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72895Re: [ccd-newastro] Re: Calculating average brightness in an area of a fits image

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  • Mark Striebeck
    Jun 2 10:29 AM
      Hi Stan,

      Thanks a lot for these instructions (I was just composing an email asking
      how to do this? :-)

      But why is it that sky glow affects star colors differently then sky color?
      In my (naive) view, I thought of sky glow as a kind of filter that distorts
      the color for everything equally in an image.


      On Sun, Jun 2, 2013 at 10:25 AM, Stan <stan_ccd@...> wrote:

      > **
      > Obviously the sky background color pollution was not handled and those
      > ratios are invalid.
      > To calculate color/filter ratios, it is imperative to first subtract sky
      > bias levels from each component.
      > To manually calculate:
      > For each filtered image, measure the mode or median of nearby dark sky (no
      > stars or nebula) and then measure the avg of a tight and uniform box
      > surrounding the calibration star (G2); subtract the background from the
      > star and use that for the ratio calculations.
      > To calculate via CCDStack:
      > Create a color image, open the "Adjust Color" form (if not already open)
      > and perform a "set background" (if not done already). Drag a tight
      > rectangle around the calibration star and press the "balance" button (be
      > sure the "factor" radio box is checked). The ratios are automatically
      > calculated and displayed. To normalize ratios to a particular color �
      > double-click the area next to the ratio display box for that color. (read
      > Help: Concepts, Color section)
      > Stan

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