CCD detector cooling for observatories

Charge-coupled devices (CCD) were introduced in the mid 1970’s. They have virtually replaced film and photographic plates for professional astronomical imaging since CCDs are more efficient at collecting light than photographic film. This high sensitivity for light makes it possible to obtain a digital astronomical image in seconds to minutes rather than hours. However convenient and efficient CCDs are, they aren’t perfect. Various sources of noise inherent in the use of CCDs can have negative effects on the obtained data.

Electrons generated by the heat produced by the system (thermal noise) cannot be distinguished from electrons generated by photons (signal). Thermal noise exists even when light does not hit the detector surface and is referred to as dark current. Dark current is a form of noise, the level of which is proportional to the length of the exposure. Given a long enough exposure time, the detector could become fully saturated with electrons due, in large part, to thermal noise.


Alma Observatory, Chili

The solution to reduce dark current is to cool the detector to cryogenic temperatures. Professional instruments are cooled with liquid nitrogen (LN2). The CCD sits in a vacuum environment with the cooling of the chip provided by a metal contact between the chip and the LN2. Because of these extremely low temperatures, that the camera operates under, a professional CCD camera is virtually free of thermal noise.

Over time the liquid nitrogen (LN2) in the CCD cryostat evaporates making refilling necessary. With the on-going developments on larger CCDs the liquid nitrogen consumption of observatories tends to increase. In the typically remote areas of the telescopes required for clear sky conditions the LN2 requirements can cause logistic problems. Furthermore the dependency and reliability of the supply becomes an even greater risk to observation down times.

The solution can be found in reliable on site production of liquid nitrogen or direct cooling of the CCD in a closed loop solution. Both solutions have been installed by Stirling Cryogenics in different observatories over the world.


For use in applications as described the following Stirling Cryogenics products may be considered:

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