Back focus

[Sparkie67]

Hi all.

a quick question, and just to clarify what’s happening in my head.

had a quick chance to observe the moon this morning, 1st time in a few weeks due to work etc.

all set up 80mm WO refractor, eyepiece in the  focus tube. Nothing, wouldn’t focus at all. Thinking it worked last time I used it.

so dragged out my reflector scope, popped the eye piece, focused without a problem.

it’s only now I remembered I had the 90 degree dielectric in last time.

this brings me to the decision that abviously my scope is not designed to have an eye piece directly in the 1.25” on the focuser without an extension piece?

is this normal for a telescope with 80mm of travel on the focuser.

going to see what happens with my dslr camera attached directly or maybe this will need an extension tube also.

just don’t fancy it sliding out and hitting the floor.

many thanks

andy

[Louis D]

Yes, focuser travel is generally setup assuming use of a diagonal.  You shouldn't have any issues with the DSLR if using a T-mount because it will add 55mm of extension which is very similar to the optical path length of a 1.25" diagonal (65mm to 95mm, depending on prism vs. mirror and brand).

[Sparkie67]

Hi, that would make sense, considering the focuser would be pointed down so the 90 degree makes up the difference.

it really did my head in this morning, I thought I’d broken something 😂

just tried my dslr and surprised at difference when using the 1.25 or 2 inch adapters.

the 2inch seems to suit the camera much better. Can maintain focus over a greater distance that I can with the 1.25” adapter.

[Louis D]

You'll also get less vignetting with the 2" camera adapter.