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Help...Cant get dob to Focus


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Everything seems slightly out of focus on my dob. The first thing I checked was the collimation and it seems to be spot on. I have taken my scope out and everything seems to be like I'm looking through a bubble almost. I know reflectors have a coma problem and I think that is whats wrong with mine. Although I don't have a clue as how to fix it. I have a Celestron EP kit that has 5 different EP's and I have tried them all and it seems to be worse on the larger mm EP like the 25mm and the 35mm. I was viewing the moon last night as when I put a 17mm in and the moon filled the EP when I get the outer portion in focus the middle seemed out and vice versa. What do I do? Should I loosen the collimation and locking screws back my mirror up and re-collimate will that fix my problem? Or is it a case of its always been a problem and now I just notice it more? Any help would be greatly appreciated. :)

Also I have the stock 2" Crayford focuser which seems to be working fine. I dont think thats the problem but if it is Im willing to adress it too.

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I'm Only guessing here but you may need to use an extension tube for your eyepieces try moving the eyepieces out of the crayford and see if that helps if so you need to work out what length you will need (you can make up cardboard tubes to do this)

Hope this helps Paul

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What you are describing sounds like classic field curvature, and this is more likely to come from your eyepieces than from your scope.

If I were you, I'd go to a local observing session and see if you can borrow some eyepieces of different brands to try in your scope.

If you think the problem could be coma (less likely in my opinion), see if you can try out a Televue Paracorr.

This may not solve your problem, but it could at least narrow down the possibilities.

Another observation,

if you can currently find focus at the center and the edge of the image--albeit with two different focal settings--you are not going to improve things by moving the mirror up or down in the cell or by getting an extension tube.

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