I've had issues with my LUM flats recently and when going through my images for answers it seems that, when taking flats after an LRGB run, the filter wheel was not returning my LUM filter to the same position it was in when taking the lights. The flats didn't follow the same acquisition pattern as the lights: the lights rotated through the parfocal filters LRGBLRGBLRGB etc., while the flats just did one filter at a time, 30xL followed by 30XR etc. I guess one solution is to rotate through the flats as well (to better replicate possible filter positions) or don't rotate at all and do all the L followed by all the L flats. Rotating filters meant I'd have a colour image of whatever duration the clouds destined me to have, while imaging all L and then all the L flats requires enough confidence clouds will stay away. I wonder what others do? My feeling was that a recalibration of the ZWO 8 position EFW would help plus rotating through the flats at the end of the imaging session, which at least would help average out any errors.
Anyway my real question, given I now have a slightly skewed flat, was if there was a way to align my errant flats with the lights? It seems fraught with issues given telescope affects on the image won't correct well, but it does seem that the filter correction is the bigger issue. Attached is the calibrated image which shows various under and overcorrected areas which I think (?) are due to the filter alignment error.
I could be wrong about this but when overlaying the master flat with an uncalibrated original sub the flat did look light it was in the wrong spot. I the attached side by side comparison you can see that the dark corners are different: the sub has a more complete dark circle on the right and there is more to the dark corners, which is the opposite on the left.
