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Pinched optics?

Equipment Reflectors Cleaning
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Michele Vonci avatar
Michele Vonci avatar
Hi everybody,
after more than six years of intense use, I decided it was time to clean the primary mirror of my 8-inch f/4 Newtonian. I did it before Christmas and I didn't have the chance to check how good my job was until tonight.  I knew from other people's experience that one of the issues when reassembling a Newtonian is pinched optics, so I wanted to check that everything was in order. Here is the result with some defocused stars. To me, it looks like my optics are pitched. 
Does anybody know a method to secure the primary mirror to the cell without pinching the optics that does not involve a tedious trial and error?
thanks!
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andrea tasselli avatar
andrea tasselli avatar
From what I can see it looks just not collimated. Maybe a tad rough but there's not much you can do about it. The usual method is to leave a small gap between the edge of the primary and the clips. Depending on the cell you have you may have to shift the primary toward the clips and looking at the in-focus or just slightly defocused star back off as soon you notice the triangular deformation which is the hallmark of trifoil i.e., pinched optics.
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Brian Puhl avatar
Brian Puhl avatar
I agree with andrea.     That just looks like bad collimation.    If i remember correctly, pinches will show up on the outside edge of the defocused star and look more like a 'dent' in the circle.  I got out of mirrors a few months back so my memory is vague right now.

I've heard of folks using a piece of paper as a guage on the clips, but personally I just went until the gasket of the clip was touching the mirror and stopped.    We don't want mirror flop, but we don't want it too tight.
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Dale Penkala avatar
Dale Penkala avatar
I agree with Andrea and Brians posts, this is a badly collimated set of optics. Also looks like there is a tube current going on and maybe the optics are still cooling as well but it’s hard to tell for sure with the collimation off that bad. I’d recollimate it and see what it looks like.

Pinched optics from the mirror clips will give you a 3 lobed pattern in the defration pattern. The harder its pinched the more 3 lobed it will look.

Dale
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Brian Puhl avatar
Brian Puhl avatar
Just a thought, but is this star centered in frame?   I'm glancing at your screenshot and I'm seeing a small bounding box in the top left corner of the larger image frame, which maybe suggest you are using a corner star? I'm not familiar with APT unfortunately
Stefan Pfleger avatar
Stefan Pfleger avatar
I also think like the rest here that this is looking like a collimation issue. In addition, i noticed that tube seeing (if the mirror is not cooled to ambient) can also produce similar artifacts. 
if you were worried about pinched optics, then I‘d imagine you know why it happens and probably you haven‘t overtightened the mirror.
greetings, Stefan
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Michele Vonci avatar
Michele Vonci avatar
Hi, thanks everyone for the suggestions! I've reseated the primary in the cell and used the "business card" trick to tighten the clips' screws. Also, I collimated the telescope again. Tomorrow it should be a clear night, I shall see how things turned out!
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