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NOTE: at present the code does not support co-added sequences of flats. We will add this very soon, there is no real technical challenge, only there are other higher priority items. In the meantime if you absolutely must have combined flats, we recommend you use IDL or another utility to combine your flats in 2D and save out to a temporary file in your Raw directory. Then load this temporary file into the Red or Blue flat box and proceed as before.
The image below shows an example flat field. The zero pixels are outside the slit boundaries; inside the boundaries look for pixel values near 1.0 +/- 0.05 or so, except for the occasional dark pixel.
