saxon FCD100 127 focuser

[skogpingvin (Bill)]

Hi all, I'm mucking about with a saxon FCD100 127 APO for my job. I've got a question for the brains trust, because I'm not sure I understand the geometry.

To focus on infinity with my little 20mm 1.25" test eyepiece and the diagonal, I needed one of the two 37.5mm spacers supplied with the scope. The focuser was racked out to about 26mm.

To focus on infinity with my DSLR (without the diagonal) I needed both spacers and even then the focuser was racked all the way out to 44mm.

I've attached two pictures showing both of these.

Perhaps I'm wrong here, but I can't see any application that would require no spacer to be in the train. A shorter (non-parfocal) focal length eyepiece might require racking in, but not by 20mm, right? Removing the diagonal and just going for the eyepiece would require more spacers, right?


Or am I completely wrong?

[casstony]

With a reducer/flattener in front of the dslr you probably wouldn't need any spacers and the focuser would be racked in a fair way.

[Ukastronomer (Jeremy)]

Surely no scope needs spacers between the diagonal and scope with eyepieces, I just place the DSLR on the diagonal

[skogpingvin (Bill)]

Originally Posted by casstony

With a reducer/flattener in front of the dslr you probably wouldn't need any spacers and the focuser would be racked in a fair way.

Aah, I hadn't thought of that - I have a reducer/flattener on my Sharpstar 107, but it still racks out a fair way (to about 68mm) for the ASI.


Having thought of that - my refractor has an OAG and a filter wheel on it between the flattener and the CMOS well - that's going to push the focus back a fair way. Perhaps I'm being overly harsh on the designers.

[skogpingvin (Bill)]

Originally Posted by Ukastronomer

Surely no scope needs spacers between the diagonal and scope with eyepieces, I just place the DSLR on the diagonal

I wouldn't want to put the DSLR on the diagonal - you get an extra element between the sensor and the objective on which dust can settle.


But the elements I'd neglected to think about were filter wheel and OAG. I think that might explain the design.