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I'd like to get a dual-speed Crayford focuser for my newly acquired a B&L criterion 8000 (8") as the mirror shift is pretty bad.

I'm really not sure which one to buy because I'm quite confused about the dimensions and potential adapters I need, and don't know how standard this config is. Let's call [A] the 4 screws-piece at the output of the optical tube, and [B] the threaded adapter that I'm holding in my hand in the 3rd picture. The ruler is in inches.

My questions:

  • a) where would a Crayford Focuser like this one go? (edited after feedback) Should it replace part [A] or part [B]?
  • b) what is part [B] officially called? and what diameter is it referred to by? (I measured 1.9in but can't seem to find anything like that on the Internet)
  • c) how standard is this mount? Should I worry about the dimensions for what might replace Part [B]?
  • d) can anyone comment with potential focusers that you think would fit this build? I'm looking for dual speed and < $150

Thanks!

The output is 1.25in enter image description here enter image description here enter image description here enter image description here

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Note that I am aware these aren't the best scopes ever made but I do wish to keep it for a while.

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Item B is called a visual back. The new focuser would just replace this, and screw onto item A. The focuser you link to is designed for a refractor, but you need one for a SCT, like this https://www.firstlightoptics.com/skywatcher-focusers/dual-speed-2-crayford-focuser-for-sct-telescopes.html There are lots of different makes and prices.

Regarding the sizing, I'm not sure about an old scope like the Criterion: it's probably the same as the other popular current models, but you might do better with cloudynights.com to get that confirmed.

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  • $\begingroup$ Thank you Dr Chuck - very helpful. $\endgroup$
    – Julien Altieri
    Sep 6, 2020 at 22:39

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