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Focus problem

gooseholla

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gooseholla

gooseholla

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Right, so when I first got my scope, the 2 inch eyepieces were sharp across the field. Now, however, they aren't sharp apart from in the centre hmmm 60%? And at the top left and left side especially they have soft patches around bright stars with two soft spikes coming out of the bottom left and top right of a star and they are out of focus and larger. However, when they get in the centre they become pin sharp. It seems to be collimated, what is going wrong?! Not letting tube cool down? Something gone wrong in the system?!! Collimation not actually in collimation?

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gooseholla

gooseholla

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12 inch scope. Bascially, if the centre is sharp, the outer most are blurry - makes everything look like you've found galaxies by the tonne! If you get the outer parts focused, the middle are just out of focus, but they don't go as blurry or distorted as the edges.

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Moonshane

Moonshane

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it will be 5.3 I suspect, OOUK longer tubes usually are. there's certainly something not right if the Nagler does it. you'd need to let your scope cool for maybe an hour or so but I can usually observe at low powers (maybe 70x) without issues more or less straight away.

I am sure whatever it is, it will be fixable.

when you moved the primary up the tube, did it happen then? how did you manage to move the primary up the tube? did you redo the bolt holes or some other means? this sort of thing is so hard to try and do remotely!

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John

John

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....when you moved the primary up the tube, did it happen then? how did you manage to move the primary up the tube? did you redo the bolt holes or some other means? this sort of thing is so hard to try and do remotely!....

Something connected with that would be my suspicion as well. If there is something putting some pressure on the primary mirror somewhere it could be warping very slightly - enough to change it's figure from a paraboloid just a tiny bit.

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gooseholla

gooseholla

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No, because the primary mirror move solved the problem of focusing. Before, if we remember, the eyepieces didn't focus at all! I am starting to lean towards scope not cooling down now. Perhaps time to add a fan?

When I moved it up, there were two holes already in the scope. I simply moved the bracket and the mirror to the already made holes. It worked fine, but only noticed it recently (the past week or so). Scope is left outside, but has end cover on.

Perhaps I shall let it cool down for a couple of hours before viewing, and see. if not, then try taking the mirror out and starting from scratch, incase something is changing its shape.

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Moonshane

Moonshane

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just so I understand, the issue of blurring etc happened after you moved the mirror but you had an initial period after you moved the mirror when the focus was spot on? if your scope is stored outside, I doubt that inadequate cooling is the ssue but give that a try. is your primary cell held in by three bolts or two? (you mention two holes).

things to check are

the mirror clips - they should have a small (1mm or less maybe depending on the cell design) between them and the mirror.

mirror supports (probably three) - are they evenly adjusted and all touching the mirror

is the primary glued in with silicone? could affect things.

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alan potts

alan potts

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If things are outside you can get issues of the sun being in a different place. Maybe two weeks before it was in the shade of a tree or something. The other thing is maybe the sun was out all day and now as it goes down temperature fall off it quicker now than in July. I have these issues all the time. Day temp 34 night 8, I often start a good two hours before I observe and make a special effort to keep the sun a bay.

Alan.

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michael.h.f.wilkinson

michael.h.f.wilkinson

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Two things spring to mind: deformation of the optics, although bad cooling and pinched optics (mirror clips/supports too tight) generally affect the centre of the FOV as well, or your eyes becoming better at spotting the problem. I remember being very pleased with my cheap and cheerful Omegon 15x70s, but after prolonged use I started spotting all sorts of optical and mechanical shortcomings. You might simply be looking at coma that you did not notice before (though you description suggests quite a severe problem). A coma corrector should sort the problem if the mirror is not deformed.

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gooseholla

gooseholla

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Shane - yes, after I moved the mirror to allow 2" eyepieces to focus, it was fine for a while. By two holes, I meant that on the side of the tube there are two different hole sets for brackets to allow three bolts to hold the mirror cell to the tube. I don't know whether the primary is glued with silicone, or how it sits in the cell! I'm afraid. All I did was bolt the mirror into the tube upon purchasing it!

Scope isn't stored outside, I usually leave it out an hour or two before observing, but with the front on to avoid leaves, dust etc.

Michael, an interesting theory. In part I probably would say, yes I am noticing things that I didn't before - happens in everything in life, once the romance wears off, we notice imperfections for the first time, doesn't it? But I believe this to be such a radical change that anyone would have noticed it from the off. As I said, off field, it looks like hubble deep field - everything is a blurry galaxy now!

So, today I am trying two/three things:

1. I've had the primary mirror bolts out and checked them all, and re-screwed them all in trying to make sure they hold the mirror evenly.

2. I have re-collimated, obviously.

3. I have my scope out there now in the shade cooling down, without the front on.

i'll let you know if I have any success or not.

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