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&amp;L criterion 8000 (8&quot;) 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 &lt; $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.