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Hello good people. So i just got myself a new SkyWatcher Newtonian 150/750 PDS along with the Baader multipurpose coma corrector. I’m using it with a DSLR with a crop sensor for now so it should work well but i still get coma in the corners. The manuals say the corrector has to be 55mm from the sensor and i measured it to be the right distance. Any ideas?? Is this normal, maybe i cannot eliminate all of the coma? Maybe i’m not collimated right? Or is there a better corrector i could use? Thank you |
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Hello good people. For starters, just to clarify: How did you measure the 55mm? and what Baader MPCC do you use (II or III)? |
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andrea tasselli:Hello good people. I got the MPCC III which according to Baader needs 55mm from the sensor. So, I researched my cameras distance from the flange to the sensor (44mm) and then added that to the T Ring and the corrector screwed on which I measured (roughly to be honest with a measuring tape) was about the right distance. is there a proper way to measure? |
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I got the MPCC III which according to Baader needs 55mm from the sensor. So, I researched my cameras distance from the flange to the sensor (44mm) and then added that to the T Ring and the corrector screwed on which I measured (roughly to be honest with a measuring tape) was about the right distance. is there a proper way to measure? My guess is you basically go it right, in principle. 44mm from camera flange to sensor if you have a Canon camera then add the T-ring thickness (best measured with a caliper to be honest) but they usually come in the 10.5/11 mm thickness range. You best bet (other than buying a better CC) is to add spacers (from 0.2mm thickness to 0.5mm thickness with internal diameter of 42mm) to see whether the field coma improves by adding some but to be honest not sure it will do. |
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Hi, I have the same problem with my Bresser Messier 10" F/5, modified Canon 6D and the MPCC III. Moreover, I also use their Canon Protective Ring, which has 11mm of the optical length. Coma still appears in corners. I cannot presume that the known professional, who modified thousands of the DSLRs, screwed the sensor in a wrong position. |
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Hi, As others have said here, the distance matters. It's best to get it within 1 mm. Also, if there are filters in the optical path, these will contribute some optical distance - random guess would be 50% longer than their thickness, but perhaps don't worry about that first. My own experiences are with the MPCC III, an f/4 Newtonian and a CCD with a chip diagonal of 22.5 mm. I had slight coma in corners depending on the camera rotation. Very slight, but I was obsessing. This led me to rigorously going over everything related to collimation and in the end it was still there (example). In your case, how much does f/5 matter with collimation? Not sure, but you should check that too. You didn't say if the coma was about the same in all four corners; if it isn't I'd venture a collimation issue. Also, if the camera chip is large, you'll certainly be seeing a degradation the further away from center you get. |
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| I forgot about my ASI2600MC-Pro, which I have used recently. The coma is also visible. Usually, it's in all corners, but sometimes it's more visible in one corner, while less in the opposite. I think that it's because the MPCC (III) isn't screwed like field flatteners to refractors, but simply inserted into the focuser that causes a bent. My Newtonian is perfectly collimated with the Farpoint Tripple Collimation kit (https://www.astroshop.eu/laser-pointers/farpoint-laser-pointers-650nm-cheshire-autocollimator-2-set/p,61481) that is confirmed by the autocollimator. Additionally, my Newtonian has a quite good Hexafoc focuser and it's modified for stiffness purposes. Anyway, The MPCC, even if it's the 3rd edition, does not have a great opinion and there are much better CC in the market., i.e. several aplanatic ones. |
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Here’s the manual just in case… https://www.baader-planetarium.com/en/downloads/dl/file/id/483/product/3004/mpcc_mark_iii_instruction_manual.pdf for back focus …  once you’ve sure collimation is correct , if your using Nina for taking images, there’s a handy app call Hocus Focus which can tell if you back focuser is correct, Here’s a video explaining https://youtu.be/M1-izvBlO44?si=NTDv5sI4Pj5v883q But before you go down that rabbit hole , collimate look up Vic Menard on Cloudynights Jim Fly of Catseye collimation there’s also a guy named Jason who’s up there with the best on cloudynights, once you’ve that down , with the correct measurement … you’ll be teaching others 😉 |
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I have the same combination of Skywatcher 150/750 PDS, Baader MPCC III and Canon APS-C camera. I have residual coma in the corners as well. Currently I have a 11mm T-Ring + a 1mm T-ring-spacer. So in total I have 56mm backfocus and still the same issues. I have found that the appearance of the coma also correlates to the exact focus point. With a Bahtinov-mask you can try to focus "perfectly" and a little "high" and "low" for the central spike. Just barely visible off center. At least with my corrector i can shift the coma shape from under- to overcorrecting while maintaining a more or less constant HFD in the center with minimal defocus. Over all I am not super happy with this corrector. I maybe change to the GPU in the next month. |
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| I bought the TS-GPU back in February. It is definitely a step up. Check out this thread on Cloudy Nights near the bottom for my experiences with it. |
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| Have you tried using the small spacer ring that comes with it? Might even be screwed in by default. |
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Padraig Farrell: Hmmm... I forgot about the 2.5mm ring while I was working with Canon 6D and modified 'Baader Protective T-Ring for Canon EOS' which gave me exactly 55mm back focus. When I moved to the ASI 2600MC, I should come back to the way. Thanks for that, I'll try it no daubt, but first I have to picture all planned wider objects with my Askar FMA230. |
