Re: [APML] M13

[Roland Roberts <roland@astrofoto.org>]

>>>>> "Tony" == Tony Hallas <tonyhallas@foothill.net> writes:

    Tony> Needless to say the seeing would prevent any pinpoint
    Tony> representation, but the star colors would be obvious. I used
    Tony> my usual reduction techniques and PS for image extraction
    Tony> from the 16 bit data...

While the picture looks very pretty, I have to wonder about the
accuracy of the colors (especially for scientific measurements) for
three reaons:

    1. Calibration.  What was done to calibrate your measurements with
       wavelength?  

       As an example of what you could do, you can use the
       manufacturers response curve for the CCD, multiply by the
       transmission curve of the filter, and integrate over all
       wavelengths.  This gives you integrated R, G, B relative
       efficiencies.  Now take your raw R, G, B images, flat field and
       dark subtract them.  Pick one of your stars and measure the
       total flux in each of the R, G, and B images, then correct it
       for the relative efficiencies of your detector/filter
       combination.  

       Now for the fun part.  You want to match those with the color
       temperture from a blackbody curve when you integrate a
       corresponding wavelength range.

    2. What is your "usual reduction techniques"?  Does it include any
       color balance or color saturation changes?  If you, you have
       distorted the colors; the result may be aesthetically pleasing,
       but is no longer suitable for trying to draw scientific
       conclusions, *especially* just by eyeballing it.

    3. The images really appear to be color saturated.  Admittedly,
       this is subjective, but it looks a lot like what I do to my
       constellation portraits in order to bring out the colors.  It
       looks good, but, see (2).

It's *still* a nice shot.  Some day maybe I can do one as nice....

roland
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Roland B. Roberts, PhD                             RL Enterprises
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