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Abstract. The Brorfelde Schmidt telescope has been modified to use a 1024 X 1024 12 micron pixels CCD camera. The field is 27 X 27 arcmin. When required, the spatial sampling is improved by co-adding multiple exposures with small offsets. The telescope is operated in service mode, with one observer carrying out several different observing programmes in parallel. The telescope is particularly well suited for long term monitoring programmes, most notably photometric monitoring of gravitationally lensed QSOs and searches for extragalactic supernovae.

We have found that many materials that are most commonly used in CCD cryostats are weakly radioactive and therefore contribute to what is rather liberally labelled as Cosmic Ray Events. Some standard optical glasses that are extensively used for lenses and optically flat windows such as UBK-7 contain large amounts of potassium 40, which renders them useless as windows in CCD dewars. Cobalt 60 is sometimes found in excessive amounts in some steel alloys, even in Covar used as a thermal match to silicon. The choice of materials that emit a minimum of ionizing radiation is discussed.