If your scope was well out of focus the central blob is likely a shadow of the secondary. It's important for flats to be taken with exactly the same optical setup as your light exposures, including focus and rotation angle of camera, reducers, filters etc. If you're taking twilight flats the white T shirt over the end of the scope is a good idea to even out the background and stop individual stars from registering. Best solution of all is to buy a light box from Peter. Highly recommended.
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