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Hello AstroBin Community I hope everyone is doing ok. I need help with my images taken with a newtonian telescope. I've been having some problem with my frames. It seems that some kind of circle is messing my images in the center of the picture. This pattern appears in flats and light, not in darks and darkflats (i don't use bias frames). I've tried to cover any light leaks, but that doesn't seem to correct the problem. My equipment is this: Skywatcher 200/1000 Newtonian Telescope QHY 294C Camera Pegasus Astro Pocket Powerbox Micro ZWO EAF ZWO ASI 290MC (guide camera) ZWO OAG ZWO Filter Drawer I use the L-Pro and L-Extreme filters (2 inch) Coma Corrector Baader MPCC Mark III If someone have been experience something similar, please share your thoughts and posible solutions. Thank you again!  |
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| You say this appears in your flats and lights; maybe you could post a stretched version of each. It would be good for the light to be a single original stretched image before calibration with flats. |
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| The only time I had a similar problem it was due to a bad flat caused by a light leak. Show us your setup and we might be of help. |
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Andy Wray: *** Hello Andy, thanks for your reply I uploaded one of each file (Light, Dark, Flat, Darkflat). I've also uploaded the integration file. Here is the link: https://drive.google.com/drive/folders/13dKFMwQPfFU5LHA7C-13aq4_nPlc8FPZ?usp=sharing *** |
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andrea tasselli: *** Hello Andrea, thanks for you reply. I uploaded a picture of the camera installed in the focuser with all the accesories. Tell me if you need something else. Here is the link: https://drive.google.com/file/d/1pactX6t9ztromg1lBbzG5U0LsLwcAn1s/view?usp=sharing*** |
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I second the light leak idea, but I think it is in the darks. CS |
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| You will always have cirular patterns in your flat, or better: your flat is basically a circular pattern. If the flat was taken with a slightly different focus than the light, it will lead to circular patterns in the corrected light as well. |
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| Ok. Looked at them files. From the imaging train pic there isn't any clue that the light leek might be there. Seems a pretty solid setup. I'd like to see the rear of the scope which is possibly another source of light leeks. Looking at dark/flat-dark nothing to say about them either. Looking at the light frame and the flat frame it seems the flat is overcorrecting the light. So the question is how do you take your flats and in what conditions (day/night/dusk, with what and with the telescope mounted in situ or dismounted and so on). |
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| Could it be dust? I have seen this before with dust and once I cleaned my sensor all was much better. |
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Robert Winslow: Very very unlikely. |
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andrea tasselli: *** Hello Andrea. Here is the link to a photo of the rear of the telescope: https://drive.google.com/file/d/1voE40M0TKhiRf_uhwZRSzOKcr4dQa2Dy/view?usp=sharing As you can see, is covered with a paperboard (i painted it black). I took the flats during the sunset when the light is enough, pointing to the sky with a white t-shirt I use the program called NINA and the Flatwizard feature (in this case the flat is 0.73 s of duration with the histogram at 50%, i have a tolerance of 10% also). The telescope is in situ always. I was thinking that something weird happened the last time that i was taking photos. I pointed to the south and the circle almost dissapears, but when i point to the southeast or east, the circle becomes very visible. I´ve only tried this once, so i´m not so sure (i'm going to try again definitely). Could it be related with the Baader Coma Corrector? I have the MKIII version and some people in others forum said that there is some problem with the coated optics. Thanks for your help!*** |
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Hello Fernando, Was the moon visible at that time (when you took the flats)? That's one source of possible light interference. This said do you have some previously saved master flats with the same set-up and do they show the some over-correction. I don't think there's any chance of light leek from the rear of the scope. I've used the mkII MPCC in the past and while not great didn't cause any issue with the flats nor do I expect it to do, to be honest. Given that the rig is fixed it shouldn't be an issue to retry flats again. Take them 5 to 10 min after sunset (the actual sunset, check your local sunset time) and do not use t-shirts (you don't need to). Point the scope at least 90 degrees away from the point the Sun has set staying as close as possible to the zenith. Try different exposure levels, say from 20% of the histogram to 50% of the histogram. At least 20 shots x master flat. See which one works the best. Obviously we can correct your final stacked up light frame, even with the circle, if you want us to. Just upload it on your goggle drive. As a fits file. |
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Hey, Fernando. My work computer can't open TIF files, so I couldn't look at the files you shared. Based on your descriptions of the setup, I'm going to go with either a light leak when taking your darks (there are too many joints in your shadowbox assembly, although I do love that idea!) or changing illumination when taking your flats. I used to get very frustrated with trying to take flats at dawn/dusk because of how fast the ADU levels changed, so I just gave up and took them when the sun was fully up. Just aim the scope well away from the sun, make sure there were no clouds, and use more layers of t-shirt. Hope some of my ramblings can help out, please let us know! |
