As I am trying to learn how to use my 216XT, I have a couple more
questions:

1) Except for the random noise, shouldn't a dark frame show up as mostly
black? My dark frames are very light at the bottom, gradually becoming
darker towards the very top. I get this effect on or off the telescope,
even with a lens cap over the CCD barrel. I've waited until the camera
becomes temperature stablized at -10C, but temp doesn't seem to
influence the overall pattern I see.

2) I've noticed a small speck of something on the bottom side of the
cover glass, over the imaging chip. When I take a flat-field image,
this speck shows up as a splotch about the size of a dime. This may be a
rhetorical question, but won't this block light from faint, deep sky
objects, or is the speck far enough from the focus plane not to be a
problem?

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