What is backfocus?
Steve says: “Backfocus refers to the distance of the focal plane from the end of the eyepiece drawtube and it is critical that the telescope’s focuser has sufficient travel both inwards and outwards to achieve focus with either an eyepiece or a camera.
Newtonian reflectors often suffer from insufficient inward focuser travel to allow a camera to reach focus, while refractors manufactured for portability with short optical tubes result in insufficient outward focuser travel.
A star diagonal for eyepieces or an extension tube for cameras can resolve the issue in a refractor but resolving insufficient inward travel in a reflector is sometimes impossible without making big alterations to the telescope.
Email Steve your astronomy queries to email@example.com and they could be answered in a future issue of BBC Sky at Night Magazine.