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    Default I can't get the telescope to focus when using prime focus.



    Hello all,

    I made my first trip to my nearby dark area today to make my first foray into photography. I have a Canon 10D DSLR camera, with a T-ring, and the cap portion of Orions universal adapter (here).

    My telescope is f/6.9. I was unable to get the telescope to get a sharp image. With my focuser in its lowest position, I was able to get the image to a small circle where I could still see the mirror and supports. Needless to say, I was a bit ticked off. I had done a lot of research to make the process as smooth as possible, but evidently not enough to overcome this problem. What can I do to ?

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    Default Re: I can't get the telescope to focus when using prime focus.

    You don't provide any information about your telescope or focuser, other than the focal ratio.

    Is it a Newtonian reflector? This would be my assumption based on your post.

    If so, is it an astrograph, or a standard Newt? Is it equipped with a low-profile focuser? Are you using a Barlow or coma corrector?


    Hard to provide any more help without specifics of the OTA/focuser...
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    Default Re: I can't get the telescope to focus when using prime focus.

    Standard newtonian reflector, rack and pinion focuser, wasnt using a barlow or corrector.

    A 5.1" diameter scope, 130mm primary mirror, 900mm focal length

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    Default Re: I can't get the telescope to focus when using prime focus.

    Almost all Newtonian reflectors, unless they are designed as an astrograph, lack sufficient back-focus distance to allow a DSLR or similar camera to be brought into focus. The focal plane is inside the body of the focuser/OTA.

    There are some possible solutions, in order of difficulty:

    - use a Barlow lens, or coma corrector (the latter is more expensive and not normally required above f/5, the former less expensive) to attempt to 'throw' the focal plane outside the focuser body

    - replace the focuser with a low, or zero-profile aftermarket focuser. May require an adapter, or modification to the OTA for mounting the focuser

    - move the primary mirror "up", or forward in the OTA, toward the secondary mirror - requires extensive, and usually permanent/irreversible modifications to the OTA
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    Default Re: I can't get the telescope to focus when using prime focus.

    Quote Originally Posted by AustinPSD View Post
    Almost all Newtonian reflectors, unless they are designed as an astrograph, lack sufficient back-focus distance to allow a DSLR or similar camera to be brought into focus. The focal plane is inside the body of the focuser/OTA.

    There are some possible solutions, in order of difficulty:

    - use a Barlow lens, or coma corrector (the latter is more expensive and not normally required above f/5, the former less expensive) to attempt to 'throw' the focal plane outside the focuser body

    - replace the focuser with a low, or zero-profile aftermarket focuser. May require an adapter, or modification to the OTA for mounting the focuser

    - move the primary mirror "up", or forward in the OTA, toward the secondary mirror - requires extensive, and usually permanent/irreversible modifications to the OTA
    Hehehe, Barlows are about $50. Focusers start at much higher prices, and I'm definitely not gonna touch the primary mirror. I think the answer is obvious. Thanks very much.

 

 

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