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Mirror Diagonals, Prisms and Optical Path

Every diagonal adds roughly 90 to 130 mm of optical path, which affects whether your telescope reaches focus — a detail that matters more than most buyers expect.

• Dielectric mirror diagonals: Multi-layer coatings exceeding 99% reflectivity across the visible spectrum, with no chromatic effect at any focal ratio. The default choice for most telescopes.
• Prism diagonals: Total internal reflection with excellent light throughput and slightly better scatter control, and they can outperform mirrors at f/10 and slower. In fast refractors they introduce chromatic aberration.
• Amici roof prisms: Produce a correctly oriented image for terrestrial use, at the cost of a diffraction spike on bright stars that makes them poor for astronomy.
• 1.25 inch versus 2 inch: A 2 inch diagonal is required to use 2 inch eyepieces and passes a wider field; a 1.25 inch is lighter and adds less back-focus.
• Compression ring clamping: Even radial grip on the eyepiece barrel rather than a single set screw, which prevents marking and keeps heavy eyepieces square.
• Back-focus consequences: Newtonians rarely accept a diagonal at all; refractors and Cassegrains generally have enough travel. Check before assuming.

Why Choose David Astro for Diagonals

A diagonal is not a neutral component, and the right one depends on the telescope in front of it.

• Focal ratio matched: We recommend a prism only where it will genuinely outperform a dielectric mirror, and never in a fast refractor where it would introduce colour.
• Back-focus checked: We confirm your telescope has the inward travel to reach focus with the diagonal and your eyepiece or camera.
• Weight considered: A heavy 2 inch diagonal plus a heavy eyepiece can unbalance a small refractor, and we will mention it before you discover it in the dark.
• Authorized dealer: Genuine diagonals with verified coatings and manufacturer warranty, in Canadian stock.
• Ships from Canada: No duties or brokerage fees on Canadian orders.

Diagonals in Canada — Frequently Asked Questions

Dielectric mirror or prism?

Dielectric for almost all telescopes, and especially anything faster than f/8. A good prism can edge ahead on planetary contrast at f/10 and slower, where its chromatic contribution is negligible.

Do I need a 2 inch diagonal?

Only if you want to use 2 inch eyepieces for wide low-power fields. If your widest eyepiece is 1.25 inch, a 2 inch diagonal adds weight and back-focus with no benefit.

Why does my telescope not reach focus with a diagonal?

A diagonal adds around 100 mm to the optical path. Newtonians generally lack the inward travel to absorb it. Refractors and Cassegrains are designed with a diagonal in mind and usually have no issue.

Is an Amici prism suitable for astronomy?

Not really. It corrects image orientation for terrestrial viewing but produces a diffraction spike across bright stars that is distracting on the Moon and planets.

Choosing a diagonal?

Tell us your telescope and focal ratio and we will recommend mirror or prism. Authorized dealer, Canadian stock.