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Patching or interpolation is the final element in the cleaning of CCD images. It is an admission of defeat, and is an illegal fix of the data; there should be justifiable reasons for its use. Large-scale surface photometry may well be one such reason. Patching may be useful for ``removing'' the occasional cosmic ray or defective column. An example of the latter is column 142 in Fig 5a, which is bright for most of its length. This is due to a single defective pixel 8 % of the distance up the chip from the readout register (bottom row). The pixel adds a huge and spurious amount of noise to each signal clocked through it. These data are lost; there is no resurrecting them. CCDs in use at La Palma are so clean that no such patching is likely to be necessary.
Fig 5b shows the effectiveness of the cleaning process desribed above. The single illegal operation has been the patching of column 142, for which cosmetic improvement is the sole (but weak) justification in the present instance.
Finally, low-light-level effects may produce severe problems in some applications such as spectroscopy or narrow-band imaging where backgound photons are at a premium. One problem in this regard - charge-transfer inefficiency - has already been mentioned. A second is caused by electron traps, substrate deficiencies which result in dark columns, down which charge transfer is severely inhibited unless a threshold of electrons is present. Both charge-transfer inefficiency and electron trapping can be minimized by careful exploration of driving waveforms and chip operating temperatures. But if these do not succeed, pre-flash may be required, a pre-illumination by a small amount of light to provide the threshold. Of course this procedure is undesirable: it adds shot noise, and it has to be subtracted precisely (as for bias and dark count) in early stages of the analysis. This different operating procedure is another aspect of chip operation with which a general purpose camera-controlling microprocessor can deal.