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23255RE: [ccd-newastro] What the heck....a donut near M81?

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  • Mark de Regt
    Feb 1, 2004
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      To add to Ron's information, I always take dawn flats, and I do my best to
      wrap dark, thick cloth (black corduroy from a cloth store is cheap and
      effective) around all places where light could enter the system (other than
      the front of the scope, of course). I actually keep it wrapped all night,
      too, since there are too many places in my system where stray light enters
      when given the opportunity.

      Mark de Regt
      Redmond, Washington, USA
      47.7 N. Latitude; 122 West Longitude
      Astro Images: http://home.earthlink.net/~akilla/MAD


      > -----Original Message-----
      > From: Wodaski - Yahoo [mailto:yahoo@...]
      > Sent: Sunday, February 01, 2004 7:27 AM
      > To: ccd-newastro@yahoogroups.com
      > Subject: RE: [ccd-newastro] What the heck....a donut near M81?
      >
      >
      > A 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
      > scope.
      >
      >
      > Ron Wodaski
      > The New CCD Astronomy Book
      > web site: http://www.newastro.com
      >
      >
      > -----Original Message-----
      > From: mike_from_tn@... [mailto:mike_from_tn@...]
      > Sent: Sunday, February 01, 2004 7:55 AM
      > To: ccd-newastro@yahoogroups.com
      > Subject: [ccd-newastro] What the heck....a donut near M81?
      >
      > Would greatly appreciate any insight regarding this gremlin in my
      > image.
      >
      >
      > http://personalpages.tds.net/~mfleenor/st10/m81_150min-Irblock.jpg
      >
      > 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
      > this before?
      >
      > Thanks,
      >
      > Mike
      >
      >
      >
      >
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