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Hi Joe:
Why did you expect the exposures to be shorter with
the SCT than the refractor? Aperture doesn't affect exposure with extended
objects, it only depends on the f/ratio. Actually, 20 minutes at f/6.5 is
not terribly short for such a bright object. You may get more detail in the
fainter surrounding nebulosity with 45 or 60 minutes but you'll have to be
careful not to burn out the central area --- maybe combine long and short
exposures with a mask protecting the inner area from burn out. If your present
slide shows more data in the faint areas but you get burn out in the center with
stretching, do a selection in Photoshop with the lasso tool, invert (so the
center is excluded) and then stretch the periphery some more.
Good luck!
Bert
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