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Hi all, I am having some issues with my images - it is obvious that I have issues with corners of my images and I was ignoring it up to know (just crop it out) - I know, not nice but I wanted too fast to get some light  Now I am trying to do globular clusters where the problem is really obvious (they are all stars right) and I have big issue with auto-focus as it is changing too much during the night.I would like some input if you think it's collimation problem, tilt, back focus problem anything else? How would one calculate if it has 'ideal focus', as I am not even sure if it's OK focus - I was just 'visually happy' with it. Equipment: - Celestron C8 SCT (quite old, 2006 year) + 0.63x reducer/corrector - Vixen SXP2 mount - ZWO asi294mm camera - ZWO filter wheel (8x filters, 1.25"/31mm) - I am using R, G, B filters for this session - ZWO OAG & ZWO asi224mc as guiding camera - ZWO EAF - For acquisition I am using Kstars / EKOS and PHD2 for guiding I think it is not guiding problem as it was really stable during the night (0.66 RMS for compared images below). The calculated back-focus is slightly larger than should be. C8 & 0.63x focal reducer should have 105mm back-focus, with my setup I am at 106mm. But if anything it seems it could still be too small (not sure but stars might be pointed toward the center, see image below). Auto-focuser seems to work OK but I think the problem is which star it auto-selects during the night all in all image quality changes. Unfortunately I don't have auto-focus runs / quadratic function to show. I 'thought' collimation is OK based on observation of centered donut shape defocused star. Today I was reading this is not necessary true and I should check both - defocused and also focused stars. I will check this tonight. Tonight I plan to try to fix to have flat frame which I think will also help with focus. I would just like to have some feedback to see what would be best course to start dealing with this (right now I think I will first check collimation and with optimistic view everything will be well after that )I attach one 'good' and one 'bad' image, comparison of both with AbberationInspector and two side-by side comparison (one close to center and one in the corner). These were taken yesterday and are uncalibrated 120s subs (I only used STF to stretch them) with red filter. 'Good focus' (focus in center so M13 is 'nice'):  'Bad focus' (focus outside of center so outer stars are 'nicer'):  AbberationInspector of both:  Zoom close to center:  Zoom to lower left corner:  |
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| After further detailed investigation of the stars (when preparing this question) I am more and more convinced it is collimation issue (maybe other things also but for sure collimation), as I can't see nice round stars, they are all elongated when focused. |
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If you can you'll need to dial in your backfocus to bang on 105 would be best. Collimation on the c8 can be an absolute pain to get bang on but it's worth getting it right. Because your telescope is oldish, this particular telescope will always have slighly bloated out of focus stars on the edges even with the flattner. Its annoying , as I found after I got my hd8. Though its not as bad as the older version. As s quick test for collimation have you tried out of focus star (the donut effect) the black middle should be in the middle of the donut. Also you can check you camera tilt in pixinsight or sharpcap, adjust accordingly if its out. Otherwise its a fun telescope and i still love it. |
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If you can you'll need to dial in your backfocus to bang on 105 would be best. Collimation on the c8 can be an absolute pain to get bang on but it's worth getting it right. I will also check the backfocus - I think I needed to add additional millimeter to get OAG guiding camera in focus. I will double check. Thanks for your reply - I did check collimation with out of focus star few days ago (a week I guess?) and it seemed nice to me. Unfortunately I didn't log it and I also don't check it every day - I will pay more attention now. I didn't check the tilt yet TBH (I wasn't sure if collimation could influence it). I show below the screenshots from ASTAP, I need to read more about this but to me tilt calculation seems different for both focuses - I would imagine it should be the same for both? I agree it's fun scope - I started last year and wanted to have something 'cheap' and universal (which I know is not best to start with, but I had a lot of fun with it). My budget will open next year for something new, for now I would really like to understand all things and really try to pull out as much as possible from this telescope Here are ASTAP tilt analyses (FWHMEccentricity script doesn't want to show graphs as it claims it's not enough stars on, I don't know any other way in PixInsight): - Focus on the corners:  - Focus in the center:  |
