Eyepieces & Optics
Star Diagonal
A mirror or prism that bends the light path 90° so you are not lying on the ground.
Specifications
| Types | Mirror (dielectric or enhanced aluminium), Amici prism |
|---|---|
| Barrel | 1.25″ or 2″ |
| Reflectivity | 96% – 99%+ (dielectric) |
| Clear aperture | 26 – 46 mm |
| Optical path added | 70 – 110 mm |
| Image orientation | mirrored (mirror) or corrected (Amici) |
Observing parameters
| Light loss | 1% – 4% for a good dielectric |
|---|---|
| Minimum clear aperture | must exceed the eyepiece field stop or you vignette |
| Amici prism caution | can show a central spike on bright objects |
| Fit check | verify focuser has travel to spare before buying 2″ |
| Best for | SCT, Mak, refractor; optional on Newtonians |
Overview
A star diagonal fits between the focuser and the eyepiece and turns the light path through 90°, which matters whenever the telescope points high — without one you would be looking straight up through an eyepiece at knee height. A mirror diagonal gives a correct but mirrored image; a prism diagonal gives a correctly oriented image but can introduce a slight colour cast.
Diagonal quality is easy to overlook and easy to get wrong. A diagonal is in the optical path, so a poor one degrades everything you own. Dielectric mirror diagonals reflect over 99% across the visible spectrum and are the safe choice; cheap prism diagonals on fast telescopes can add a visible spike of chromatic aberration.
Note that a diagonal adds optical path length — usually 70–100 mm — which some Newtonians cannot accommodate without moving the primary.
Where it fits
Practical notes
- Buy a dielectric diagonal once; it outlasts several telescopes and never needs cleaning.
- Check the clear aperture against your widest eyepiece field stop — a 27 mm aperture chokes a 2″ wide field.
- On a Newtonian, confirm you can still reach focus with the extra path length before committing.
- A 2″ diagonal with a 1.25″ adapter gives you future upgrade room without re-buying.
Pairs with
- Plössl eyepiece
- Wide-Field Eyepiece
- Barlow Lens
- Lens Cleaning Kit
Related tools
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