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Post by paddyman on Feb 10, 2010 3:23:56 GMT
Hi, im wondering if anyone has ever adjusted the positioning of a secondary mirror on a SCT.
The reason im asking is that i cannot get a planetary image just as sharp as i would like although i know seeing condition haven't been good lately here. I have done a fine collimnation at about 700x and nearly have a perfect airy disc but couldnt just quite get it finished even after lots of tiny tweaks.
After a few days I thought of checking other things and found that the secondary mirror isnt quite centered on the corrector plate, its 1.8mm off centre horizontally and .9mm off vertically. Is this a design factor or an error is what im trying to figure out, and if its error is it much?
From what i have learned about how an SCT work i would have expected the secondary mirror to be exactly centered, it seems fairly straight forward procedure to do this if needed, but i would like to ask if anyone knows if the secondary is delibrately off centre.
I suppose i could always try moving it, checking results and return it to current position if there's no improvement but it seems much easier to ask if anyone knows if the secondary mirror should be spot-on centre or if its ment to be slightly offset.
The scope is a celestron nexstar se6.
Thanks, Paddyboots
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Post by brianb on Feb 10, 2010 10:15:52 GMT
If you have a "nearly perfect" Airy disc at x700 it's good enough. Minor disturbances caused by changes in the air temperature will prevent you from ever seeing a "perfect" pattern - at focus.
Not sure about the off centre but I would not reccomend messing with it ... the secondary mirror being centralised on the axis of the main mirror is what matters optically, and you can't measure that any more accurately that the star test.
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Post by paddyman on Feb 11, 2010 23:54:58 GMT
Thanks brianb for the advice, I was really considering moving it but I think your right, if its this close to a good airy disc I will leave well-enough alone. I had also sent an email to celestron support asking the same question, their reply was that mirror should be centre but if collimnation looked ok then not to worry about it. I think I will leave it alone for now, at least until I get a bigger scope Something above 15" would be very nice, lol, better start saving up. Thanks for your quick advice, it probably saved a lot of hassle. Cheers, Paddyman
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Post by paddyman on Feb 20, 2010 22:15:56 GMT
Hi, Well after thinking about it for a few weeks and talking to David at the last IAA observing night I decided that I would try moving the secondary mirror. Before starting I took lots of measurements and noted them all down just in case I wasn't happy with the results and had to undo everything, but lucky enough it worked out good. While taking more measurements I soon realized that the mirror was even further off-centre than I realized. It was as much as 2mm in one place, so then decided it was time to do something about it. I can go into detail if anyone needs but to keep long story short, I made lots of alignment marks, removed corrector plate, made more marks on secondary housing and removed secondary housing from lens. Measured everything carefully, made calculations to where the centre was and started rebuild (wearing gloves of course). It was tricky to tighten secondary housing on corrector plate without it moving but after several attempts got it sorted, then put whole assembly back in and lined it up. After doing a collmination I was quite happy with the improvement, it was much easier to get it aligned. The image on left was taken about a week before the adjustment and the image on right was from a few nights ago. Both were 1000 frames stacked in registax on clear cold night. I think it was worthwhile, what does anyone else think? Attachments:
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