Star diagonal light path?

[Tiki]

What is the difference in optical length between a typical 2" diagonal for a refractor and a 1.25" ? I need to gain at least 23mm of in-travelfrom my 2" focuser to a 1.25" visual back (or 20mm to a t-thread).

Thanks in advance.

[John]

I found a thread on this which might help. It was posted in 2011:

[Tiki]

Thanks for the link John. Do you know how i might measure my own diagonal without touching the mirror surface?

[John]

What I'd do, if it's possible, is to view something like a distant tree top or similar with no diagonal in place, just the eyepiece (ie: using the scope "straight through") and note where the focus point is (mark the drawtube ?). Then add the diagonal and use the same eyepiece to view the same object and note where the new focus point is. The difference between the two focus points should be the light path through the diagonal.

[Tiki]

What I'd do, if it's possible, is to view something like a distant tree top or similar with no diagonal in place, just the eyepiece (ie: using the scope "straight through") and note where the focus point is (mark the drawtube ?). Then add the diagonal and use the same eyepiece to view the same object and note where the new focus point is. The difference between the two focus points should be the light path through the diagonal.

Of course. All nice and simple.