Pre-flashing

The transfer of charge between pixels (and hence along columns) suffers from inefficiencies. Usually this amounts to a charge loss which is never read out from the CCD well - this level is often referred to as the `fat' or `skinny' zero to confuse matters; I refer to it as the deferred charge value. When observing objects with low sky backgrounds (and/or low counts themselves) this loss of charge may be significant (at least in some older CCDs). To overcome this CCDs can be pre-flashed. This amounts simply to illuminating the CCD with a uniform light flux just prior to the actual object exposure. The object counts are then simply added to this pre-flash level of charge in the CCD wells. Note, however, that this method is of no use for very low counts as the signal to noise level which is required after pre-flashing is higher than before (the noise from the pre-flash photons adding to the noise of the object photons). Correction of data for pre-flashing is achieved by subtracting the pre-flash ADU count from the final data (before flatfielding).