Thanks Paul,
How do you calibrate defocussed images? Presumably you have to take all the flat frames at that the same defocus?
Not really. All flats are completely out of focus anyway!
That is the whole point of flats: they should have no extrinsic structure whatsoever and all non-uniformity over the flat image arises from sensor sensitivity variation, dust rings, vignetting, etc, and not from anything outside the optical chain.