Back Focus - What does it mean ?

[johnturley]

I always find the term 'Back  Focus' a bit confusing, and is frequently used when what people actually men 'In Travel' on the focuser.

At first glance some people (like me) might think that it means the amount you can rack out the focuser by, i.e.  distance of travel on the focusing mount, but I think it means something completely different. What I actually think it means is the amount the focuser can be racked in from the focal position. For example most Explore Scientific Refractors state in their specification 150mm Back Focus, and yet only have a miserly 45mm of travel on their focusing mounts, and come with two 40mm extension tubes, yet to reach focus with most eyepieces without a star diagonal you still need a further extension tube of about 50mm, as a simple eyepiece (not one with a built in barlow lens) will come to focus at a distance further out from the focal point equal to the focal length of the eyepiece. Incidentally back in the 1980's most refractors had focusing mounts with travel distances of  around 125-150mm, but now the average is around just 80mm, or even less in the case of ES Refractors. 

Personally I would find it less confusing if the words 'In Travel' and 'Out Travel' were used instead, I think Back Focus is used to determine whether accessories such as binoviewers, can be successfully fitted.

John 

Edited March 1, 2020 by johnturley

[vlaiv]

Back focus is distance of focus plane to telescope - or how far back from the tube is your focus plane.

It has nothing to do with moving focuser in or out. It does however have meaning in context of using certain accessory and it's optical path.

In order to use accessory, total optical path must be shorter than back focus.