Michael,
I think you are referring to the large annulus-shaped dark circles. These are
called dust donuts. There are two different sizes of donuts in your image,
meaning that there are dust particles on two glass surfaces at different
distances in front of the sensor. The smaller ones could come from something
on the sensor cover-glass itself, and the larger ones result from dust on a
filter. The holes in the donuts are caused by the secondary obstruction, or by
the diagonal in the case of a Newtonian. Pre-processing with good flats for
each filter helps eliminate these, but only if all the dust follows camera
rotation.
Paul Lind
----- Original Message -----
From: Michael McDonald <mikemac@xxxxxxxxxxx>
To: az-observing@xxxxxxxxxxxxx
Sent: Fri, 25 Jan 2019 12:06:41 -0500 (EST)
Subject: [AZ-Observing] Water spots?
I was playing around last night. Everything was going great until around
midnight when I tried to image the Flame Nebula. Other than Alnitak causing
contrast and glare issues, I got a bunch of donuts in the image. They’re big
and dark as shown here:
http://www.mikemac.com/Astrophotos/NGC2024_Light_L_2019-01-25_00-06-57_Bin1x1_90s__-1C.png
Are those water spots from dew? I didn’t see any dew on the corrector plate on
my C6 SCT. Could they be on the image sensor its self? If so, how do I prevent
that from happening?
If they’re not water spots, what could they be?
Mike McDonald
mikemac@xxxxxxxxxxx
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