Strange Blue Flat

Below is a set of flats taken with an ST10xe, Cfw8 (older model with new IR-blocking SBig filter set), William Optics minus-violet filter and Synta 102mm f5 achromat, using a home-made light box per the AIP4Win design.

They all look reasonable enough, except for the Blue Flat at the bottom. Does anyone have any ideas as to what might be causing the bright glow in the upper-left corner? And the strange horizontal line near the top of the frame?

Why would this only be happening to the Blue Frames? Could it be related to the minus violet filter? I haven't tried removing it yet but will.

I've been having this problem for a while now, and have been working hard to remove it during processing, but the results were good enough (and the other problems numerous enough) for me to leave this particular problem until later. But the time has now come to solve this issue.

Anyone have any ideas? Please email me at

Ha Flat. 5 seconds

Red Flat. 2 seconds

Green Flat. 5 seconds

Blue Flat. 10 seconds