Hello community from Astrobin: I have a question that I hope you can answer. Recently I bought a Newtonian reflector (GSO 200/800 F4) and before that I had a coma corrector Baader MPCC Mark III. I run a test to see if there is any problem with the equipment and the adquisition. I've noticed that in the right side of my lights there is a sort of "coma" but in the left side there is none, or almost nothing. I've attached an image so you can see the problem. I initially thought that it might be pitched optics or bad collimation, but I ask to this community if there is something else that I should consider. Thanks in advance and clear skies!  |
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In my experience is this a collimation problem. The Collimation must be very accurate at F4. What do you use for collimation?
CS Haakon
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The Baader MPC III isn't terribly good at f/4, am afraid. Tried it and ditched for a Sharpstar CC. What you see in your image is tilt toward the right hand side of the image which needs to be corrected in order to achieve a uniform a field as possible. You'll be still left with some at the edges but maybe you can live with it.
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Collimation and/or tilt. Tilt is tricky to diagnose unless the collimation is always spot on. The Baader MPC III isn't great on fast scopes (F5 or slower is preferable) but that's not your problem here.
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can confirm andrea's experience with the MPCC, the baader corrector is simply incapable of proper correction below F5. ditched for a ts optics paracorr 0.95x and got much better results.
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Ive had a huge problem with tilt causing by the locking screws and the the bracket holing the coma corector. I installed the baader click lock, checked the woring distance mm exact and collimation. It was gone. Ive heard the baader mark 3 is very sensitve to tilt on the focuser
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Yeah, if there's coma on side of the image, it could be that something—an accessory on your focuser or your focuser—was tilted a little. Like it was maybe a little wobbly.
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Marc Fischer:
Ive had a huge problem with tilt causing by the locking screws and the the bracket holing the coma corector. I installed the baader click lock, checked the woring distance mm exact and collimation. It was gone. Ive heard the baader mark 3 is very sensitve to tilt on the focuser Before I switched to a threaded connection (feathertouch 3" focuser), I would also have tilt issues using the locking screws. Coma correctors in general compound errors so the tolerance decreases more that if you didn't have a coma corrector.
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Tilt, collimation and probably some backfocus issues. However, even if you fix all these, you will see noticeable "mushroom" shaped stars and spherical aberration at f/4. Try mpcc from SkyWatcher or Paracorr. Even a cheaper Shaprstar 0.95x can provide better stars than baader's mpcc…..
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Jingquan Yan:
Tilt, collimation and probably some backfocus issues. However, even if you fix all these, you will see noticeable "mushroom" shaped stars and spherical aberration at f/4. Try mpcc from SkyWatcher or Paracorr. Even a cheaper Shaprstar 0.95x can provide better stars than baader's mpcc..... I hope you are not saying the Baader is not good, because I already preordered mine a few weeks ago for my F/5 newt. I don't have any coma corrector yet.
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I hope you are not saying the Baader is not good, because I already preordered mine a few weeks ago for my F/5 newt. I don't have any coma corrector yet. It will work at f/5 up to 22-24mm diameter. It will stil introduce spherical but I suspect not as noticeably as at f/4. AFAIK, only the Paracorr type II does not.
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i have the same scope. i use the skywatcher gpu corrector,its expensive but really good. indeed collimation problem or serious tilt. Was it a heavy camera you had on ? i use a baader laser collimator,its good and brings it almost close, the rest i adjust when the camera is in,and go to a star nearby my object that night. so good tips here. https://stargazerslounge.com/topic/283718-is-it-even-possible-to-collimate-an-f4-newtonian-telescope/ |
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andrea tasselli:
I hope you are not saying the Baader is not good, because I already preordered mine a few weeks ago for my F/5 newt. I don't have any coma corrector yet.
It will work at f/5 up to 22-24mm diameter. It will stil introduce spherical but I suspect not as noticeably as at f/4. AFAIK, only the Paracorr type II does not. Ok, thank you. I use a full-frame camera, so I need to use aps-c mode for the baader right?
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In my long experience with a 200mm f/4 newtonian (Quattro), I can say that the TSGPU corrector is a perfect fit - unless there is some tilt, of course, which can be tricky. I use SBIG CCDs and ASI CMOS cameras.
Philippe
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Ok, thank you. I use a full-frame camera, so I need to use aps-c mode for the baader right? That's right.
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@andrea tasselli Do you think I should get the Sharpstar 0.95x reducer instead? It has a back-focus of 56mm, so it will still work for my current spacers. Idk, what would you get?
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@andrea tasselli Do you think I should get the Sharpstar 0.95x reducer instead? It has a back-focus of 56mm, so it will still work for my current spacers. Idk, what would you get? That would be a better choice. Same as I did.
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andrea tasselli:
@andrea tasselli Do you think I should get the Sharpstar 0.95x reducer instead? It has a back-focus of 56mm, so it will still work for my current spacers. Idk, what would you get?
That would be a better choice. Same as I did. And there is no problem in using the 0.95x reducer for visual astronomy too, or is it just for AP?
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And there is no problem in using the 0.95x reducer for visual astronomy too, or is it just for AP? It would need a tuneable top to work, which it doesn't have. So, no, it won't work for visual use except for few selected EP cases. If you want a CC doing double duty you'd need the Paracorr.
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I am not in full agreement with you all on the baader. I've been using it on my 13 f/4.5 for 9 years and 7 with my full frame. Nothing but round stars to the very end. As far as the f/4 goes, I don't know. But it surely works at f/4.5!
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@Tom Marsala Could you post (on this thread) a few uncropped and stretched subs taken with your baader? It would help a lot.
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Yeah, I'll see what
can find
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Putting my money where my mouth is, here are some five minute subs shot with my full frame and the MPCC. They may not be acceptable to some, but considering my rig I'm really happy with the way the stars come out! I've seen better and I've seen worse, haha. Blessings and abundant clarity,Tom    |
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@Tom Marsala The stars look perfectly fine to me. Only the bottom image has some slight elongation of the stars at the far left, but that doesn't bother me. The vignetting is natural. And you're using full-frame and they aren't cropped?
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