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Something has been bothering me when imaging bright stars. It's not an issue for most/normal stars, but it is visible for the bright ones. It is visible in other filters too, the image is in a ZWO Blue filter. But the same is visible in Green/Red/H/etc. Part of it are just halo's of the filter, perhaps ZWO isn't the highest quality filter out there, so.. understandable. But the main issue is the spiking, which mainly seems to be going to the right. What is this? Pinching? Tilt? And how do I fix it? I'm using a ZWO 183MM, Skywatcher Esprit 80ED. Thanks  |
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| Show us a picture of the Fresnel rings and we might be able to help you. No filters. And with filters. |
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andrea tasselli: Not going to lie, didn't really have a clue what those were.. And probably still don't know 100% how it all works. But, it would appear to just be a look at an unfocused star to see what shape you get.. so, I did create a little test at home.. Aluminium foil over a flashlight with a pinprick in it, to create an artificial star (instead of buying one or waiting for clouds to clear).. put that at one end of the hallway, scope at the other. And did a quick test to see if I could see my fresnel rings.. bit of a low budget test, and maybe not perfect, so probably not very accurate... But.. here we are..  At least the rings seem pretty centred if you ask me, so at least that would mean that collimation is fine? I did find this post on the internet: Esprit tuning Which might be the case for mine, not sure how to best test that though without opening up the scope.    |
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| The approach you followed is about right. I'll venture to guess that if your artificial star to scope distance is greater than 16 meters you'll be all right. Assuming this is the case then what I see might be zonal error and turned down/up edge somewhere in the optical train. I'll have to run through some simulations to be more certain. |
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andrea tasselli: Thanks for the reply Andrea. Couldn't quite get 16 meters, I reckon it's about 11-12 meters. I live in an apartment, not a mansion .So, like I said. far from a perfect test. I would get the scope tested officially by a reseller, but most are at least 3-4 hour drive away. And want to at least try and figure some stuff out before making that leap. (+ cost), but I might have to do that at some point if I can't figure it out.. But I reckon the issue might be similar to the "Esprit tuning" post I found. I appreciate the help and comments. Thankfully, So far it's not a huge issue, it's only visible on the larger and bright stars, but it's not like it's impossible to deal with that in photoshop. it would just be nicer if I didn't have to. |
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| Well, after much head scratching I came at the conclusion that while the test is imperfect any aberration that might be there of the sort I have listed are unlikely to give unsymmetric star shapes the way shown. I put my bet on something in the light path (e.g. a lens cell clip) that affect only one side of the optical path. |
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| The smaller stars look fine. If it were a collimation problem, I would expect the problem would also appear on the smaller stars. Good luck |
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There‘s either something ranging into the light path (possibly the lens spacers, otherwise there shouldn’t be much other stuff) or you have slightly pinched optics (my guess). Did you notice if the effect is temperature dependent? Should be worse on cold days. If you have a Cheshire eyepiece, you can double check collimation. If you shine light into the Cheshire or point it to a bright surface, you should see concentric reflections cause by the lenses. The test with an artificial star is a good idea. However, don’t defocus that much. The image near and in focus are telling the most. Examine the star image in focus. You should see an evenly illuminated airy pattern. Take a look at the star with a very high magnification (I‘m using a short FL eyepiece with a Barlow lens). This will show you if something is wrong. If the lenses are pinched then the diffraction pattern close to focus won’t be circular but will have a triangular shape (if there are three adjustment screws at 120 degree angle. If there are more in a different pattern you usually see this symmetry in the diffraction pattern) Cheers, Björn |
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andrea tasselli: Hope I didn't cause too much head scratching. Thanks though. Yes that's the weird thing, I mean, the spikes are also a little on the left side, but far more on the right, could be a bit of tilt or pinching too. Or a combination of things. Either way, thanks very much for your assistance. I'll keep testing things here and there. And I've emailed a large reseller that should be able to do bench testing and possibly tuning.. Maybe I'll just get the scope fine tuned by the pro's (shame they're 3-4 hours away) Or I'll just leave it (for now).. fine tuning might come at quite some cost, and it only affects a handful of stars, so no idea if it's actually worth it. The smaller stars look fine. If it were a collimation problem, I would expect the problem would also appear on the smaller stars. Good luck Thanks! Yes it's only the big and bright stars where it's actually an issue. the small ones are fine. And details in nebulae tend to be fine too. |
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There‘s either something ranging into the light path (possibly the lens spacers, otherwise there shouldn’t be much other stuff) or you have slightly pinched optics (my guess). Did you notice if the effect is temperature dependent? Should be worse on cold days. Thanks for the comments Björn, I'll try and do some of these things in any next test that I do. Next time I have time I'll unscrew parts of the image train first. I did notice a ring on the flattener being quite stuck (couldn't quickly undo it when I tried today) If there is any pinching, it might be there. We'll see, thanks for the tips. Testing continues  |
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... Wait a second. Don't do star testing with corrective optics attached. First test your triplet without any flattener, reducer or whatsoever. You should first identify if it's the objective lens or the corrector before doing anything. Björn |
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... Thanks, That is already one of my plans yes. To also test it without the flattener to try and rule out issues. |
