ColorCalibration vs PhotometricColorCalibration for Fireworks Galaxy

[KuriousGeorge]

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?

[dld]

If I remember, the Fireworks Galaxy is bright in infrared. Thus the PCC result might be more reasonable. This is one of the advantages of PCC over using the whole galaxy as white reference.

[ngc1535]

George,

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?
-adam

[dave_galera]

Adam,

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

[ngc1535]

*laugh*
Dave, that is exactly the issue.

- 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.
-adam

[dave_galera]

LOL.......OK thanks Adam......perfect

[KuriousGeorge]

Great feedback Adam! I suppose I didn't realize the star catalog doesn't compensate for dust. Nice to know we can have both images.

[wvanreeven]

Astrophysically speaking the catalog *does* compensate for dust because the stars are affected by dust as well though mostly less than the galaxy since most of the dust lies behind the stars. This is why the galaxy appears as red. It is counter-intuitive though since “compensating for dust” implies “removal of the effect” while that is not the case in this case. Anyway, that’s all semantics of course 

[KuriousGeorge]

Want to make sure I get this right.

Is PCC doing its best to make the "cataloged stars" appear as they should if there were NO dust. So that shifts the color of the galaxy?

Or does the catalog include the color of the stars with the dust, so EVERYTHING you see in the PCC version is as an observer on Earth should see it?

In other words, is PCC 100% correct for what we should see on Earth, both stars and galaxy?

[wvanreeven]

PCC uses the star colours as seen from Earth so it shows things as we should see it from Earth. Whether or not you like it is up to you 

[KuriousGeorge]

Here's an example of PCC vs CC for these two "hidden" galaxies. This is 2-3 hours in each RGB channel in clear 21.5 SQM skies with a Planewave CDK24.