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The standard Celestron .63x isn't a good FR at all. You need to up your game and get a fancy Nexus: https://starizona.com/products/starizona-sct-corrector-63x-reducer-coma-corrector |
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andrea tasselli: Do you think I have these issues mainly because of bad FR? I will look into Nexus and buy one but would also like to understand what others issues I am having right now |
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Jure Menart: You have a collimation issue, from what I can see. But that is par for course in an old SCT. I used to collimate it every time I used it, which was a pain in the backside to be honest. Are you using an external focuser? If you don't start thinking about getting one. |
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andrea tasselli:Jure Menart: Thanks for your answers! I don't have external focuser... I will look also into that |
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HelloI checked your photo completely. Your focus is different when it is in the center of the image than it is in the corner of the image.This problem has nothing to do with an Autoguider, telescope tracking system and mechanical parts. The problem and error is in the optical system of the telescope itself.C8 telescopes have spherical and optical defects. When you use 0.63x reducer/corrector and back focus this problem is partially solved, but not completely.I have this telescope and had the same problems with the crop sensor. The final image after flattening in this telescope is 60% of the 294 sensor.I don't see a problem with the tilt sensor. PLZ check Your Back Focus... Regards |
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Khosro Jafarizadeh: *** Hi Khosro, thanks for feedback! Yeah after some additional reading I saw this is the case. I ordered Starizone 0.63x reducer/coma corrector which should help with this, let's see once I receive it |
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Jure Menart:Khosro Jafarizadeh: Very Well, You did the best job...  |
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Hi Jure, Nice write up of what you're hoping to achieve and I have a similar approach. There are still some of us who like to squeeze as much out of these scopes as we can. I love my C8. Many times I've dreamt of swapping for a 4 or 5 inch expensive refractor but I love how I can image nebula, galaxies and planets. Give friends and family a look through the eye piece at Moon and Planets and with a Hyperstar grab F2 390mm mosaics. I too am striving for the flattest field I can achieve with my APS-C sensor, not easy. I have found the Starizona reducer to be a great improvement but beware, there can be a greater risk of internal reflections. I have also gone down the route of external focuser which does give a finer result and eliminates mirror shift but you will still be left with mirror flop after a meridian flip. I have struggled with collimation. The main difficulty I have found is the seeing. I just can't get a good enough airy disc at a high enough magnitude for great collimation. Meta guide has given me the greatest level of accuracy but is another set up to master. I have centered my corrector plate as best as I can also. At 1280mm, or more like 1400mm or so with your new reducer, guiding errors will start to make more of a difference on your star sizes and shapes. Are you more accurate in RA or Dec etc. I wouldn't worry about tilt until you are happy with back focus and collimation. Many are using the Hocus Focus plugin in N.I.N.A. It attempts to take a tilt measurement based on the fwhm of stars across a focus run. I have tried to use this but with the C8 I get inconsistent results across subsequent focus runs of the same target and so have reverted to adjusting tilt by eye. I may be doing something wrong. You will encounter diminishing returns the closer you get to dialling everything in, but like you, that doesn't stop me from wanting to get the best out of my rig and learn the skills in the process. I don't think for a minute that I have achieved that yet. All of these things take alot of time and sometimes I have to remind myself that getting too obsessed will stop me from just imaging and observing what's out there. Modern processing techniques can also provide great results that will reduce star abberation, depending on your opinions on these. I wish you all the best in your endeavours and hope you keep enjoying yourself. Great Pinwheel by the way and congratulations on bagging yourself a Supernova, in a distant Galaxy with your old C8, in your own back garden 😁 CS Steve |
