Experimenting with diagonals for visual back focus

[JimsShed (Jim)]

I read somewhere the other day about the length of a light path through the diagonal can affect the focal length of a scope, and consequently the amount of coma observed at the edges when looking through an eyepiece.
I have a C9.25 XLT SCT. A suggestion was made to use the original 1.25 diagonal when using the f6.3 reducer/corrector.
Clear skies last night so I did a diagonal comparison. Eyepieces were Delos 14 and Ethos 13 as they both accommodate 1.25 holes.
The outcome was better coma correction when using a 2" visual back (Baader click-lock) and 2" diagonal.
This indicates to me that on this scope, with these eyepieces, the factory default back focus is too short.
I'm actually happy with this result because who wants to live with a 1.25 diagonal!

Jim

[Merlin66 (Ken)]

Changing the backfocus will impact on the effective focal length/ focal ratio.
On my C11 when the spectroscope is used the f10 becomes close to f11.
Here's the spreadsheet.

[JimsShed (Jim)]

Thanks Ken. That is one handy spreadsheet. Must confess it never dawned on me that the FR would have a focal length, but makes sense now.
It also explains why the visual quality improved when I increased the back focus spacing because I was actually getting closer to the optimal reduction.