STT 8300m FW8G back focus

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

  1. ftheadley

    ftheadley Standard User

    Joined:
    Oct 11, 2014
    Messages:
    3
    Location:
    Discovery Bay, CA, 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,
    Frank
     
  2. Colin Haig

    Colin Haig Staff Member

    Joined:
    Oct 27, 2014
    Messages:
    2,287
    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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