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| From the last image I'd venture to guess it is a defective coating within a curved surface, so in all likelyhood the flattener. |
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| I reached out to Sky-Watcher support. |
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I'd like to add my experience to this thread and see if others find it helpful. The images discussed here were taken with an SVX140-T using the mfr's reducer. Without the reducer, these artifacts didn't show up. I found this artifact in the first test exposure using the reducer. It reminded me of a similar artifact I saw briefly looking at Mars, so I went back to Mars to see if it was still there.  It was - and it was very similar in shape and orientation to the M42 artifact. I went through a number of changes such as backing off the lens shade and using a spacer to change the camera orientation. The artifact was related not to the camera, but to the optical system in front of it. I had just put in a reducer that brought the scope to f5.11 and wondered if the OAG pickoff prism may be in play. I have the OAG-L with 12x12mm prism that had been adjusted for f6.7 and f7 scopes. I moved it quite a bit upward, losing 40% of the pickoff's vertical dimension and got this image.  No need to highlight this one an inch from the left middle. The reflection is still symmetrical across the optical axis at the same distance as the brightest star, although its morphology is much different than above. The prism is in play or the cause. Note that the fairly bright star above-right does not have a reflected counterpart on the left. When I turned to Mars the artifact was similarly changed. I noticed that, both before and after the adjustment, the reflection only occurred when the bright object was lower-right, never above the middle. It had a field of sensitivity where the artifact was brightest, falling off as Mars was moved from the locus of reflection. I don't know if the prism is the complete cause of the problem. Moving it higher didn't seem like a nice option, but that's the next test I'll do. Right now I have the 150ED on the mount with its .77 reducer and the prism back to fully-illuminated. I don't see the artifact and thus suggest that other 150ED users - especially with OAG-L - try adjusting their prism and let us know what you find. [EDIT] Hyper-stretching the second M42 artifact shows an interesting feature.  Its shape changed during the one hour exposure set, even though the star field was guided well! The shape can be mentally deconvoluted and it seems to hold insightful information. |
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andrea tasselli: If you twist the camera and OAG a bit relative to the flattener, a reflection from the flattener can be differentiated from a reflection by the prism. I used a thin spacer to keep things orthogonal, but as a test it doesn't matter much. |
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David Andra: I cited this thread when I contacted Sky-Watcher support and here was their reply: "My 1st thought is the Filters." Okay, so I removed a filter and shot a 60s exposure. The reflection persisted in the absence of a filter (below). The offending part of the image train must be the Sky-Watcher flattener since removal of the flattener was the only test that removed the reflection. Disappointing since the flattener is "stock" and I'm left to post-process out the reflection, try a reducer-flattener, or see if I can order a new flattener. David  |
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This is an older post, but I have almost the exact problem with my Tak TSA-120 refractor. Did you ever find the source of the problem? Thanks!! CS - Larry |
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Larry S: I have not seen the reflections since I changed the distance of the OAG prism, making it farther from the image's optical cone. I saw a photo of a "sock" someone made that covers the edges of the prism, which I believe is the source of the reflections. Note that every telescope here is fast (assuming the Tak has a reducer). This is when the initial problem showed up for me, switching from f6.7 to f5 with the SV reducer. It's easy to determine if the prism is the source. Expose an image with a reflection... rotate the OAG relative to the telescope, leaving the camera unrotated relative to the OAG. A quarter-turn will be sufficient, and while the connection isn't rock-solid, the results will isolate the fault to prism or filter set. It's hard for me to imagine these various scope and filter combinations all being related to filters. EDIT: I should note that this reflection always appeared opposite the optical axis from the bright source. |
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| Thanks for the reply! Unfortunately, I don't use an OAG. I do have a reducer, but I have the same issue with the reducer and the flattener, and they have large differences in position relative to the filters and camera. I have it with two different cameras and filter sets (Chroma and Astrodon). I'm thinking it must be some reflection at the objective end. I bought a dew shield extender but have not had any clear skies to try it yet. |
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| Duplicate {oops} |
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No OAG does speak to filters - as well as the other potentially reflective surfaces. I use Chromas on the CDK14 and Astronomiks on the refractors, as they all have 55mm back-spacing and 48mm mating threads. I assume you've followed the Chroma guidance and have all filters' shiny side away from the camera. An Esprit 100ED f5 had ballooning red and blue stars, and Rouzbeh suggested I try an L3 luminance filter as I was using an OSC. It helped quite a bit with the 100ED and 2600MC, and there are no reflections with the SVX140-T with its excellent FF and reducer. I just looked at the 100ED FF's camera side and it appears to have minimal or no anti-reflective coatings. Check out the back end of your reducer and FF - do they appear to have good AR coatings? The 100ED FF has a curved final surface, which would result in radially offset reflections, with offsets that vary across the image field. If an AR element was inserted between the filter wheel and FF/FLR in place of the OAG, this theory could be directly tested. ZWO makes a filter drawer, perhaps that can replace a spacer? If so, any [broadband] 48mm filter with full AR on both sides would work for the test, assuming one can be found. Does that make sense to you? How do your FF/FLR rear surfaces look? [EDIT] Do you see reflections in the G band? |
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| Here is an Anti-Reflection Window Filter, coated both sides, 0.5% reflectance. Sadly, a 48mm filter is $221, but this is the first vendor I've found offering such a product. |
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Thanks for the suggestions. I get the reflections with all filters including G (also SHO). I have flipped all the filters. I will look at the rear surfaces of the FF/FR. I seem to recall that they are both flat. Also of note, this started recently (last September), I used the same combo of equipment since January 2022 with no such issues. Quite the mystery... CS - Larry |
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Sudden thought: could it be dewing? Maybe the desiccator needs to be heated in an oven. Uh... I have a QHY268M and it has a removable desiccator. Don't know about the 2600MM, but perhaps worth a search if you live in a humid area. It's a surprise that you'd have no problems, then have them reliably after period of time (or an event). Would be nice to have a second camera, even an OSC. You said you had "almost the exact problem". Several problems were discussed. Are you seeing blooming stars, offset reflections, or what? I got past my problems and would like to help if possible. |
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I have the same problem with two different cameras, ASI294MM with Astrodon 31mm filters, and ASI533MM with Chroma 31mm filters. So, I don't think it is dew in the camera. I also have ASI2600MM with Astrodon 36mm filters, but I have not used that one with the TSA-120 since this problem surfaced (it has been on the FSQ-85EDX). That one has its own dew heater. I will try it when I have the opportunity, looks like maybe a clear night on Monday. The problem I am having is offset reflections. I have a forum thread I started with images here Ghost Reflections from Refractor Thanks again! |
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| David, here is my case study - I also have the same issue, but I have observed during the same night some images have them and some do not. after careful observation I figured the presence of the moon was contributing to the glow. since my setup is remote my best guess is moon light is reflecting from my flip-flat and causing the Halo. |
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Randy Pfeiffer: The surfaces look pristine, like new, very little dust. Whatever original coatings are intact, no smears or blots. The rear (camera facing) surface of the FR is slightly concave. The FF is flat. The distance to camera from rear surfaces are not the same, the FF is about 50mm more. Both are within 0.5mm of Tak required metal back distance. I am having the problem with both FF/FR and two different cameras/filter sets. This would seem to indicate something in the telescope objective to me. Thanks, Larry |
