For the last 9 months or so I've been having on and off problems with my
flats. The donuts just didn't always line up exactly. I had to use
pseudo flats in Photoshop to remove the donuts this left. Lately that
problem was worse plus there was a change in the vignetting. It was
totally dark in two corners.

Tonight it was so bad I took the camera off the scope and had a look
inside. I've done this many times before but other than a too tight
axle screw for the filter wheel I found nothing. When I opened it up I
found the IR block filter position (use for L images) was rotated far
enough it was blocking two corners of the CCD. No wonder those corners
were totally vignetted!

But why? The wheel turned freely -- too freely, I didn't feel the
resistance of the drive motor. The drive motor is mounted on one screw
that screws into a threaded hole in the front (scope side) of the
camera. A spring uses this as a pivot to push the drive against the
filter wheel's rubber ring. The problem was the screw wasn't just
loose, it wasn't in the hole at all!!! The motor was just held against
the wheel by the "spring" in the wires running to the motor. Once it
was screwed back in everything worked fine. Just a 1/10th rotation from
tight (not real tight as that locks the pivot so it won't swing) allows
the motor to rock quite a bit. Obviously this was the cause of my
misaligned flat donuts. As the screw worked its way out the
misalignment went from minor to severe as the wobble in the motor
increased with each fraction of a turn I unscrewed the screw.

I have wide temperature swings in winter, 30C swings day to night are
the norm up here. That started the screw coming loose. What kept
slowly unscrewing it I don't know. Maybe there's a vibration as the
scope slews rapidly.

If you have an STL series camera (George N does I know) and the flats
show this problem, tighten (but not too tight) that screw. I'll be
checking it each time I open the camera as preventive maintenance.

Rick

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