What is a diagonal on a telescope used for?

BBC Sky at Night Magazine's Scope Doctor Steve Richards solves your astronomy ailments.

Celestron 2-inch diagonal

What is a diagonal used for?

Steve says: “The eyepieces on Newtonian reflectors generally remain at a reasonably comfortable viewing height irrespective of where they’re pointing.


But as refractors and Cassegrains tilt up in altitude their eyepieces get lower and lower, making them harder to look through. This is where a diagonal comes into play.

A diagonal is a small triangular attachment, usually incorporating a small surface-coated mirror set at 45°.

The diagonal bends the light collected by the telescope through 90° before projecting it into the eyepiece.

This simple attachment makes it much easier to observe because now you can view looking down into the telescope rather that up into it.”


Email Steve your astronomy queries to contactus@skyatnightmagazine.com and they could be answered in a future issue of BBC Sky at Night Magazine.