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72897Re: [ccd-newastro] Re: Calculating average brightness in an area of a fits image

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  • Mark Striebeck
    Jun 2, 2013
      That makes sense - thanks!

      On Sun, Jun 2, 2013 at 11:01 AM, Stan <stan_ccd@...> wrote:

      > **
      > --- Mark Striebeck <mark.striebeck@...> wrote:
      > > But why is it that sky glow affects star colors differently...
      > > thought of sky glow as a kind of filter...
      > Sky transparency is a filter that can vary by color. Thus it may be
      > necessary to adjust ratios based on air mass (sky alt).
      > But sky glow is not a filter, the glow does not absorb or block photons.
      > Sky glow is an imposition that is equivalent to a double exposure of the
      > astro objects and a colored screen. So the way to neutralize sky glow is to
      > subtract its contribution. Messing with color ratios to "balance" sky glow
      > will necessarily distort the colors of the astro objects. (This is why most
      > terrestrial photo software cannot correctly deal with astro image color.)
      > Stan

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