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Thread: Begginer Astro-photography question, Trouble getting it focused

  1. #1
    Join Date
    Feb 2011
    Posts
    101

    Question Begginer Astro-photography question, Trouble getting it focused

    I have an 8-inch Dobsonian Orion Skyquest and I recently purchased a Cannon T2i DSLR camera. I have been experimenting with taking photo through the scope by connecting the camera via a t-adapter and t-ring.

    I have had some success taking some short exposure photos but only through a 2x Barlow lens. When I insert the T adapter directly into the scope I cannot seem to get the scope in focus. It is blurry no matter how much I turn the focuser.


    Is there an extension needed? What am I missing here?

    Thanks in advance for any help,

    Chris

  2. #2
    Join Date
    Dec 2006
    Posts
    3,275
    Probably your problem is exactly the opposite. It likely doesn't have sufficient in focus. Use a piece of tissue paper and a bright star or the moon to see where focus is. You can see the image through the tissue paper which acts as a screen. As you get closer to the focuser the star or moon should get smaller until it is in focus. Likely the focus point will be closer to the focuser than the DSLR chip can be placed. It may even be inside the focuser. Likely you will have to either get a shorter focuser or move the main mirror forward. Moving the mirror forward will change balance and may require some rebalancing.

    Dobs are designed for visual work not photo work at prime focus. Keeping the focus close to the tube allows a smaller secondary for a sharper image that is still fully illuminated. But what is good for visual work isn't so good for photographic work. Without an equatorial drive they are pretty well limited to imaging the moon and bright planets which usually requires a barlow. Since the barlow lens is inside the focuser it can usually reach focus, as you've discovered. Except for the full disk of the moon little else is suitable for prime focus work with simple Dob. Moved to a capable (read expensive) equatorial mount they can do deep sky imaging the scope is then up to the task once focus position is adjusted to where it is reachable.

    Rick

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