ccd detector cooling | Stirling Cryogenics
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 CCD’s are, they aren’t perfect. Various sources of noise inherent in the use of CCD’s 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.
The first solution to reduce dark curent 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 CCD’s 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.
LN2 production plant at ING, La Palma (Spain)
Mr. Alan Chopping, head of site services of the Isaac Newton Group of Telescopes (ING), wrote an article about their on-site Liquid Nitrogen Production Plant. Some quotes are:
“As the nearest commercial supplier for liquid nitrogen for the Observatory is located at Tenerife, ING produces liquid nitrogen on the Roque site to provide cooling for all its CCD’s. This has enabled us to eliminate the logistic problems of transportating large liquid volumes of nitrogen between islands and has been found to be more economical. Also from the surplus produced we have been able to supply our neighbouring institutions".
"While this arrangement has worked successfully for the past decade over the past couple of years demand for liquid nitrogen has grown to accommodate development work, larger CCD’s and instrumentation at all the ORM institutions.” Therefore ING, together with its neighbours TNG and NOT, decided to purchase an additional StirLIN liquid nitrogen production plant, which was delivered in March 2001. Back to Mr. Chopping: “Since then the plant has been installed and connected to a new 3000 litre storage vessel. This unit in tandem with the original plant now ensures sufficient liquid nitrogen for our entire site needs at the ORM.”
Since 2003 ING has decided to have the maintenance for both plants done by Stirling service engineers, covered under a fixed-sum maintenance contract.
Supernovae and gamma ray bursts at the world’s top level!
Hanle observatory is the highest optical infrared observatory in the world at 4500 meters above sea-level (14,763 ft) in the Ladakh region of the Himalyas. The extreme location requires low maintenance and highly reliable technical systems. Therefore the Indian Institute of Astrophysics has chosen for Stirling Cryogenics BV stand-alone on site liquid nitrogen production plants. Nice to know: Hanle uses the oxygen-enriched exhaust gas from the PSA (air separator) for easy breathing in the sleeping cabins of the crew!
Products
For use in applications as described the following Stirling Cryogenics products may be considered:
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