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Hi Steve, Thanks for nice and positive message. I agree with your view it's nice to squeeze as much as possible from existing equipment. Next year I plan to buy smaller APO to get wider field (I gave myself a rule to buy one bigger piece of equipment per year). This way one has time to get to know each part separately. But even after that I want to keep my C8 and use it periodically - for sure during galaxy season I think it's good choice I am not sure if I'd like to try Hyperstar F/2 390mm - I think I will not do it for now as I think I will have all other problems with this speed and focal length and will just lead to buying more equipment Maybe in few years...I was reading also about external focuser, but I read one can have problems with having both Starizona 0.63x R/C correcter and reducers. So for now I put this on the side - I hope I will not struggle too much with automated focuser on original focuser. Which focuser do you have and how do you combine it with Starizona? Right now (@1280mm) I am quite happy with my guiding - since having new mount (Vixen SXP2) and switch to PHD2 (from Ekos guiding) I really like how repeatable and stable the guiding is. Let's see if I can/need to improve it in the future. Regarding the collimation - I am still not sure if I know how to correctly collimate. Yesterday I was trying it and original collimation wasn't too bad. I intentionally put it out of collimation and try to put it back in. I was doing it as described in Celestron documentation (with bright out of focus star). I am lucky Arcturus is bright and early star for me and I was using that for collimation. After collimation clouds came so I didn't do any real imaging. Today I will try to read some more on collimation and try to repeat it - any advanced SCT collimation documentations/videos are welcome My yesterdays collimation images (now that I am thinking, maybe it was a little bit too bright?). I also usually don't notice the strange light disturbance seen on the donut below, so I am not paying too much attention to it until I see it on focused image. - Original collimation which is the same as I've observed in the past and concluded that collimation is good enough - the donut is slightly larger on lower part and I wanted to correct that (I am not sure how good must the collimation be):  - Intermediate collimation, where it got worse:  - Final collimation, I will repeat it today and try to be more precise (yest clouds were coming in and they covered the star sometimes so I didn't want to spend too much time and rather spend it for some more sleeping that was quite rare last days  ) - I actually see it now, it's not ideal, but at least it's a start to do collimation from my side: |
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Hi Jure, I was lucky and bagged a second hand 2" Esatto focuser. I have a 2" Baader click lock attached to it and that takes the Starizon Reducer and it fits along side my Celestron Focus Motor so I can use both. But I think you are right, the stock focuser with a motor and autofocus software will get you good enough results. I wanted to try it as much to see if it would improve consistency with Hocus Focus to work on my tilt error but I going to put this off now until later in hte year. Your collimation looks good to me for using the doughnut technique. There are greater degrees of accuracy that can be achieved by collimating visually at higher magnification but I've never had enough luck with seeing. The scope needs to cool also which may explain the disurbance you are seeing. I find Al's Collimating Aid a handy little program for checking using the Doughnut method if you've not seen it before. It is a free download and takes some subjectivity away from the process.  Metaguide is a better was of collimating using a camera but there is not much in the way of tutorials. It does offer the opportunity to collimate off axis if that provides better results. As mentioned here before, the C8 is not designed as a flat field astrograph and will always produce abberations of some kind across a larger sensor. All you can do is tighten each aspect up and find the best balance, which it seems you have a very sensible approach to doing. From my experience, I am happier with the results after all of my efforts but the returns are not as great as the effort I've put in. I'm OK with that, others might not be yet others may yield better results. I have also heard said that on axis collimation is more important for visual and planetary imaging. For DSO it's not so important and with astronomical darkness starting at nearly 2300hrs here in the UK, it's not the best time of year for tweaking! I'd be interseted to see how you get on with the Starizona reducer. At least with that, the Back Focus is accurately documented. I always found that the Celestron back focus was hard to get right whatever i set it to because it just doesn't correct as well. Hope I'm not giving you duff advise, I'm still learning too Cheers Steve |
