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Color-correcting a flat panel
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Color-correcting a flat panel
Michael A. Covington
It all started with a red galaxy. I took a picture of M61 with my DSLR and calibrated it with PixInsight. It puzzled me that the corrected, calibrated image came out strongly red, like this:
But at first it didn't puzzle me as much as it should have. Of course the color cast was easy to correct by stretching the R, G, and B layers differently. I figured the red came from light pollution.
But wait a minute. The uncalibrated images from the camera, both raw and JPEG, were not red. They had more of a golden-brown color that really was explainable as light pollution and resembled results from many other cameras and processing workflows at the same site.
Then I figured it out. My flat-field frames had a strong cyan cast. Here's how one of them looked on the camera's display:
Because of the way flat-field correction works, any color in the flat frame will produce the complementary color in the resulting picture. And red is the complement of cyan. Mystery solved.
Mystery solved, but not problem solved. How could I fix it? I'd rather not do heavy color adjustment in software because that would imply I wasn't using the full dynamic range of the camera in the original image.
Here's what I did. I shoot flat fields by holding an AGPtek artist's tracing panel in front of the telescope. (The same panel is sold under other brand names.) It's uniformly bright, very lightweight, and USB-powered (in fact I Velcroed a small rechargeable USB power pack to the back of it). Handy and easy to manipulate...
...and obviously cyan-colored, once you look at it and ask yourself the question.
Cyan is the absence of red, so a very pale red filter would do the job, but it would need to be big enough to cover the whole panel. Fortunately, filters this size do exist â€” "gels" used for theatrical lights. And color adjustment of theatrical lights or floodlights is very important for moviemakers.
A web search led me to Stage Lighting Store, which sells a huge variety of gels for a few dollars each, with free shipping. But how could I decide which one I needed?
For that, I drew on my experience with color photography in the film era. I knew we needed to transmit red and cut green and blue slightly. To my aged eye it looked like I needed about CC10 or CC20 worth of red (film-era photographers will know what that means).
To my further delight, gels corresponding to CC filter numbers are available. They are the Rosco CalColor product line, and the CalColor 15 Red is what I ordered (for slightly more than $7, 20 by 24 inches). The only way to know if it's the right one would be to try it.
So I tried it. I cut out a piece of appropriate size and taped it to the panel. As you can see, the panel looks very pink when turned off, but pure white when glowing.
To test the correction, I took flat-field frames with it the same way I had done before. Here's how they looked on the camera this time:
Just a little strong on blue, but much closer to pure gray. And sure enough, using this kind of flats to calibrate astrophotos, I get the same color rendition that the camera would give without calibration. My light pollution shows up as golden brown, not vivid red.
An avid amateur astronomer for more years than you care to count, Michael Covington is the author of Digital SLR Astrophotography, Astrophotography for the Amateur, and other books. By day he is an artificial intelligence researcher, retired from The University of Georgia and now working in industry.
- WarrenP2, mrlovt, Bill Schneider and 1 other like this