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Hi, I have uploaded my quick and dirty image of ic1795. Issue is, smaller stars are more or less oblong and larger stars seems be mostly ok. In the upper left corner this is very prominent. I am way out of my depth on this. Not sure where to start. Please help! Thank you! Jacob _______________________________________________ Acquisition conditions: - seeing good (about 1.5, estimated) - guiding good (error overall RMS 0.42 px/s) - moon 84 % - fresh collimation - good polar alignment (with polemaster) Equipment: - C9.25 Edge HD - 0.7 FR - Moonlite Nitecrawler (Rotator, Focuser, motorized) - Spacer (for correct backfocus of 146.05 mm) - Celestron OAG (with ASI ZWO 174 mm) - Spacer - ZWO ASI 2600 mc Image link: https://www.astrobin.com/gsubxl/ lower right corner:  upper right corner:  Center:  |
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Hi Jacob, I have a similar setup but I have an EdgeHD 11", Moonlite Edge CHL focuser and ASI 2600MC Pro. When I look at your images, even the larger stars are oval; it is not limited to the smaller ones. It is just easier for you to see on the smaller ones. If your backspacing is correct, most likely you are dealing with sensor tilt and potentially inaccurate collimation. The reason why I think it is mostly sensor tilt is, firstly, that ZWO has had major issues with this, and secondly, all your stars are oblong in a similar direction. Collimation may not be perfect because even the middle of your center image is affected, and typically with sensor tilt, stars dead center of the sensor are spared from tilt. What process did you use for collimation? The defocused star method (donut) is not ideal but yet highly publicized. I would strongly suggest using the free program Metaguide, which has a bit of a learning curve, but is well worth it. Check out this thread: https://www.cloudynights.com/topic/659381-need-help-collimating-edge-hd/ This YouTube video from the creator of Metaguide also is phenomenal (he is also PhD in optics, so reliable source): https://www.youtube.com/watch?v=hZV43ZHNUKM&t=1475s Once you have gotten collimation as good as you can get it there, then comes the issue of sensor tilt, which I personally found much harder to learn conceptually and is a known problem with ZWO cameras across the board. Check out this thread for details: https://www.cloudynights.com/topic/775953-asi6200-tilt-adjustment/ The program used in that thread to detect sensor tilt is called CCD Inspector. It has a 30 day trial period, but after that is quite expensive. Similar methods can be done using PixInsight and ASTAP, the latter of which is free. Major pitfall though is ASTAP is significantly slower to compute the tilt than CCD Inspector so the process may take much longer. I went so far as to build the laser jig referenced in that thread, but in hindsight, iterative star testing was still the best way to work it out. Be prepared to sacrifice a couple clear nights to work out the technicalities. This can all be a bit overwhelming for sure. I keep trying to work out the details on my own setup too. The pursuit of perfection is never-ending! If you need more help, I can try to assist. Clear skies, Ben |
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Jacob, I agree with everything Ben has said above. I wasted countless hours trying to work out what was up with my Edge HD 8 and the 0.7 x reducer. I tried OAG, a myriad of guide scopes to rule out guiding and then focused in on tilt or collimation. I found it almost impossible to tweak the tilt using the ZWO tilt plate and CCD inspector and again waisted countless hours of my very scarce clear skies (note there are better off brand tilt plates for the camera but I didn't take the plunge). Then one evening I ditched the focal reducer and started shooting at f10 and everything worked fine. So for now I'm going to shoot at f10 and BIN 2x2. I use and ASI 2600 MM Pro so the field is still pretty wide and as I only use the Edge for small targets that's fine for me. I do plan to give the reducer another go and I don't want to blame the oblong stars on the optics it's most likely just introducing another tilt component into the system and at 1400 mm focal length the image scale is not forgiving! Also compared to my William Optics FLT 132 the image train on the HD 8 is much longer and far less robust so all the more opportunity for tilt/flexure. The reducer wasn't a total waste I still managed some pictures of M57 and M27 but I had to correct the stars using an offset filter which is far from ideal. Let us know how it works out. Thanks John P.S my stars looked exactly the same as your - if I recall I needed a 2 by 1 or 3 by 2 (faded) pixel offset filter to correct them |
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John, I had similar experience: I only realized how bad my imaging train was once I got good guiding down pat. I have a feeling that the Celestron 0.7x reducer is contributing to my field curvature problem since my backfocus is spot-on. I had the same thought as you to remove the reducer and take a stab at shooting f/10 while binning 2x2. Haven't tried yet though. Regarding the tilt plate, the one that comes with ZWO is garbage. The included screws are fairly soft metal, so it is very easy to strip them with a quality Allen wrench. Unfortunately, when shooting at f/2 with Hyperstar, I have no other choice but to use it. However, at f/7, I was able to correct the tilt in about 30 minutes using a Gerd Neumann M48 CTU. It is expensive for what it does, but if you have enough backfocus to accommodate it, there is no better way to correct tilt (except arguably Octopi Astro but I have no personal experience with that). Ben |
