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Old 04-10-2019, 05:42 PM
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codemonkey (Lee)
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PCC - does it give *correct* results?

I created a thread recently regarding PCC and the impact that deconvolution has on it, but Colin's recent NGC 253 image has inspired me to make another related topic.

In my previous thread I noted that PCC gives different results with the same settings, on the same data, if you but deconvolve the data before applying PCC.

Looking at Colin's image, I noticed it was very orange. I know that my image was much more blue, but I used the old ColorCalibration script plus a few tweaks so that's to be expected. Even though I used ColorCalibration in my image, I did try PCC first and didn't think the result looked like Colin's image so I tried it again... the result is attached here, see Colin's thread for his. Both used PCC with average spiral white reference.

I tried to very roughly approximate Colin's processing in the image I present here, but of course they're non-linear images that were processed differently. In addition, Colin's was OSC and mine was mono RGB.

So... should PCC give the same colour for the same target? It's supposed to be an objective method of colour calibration, so what's with the variations? I used to think PCC gave the correct results and that maybe they weren't what we're used to, or what we find most aesthetically pleasing, but they were correct... now I'm not so sure.
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  #2  
Old 04-10-2019, 05:57 PM
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Slawomir (Suavi)
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So far I have been unsuccessful with using the PCC on my RGB images. The colours are clearly not right, often strongly skewed towards one colour or another (all stars in the image have very similar cast - definitely not what I see with an eyepiece through the same telescope), even if the graph from PCC shows a nice tight trend with only a few outliers.
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Old 04-10-2019, 06:23 PM
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Atmos (Colin)
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I tend to agree with you Lee, I don’t think PCC does the best of jobs with the white balance from my experience.

It will do a different job with OSC sand RGB also due to the different band passes. RGB is more discrete and usually has drops around the sodium vapour lines (light pollution) where as OSC has band leakage, more green than red or blue and designed more so for daylight photography where it isn’t as important.
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Old 04-10-2019, 08:54 PM
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codemonkey (Lee)
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Cheers guys.

The difference in bandpass between OSC and typical RGB filters is exactly the sort of thing that I (perhaps naively) thought PCC would correct...

Either way, even if the intended purpose of PCC is to make objectively correct images, it seems to me that it's at least easy to fool.

Oh and btw Colin, I should be clear in that I'm not intending to say your image looks bad or anything, it's just different than what I get with PCC on the same target. I actually quite like your colour.
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Old 05-10-2019, 11:46 AM
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Atmos (Colin)
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I agree with you on the colour, it doesn’t look quite right

I’ll try a few different versions when I get some time for a comparison.
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