Fourth a baking procedure is described which will be done after all integrations and tests and before shipping the cryostat to a consortium or to the telescope and which can be done periodically in the observatory, if a problem occurs. 2 Introduction Contaminations on CCDs which are reported from many observatories in the world made it necessary to develop a more stricter cleaning, handling and baking procedure with scientific CCD cryostats and its components. Contamination on CCD detectors first is showing up as a loss of transparency in the UV and blue wavelengths bands. CCD Cryostats One should not only concentrate one's forces to fight against the symptoms like baking the contaminated cryostats but also reduce the sources of contamination, which means understanding the process and improving of materials inside of the cryostats. It is a physical law, that under atmospheric conditions all surfaces are covered more or less with gas, which consists mainly of water molecules.
Water molecules cause no critical contamination like oil, but they are hard to pump.
One should avoid anodised surfaces and plastic materials if possible. Nevertheless for the temperature range envisaged for a dewar, the choice of material is not too difficult. One should not hesitate to test suspicious materials before using it at the cryostat tank and head, especially if one find signs of contamination near suspicious materials. Tests are possible in a very clean cryostat monitoring CCD contamination over 1 - 4 weeks or with a mass spectrometer combined with a gas chromatograph. All used self-gluing tapes should be avoided from the design.