STT 8300m FW8G back focus

Discussion in 'STT Series' started by ftheadley, May 15, 2019.

  1. ftheadley

    ftheadley Standard User

    Oct 11, 2014
    Prescott, AZ, USA
    Greetings Group,
    After two years of storage I pulled out my STT 8300 M with FW8G-STT and attached it to my Celestron C-11 SCT. Once all was connected to CCDOps5 I found that I could not get fine focus on a mountain top 2 miles away. Once it was dark I aligned the mount and centered on arcturus. Focusing through the clear filter I could not get fine focus again. Only an odd shaped star that was not bright enough to use the Bahtenov mask. I thought that it was a massive collimation error so I replaced the camera/filter arrangement with a 2" diagonal and 40mm eyepiece. With the eyepiece I am able to get fine focus so collimation seems not a serious problem but back focus may be. Should I be adding the equivalent distance of the 2" diagonal or is there some more basic arrangement of C11/STT/FW8G?
    Thanks in advance,
  2. Colin Haig

    Colin Haig Staff Member

    Oct 27, 2014
    Hi Frank,
    The back focus on C-11 is around 5.5 to 6 inches (125-130mm) if I remember correctly. The vaious models differ a bit.
    A cheat that I use is to point the scope at the moon, and project it's image on my hand or a piece of paper, and measure the distance to where it comes to focus.
    Because SCTs have a wide range of focus travel, you might want to set it at the midpoint for the test.
    You likely need a spacer / extension tube.

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