72895Re: [ccd-newastro] Re: Calculating average brightness in an area of a fits image
- Jun 2 10:29 AMHi 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)
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