I was using the Zuiko 135mm telelens as an eFinder for many years...only replaced recently with a 60/240 finder.
I made the "adaptor" from a Oly spacer and epoxy glued a 1.25" adaptor into it.
(see Astronomical spectroscopy for amateurs, fig 12.13)
That's brilliant!
Can you also use a DLSR for the guide camera? I have several spares floating about. It'd be amazing to use an old camera plus my 100mm macro as a guide system
Adam,
I don’t think a DSLR is capable of acting as a guide camera.
A smaller CMOS with a good QE and fast frame rate is usually used with guide software like PHD.
Like Ken I used an old 135mm t2 lense as a guide scope while I was trying to get my OAG sorted on a new scope. It worked fine. Some of the old lenses have bad coma which really shows up on a small guide camera but phd still works.