Old 03-09-2019, 08:12 AM
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codemonkey (Lee)
Lee "Wormsy" Borsboom

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Join Date: Jul 2013
Location: Kilcoy, QLD
Posts: 2,055
PCC and deconvolution

Greg Bradley tactfully pointed out that my recent NGC 253 could do with a bit more sharpening. I knew my luminance data was pretty good (the integration was 1.9" FWHM) but my colour data was sparing and probably not as sharp, in fact I probably even blurred it in order to mitigate the lack of integration somewhat. I wondered whether pushing the saturation so much with less than great chrominance data might compromise the apparent sharpness of the image. If I improved the colour data, or boosted the saturation less, would it look sharper?

Last night I decided to get some more colour data. Unfortunately the seeing was pretty poor and I ended up with the following FWHM on the integrated masters:

Blue: 2.79"
Green: 2.65"
Red: 2.56"

So now I had some (relatively) much less noisy data, but it was pretty blurry. I then decided to do something that I don't think I've ever done before; I deconvolved the RGB masters individually. After doing that I noticed something that I didn't expect: after calibrating the images with PCC using the exact same settings, the colour balance is wildly different.

After deconvolution, the images were:

Blue: 2.28"
Green: 2.21"
Red: 2.35"

I deconvolved the red data less to try and get star sizes similar, but clearly could have deconvolved the red a bit more to even it up. Still, the stars are closer in size after decon than before.

I often use PCC and then pull the red back a bit because the result is more red / magenta than many other images around, in fact my images are frequently significantly more red / magenta after pulling the red back some. I had been thinking that PCC must be technically correct and the other images around whilst looking prettier may not be as "correct" Discovering the impact of the star profiles on the resulting colour has shaken that belief and now I think I need to pay very close attention to getting similar star sizes in order to get the best colour.
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