Hello folks,
I picked up a previous generation 14in RC from a friend and it seems place solved focal length comes back to 2781.39 mm, the baseline reported “should be” 2848mm.
Please share your direct experience with a similar issues and how you resolved it.
P.S: I picked up a Gerd Neumann photographic Ronchi eyepiece (10 lines per mm) for a later star test using it.
What’s the specified back focal distance?
Tony Gondola · Sep 21, 2026, 11:43 PM
What’s the specified back focal distance?
I believe the backfocus distance on these scopes is 10” (254mm) from the rear of the mirror cell..
Essentially, If you are plate solving your image at less focal length (mind you, ~60mm off on a scope with a 2848mm f/l is VERY close, and it will be fiddly to get it right) will require you to move the secondary mirror further away from the primary.
The caveat here is this.. Once you do that - you will need to re-collimate the scope. The process of collimating the scope will likely change the set distance the mirrors are appart unless you are very careful about how you use the push/pull screws for the primary. If you get the mirror spacing 100% perfect, then recollimate the scope, only pushing (turning the collimation bolts counter clockwise) the primary into alignment, you will move the mirror spacing too close together again, alternately, if you get the spacing perfect then only use screws clockwise to collimate the primary, you’ll move the mirrors too far apart.
Finding the balancing act will be tricky, especially at the level of precision you’re already at (~2.11% away from perfect).
This scope was bought in 2016 with the badge “HIGHPOINT”, this is the iteration right after GSO decoupled the focuser from the primary. I have email GSO requesting a drawing.
I have found this diagram where all the measurement are detailed well. I am hoping this actually accurate
📷 image.png