Relationship of Eyepice focal length to needed back focus?

[Mark D]

Hello all,

perhaps someone could help a complete beginner with a question.

I just upgraded my Celestron C6R's stock R&P focuser to a GSO linear bearing Crayford.  Knowing that the Crayford had less back focus than the stock unit I ordered a 2" extension tube.   With my only eyepiece (the stock 20 mm Plossl ), the 2" extension and full back focus I can just get the focus to come in on nearby terrestrial targets.

If I were to switch to shorter focal length EP am I likely to need more or less back focus? 

I think I am going to eventually need a longer extension and I trying to determine if I need a 3" or 4" length.

Thank you,

Mark D.

[Earl]

It depends on the type of eyepice rather than its focal length iirc.

[John]

If you can just get it to focus on terrestrial targets with the focuser fully racked out, you should be OK on astro targets as they are at infinity (effectively) and will need more inwards movement of the focuser. I don't think you will need a longer extension unless you want to focus on terrestrial targets that are even closer - the closer the target, the more the focuser will need to be racked outwards or another way of looking at it is that the eyepiece needs to be moved further from the objective lens.

Eyepiece focus positions depend where their focal plane is. Often this is located around the junction of the eyepiece body and the 1.25" or 2" barrel but this does vary with different designs and manufacturers. It's not automatically the case that shorter focal length eyepieces will need more or less back focus.

[Peter Drew]

In general, the shorter the focal length eyepiece the less back focus you need, however some designs differ for the same focal length.