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Optolong L-PRO 2in filter causing red halos around bright stars

Optolong L-Pro 2"
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Steve Eltz avatar
Steve Eltz avatar
Has anyone else experienced this? I have a William Optics Zenithstar 73 Doublet APO and I using the William Optics Flat 73A flattener. My camera is Canon T5i (700D) The 2 inch Optolong l-pro filter is screwed to the back of the flattener so it is positioned between the flattener and my DSLR's sensor. You will see in the attached photo that the very bright stars have a red halo around them. Is there something wrong? I just recently purchased the filter and did not have these issues before the filter. I was wondering if anyone else had these issues with this filter or maybe, I hope, that I just have a bad filter. Please let me know. Thanks.

Engaging
D. Jung avatar
D. Jung avatar
This is fairly normal. I have that with all of my filters on bright stars.
My best guess is some double reflection causing this secondary out of focus star halo.
This filters don't block 100%, so there's always some leakage that can bounce around.
Concise Supportive
Steve Eltz avatar
Steve Eltz avatar
OK thanks. Is there a way to remove them in processing?
D. Jung avatar
D. Jung avatar
Sometimes I use clone stamp to remove them. If you get reflections on background this is a good method, but in your case it will probably look worse than just leaving it as is.
Maybe there are other methods, but I don't know of any.
Steve Ludwig avatar
Steve Ludwig avatar
Steve Eltz:
OK thanks. Is there a way to remove them in processing?


As they are red, you could extract the red channel and use "range selection" to generate a mask that contains all halos. Areas you don't like to have in that mask can be removed with the "clone stamp" tool. Then, I would apply the mask to the color image and use "curves transformation to get rid if the halos. Maybe by decreasing saturation of the red channel.
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Gustav Lundby avatar
Gustav Lundby avatar
I use Optolong L-Pro too, but so far have not seen this. It might be back and forward reflection. I can see that both my full spectrum Sony 5000 sensor and the Optolong L-Pro looks like shining mirrors!
Bruce Donzanti avatar
Bruce Donzanti avatar
I also have used the Optolong L-Pro but have never seen this red halo.  Perhaps it is scope dependent?   I 've used it on an 11" SCT and an 80mm APO triplet with no issues.   I'd be curious to learn if you ever figure out what is causing it.
Well Written Respectful Engaging
D. Jung avatar
D. Jung avatar
I have seen this issue on 2 different telescope types and camera lens. Severity seems to depend on position in the optical path as well as type of optics. I have the biggest issues when the filter is close to the sensor or to another lens element. So my guess is, this is defocused double reflection.
Ian avatar
Ian avatar
Steve Eltz:
OK thanks. Is there a way to remove them in processing?


 I remove chromatic aberrations around stars using "Defringe" in Adobe Lightroom, it works well for purple and green fringing most of the time, it might help here.  I think Adobe camera raw may have something similar but I have not tried it.
Concise
Lodewijk van Pol avatar
Lodewijk van Pol avatar
It is caused by the filter, in particular when You use flattener/reducers. Check this one:

https://www.stastrophotography.com/the-source-to-the-halo-around-bright-stars/