Refractor with plenty of back focus.

[ayoussef]

I am selling my Esprit 100mm and trying to get something smaller and easier to travel with, so I am looking for a small refractor with at least 170mm of back focus 'for future upgrades to my imaging train' ,and F less than 6,  the only small refractor I know about is the FSQ-85, I think it has 200mm of back focus, but until I find a used one its not currently an option.

The other option is to get a refractor that can be customized in away that its body can be shortened and its focuser replaced with a lower profile focuser which will give me extra back focus, but I really want to avoid that route.

What do you think guys?

[MARS1960]

Ive been looking at this but it only has a modest 85mm of travel but i dont mind adding an 80mm extension, better than seriously limiting your options with wanting a focuser with that much back focus IMO.

http://www.365astronomy.com/TS-INED-APO-70-420mm-Carbon-Fiber-Tube-2-Micro-Crayford-Focuser.html

[ayoussef]

Thanks Mars1960 but isnt adding an extension or installing a lower profile focuser will not incease the back focus unless you get the focuser it self closer to the doublet lens group? My understanding is that the focus length for this teleacope is 420mm now lets say that the distance from the lens to the focuser end is 320mm that gives you 100mm back focus left, assuming we didn't add any optical element along the focal length that will change it.

[MARS1960]

I'm new to all this, i think i may have misunderstood your original question, maybe someone else can help.

I simply think of my focuser as a variable extension tube that just alters the distance between the eyepiece or camera lens and the telescope lens, i assumed adding an extension tube IE: increasing the length of the focusing tube was achieving the same thing as having a longer focusing tube,  see how new i am to all this :).