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Something came up in another thread that I thought might be applicable here: by inserting a guide camera with a small lens attachment into the focuser, it can help identify light leaks in the OTA. The one I found most interesting is the light leak around the focuser tube, which might be relevant in this case. If there is movement in the focuser tube, that could be the source of the ring in your light/flat frames. The shift in the position of the secondary may also influence how this appears when the OTA is positioned in different directions. Austrian Astrophotographer Daniel Nimmervoll - who has a nice German language youtube channel ((1) Daniel Nimmervoll - YouTube) - uses this cheap ZWO lens for such purposes: ZWO ZWO New CS lens 2.8mm-12mm F1.4 C Mount Lens - Camera Concepts & Telescope Solutions (shoplightspeed.com) Clear skies Wolfgang |
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| @R8RO , any news about the issue? Did you address it out? Ciao. Lorenzo |
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Lorenzo Siciliano: Still waiting for the Artesky stable secondary spider. I've been looking into getting a bigger secondary mirror if the spider doesn't fix the issue. I suspect there is something along the edge of the mirror that is causing this so my thinking is: a large secondary might push the issue out of the imaging circle of my camera. I will update as soon as I have more results to show! |
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| I personally think you should give up on this. There are hundreds, if not thousands of us using this OTA in its standard configuration with no problems. Maybe you have got carried away with all your flocking and modifications. |
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Andy Wray: Can't give up now, I'm in too deep the alternative would be to spend 4x the money to get a decent refractor at the same focal length. I'll see you on the other side o7 |
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Andy Wray: Well, I think he should not give up lol. I am having the same problem with the same standard configuration and use. It's a real headache for shooting LRGB. As I've said, some live under less polluted skies and even if the scope presents the problem, they won't be affected because of the sky. Plus it is a tough problem to solve, it would be interesting to understand what causes it optically. I ordered the spider too, but don't tell anyone  .
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Alright, latest update! My Artesky stable secondary mirror holder/spider came a few days ago and last night I had a 20 minute window to test it. It works!  I still have no idea what is causing the ring, it's still very visible in lights and in uncalibrated stacks but at least it looks consistent between flats and lights now, no more movement. These flats were even taken the day after imaging so that goes to show how much more stable the secondary is. Uncalibrated stack with a crop on the ring:  My next step is probably to to buy a larger secondary. The one I have now is 58mm and I am thinking either 63,5 or 70. |
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Dang im late to the party. i want to add a few things I have found. You definitely do not under any circumstance want the drawtube obstructing the light patch. The face of my drawtube facing the secondary is very shiny the edge of a secondary is not turned down and creates stray light an ed turco light trap can help decrease stray light, big mod though. try taking iterative flats. Does your donut improve if you take 20 flats vs 5? My first choice would be to do 100 flats, at around 1/250 of a second. Ideally with a dead center histogram. Your secondary could be shifting at different angles, the springs are under compression Your primary springs could be shifting and compressing. I replaced mine with silicone 3d printer bed springs. You can practice flats on any data indoor to try and resolve the shadow issue. It wont fix dust of course but will fix the shadow |
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Wes Schwarz: Thanks Wes, care to elaborate more on these two? With the help of Teleskop Service customer support I think I've narrowed the issue down to something to do with the secondary reflecting. Appreciate the input. |
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Wes Schwarz: The secondary edge is usually sparkly after a coating, flocking this reduces stray light. If the edge is sharp and not turned down, which btw is pretty unusual on a Newtonian secondary, it can create alot of scatter. The late ed turco invented the dark bucket / light trap. Its for visual but essentially increases contrast by capturing stray light at the top of the tube before it back illuminates the secondary https://www.cloudynights.com/topic/639438-show-us-your-tdb-turco-dark-bucket/ |
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Wes Schwarz:Wes Schwarz: Thank you, I've since a couple of weeks back flocked the secondary to reduce as many reflections as possible. Never heard of the light trap before, feels a bit extreme of a mod for me to do at the moment but might be something to look into in the future. |
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Alright, latest update! Ok, glad it works to you too! I'm very happy that you eventually address that annoying issue out. Clear skies and stable secondary mirrors! Ciao. Lorenzo |
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Lorenzo Siciliano: Grateful for your advice! Still considering a larger secondary, but that's in the future for now! Clear skies! |
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Alright, latest update! how many flats do you take? |
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Wes Schwarz: 20 flats and 30 dark flats at 0.03s with an ADU of 37k |
