Problems with the blue color of stars - Forums

Topic starter

Oct 4, 2023 · Edited: Oct 4, 2023

Hello Everyone!
I would be very grateful if you could help me with the stars in my image please. As you may notice, blue stars are not completely blue, in the center they look like yellow. Others have one part blue and the other yellow and some that should be blue only have a blue dot and the rest yellow or white. I don't know if it is a problem in my setup or in my processing or in both ='(. My setup is: William Optics FLT91, Hotech flattener, ZWO filter wheel, ZWO LRGB filters and Atik 414 ex mono camera. The image is composed of 20 x 300 for RGB and 120 x 300 for L (Drizzel integration), in addition to Darks, Bias and Flats (Sky Bortle 2). The camera temperature at 5°C. I did not use any additional filter. The processing is in PixInsight. As I see the problem is only with the blue stars. I have already tried several color calibration methods but without success. In the RGB master, before performing the color calibration, I can see the detail in the blue stars that I mentioned, which makes me think that maybe it's my Set Up. The truth is I don't have much experience that's why I come to you. I have already investigated possible solutions but without success. Thank you very much!

Helpful Respectful Engaging

Sean Mc

Oct 4, 2023

Is this a brand new scope?

andrea tasselli

Frequent contributor

Oct 4, 2023

Difficult to say without accessing the raw RGB frames but I expect LCA being the culprit.

Enrique Ojeda

Topic starter

Oct 4, 2023

Sean Mc:
Is this a brand new scope?

WO has been selling it for almost 3 years. Thank you

Enrique Ojeda

Topic starter

Oct 4, 2023

andrea tasselli:
Difficult to say without accessing the raw RGB frames but I expect LCA being the culprit.

LCA? Is it a PixInsight process? Thank you

Sean Mc

Oct 4, 2023 · Edited: Oct 4, 2023

You also have star spikes. Did you add those? If not, I ‘d say the scope either needs to be fixed or returned. The aberration is affecting all the stars.

Enrique Ojeda

Topic starter

Oct 4, 2023

Sean Mc:
You also have star spikes. Did you add those? If not, I ‘d say the scope either needs to be fixed or returned. The aberration is affecting all the stars.

Hello! The spikes are natural, I didn't add them. Is that bad in a triplet refractor? I also have an ZWO ASI 1600 mono and I use it with a WO reducer flattener and I don't get spikes. They only come out when I use the tube with the Hotech flattener and the Atik 414 EX. I'll ask William Optics and see what they tell me. Thank you.ur reply here ***

Sean Mc

Oct 4, 2023 · Edited: Oct 4, 2023

They are hopefully going to ask for some pics. Take off the flattener and shoot a few at zenith and slewed east and west. That will also rule out the flattener. I returned two flt91’s. for the money, the stars should be small and virtually perfect.

Enrique Ojeda

Topic starter

Oct 4, 2023

Sean Mc:
They are hopefully going to ask for some pics. Take off the flattener and shoot a few at zenith and slewed east and west. That will also rule out the flattener. I returned two flt91’s. for the money, the stars should be small and virtually perfect.

My tube is still in the warranty period. I'll do the tests you mention...I think maybe it could be the Hotech flattener. Anyway I will contact WO. One question: when I shoot in narrow band I don't have this problem, but I don't know if this detail doesn't appear due to the type of filters. Thank you for your comments. Clear skies.

Sean Mc

Oct 4, 2023

You might not notice CA in narrowband single images, but when you combine rgb, you’ll notice. See what happens without the flattener, you have to start ruling things out now. The camera you’re using seems to have very large pixels so it should be way more forgiving than something like an asi2600.

Enrique Ojeda

Topic starter

Oct 4, 2023

Sean Mc:
You might not notice CA in narrowband single images, but when you combine rgb, you’ll notice. See what happens without the flattener, you have to start ruling things out now. The camera you’re using seems to have very large pixels so it should be way more forgiving than something like an asi2600.

Thanks a lot Sean! Clear skies!

Rich Sky

Oct 4, 2023

Perhaps, a focus problem followed by stacking method. RGB Channels mis-alignment, or curvature caused by filter or scope.
All need to be investigated. Good luck.

andrea tasselli

Frequent contributor

Oct 4, 2023

Enrique Ojeda:
LCA? Is it a PixInsight process? Thank you

LCA stands for Lateral Color Aberration, the variation of focal length with wavelength and field angle. However, in your case, it seems pretty constant across the field so it is unlikely to come from the main objective. Some other refractive components must be adding this.

