<p>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
<a href="https://www.firstlightoptics.com/skywatcher-focusers/dual-speed-2-crayford-focuser-for-sct-telescopes.html" rel="nofollow noreferrer">https://www.firstlightoptics.com/skywatcher-focusers/dual-speed-2-crayford-focuser-for-sct-telescopes.html</a>
There are lots of different makes and prices.</p>
<p>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.</p>

CC BY-SA 4.0
2020-09-06T22:39:49.633Z
Thank you Dr Chuck - very helpful.
Julien Altieri
CC BY-SA 4.0
2020-09-06T13:47:25.750Z
2020-09-06T13:47:25.750Z
Dr Chuck
<p>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.</p>
<p>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 <code>[A]</code> the 4 screws-piece at the output of the optical tube, and <code>[B]</code> the threaded adapter that I'm holding in my hand in the 3rd picture. The ruler is in inches.</p>
<p>My questions:</p>
<ul>
<li>a) where would a Crayford Focuser like <a href="https://agenaastro.com/gso-sct-dual-speed-crayford-focuser-schmidt-cassegrain-telescope.html" rel="nofollow noreferrer">this one</a> go? (edited after feedback) Should it replace part <code>[A]</code> or part <code>[B]</code>?</li>
<li>b) what is part <code>[B]</code> 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)</li>
<li>c) how standard is this mount? Should I worry about the dimensions for what might replace Part <code>[B]</code>?</li>
<li>d) can anyone comment with potential focusers that you think would fit this build? I'm looking for dual speed and &lt; $150</li>
</ul>
<p>Thanks!</p>
<p><a href="https://i.sstatic.net/Z4Oq6.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/Z4Oq6.jpg" alt="The output is 1.25in" /></a>
<a href="https://i.sstatic.net/YbKvo.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/YbKvo.jpg" alt="enter image description here" /></a>
<a href="https://i.sstatic.net/ZGuoy.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/ZGuoy.jpg" alt="enter image description here" /></a>
<a href="https://i.sstatic.net/YkeHN.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/YkeHN.jpg" alt="enter image description here" /></a>
<a href="https://i.sstatic.net/RXnLx.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/RXnLx.jpg" alt="enter image description here" /></a></p>
<p><a href="https://i.sstatic.net/3X9hH.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/3X9hH.jpg" alt="enter image description here" /></a>
<a href="https://i.sstatic.net/WnRpc.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/WnRpc.jpg" alt="enter image description here" /></a>
<a href="https://i.sstatic.net/JkQ9s.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/JkQ9s.jpg" alt="enter image description here" /></a></p>
<p>Note that I am aware these aren't the best scopes ever made but I do wish to keep it for a while.</p>

<telescope><refractor-telescope><cassegrain-telescope>
What Crayford focuser to buy for a B&L Criterion 8000?
Julien Altieri
# What Crayford focuser to buy for a B&L Criterion 8000?

(Asked by: username_0 on 2020-09-05)

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](https://agenaastro.com/gso-sct-dual-speed-crayford-focuser-schmidt-cassegrain-telescope.html) 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](https://i.sstatic.net/Z4Oq6.jpg)](https://i.sstatic.net/Z4Oq6.jpg)
[![enter image description here](https://i.sstatic.net/YbKvo.jpg)](https://i.sstatic.net/YbKvo.jpg)
[![enter image description here](https://i.sstatic.net/ZGuoy.jpg)](https://i.sstatic.net/ZGuoy.jpg)
[![enter image description here](https://i.sstatic.net/YkeHN.jpg)](https://i.sstatic.net/YkeHN.jpg)
[![enter image description here](https://i.sstatic.net/RXnLx.jpg)](https://i.sstatic.net/RXnLx.jpg)

[![enter image description here](https://i.sstatic.net/3X9hH.jpg)](https://i.sstatic.net/3X9hH.jpg)
[![enter image description here](https://i.sstatic.net/WnRpc.jpg)](https://i.sstatic.net/WnRpc.jpg)
[![enter image description here](https://i.sstatic.net/JkQ9s.jpg)](https://i.sstatic.net/JkQ9s.jpg)

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

**Tags:** telescope, refractor-telescope, cassegrain-telescope


---

## 1 answers

### Answer by username_1 on 2020-09-06. Score: 2 | Accepted answer

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.

> **Comments:**
> **username_0**: Thank you Dr username_1 - very helpful.  

---

