Good discussion on the Pixinsight forum regarding using Photometric Color Calibration on objects like IC342 and this Fireworks Galaxy. When I saw how different PCC was compared to the standard CC method I raised the question.

Shown here is how the Fireworks Galaxy color may appear to an observer outside our Milky Way Galaxy. Contrast this to how it appears as is...

http://www.astrobin.com/419052/B/

Here's a few excerpts from the Q&A discussion on the PI forum with Adam Block...

Q1: Typically when I use PCC I notice a bit of a shift towards red on galaxies. I first noticed this with IC342. But now when I reprocessed the Fireworks galaxy from CC to PCC I see a huge difference in CC (left) vs PCC (right). For CC I use most of the galaxy core shy of the dimmer arms for a white reference. For PCC, I tried Average Spiral Galaxy and Sc Galaxy. Both yield the same results. Is this PCC really what it should look like?

A1: The question you are asking deals with what to do with reddened objects. Both IC 342 and NGC 6946 are *severely* reddened by intervening dust. So they will appear quite red when you produce an image that does not compensate for this effect. PCC will not compensate since you are using color indices from a star catalog- the color channel weights you derive are independent of the galactic extinction. When you use CC, you are correcting for the galactic extinction (or at least making it less red) by shifting the white point as measured by the color channel signal strengths in your image. Said another way... if you had used CC on an *un-extincted* spiral galaxy (say from another image)- your PCC and CC results would agree much more closely (being red).

Do you think I am on track with my answer?

Q2: So are you saying that in this instance CC would be more correct?

A2: Some people would argue seeing a blue extincted (reddened) object is correct since this *is* what it looks like from our vantage point. We are getting fewer blue photons from the object than we otherwise would... so it should look redder. Other people would say... HEY! We are getting fewer blue photons...that isn't fair, we need to raise the blue signal (remove the "excess red") to compensate... this is how the galaxy would appear outside of our Milky Way (or away from the plane). I tend to find myself in the first camp since it is difficult to know exactly how much an object is being reddened. However, for artistic reasons, some objects are so red that there is a complete loss of color contrast- and so adjusting the white point does certainly bring them to life.