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At low light levels the charge transfer is sometimes imperfect and the bucket brigade spills some electrons along the way during read-out. This results in slightly deformed images, showing a trailing along the rows and/or columns of the CCD. Faint asymmetric wings may be generated around bright emission lines. To improve the performance one can preflash the chip by pre-illumination with a fixed amount of light. This is, of course, undesirable because it adds shot noise to the data. Although all the chips currently in use on La Palma have good charge-transfer characteristics and are said by the manufacturers not to require preflash, the charge-transfer is not perfect. Therefore we recommend that users consult their support astronomer over whether a preflash is needed.