.com)