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I am using EQR-6 mount with Williams gt81 and asi533 with asi290mm camera with 50mm guidescope. My guiding is very good, I am under .4" for 5-10 minute frames. My stars are a bit oblong streaking to the upper left. Uniform thru-out frame so not a back focus issue I don't think. I should have round stars.... |
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If only 1 corner, usually tilt in your imaging train. If its a reflector could be collimation too. You really should post an image for people to see. Rouz, |
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| Sounds like flexure between your scopes |
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if it's only one corner, it is most likely tilt. this article seems to do a good job at explaining it. See if it matches your experience. best of luck |
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| ...and if it is tilt, consider dissambling your image train and carefully re-assemble. It might just be a small problem with tensions and such. Otherwise, there are tilt adapters available to fix this. |
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Besides the imaging train, the sensor its may not be square. Nina has a tilt measuring tool now. CCD inspector may help too. |
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Dan, If you have uniform oblong stars throughout the field, it's most likely due to a guiding related problem. Since you didn't show any data and no photos of your system, it's hard to say much more than that. However, I agree with Peter that you likely have flexure between your imaging scope and your guide scope but oblong stars can also be related to polar alignment issues...along with a LOT of other possible causes. I see a lot of questions on AB that are similar to yours so forgive me for making this comment--I am not picking on you. Here is my gentle advice: If you want quality help with a problem, you have to ask the right question and provide detailed information on your setup, tell what you have done to investigate the problem, and provide detailed data. There are a LOT of things that can cause oblong star images and without more background, you are going to get a lot of answers but most of them won't have anything to do with your specific problem. I normally ignore poorly asked questions even though I'd like to help out. It's just not worth my time to fish around playing 20 questions to resolve most simple problems that are poorly framed. Slow down, ask a good question and you'll stand a better chance of getting a good answer. John |
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I took a look at some of your images and it appears you are using a 135 Rokinon camera lens. This is a popular lens for astrophotograhy but your specific lens seem to show noticeable CA, (a teal shadow image). You might try shooting a few test images with a RED 25 filter. If the oblong stars go away your lens will need some serious service to remove the CA. BTW John has a valid point. A sample image with full parameters (mount, exposure times, equipment details) is required if you expect real help with your problem. David CS |
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Uniform thru-out frame so not a back focus issue I don't think. If it's indeed identical-looking in the center, corners, everywhere and your guiding is good but not perfect (for the latter you're likely guiding on a hot pixel and not a real star) then that means that your guide scope's tracking does not match your main scope's tracking. This happens, but is more common with longer focal length scopes and SCTs in particular. In an SCT this can be due to mirror flop for example. But in your case it's likely differential flexure. Check your guidescope, you'll likely find it can "wiggle" relative to the main optics. |
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This sounds guiding related. I know I had issues where I thought I had a good polar alignment. Also, the guiding software, if I even move the scope to a new position, well I need to re-calibrate the guiding software. |
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Thanks very much for the responses, I have tightened everything, re-adjusted the reducer and will wait for the next clear night. I did state I was using a Williams 81 scope with an asi 533 and 290mm guidescope on an EQR-6. I stated the time (5-10 minutes) I should post a photo but it just shows what I described as uniform across frame, not corners. I am not sure what other info I could add, it seems some did not read my post but I appreciate the responses anyway, I will look into everything. |
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Some of this has been mentioned a bit, but I want to make one important point about things like this that could be helpful for others who come across this post later. First off, yes if you have the same trailing throughout the image, that eliminates coma, tilt, and many other aberrations of the optical system. If the trailing is in the same linear direction (not swirled or radiating out from the center) that eliminates astigmatism. Options that remain; mirror flop/shift, poor polar alignment/PEC/other poor guiding issues, differential flexure, or incorrect guiding parameters. You don't have a cassegrain, so eliminate flop/shift in the mirror (I had this issue on my SCT when guiding with a separate guide scope, the guiding RMS values were good, but the images contained trailing nonetheless). Eliminate (for now) poor polar alignment/PEC/other issues too. Why? Because you said you have good guiding numbers. These issues lead to poor guiding, not trails during good guiding. You likely have differential flexure (mentioned before) or incorrect guiding parameters. I am almost certain you have the latter as well, and that's because of the point I wanted to make: The atmosphere and optical setups we have can only guide so well, and the absolute best we could expect to do for composite RMS values, under perfect conditions, doesn't fall much below 0.4" RMS anyways... When people report getting guiding of, say, 0.5" RMS or better, I start to think they might have their PHD2 guiding parameters wrong (I'm referring to focal length, pixel size, and the mount's step size values, etc.) Why? Because if you guided on literally a hot pixel, which shouldn't move at all, you will still get errors of easily 0.3-0.6" RMS, for example. Any guiding better than, say, 0.5" is unlikely to actually be obtained, even in the best seeing with the best mount, with properly set up parameters. It's just not reasonable. Check these carefully; I once caught an error in my PHD focal length parameter while using a reducer that I didn't initially catch, and I was off substantially. Maybe your actual focal length isn't quite what is advertised. Maybe the camera pixel size is wrong. Etc. Why does this matter? If you find an error here, it will likely increase all of your RMS values, showing the guiding wasn't as good as you thought it was. Then, you can open up some of the possibilities eliminated above, such as poor PA, PE, and the performance of the mount! After all, RMS values are only as useful as the parameters you input, since the latter are what the program uses to convert from arcseconds/px to arcseconds outright. |
