Back Focus

[25585]

When is back focus usually needed and when there is not enough, how can it be compensated for without increasing magnification?

Which telescopes; design, focal length, focuser types are most & least likely to have back focus requirement issues? 

[baggywrinkle]

When used with their eye pieces most telescopes attain focus easily.

The back focus issue kicks in when you start using cameras of various kinds. The point of focus for your ep , the focal plane, can then be beyond the camera chip.

Then you need an extension piece to add some extra distance allowing the scope to focus.

I have this with my AA152mm refractor and a video camera. Easily solved with an extension tube.

My 9.25 and 8" SCT are very forgiving and I have not had an issue.

I don't have a Newtonian but from reading i have seen quite a bit written about back focus on these.

[Geoff Lister]

With my Skyliner 250PX (Newtonian OTA on Dob. mount), I have the opposite problem when using a camera or binoviewer. Even with the draw tube fully "in", I cannot get focus, but fitting a X2 Barlow solves the problem. However, when I use my 2" EP, I have to add the extension piece.