Focuser position and travel guidelines and advice

[CharlP6]

I've mostly finished the bulk of my scope's parts design but I'm stuck on the focuser now; specifically on where it should be along the optical axis and how much travel to allow for.

My main concern is, when the focuser is fully racked in, how far outside of the focuser tube should the primary focal point be, and how far inside should it be when fully racked out?

Some info that might be useful: I'll be using it for astro observing, no imaging or terrestrial viewing. It's a 150mm f/5 scope and I'll use a range of EPs and Barlow with it.

Do you have any guidelines for me on this topic? Some further reading references to theory and literature will also be helpful.

 

[Merlin66]

When fully racked in, you should allow enough space for a diagonal and the eyepiece.

This usually is around 80-90mm for 1.25" fittings or 110-120mm for 2".

the focuser travel on refractors seems to be around 75mm

 

 

[CharlP6]

 

27 minutes ago, Merlin66 said:

When fully racked in, you should allow enough space for a diagonal and the eyepiece.

This usually is around 80-90mm for 1.25" fittings or 110-120mm for 2".

the focuser travel on refractors seems to be around 75mm

 

 

Thanks. I probably should have mentioned that it's a newt, so I don't think I'll use a diagonal.

I guess I can always low-ball the tube length and make some extensions, but that seems redundant. 

[Merlin66]

OK

If you design the tube length and position of the secondary to bring the focus say 55mm (to allow for a DSLR camera, just in  case) above the focuser when racked fully in, you should be OK