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| Thanks Ben I’ve always been a fan of Gerd I think I bought some of his first filters way back in the early 2000s. Perhaps I’ll donate some more for the tilt plate!!! |
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Seeing this post and the replies is quite a relief for me. I have been battling an Edge 9.25 and the .7x flattener since day 1. I have spent many clear night hours as well constantly tweaking and experiment. Thinking it's the mount. Thinking it is backfocus. Thinking it is tilt. Experimenting with OAG and buying way too many adapters to adjust backfocus. I also have found not using the reducer has helped, but F/10.... combine with narrowband... just makes me want to go through the punishment all over again with the .7x reducer. Seems to me the marketing that flat edge to edge plus the reducer is an automatic easy thing felt a little misleading. The threads on the OAG full frame connector aren't the same measurement as the m42 visual back adapter and that's a whole multi thread/page discussion on Cloudy Nights in itself. My next experiment is to just tighten the 2600's tilt adapter as tight as reasonably possible on all the screws and see what the short 1-5s exposures look like to rule out RA tracking on the mount. I'd love to not have to crop so much on every single pic from an APS-C. I can say that the 2600MM with filter wheel screwed directly into the backplate of the camera(no tilt plate) looks decent in all of the corners so it does lead me to believe the tilt adapter is the culprit. The fact the one corner looks perfect give me hope that this may be the solution. Once you get one corner looking good, there's a bit of relief that you are on the right track, but maybe I'm just being too optimistic  For collimation, a Tri-Bahtinov mask was very helpful along with the usual suggestion of Bob's Knobs. I have spent additional hours with a fancy laser collimator, but the real world star test has been more productive time wise. |
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Hi Jacob, Hi Ben, Hi all, thank you for the support. Especially two observations are quite vexing for me: - a few days ago a did a fresh collimation with the large donat method as good as i can manage. Before that, collimation was slightly off. Interestingly pictures were better before that, because oblong stars are only detected more or less are detectalbe in two corners instead overall. - prominent oval stars seem to differ slightly from image to image. Mostly in upper right corner it is very bad. Sometimes more or less equally distributed at upper border, sometimes lower picture area. To make matters worse, some condensation has been cultivated on the interior side of corrector class. Hence i am doing this only for 1,5 years all this equipment is fairly new and i am not that experienced with it. I deducted following procedure: - careful cleaning job - new collimation with Metaguide - image with 0° rotation (via rotator) - image with 180° rotation - compare with ccd inspector (if more or less identical equals sensor tilt, of not it must be something else) - if so, remove FR and start over ... I recon (after some reading) sensor tilt might be curable. What would you recommend if FR is the problem like others reported. Using our gear at f10 instead of f7 word double aquisition times. I am not sure this is acceptable because we live in an area with sparsly clear skies... Thanks and i will report. Cheers Jacob |
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Hi Jacob, If after collimation your stars were worse, it means either that the donut method wasn't sufficient or that now you are better able to detect the tilt since collimation is improved. Only real way to tell is to use Metaguide to collimate on a focused star, which is far more reliable. If you have condensation on the inside of the corrector glass, you could also be dealing with temperature equilibrium problems within the OTA. That may cause issues with the accuracy of your collimation. I personally would be very careful removing the corrector glass: often times these are aligned in the factory to the primary mirror for best possible positioning. Many folks on Cloudy Nights have removed the corrector successfully, but be sure to mark the edges of it before you remove it so you know what orientation to put it back in. You will also need to recenter the corrector glass/secondary mirror, or you will never get accurate collimation. I was never brave enough to remove the corrector myself, but also haven't had condensation problems. If it were me, I would probably contact Celestron first for advice. Rotating the camera can definitely be helpful to determine if your problem is related to optical train vs. sensor tilt. I'm struggling with the idea of the focal reducer being the problem on my own setup. I also don't have that many clear nights and cringe at the thought of doubling acquisition time. However, if you are having a very hard time with collimation and tilt after trying what we already discussed, it would be best to remove the focal reducer to eliminate that as a source of your problem. You will also get more accurate collimation without the reducer than with the reducer on there. Regards, Ben |
