On a refractor or a folded telescope, a diagonal is what makes anything above the horizon comfortable to look at. A dielectric mirror gives the highest and most even reflectivity; a prism can suit a slow refractor and is more sensitive to focal ratio.
A diagonal is a small accessory with a disproportionate effect on whether an evening is comfortable.
Why it exists
Without one, a refractor or a folded telescope pointed high puts the eyepiece near the ground and the observer on their knees. The right angle moves it to a position a person can actually use.
Mirror
A dielectric coating reflects a very high and very even percentage across the spectrum, and the best figures on this shelf belong to mirrors. Where a maker publishes a reflectivity figure, that is worth comparing; an absent figure is itself an answer.
Prism
Total internal reflection rather than a coating. It can be slightly better on a long-focal-ratio refractor, where it also corrects a little spherical aberration, and it is more sensitive to the ratio it is used at.
The detail that saves your eyepieces
The clamping method. A compression ring grips a barrel without scoring it; a bare thumbscrew marks every eyepiece it holds, permanently and visibly.
The questions that keep arriving
Does a diagonal flip the image?
It produces an image the right way up and reversed left to right. That orientation is what makes printed star charts confusing until you get used to it, and it is normal rather than a fault.
1.25 inch or 2 inch?
Match the focuser. A 2 inch diagonal passes the wide fields that 2 inch eyepieces exist to deliver, and most come with an adapter for smaller barrels, so it is the more flexible choice if the focuser accepts it.
Last reviewed 17 September 2026