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Hi Fernando, Did you change the rotation of your camera between flats and lights? That has burned me before... Jason |
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After processing the final image I'm leaning toward an issue with processing rather than fundamentally wrong flat-fielding. Here is the stretched final image once cleaned up in PI. What program was used to process it?: |
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| Incidentally, the bluish streak on the upper left quadrant is an odd ghost reflection from the MPCC. You might also want to check collimation, especially centering of the secondary, as there is significant flaring in the brighter stars. |
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And, finally, unprocessed but auto-stretched in PI: |
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I would agree with Andrea. I looked at all your files (flat, light, dark flat, dark) and they all look like I would expect. A couple of dust bunnies and vignetting in the flat (normal) and amp glow in the dark (also normal). I think you did your stacking in DSS, so wondering if you have altered your settings in there somehow to cause the problem. I used to use DSS and StarTools and quite frequently ended up with results like your first image. Right now I'm just using Pixinsight for everything and am finding the whole process a lot more satisfying. I also had a quick go at your integration frame in Pixinsight and ended up with something similar to (although not as good as) Andrea.  |
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Hello everyone! i read all your replies and definitely i'm going to try all your suggestions. An stunning processing from Andrea and Andy, congratulations both of you. Wich tools did you use in Pixinsight to get rid of the circle? Some of you left questions. I'll try to answer them: 1-. Was the moon visible at that time (when you took the flats)? No, it was new moon. i have to add that there are some street lights (LED Lights) at 50 m from my imaging position. But, as you already saw, i've tried to cover all possible light leaks in my OTA. I've also added a dewcap in the front of the scope. 2-. What program was used to process it? I've integrated the files with Deep Space Stacker. After that, i process the image with PixInsight. Now i'm trying to do all with Pix. 3-. Did you change the rotation of your camera between flats and lights? No, the camera is always fixed. Every time that i select a new target, i renew all my calibration frames (Flats, Darks, Darkflats). I don't use Bias Frames. Recently some people in other forums told me that maybe the Skywatcher 200PDS could have a smaller secondary mirror, and this can cause problems when you have to illuminate the whole sensor (my sensor size is 4/3"). Could this be related with my issue? The rest of the week i'm going to have some clear nights. As i said before, i'll try all your suggestions. I hope i can point the problem soon, because i'm struggling with this for some time. Thanks to everyone! |
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| I had this problem for two years and its easy to solve. just cover up your primary mirror from back of the telescope and it'll be ok. light leaks from back of the primary holders |
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Ref a couple of your points: * I used ABE with a very large number of small sample sqaures to extract the background; I tried DBE first on it, but found ABE did a better job in this instance. * I also have the 200PDS/4/3" sensor combo and do not have the circle issue. I do, however have some pretty significant vignetting, but that is easily corrected with flats. |
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| I used 6 iterations of DBE with 55 samples per row, initial sample weight 0.25, increasing to 0.35 for the subsequent iterations. Correction set to subtraction all the times. I can send you the corrected light frame, processed and unprocessed, if you need them. I strongly advise you to ditch DSS and use PI for ALL your processing. It is going to be well worth your while. |
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| One last thought as someone who transitioned from DSS to Pixinsight: the Weighted Batch Preprocessor script was a very natural way to move from DSS to Pixinsight as it covers most of the same functions as DSS. You will clearly want to learn the individual steps involved downstream, but it would be a quick way to start. |
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Hello everyone! Tonight i'm going to test again and see if i can find the source of the reflections. I did some cleaning before and i found an odd circular reflection on the coma corrector. i posted an image here: https://drive.google.com/file/d/1wj3tEGDXVFxsyxsiBJpzr3TQgW66B8ca/view?usp=sharing and here: https://drive.google.com/file/d/1wmlawCWoojwYIqvcJr8VmQoJawlyNFyH/view?usp=sharing Could it be related? I hope that i can find out. Clear skies to everyone! |
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I've wanted to add that i talked with a dealer here in Chile and he mentioned me that the cause of this circle could be related with the size of the secondary mirror. The optimal size (diameter) for an astrophotography secondary mirror needs to be no less than 63 mm in the semi-minor axis. What are your opinions about this? P.S.1: I've tested if the coma corrector was the origin, but without it the problem persists. P.S.2: Respecting what i've mentioned above, i measured the size of my mirror and is smaller than 63 mm (54 mm) Thanks for all your help guys, i really apprecciate! |
