23245RE: [ccd-newastro] What the heck....a donut near M81?
- Feb 1, 2004A bright, offset circle like that usually indicates that, at the time the
flats were taken, there was some back illumination from the focuser/camera
end of things. This causes a reflection off of some dust (fairly far from
the camera in this case because it is so large), leaving a bright spot when
the flat is applied.
This typically occurs either when flats are taken in daylight or dusk
conditions, or any time the light source can enter the back end of the
The New CCD Astronomy Book
web site: http://www.newastro.com
From: mike_from_tn@... [mailto:mike_from_tn@...]
Sent: Sunday, February 01, 2004 7:55 AM
Subject: [ccd-newastro] What the heck....a donut near M81?
Would greatly appreciate any insight regarding this gremlin in my
This is 15 x 600sec with a Schuler Ir block.
ST10 XME/ Intes MK69
I took a good set of flat frames and checking the individual flats
against the master doesn't reveal anything however after calibrating
the individual image frames I get the donut pattern. Anybody seen
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