where GAIN is the detector gain parameter from the image header, for a single image. NCOMBINE is the number of images combined. Feeding GALFIT an image in [counts/sec] and asking it to create σ-image internally, bad and strange results may happen, even if the residuals may look fine, because the relative scaling of noise between the background and the data could be wrong.GAIN [e-/count] × ADU [count] × NCOMBINE = [electrons],
If one wants to work with images in [counts/sec] or other units, it is not advisable to let GALFIT create the sigma image internally. See here and here for more details.
Otherwise, GALFIT will not know how to calculate a proper sigma image. Here, again, GAIN is that for a single image, and NCOMBINE is the number of images combined to create the image.GAIN × ADU × NCOMBINE = [electrons].
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