The flat field of this CCD appears to have changed as of 15th Feb 1999. However, as of February 1999, an unusual pattern has appeared in the upper right corner of the CCD flat field. Its qualitative appearance suggests that it may be the result of contamination of the detector. At the time of writing, we are investigating possible causes. The affected area is small and unlikely to affect observations.
Pixels have very slightly different dimensions in the horizontal and vertical directions.
Failed O 2 Soak Because this device uses the PPt passivation technology, it requires soaking in oxygen before cooling in order to achieve the best possible QE sensitivity and uniformity. Occasionally, this can fail. In this case, the flat field will be very non-uniform with a number of sensitive patches distributed over a much reduced overall sensitivity. For sample images, look here . Normally this will be caught before the observer arrives at the telescope to use the instrument.