Focuser measurements- what do they mean?

[StuartT]

I see focusers designated as 2” or 3.5” etc, but I’m not sure what is being measured. If it’s the diameter at the front, surely that would depend on the size of the OTA it’s fitting into? But if it’s the diameter of the back end then surely that would need to match a standard eyepiece size (1.5” or 2”)

Can anyone help?

thanks

[michael8554]

Hi Stuart

Typical descriptions to obfuscate everyone.

Here's part of the spec of a FeatherTouch 3" Focuser:

Draw Tube Thread  M75X1 Female Thread

Thread on entry (OTA/front) side of drawtube  M109x1 Male Thread

So the eyepiece end has a M75 x 1mm connection (roughly 3").

And the end that attaches to the scope has a M109 x 1m connection.

You'll just have to look through the specs for the truth.

Michael

[CraigT82]

It generally refers to the drawtube diameter. 

A 3" focuser like the one below requires a thread on adaptor to step it down to 2", and then an additional adaptor to step down from 2" to 1.25".

You can see the included adaptors in the smal image on the right. 

 

Edited May 26, 2021 by CraigT82

[Robindonne]

Keep in mind that besides the drawtube diameter, the drawtube length can also have dimensions like 2” or 3,5”.   Its probably mentioned later in the list and revered to as focus travel!