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I, like many others, have struggled to get good star profiles with the SCT, especially with the larger ones like the C11" EdgeHD, which I have, and the C14" EdgeHD. I cannot disagree with much that has been suggested but from my own experience and those from others who also helped me, I would ditch the Celestron focal reducer. Maybe it is not as much an issue on the C9.25 but it is essentially frustrating to use it on the C11 and C14 unless you are willing to do significant cropping. And just because you have carefully measured to get the backfocus correct per Celestron specs, does not mean it is accurate for your setup. You may still need to tweak it by fractions of a mm to get it spot on, if you want to. I am under Bortle 7 skies and only do f/10 imaging even when I only have average or below average seeing conditions and it is fine. Acquisition time does not have to be double vs. that when imaging at f/7 to get a decent image and I also have minimal clear nights per year. So, once you have good polar alignment, good guiding, and the best collimation you can do (I only use the eyeball test on a decent star with the camera- no softwire), you will get nice round stars at f/10. To improve the image at f/10, I dither each frame and sometimes do deconvolution, but only on those nights where I am oversampling due to below average seeing. I realize camera tilt is a whole other nightmare to deal with as I have had that problem as well, but you don't need perfection. Currently, the tilt on my ASI6200mm on my C11" EdgeHD varies from 1-3%, which I can live with as you hardly can notice it. And the little bit you can see essentially goes away as you have to crop a tiny bit due to the dithering effect of the stacked frames. Best of luck! This can all be frustrating but you will reach a happy point as we all eventually do. Bruce |
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Bruce Good post - just out of interest what are your thoughts on binning 2x2 withe the 6200 or the 2600? I realize with CMOS it’s a software thing unlike with a CCD. For me there was some satisfaction in the inherently brighter image 2x2 gave even though I know I could do the same in post processing. Thanks john |
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Thanks, John. I tried that a few times, but frankly, I could not tell much of a difference with either camera. So, maybe it is just my not-so-perfect eyesight. |
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Hi there, this is just a short head's up. I am well aware weeks have gone by. But only one good - or better put clear - night so far. Hence fooling around with collimation is still ongoing. Metaguide seems to run ok, handling requires a learning curve but we manage, i assume. What is strange though, we manage to move the red collimation dot near the centre of the star. But still, star is oblong and FWHM-Graph does not fit at all ... So we will try again with peak instead of centroid setting. Coming to my question after all. Metaguide manual suggests a large F-ratio, say f10 or higher. " Typical f/numbers for a good view of the Airy pattern are in the 25-50 range, and may require a Barlow". For our upcoming tests it is vital to use the equipment as described earlier (f7). In your experience, it is possible to use metaguide for collimation with an f-ratio of f7 succesfully or is it just compromised ... In our case an airy pattern is not really there (just a roundish spot). Furthermore some of you suggested Celestron 0.7 reducer might cause the problem. With decent collimation done it should iron out some of this, or not? Sorry for the slow reporting in this. Your insight is much appreciated. Thank you! Cheers Jacob  |
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Jacob, In my case I know adding the f7 reducer was the cause of the problem. I just switched back to my Edge HD8 for Galaxy Season and ran it at f10 no issues perfectly round stars. Oblong stars at f7, but I don't think its the reducer I think its the image train flexure or tilt. Though why I'd get more flexure with 105mm than 133mm I don't know but I do! Anyway I just ordered an more sturdy spacer so let's see how that goes I will report back as soon as I have the results! Give f10 a try and BIN 2x2 even from my Bortle 7 backyard there was a lot of data on M1 so I'm expecting decent images on brighter targets! John |
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Hi Jacob, Looks like you are on the right track though. I just set my scope up at f/10 and ditched the 0.7x focal reducer while I was troubleshooting other problems. To my pleasant surprise, eliminating the focal reducer also fixed terrible field curvature and made sensor tilt a lot less critical too. Tilt could still be your problem though. Based on my own experience of wasting easily 15 clear nights fighting with tilt and field curvature with the reducer vs. the awesome results I'm getting at f/10 now, I would suggest first eliminating the focal reducer before you do anything else. If your oblong stars are still present even without the reducer, then you may have a very nasty case of flexure or tilt. Ben |
