How do I separate stars from target for color saturation?

[arrowspace90]

I have a problem child RGB project, M63.
It has good data, but somehow the channels are not well color balanced/calibrated.
I attacked the problem in PixInsight for a week, learning lots! I took the pink stars and inverted the image and ran SCNR on it. That worked well, but still, the galaxy itself wouldn't saturate correctly, despite many efforts.

Using Star Tools with this same data, the color of the galaxy comes out AWESOME! Exactly what I want, with me doing nothing.
But the stars are bright purple. (What causes this with RGB images?)
How can I separately color the stars and retain the galaxy result? Mask the galaxy somehow? I'm ready to get better with Star Tools. I consider it a fine resource, though I have worked hard to develop some skill in PixInsight.

[arrowspace90]

I hand drew a mask over the galaxy (is there a better way?). ST did a fine job of saturating it, better than I could coax out of PI in numerous tries (but I am less than a year in PI).
I inverted the mask and the best I could do was nudge the stars away from bright magenta.
I was having similar color issues in PixInsight, so there was "something" about this RGB data that was scrambling the picture.
It was great practice to begin manually using masks in ST when things are not going right.

[happy-kat]

There is a trick with the layer module.
viewtopic.php?f=10&t=2029&p=8940&hilit= ... rple#p8940

[admin]

Hi,

While the M63 galaxy in your image is indeed showing colors as expected, it is probably best to establish what exactly is causing your stars to be purple.

The most typical cause is an issue somewhere in your optical train - usually chromatic aberration. What are the specs of your optical train and camera?

If indeed chromatic aberration, and it cannot be addressed during acquisition, there are a number of different ways to tackle this in ST.

[arrowspace90]

Yes sir, it's a puzzlement.

But I thought that chromatic aberration is something that mostly happens to refractors.
I am using the F/2 RASA 8, teamed with the ASI2600MM. Technically, the RASA was designed for OSC. I am among the users that also do,
or attempt to do, RGB with it.
With my OSC, a ASI2600MC, I do not seem to have this problem. In fact, I had taken a OSC image, also of M63, a few nights earlier, and though I did not denoise it as well (I still could of course), it frustratingly seems to look better for about 5 hours of data than the 16 hours of RGB. I will post that one here as a comparison.
The OSC seems to have beautiful star colors AND great galaxy color. My RGB filters are Optolong 2in. Does a person need to drop a thousand bucks on Astrodons to get acceptable RGB?

[admin]

Yes sir, it's a puzzlement.
But I thought that chromatic aberration is something that mostly happens to refractors.
I am using the F/2 RASA 8, teamed with the ASI2600MM. Technically, the RASA was designed for OSC. I am among the users that also do,
or attempt to do, RGB with it.
With my OSC, a ASI2600MC, I do not seem to have this problem. In fact, I had taken a OSC image, also of M63, a few nights earlier, and though I did not denoise it as well (I still could of course), it frustratingly seems to look better for about 5 hours of data than the 16 hours of RGB. I will post that one here as a comparison.
The OSC seems to have beautiful star colors AND great galaxy color. My RGB filters are Optolong 2in. Does a person need to drop a thousand bucks on Astrodons to get acceptable RGB?

Thank you.

Chromatic aberration is sort of a "catch all" for anything that affects light differently at different wavelengths.
Subpar or damaged filter coatings can indeed also cause differences in how light is diffracted at different wavelengths.
Finally, it is possible the atmospheric conditions changed between acquiring the different channels, causing discrepancies in the point spread function between the channels. Indeed the stellar profiles seem somewhat "soft".

StarTools uses your stars by default to calibrate colors against. The stars are obviously not doing the job due to the discoloration - you correctly used the galaxy instead to calibrate agains,t as an alternative white balance source.

This then leaves you mitigating the discoloration in the stars;

• You can use the Filter module's Fringe Killer feature to remove any coloring (e.g. pink or purple) that does not match reality; create a star mask and make sure all the stars that show CA are selected in that (e.g. Grow once or twice). Back in the Filter module, set the Filter Mode to "Fringe Killer". Now zoom into one of the stars and keep clicking on the purple (or any other offending color) halo pixels around the stars. They should start to disappear.
• You can use the same sort of trick as you used in PI in ST on just the stars (use ST's Cap Green equivalent of PI's SCNR). Make sure Tracking is off though (otherwise it will keep using the Tracked colors and the the colors as-they-are in the image).
• You could also try the Shrink module on those stars, with Color Taming increased.

Hope this helps!

[happy-kat]

Another option is keep a mask excluding the offending stars and leave then in mono.