Focus straight through refractor tube (i/e/, w/out diagonal)

[Tempusfugit]

I'm pretty new at astronomy, and not totally knowledgeable about relevant optics....but, can anyone tell me why

• I can't focus my refractor using an eyepiece straight through the back (i.e., no diagonal), but...
• I can focus my mak using an eyepiece straight through the back, insead of using the flip mirror.

The refractor is a 80mmED.
The mak is a Vixen 110mm.
Would an "erect image" device make a difference? (I really don't know what they are, but I see them listed on dealers' websites)
Thanks for any help......

[mrsnipey]

It sounds like you just need an extender between your ep and the telescope to come to focus.

A diagonal will work as well.

[Merlin66 (Ken)]

The typical star diagonal takes about 50mm of back focus, so if you view straight through you'll probably need a 50mm extension tube. They're available in both 1.25" and 2" sizes.

[leon]

Yep, that will do it, you may even have to go longer , but I would start with a 50mm.

Leon