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Hi Ben and John, thank you very much! What do you think; does this distortion come with every Celestron FR? I am just wonderung .... Cheers Jacob |
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Hi Ben and John, No. Mine works fine. |
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| I hope not but if it is it doesn’t appear to be systemic there a plenty of images here taken at f7. Much more likely to be set up, it’s possible that at f7 the system is just more sensitive to flexure/tilt. |
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I would agree with John and Olaf. Not all focal reducers are affected, but I think mine is. As your focal ratio gets faster, your system also becomes more sensitive to more minute issues with back-focus and sensor tilt. When I shoot at f/2 with Hyperstar, if I'm off by an 8th turn of a screw on the tilt plate (microns), the whole thing goes down the tubes with tilt. Critical focus zone with the Hyperstar is also on the order of microns. So my understanding is that at faster focal ratios, everything is less forgiving. Ben |
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Hi all, this is a head's up on our oval star issues. Please find attached a short documentation (details. ccd-inspecter plots). I am not sure what to do now. F7 and F10 are not ideal. Next step will be another collimation attempt. Your insight would very much appreciated. Thank you! Cheers Jacob 2022-02-19_whatnow.pdf |
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Supplement: https://hidrive.ionos.com/lnk/VTyTAsxf |
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Hi Jacob, Looks like you have put A LOT of effort into this. Very good work though. A couple of thoughts. 1. It looks like you are trying to run CCD Inspector on images of an open cluster. While not as bad as a globular cluster, that may still throw off the algorithm CCD Inspector uses to make its calculations. When trying to get a good read on tilt and field curvature, it is usually best to use a more uniform star field. Finding a rich star field anywhere in the galactic equator is usually a good starting point. 2. If we assume that the open cluster did not throw off your analysis, then it looks like the most major problem you have across your samples is a large degree of field curvature. Most common reason for this is inaccurate back focus distance. What did you use as your starting point for measuring your back focus? There are tons of posts on Cloudy Nights over this issue with the EdgeHD series. Starting point needs to be the flat portion of the end of the threads (rear cell for f/10 or end of threads of focal reducer at f/7). I personally found that the only way to get an accurate read on back focus was to measure the distance from *inside* the focuser baffle tube rather than use an external part of the scope as a marking point. In the attached picture, I measured from the flat outer portion of the threads of the focal reducer to the most outside portion of my focuser. Once I knew that distance (in my case 64mm), I subtracted that distance from 146.05 and got 82.05. I added one more millimeter because of my filter thickness, and then used 83mm as the final distance from the rear of my focuser to the camera sensor. That substantially improved field curvature. 3. Once you have your back focus spot on, point the scope to the zenith. First focus the primary mirror to get the lowest FWHM/HFR you can WITHOUT changing your backfocus distance (in other words, do not use the Nitecrawler for this part). Make sure your last mirror focus adjustment is turning the focus knob counterclockwise. This pushes the mirror away from the rear of the scope and against gravity. This will minimize issues with mirror flop. Once you have that focused the best you can using the primary mirror, LOCK THE MIRROR. You can then do your final focus using the Nitecrawler. 4. Consider redoing your collimation once you have completed #2 and #3 above. Hope this helps! Ben  |
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Hi Ben, Thank you for your input! A measurement revealed a backfocus of about 151 mm. So 5 mm off… Reason is, celestron OAG is quite voluminous and I have reached the minimum of backfocus already. What do you think; does an offset of 5mm would cause this behavior I am describing?  |
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Hi Jacob, following up on this; a short read caused more confusion (as most times). According to https://s3.amazonaws.com/celestron-site-support-files/support_files/edgehd_whitepaper_final.pdf backfocus distance should be placed within 0.5mm. But this https://www.innovationsforesight.com/support/celestron-edgehd-back-focus-tolerance/ stated, an offset 20 mm with average seeing should acceptable. Reason I am asking is, adjusting or better put mitigating backfocus distance from 151 mm to 146 mm would cause som trouble, for instance purchase of an thinner oag … thank you again cheers Jacob |
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Hi Jacob, The back focus tolerance for the EdgeHD is definitely 0.5mm. Go by the white paper. 151mm could definitely be enough to cause the field curvature. I ran into the same thing. Ben |
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Hi Jacob, Hi Ben, thank you very much. We have ordered a qhy OAG, which is only 10 mm wide plus a bunch of adapters. Let's see how that works. Will report on th matter. cheers Jacob |
