[Aavso-photometry] Banding in flats

[robertjmodic at att.net]

Does anyone know what causes the horizontal light and dark bands seen
on the left side of this image?

http://chagrinvalleyastronomy.org/images/mflatsky1s-1-10Rc%20MinMaxClip.fts

This is a master twilight flat made with a Rc filter.

The banding is present all the time with all my filters and with light box flats as well.

I read somewhere that the banding may be an interference pattern caused by
the coverslip not being parallel to the CCD.

The reason I ask about the banding is that it does not always flat out properly.
I have already checked the scope for and eliminated any scattered light in the 
system.

Bob