Problems with blue stars when adding Ha to LRGB.

Thanks for taking the time to reply. I stretch my RGB, L, and Ha master. Then I generate my LRGB. After that, I remove the stars and then add the Ha to the starless image. I also tried the same thing but without adding the L. Since I have my RGB_Ha and I add the stars from RGB, then I add the L master to my RGB_Ha. I use Pixel Math to add the stars back to my starless. In both cases, I have this problem. I don't know if it's because of the amount of Ha around the blue stars or if I'm just doing things wrong🙃. Thanks. CS!

I think, when you “screen” the stars back, they are not “blended” in and would maintain their looks from the stars image

But maybe some underlying color makes them look slightly red ish.

Someone with more insights into those details can most likely chime in on that.

I think we need a little more detail in order to be of help.

For example, when combining your Ha are you doing continuum subtraction first?

How are you combining your Lum with the RGB - are you using ImageBlend, or LRGBCombination, or something else?

And when you use PixelMath to recombine the stars, are you doing a straight addition or a re-screen? (assuming you unscreened when removing them)?

It might be worth trying one or two different ways to combine, such as NBRGBCombination and compare results.

Christian Bennich · Jan 9, 2026, 05:39 AM

I assume you extract stars with starxterminator?

Personally, I prefer using Screenstars from https://cosmicphotons.com to reapply stars.

Arguably it’s “just” pixelmath - but it works for me.

Interesting. I use StarXterminator. I'll try the script you mentioned. Thanks! CS!

Peter Hannah · Jan 9, 2026, 09:24 AM

I think we need a little more detail in order to be of help.

For example, when combining your Ha are you doing continuum subtraction first?

How are you combining your Lum with the RGB - are you using ImageBlend, or LRGBCombination, or something else?

And when you use PixelMath to recombine the stars, are you doing a straight addition or a re-screen? (assuming you unscreened when removing them)?

It might be worth trying one or two different ways to combine, such as NBRGBCombination and compare results.

Thanks Peter.

“For example, when combining your Ha are you doing continuum subtraction first?”

I simply add the starless from Ha to my RGB starless using the formula I mention in PixelMath.

“How are you combining your Lum with the RGB - are you using ImageBlend, or LRGBCombination, or something else?”

LRGBCombination

“And when you use PixelMath to recombine the stars, are you doing a straight addition or a re-screen? (assuming you unscreened when removing them)?”

I simply add my stars to my RGB_Ha starless.

I'll investigate more techniques to see if I can find the problem. Thank you very much. CS!

andrea tasselli · Jan 9, 2026, 02:56 PM

If you used SXT with "unscreen" tab selected (as it should) then that's the reason why. The pixelmath formula is ~((~starless)*(~stars))

Hi! That option isn't selected by default, and I always run SXT without enabling it. Do you suggest enabling it before applying SXT? Thanks! CS!

Enrique Ojeda · Jan 9, 2026, 04:30 AM

Hi! I would greatly appreciate your help. When I'm processing my LRGB_Ha image without stars, and then I want to add stars, I see that the blue stars change color slightly; they turn violet with a red center. This only happens when the Ha is added to my LRGB. In my LRGB, they appear perfectly blue.

Hi Enrique,

I think the issue arises because you are adding a red channel (Ha) to an image with red already in it.

When you stretch your RGB and Ha data, aim for a 50% background count, then when combined they don’t swamp eachother. You could also try ImageBlend, varying the Ha to suit.