Hi, I've recently got a Skywatcher F4 200P reflector used but have some problems collimating it. I used ocal for collimation inside room, but after I take it out and put on the mount, I find stars are always not satisfying. I tried the next night but still didn't help (so it's fully cooled). I did different crops on M81 and even on 4/3'' sensor I think the star still shows coma. I really don't know if that's collimation problem as I've collimated several times. For imaging, I use skywatcher quattro MPCC, then a 55mm 5mm tilt plate+OAGL+2''EFW+12.5mm camera as ZWO suggested. Any advice is helpful, thank you! CS, Yingtian      |
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Or poor seeing and caused poor guiding is another reason? But I see stars are varying in all direction, so I think remaining comas are still damaging quality. Back focus may be a problem but I just followed 55mm and there are at most 0.5mm error from filters.
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Poor, actually quite poor, collimation. The 55mm backspace is quite right (I have the same CC), give or take 1 mm so don't fret over the filters'. Don't use the OCAL other than for initial centering. The gold standard is auto-collimator(s) and star test. A good auto-collimator is key for an hassle-free relationship with fast newts so you might want to splash the cash for a good one. After collimation is sorted you'll need to sort out the tilt issue which is undoubtedly there.
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I see guiding error and coma. I'm not sure why you think you need an OCAL at F/5.6 but ok. Have you looked at a bright star far out of focus to check collimation? It just takes a second to make sure it's not the issue.
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 Thank you guys! I rechecked the collimation out of focus, in-focus is still a round but out-focus is definately showing the collimation error. I'll try to fix this now. Thanks!
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Looks like a combination of guide error, coma and astigmatism. I got diamond-/triangle stars with my TS UNC newton once, because the push screws for fixation of the mirror cell were screwed in extremely tight. This caused the primary mirror to flex and produced triangle stars/astigmatism. On top of that the look of your stars is different corner to corner, which could be caused by tilt and/or misalignment of the mirrors. Newtons are not hard to collimate, for my UNC I use a laser (which can be collimated, too), cheshire and stars.
On newtons coma related issues are backfocus issues most of the time. Buy a digital caliper and make sure you are spot on with your imaging train. Sometimes the threads can be pretty short and your adapter/FW/camera/etc will not screw in completely which adds up additional backfocus. Depending on the corrector additional 1-2mm can mess up your stars already.
Regards
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YingtianZZZ:
Thank you guys! I rechecked the collimation out of focus, in-focus is still a round but out-focus is definately showing the collimation error. I'll try to fix this now. Thanks! If your star was at the center of the FOV then yes, you are out of collimation. A slight adjustment is all it needs. You can quickly do it with this view. Just make sure to re-center the star in the center of the FOV after every adjustment. You're just working with the primary here.
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I have a Skywatcher 200PDS F5 and it took me a really long time to solve almost all issues (over 2 years). The problems I had were a combination of bad collimation, focusser alignment, incorrect backfocus, pinched optics and also the original secondary mirror spider wasn't great. I bought a CNC milled secondary mirror spider from backyard universe, so this improved my weird looking spikes. As for the collimation it was important to get the secondary mirror in the correct location. In my view that step wasn't documented well in the ocal guide (I have the ocal, too). I have the Baader MPCC Mark III and at least for my unit the 55 mm backspace is not enough. I ended up buying the Askar backfocus adjuster to have some flexibility and iirc I'm around 58 mm now (incl. filter). For a proper collimation I had to tilt the focusser a bit. It's not at a 90° angle to the OTA but at something around 89°. I also reworked the primary mirror cell and clips. I still have some issues on the first few subs. Not sure if it has something to do with the temperature or altitude of the target. And I still can see some very light form of astigmatism on all subs. So I simply remove the first few subs with the problem (bright stars have a small additional spike) and BXT corrects the slightly deformed stars without issue. I could spend more time on this but after all the time spent on the issues over 2 years I prefer imaging now  |
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I have seen these same issues with my R200SS (an 8" f/4 newt). The trefoil shaped stars were due to a pinched primary mirror, the mirror cell screws need to just barely touch the mirror. Any tighter and they will distort the primary mirror and cause those 3- or 4- spot shaped 'trefoil' stars. I also had them caused by the primary mirror mask being screwed too tight against the mirror, again the screws need to just barely be tight enough to stop things moving but no further. The collimation adjustment screws should also avoid being overtightened for this reason.
I would also strongly recommend getting an autocollimator system like the Catseye Blackcat/XLKP combo kit. The Catseye system is by far the most reliable method I have found for collimating my Vixen.
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