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.

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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.”

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