Because a diagonal adds optical path — typically several centimetres — and some telescopes do not have the focuser travel to compensate. Reflectors are the usual case, refractors the usual exception.
A diagonal is a right angle and a length, and it is the length that causes the trouble.
What it adds
Several centimetres of optical path, depending on the size and design. The eyepiece now sits that much further from the telescope, and the focuser has to run that much further in to compensate.
Why some telescopes cannot
A Newtonian brings its focus just outside the tube with very little to spare. Add a diagonal and the focus point is inside the drawtube, where nothing can reach it.
Which designs have room
Refractors and folded telescopes, both of which place their focus generously far behind the optics. A folded telescope focuses by moving its primary mirror, which gives it a great deal of adjustment.
The other direction
The same arithmetic explains why removing a diagonal is the first thing to try when a camera will not reach focus: it frees exactly the length the camera needed.
The questions that keep arriving
Why do refractors expect a diagonal?
Because their focus point sits well behind the tube, leaving room for one. Their focusers are often designed on the assumption that a diagonal will be fitted.
Can I use a diagonal on a Newtonian?
Rarely, and there is little reason to: the eyepiece already points out of the side of the tube at a comfortable angle, which is the problem a diagonal exists to solve.
Last reviewed 17 September 2026