# What Crayford focuser to buy for a B&L Criterion 8000?

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!

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

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.

• Thank you Dr Chuck - very helpful. Sep 6 '20 at 22:39