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Thread: 8" Dobsonian with 2x Barlow will not focus.

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    Default 8" Dobsonian with 2x Barlow will not focus.



    I had a look at Jupiter early this morning and with the 2x Barlow and a 10mm lens the image was so out of focus that Jupiter wasn't a perfect circle, I was wondering if a 5mm lens would work better than the 2x Barlow.

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    Default Re: 8" Dobsonian with 2x Barlow will not focus.

    Go the other way and try it without the Barlow. See if you can attain focus with just an eyepiece. Then you can figure out where the problem is.
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    Default Re: 8" Dobsonian with 2x Barlow will not focus.

    You may also be pushing up the power a bit above what conditions allow, agree with the eyepiece suggestion.
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    Default Re: 8" Dobsonian with 2x Barlow will not focus.

    I can make out the bands of jupiter without the Barlow its just a lot smaller,
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    Default Re: 8" Dobsonian with 2x Barlow will not focus.

    Just FYI, Jupiter is not a perfect circle. Its diameter across its equator is noticeably greater than from pole to pole.
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    Default Re: 8" Dobsonian with 2x Barlow will not focus.

    A 2x barlow and a 10mm is really pushing things. Now you know what happens if you push it too far.
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    Default Re: 8" Dobsonian with 2x Barlow will not focus.

    Would a 6/7mm lens push my telescope to far?

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    Default Re: 8" Dobsonian with 2x Barlow will not focus.

    Quote Originally Posted by SiriusB View Post
    Would a 6/7mm lens push my telescope to far?
    A 6mm EP is about as far as you will want to go most of the time. I have found that an ES 6.7mm 82 works very well with that scope. There have been times when the ES 11m 82 Barlowed well for an effective 5.5mm, but this is not regular.
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    Default Re: 8" Dobsonian with 2x Barlow will not focus.

    You generally want to keep your eyepiece focal lengths between the focal ratio expressed in mm and 6-7 times the focal ratio in mm. So for example, on an f/6 scope, 6mm to 36mm/42mm. There may be one or two nights per year when you can really push beyond 6mm but those will be the exception rather than the rule. So if some night, it's looking pretty steady, I'd try the 10mm in the barlow again. You never know.
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    Default Re: 8" Dobsonian with 2x Barlow will not focus.

    I use a 3X and have never had a problem with focus with my EP's.
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