Rear assembly removed cleanly:
The 77mm P0.75 male thread was left:
I'm getting excited! With the M77-M58, M58-M42, M42-EOS adapters in place, we have the following compact 4" refractor with 300mm focal length!
The amount of 77mm thread is enough, on the contrary of my previous imagination! Next, the flange distance is correctly made for EOS with a single extension tube. This becomes a very fast f/2.8 compact refractor! Unluckily, when all the adapters were screwed together, the orientation of the built-in tripod socket was in a bad position, I have to find a solution for that.
The above configuation allows infinity focus without any problem! The built-in focuser is butter smooth despite it is a very aged lens. Canon L series is really really serious!
To switch it into visual configuration, the extension tube should be removed, and a EOS-1.25" adapter is used. Unluckily, the amount of optical path length is insufficient for a star diagonal. The following picture shows how I could reach infinity focus with an eyepiece.
This is a 300mm wide field refractor, with my 20mm widescan, it yields 15x with 5.3+ degree field of view, very sharp and respectable!
To use a star diagonal, I must insert a 2x barlows in front of the diagonal, so it becomes a 900mm focal length telescope. The exact focal length must be measured. I must try to use it for planet imaging soon, and if it's good enough, I shall sell my C5.
Keywords: Canon FD 300mm f2.8 conversion EOS digital EF mount astrograph telescope flange distance eyepiece M42