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23257Re: What the heck....a donut near M81?

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  • rdcrisp
    Feb 1, 2004
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      --- In ccd-newastro@yahoogroups.com, "Wodaski - Yahoo" <yahoo@w...>
      > A bright, offset circle like that usually indicates that, at the
      time the
      > flats were taken, there was some back illumination from the
      > 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
      > conditions, or any time the light source can enter the back end of
      > scope.

      I've had the same problem as Ron described. I cured it by wrapping
      things with aluminum foil in the back when I shot my flats (avoiding
      covering the cooling ports for the cameras). One handy way to be
      assured your flats are free of such spurious light is to shoot a dark
      of 5-10 seconds with the aperture cover on. If you see things that
      should not be there, you have some light leaking from someplace.

      I always shoot a few darks like that before starting my flats. If I
      can't get a clean dark that way, I keep adding or adjusting the
      aluminum foil until I do. If the darks suffer from light ingress, you
      will never get a decent flat if you are shooting them during twilight
      (evening or morning)

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