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If the chip used needed preflash, you would have obtained some preflash frames at the beginning and end of the night, plus one or two flash maps (see Chapter 4).
Proceed likewise with the preflash frames. The ratio of mean values of these two frames should be similar to the ratio of flash units used. You can divide the flash map by this ratio and use this ``scaled'' preflash as the one to subtract from the data frames. Use the preflash frames to check for consistency.