CCD color calibration Re: [APML] M13 image

["Philipp Salzgeber" <philipp@salzgeber.at>]

Hi Tony,

True color seems to be the holy grail of color astrophotography...

Professionals have been using that term, but for the most part have proved wrong. Maybe there is no such thing as true
color in an image? Because the human eye works very differently from the devices used for capturing and presenting
images?

well, enough of that...

> I guess what I will do is shoot a 21 step grayscale tablet in the moonlight at an f/stop that will allow
> at least a 20 minute exposure... here we are using a known constant with reflected sunlight. If something
> is wrong this should show it.

With your test you will surely find out if the white-point of your setup is correct. TV cameramen do this all the time
using a white piece of paper...

But you will not see, if colors are rendered correctly across the spectrum. For this reason I would include a color
calibration chart also. With that  you should be able to see, if the color representation is correct (for black-body
radiators at least, line-sources are a different problem - Chuck?).

Another method of calibration could maybe done with spectra. Take a spectrum (spectra? spelling?) of a sun-like
reference star through each filter and calibrate your combined spectrum to the known spectrum of the star. I think
professionals have to do this all the time to get valid spectroscopic data (without the color filters of course...)

cheers

Philipp



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