Well written Insightful Concise

Enrique Ojeda

Topic starter

Oct 4, 2023

Rich Sky:
Perhaps, a focus problem followed by stacking method. RGB Channels mis-alignment, or curvature caused by filter or scope.
All need to be investigated. Good luck.

Thanks a lot Rich. Clear skies!

Enrique Ojeda

Topic starter

Oct 4, 2023

andrea tasselli:
Enrique Ojeda:
LCA? Is it a PixInsight process? Thank you

LCA stands for Lateral Color Aberration, the variation of focal length with wavelength and field angle. However, in your case, it seems pretty constant across the field so it is unlikely to come from the main objective. Some other refractive components must be adding this.

Thanks a lot Andrea. Clear skies!

Marc Monarcha

Oct 4, 2023

Not sure what the problem is but fix it quick! Your picture is looking absolutely stunning

Enrique Ojeda

Topic starter

Oct 4, 2023

Marc Monarcha:
Not sure what the problem is but fix it quick! Your picture is looking absolutely stunning

Thanks a lot Marc!

Johnny Qiu

Oct 5, 2023

It looks like it is because the stars from R/G/B channels have different aberration so they don't exactly align. It may be caused by the optical system itself. But sometimes it can be improved by acquisition strategy. For example, I am using parfocal LRGB filters and don't redo auto focus when switching filters. Then I loop RGB instead of taking them separately during different time. If you take R then G then B, during different hours / days, they may have slight different / imprecise focus which causes the aberration on different orientations. Seeing, guiding and wind may also vary. If you loop RGB with parfocal filters, these difference can be averaged out.

Helpful Concise

Enrique Ojeda

Topic starter

Oct 5, 2023

Johnny Qiu:
It looks like it is because the stars from R/G/B channels have different aberration so they don't exactly align. It may be caused by the optical system itself. But sometimes it can be improved by acquisition strategy. For example, I am using parfocal LRGB filters and don't redo auto focus when switching filters. Then I loop RGB instead of taking them separately during different time. If you take R then G then B, during different hours / days, they may have slight different / imprecise focus which causes the aberration on different orientations. Seeing, guiding and wind may also vary. If you loop RGB with parfocal filters, these difference can be averaged out.

Very interesting! I get the RGB separately and yes, I think I took at least one of them on another night. I also refocus every time I change the filter. I am going to test all the suggestions that everyone who has commented here has kindly given me. I hope the culprit isn't my tube! Thank you and clear skies, without wind, without moon and with good seeing!

Respectful Supportive

Sean Mc

Oct 5, 2023

Do you have or can you borrow a color camera to try on the scope? That will rule out LCA due to stacking if you have aberration in one image. Unfortunately both of my returned scopes showed this in single color images. I wish you luck too!

Supportive

Enrique Ojeda

Topic starter

Oct 5, 2023

Sean Mc:
Do you have or can you borrow a color camera to try on the scope? That will rule out LCA due to stacking if you have aberration in one image. Unfortunately both of my returned scopes showed this in single color images. I wish you luck too!

Yes I have a DSLR. I will try your suggestion. Thank you so much. Clear skies!

Well written Respectful

Joon Ren

Oct 6, 2023

Looks like chromatic aberration from the scope. Its fixable in post-processing by running something like Star Alignment in Pixinsight to remove slight differences and distortions between the R, G and B frames. There's a fast tutorial here https://www.youtube.com/watch?v=UCKoJ1ticHI

Helpful Concise

Enrique Ojeda

Topic starter

Oct 6, 2023

Joon Ren:
Looks like chromatic aberration from the scope. Its fixable in post-processing by running something like Star Alignment in Pixinsight to remove slight differences and distortions between the R, G and B frames. There's a fast tutorial here https://www.youtube.com/watch?v=UCKoJ1ticHI

I really appreciate your help Joon. I see it right now! It is truly a blessing to have this platform to ask for support and even better to have people like you and those who have helped me with their suggestions. Blessings to all and what better than clear skies!

Respectful Supportive

Related discussions

Problems generating my flats

Hi! Does anyone know why my flats shots have two circles in the center of the image? My equipment is a WO RedCat v3, and in the optical train I have a ZWO filter drawer, a filter wheel, and my ASI 294mm Pro camera. I have the filter drawer because I ...

Flat calibration issues can cause color fringing and uneven star coloration.

Aug 5, 2025

Flat Calibration Problem

Hi everyone, I'm having an issue with flats/calibration in my setup. My setup consists of a Carbon TS Newton 200/800 with Starizona Nexus (effectively 200/600), QHY ImageTrain (QHY 268M with QHY CFW), and AstroWanderer Rotator. A screenshot from ...

Flat calibration problems directly relate to star color and processing artifacts.

Mar 11, 2025