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Hi All, I have seen this issue before, a distant star that is not in the picture, causes reflections (or shines into my system). Here, in my case it is Deneb when trying to image Sh2-115 (RGB starless image below). Equipment Skywatcher Esprit 100ED, QHY 268C. Is there a way this can be avoided? Or do I need to re-frame until the reflection is gone? Or are there any other tricks? CS Rob  |
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It cannot be removed or ameliorated in any meaningful way for a RGB image afaik. Never occurred to me but I don't use refractors.
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https://www.istarshooter.com/article/detail/141 I hope this article can help you well. |
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https://www.istarshooter.com/article/detail/141 Thanks, I will give this a try using an online translation tool. As well I will try to change the framing bit by bit until the reflection is gone. |
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I wonder if a lens hood extension might be helpful here. One that goes right to the edge of vignetting. If the light outside of the field can hit the objective the no halo. I would also reframe so that the long axis of the sensor is facing the star.
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Shine a flashlight down your OTA. Look for the shiny spots. Your culprit should be very obvious. My guess is it's the edge of your filters, the Esprit is otherwise very well darkened. To fix it, make it not shiny anymore. I used a 'vanta black' type paint I found on amazon as my culprit was my filter masks. In the temporary, you can try to shift your frame either way and get the star off that edge, you might have to go pretty far for that one. |
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Hi Rob, I know I have imaged this same area with similar gear and framing without issue. This looks like an issue I had with my QHY268M when a rubber plug came out and I did not know it causing a bad light leak. Is there any chance the 268c has the same port near the base and it is open in your case? Sorry I do not own the color model to know for sure. 268m.jpg CS, Jeff |
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Hi All, Thanks a lot for your recommendations. I checked the OTA and I could not see any obvious shiny surfaces (some slight variations are, however, present). I will make a few tests, like removing the filter and make some photos, put my mono system on (QHY268M, so same camera system) and try with LUM. If it still occurs, then I know that it should be within the OTA and most likely not the camera. I doubt that it is light leakage around the camera, I moved to NGC1333 at the same night and absolutely no issues. In addition, the radiation lines point straight to Deneb. I was thinking of trying to extend the lens hood somehow, so I will play around with this as well. Again, thanks for your suggestions. Rob |
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Stray light like this can happen in any optical system, whether is a refractor, reflector, or catadioptric system. It's caused by light from an out of field source reflecting from some internal surface in a way that the scattered light has a path to the sensor. Stray light like this in an image is almost impossible to remove with processing. The best thing to do is to figure out how to combat the problem in the optical system itself. Higher end telescopes often use a combination of glare-stops, high performance anti-reflection paint, and sometimes flocking to block and minimize stray off-axis reflections. If your system doesn't have glare-stops, you might consider using black flocking paper to fix the problem. The first thing to do is to point your scope out a window at a distant scene in the daytime. Pull the camera and put your eye where the sensor goes and look backward through the system to see if you can spot any shiny surfaces. Those are some of the reflections that you need to kill. If you can't see any strays then it becomes a game of blackening everything you can find inside the scope. It does not take much light to cause the problem that you've shown so you have to become VERY OCD about blackening EVERYTHING. Good luck with it! John PS. The other alternative is to move on to another target! That's what I had to do with my previously owned C14 Edge. It simply could not image M109 without a giant stray so I had to give up on it until I got another scope. |
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John Hayes: I'm curious John as to why extending the dew shield wouldn't help with the issue? I'm sure there's a reason, I'm just not seeing it. |
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Tony Gondola: *Because a dew shield isn't a field stop/cut-off. |
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Ahhh, meaning it would have to be at an unworkable distance from the objective to work in that capacity?
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Yes, it would need to be quite very very long.
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Tony Gondola: Andrea beat me to it and he is 100% correct. John |
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John Hayes: Hi John, Thanks a lot for your explanation and giving the issue the correct name. I cannot locate a clear problem except at the edge of the front lens when I point a light at an angle at the telescope (some dark painted parts shine a bit under strong light, and might add to the issue). This might cause some stray light. I am a bit hesitant taking the Esprit apart and trying to flog or re-paint the inside. I might end up with a miss-collimated refractor and that is not worth it. I rather point to a different target and accept that there might be some issues with some targets. Thanks again, Rob |
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Do you have an OAG? May be a reflection from the prism. I has a reflection that looked similar and I had to move the prism outboard a little more.
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I had a similar problem with an image just the other day with my 12” newt except my reflection was a very distinct defration spike that you would get off of a bright star. I imaged the object on the east side of the meridian no problem. Once I did a meridian flip I had that spike in the upper left hand corner of the image sticking right down through half of the frame. I spend a lot of time trying to track it down from internal reflections, external reflections and for the life of me couldn’t track it down. In the end I did what @John Hayes mentioned in his first post and I ended up not shooting it on the west side of the meridian. Just collected data on the east side. I’m still baffled at where its coming from! Dale |
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Rob Kiefer: Rick's suggestion to look for any stray light that might be coming from an OAG mirror/prism is a very good one. As long as you can remove the telescope objective cell in one piece without removing any of the lenses, you won't "mis-collimate" your system. I personally wouldn't hesitate to pul the objective apart to black things but unless you know how to properly handle precision optical lenses, I wouldn't recommend doing that. The objective assembly in a refractor is (or at least should be) well corrected over a wide field of view so there is no such thing as a "mis-collimated" refractor. The light that is striking your sensor is most likely coming from a pretty big angle so it may be hard to tell precisely where it's coming from. That usually means that you just have to blacken or flock everything that you can get to. Stray light problems can sometimes be a real bear to fix! John |
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Rick Krejci: Hi Rick, No, I am using a guide scope. |