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Wes Schwarz: id really be curious to know if 100 flats help. For me the difference in flat integration between 25, 50, and 100 with fast optics using no filters was the difference between horrible flats and perfect flats |
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Wes Schwarz: Flats are correcting perfectly now, it's just the the reflection itself isn't gone. It is no longer a flats issue and a: hunt down a ghost reflection issue |
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Wes Schwarz: maybe I’m confused about the issue then because I am looking at that last post where there is a dark circle in the center of the image. What is the reflection that you were talking about is that the same image? Because that is most certainly a flat correction issue |
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Wes Schwarz: Both of the images are the same stack. One is calibrated with flats (the top one) and the second is not. The dark spot you are seeing is a result of a reflection in my lights that I previously had issues calibrating out. With my new stable mirror that no longer moves the flat works perfectly as seen by the above image. However, the lights still have this reflection in them, so I haven't solved the problem fully, I've just made sure that the flats now apply correctly. |
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It is interesting to look back through this thread to see all the things that were tried to fix this problem. Lorenzo’s idea of replacing the spider merely makes the mechanics more stable so that the flats are better aligned. There is no way that the the entrance pupil can be “imaged” in the image plane as he originally suggested. Without any optics between the primary and the sensor, both the entrance and exit pupils in a Newtonian are located at the primary mirror. Further more, the image of the secondary itself is virtual and lies way out in front of the telescope. If a zero power, coma corrector is in the system that won’t change anything. Changing the size of the secondary could do a number of things. First, it may do a better job of blocking light that is getting around the edge of the secondary mirror. This could be easily tested with a piece of black cardboard behind the secondary. Second, a new secondary will have a different coating. It is possible that the a coating problem could cause this sort of problem; although that’s highly unlikely. At this point, you’ve got something that works and maybe that’s all that matters, but it’s not a very satisfying solution simply because it doesn’t make much sense. I’ve run into similar issues with calibration due to vignetting and stray light issues so I understand that these kinds of problems can be difficult to explain. Optical problems may come from unexpected sources but optics isn’t Voodoo. The symptom itself strongly suggests an issue within a spacer tuber or with a ring of stray light being generated precisely on-axis. You’ve tried a lot of stuff, but clearly you haven’t found the culprit. I assume that you were very careful but I also have to emphasize that when you are looking at stretched data, it doesn’t not take very much to cause a problem. One part in a thousand or even ten-thousand can be very noticeable. Did you ever pull the camera and look backwards through the system? That is generally the most effective way to ID the source of problems like this. This post shows what I’m talking about: https://www.astrobin.com/xfedon/I/. John |
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Hi folks! I know that this is an old topic but I want to share anyway my solution to this problem and maybe it can be useful for someone. After I saw this topic I suspected that in my case there was some unwanted light that was coming from focuser and from primary mirror support. I did some tests in daylight time using one of the most cheap and powerful instrument that I have in my personal gear: the eyes!!! Thanks to @John Hayes that inspired me with his very useful post about this kind of problems, I put my eye in the 2" focuser hole and I saw that there were a lot of unwanted lights. I noticed that there were concentric circled lights infiltrations and with the help of fingertips I have identified where they come from. 1. Around the secondary mirror (@Lorenzo Siciliano with the replacement of secondary support solved it) 2. Around the focuser tube attach. 3. Around the 2" focuser tube. 4. Around the primary mirror. Thanks to an art attack that I had after the discovery I solved it with some black paper...this time everything was black with no unwanted lights. Now the effect is very mitigated and I can solve it with flat frame applying. I have attached some picture to better understanding.   Before and after photo example... https://www.astrobin.com/fz5m52/B/  Crescent nebula stack of three 180" frames just calibrated.  Carlo |
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Carlo Caligiuri: Also check the primary isn’t moving, I have seen this solved with a few blobs of low modulous silicone on primary mirror to holder to stop mirror shift as telescope tracks . |
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Carlo,but I want to share anyway my solution to this problem and maybe it can be useful for someone. Thanks for following this up Carlo. Very useful to see what worked in the end plus some nice images |
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| I think you should watch the excellent videos from Cuiv the Lazy Geek on the topic, he struggled with similar issues |
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Hi everyone i spent last night reading the entire thread and i waas releaved that someone had a solution. I just got my quattro 150 and i tested it out last night. i had a ring artifact but its not the same as the one shared here. I even tested collimation which was perfect and the collimation does not shift during movement.  |