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I find Al's Collimating Aid a handy little program for checking using the Doughnut method if you've not seen it before. It is a free download and takes some subjectivity away from the process. Hi Steve, thanks for letting me know for Al's Collimating Aid - it's indeed very useful And yeah - I know the pain of needing to deal with this at this time of year, it's always sooo late CS, Jure |
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Be advised that, in general, looking at the exit pupil image as done here is not (always) the best way to judge collimation. Visually, you would move slightly out/in of focus just enough to show 3-4 Fresnel rings and judge the concentricity on that. You can try to replicate that in imaging mode using the video capability of your camera (most if not all astro-cameras have it and quite few DSLRs too). Or use Metaguide if you sample the psf well enough.Steve Spring: It might not be as important but if you want a large corrected field is important enough if your imaging system isn't a well corrected one (basically, just APOs, RCs and Maksutovs qualify for the accolade, with caveats [APOs have field curvature, RC astigmatic field curvature and Maksutovs have lateral chromatic aberration]). As for the UK having dark nights (of any lengths) this time of the year I very much doubt it anywhere in the mainland. |
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Thanks for the info @andrea tasselli, I'm glad someone knowledgeable can advise Jure than myself You're right about darkness in the UK, three hours last night good enough to catch any photons ..........well good enough for my standards
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Short update from my side: - I ordered Starizona 0.63x R/FC and they were super fast with sending it - so I received it already on Friday - I set it up yesterday and I did see immediate improvement - I also noticed the flare from nearby bright star (Arcturus is the playground for me right now, as it's the first star I see it and I start to 'play' with setup early - already at 10pm :/) - so yeah, I noticed the flares even if the Arcturus was outside of the image, one thing more to be careful about - I also got logical but unanticipated issue with guiding camera - I am using OAG so it makes sense that guiding camera was now much out of focus (I fixed the field on the corners so it makes sense it was much affected) I still see back focus and tilt issues but all in all image now is much flatter (especially right side), it will take few days to fix these (I am waiting for some equipment and tilt adjuster) but I think already with first light the improvement can be seen. Unfortunately I don't have 1:1 comparison with M13 so I show some other images. Random star-field:  Tilt calculation from ASTAP for random star-field photo:  M24 5 minute sub with Baader 6.5nm hydrogen alpha filter:  Tilt for the same frame:  Tilt calculation shows consistency on all test frames (it didn't before - with bad corners it had issue to calculate the tilt), which is good and I can start working on it. One 3.5 hours integration of Ha M27, let's see if I can make nice image out of it in few days  |
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Update (mainly for myself and if somebody will be on similar journey) I got tilter so I added it to my optical train - when doing that I noticed I actually had accidently 2mm longer optical train (there was 2mm adapter on my OAG which I counted as part of OAG last time). I added ~0.7mm additional space now to compensate for 2mm thick filters (90.3mm original backfocus, I have it at 91mm now). I think my frames are getting nicer, there is still slight issue in lower right corner which I still need to understand whether it's tilt or something else (ASTAP calculated tilt is very small but does hint it could be tilt). Also this time I did manual focus with Bahtinov mask. I did my collimation and was quite happy with it (I did it on two defocused stages with two sizes of 'donuts'):  M13 90-sec sub (uncalibrated, just stretched in PixInsight) via UV/IR-cut filter:  The tilt doesn't show too much of a tilt:  Next 1 M27 60-second sub over R filter (uncalibrated again, just stretched):    |
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Hi Jure, Great work in very little time. It's taken me a lot longer to get to grips with these adjustments. I too have just one corner that is slightly out and no amount of adjustment seems to take care of it. However, I can live with it and look forward to getting on with some imaging. Looking forward to seeing more of your images CS Steve |
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Steve Spring: Hi Steve, thanks - indeed it was faster then I expected. Without this community it wouldnt be possible to be so quick I really find AB great place to get info and see really nice pieces I hope I will be able to contribute back to it quickly CS, Jure |
