Celestron 93573 Dielectric Star Diagonal 2 inch with Twist Lock and 2 inch to 1.25 inch Step-Down Adapter Review (2026)

Who a 2 inch dielectric diagonal is for

This diagonal is built for owners of refractors and Schmidt-Cassegrain telescopes with 2 inch focusers who want a single accessory that handles both 2 inch wide field eyepieces and standard 1.25 inch eyepieces without swapping diagonals mid session. Because it bundles a dedicated 1.25 inch step down adapter with 2 inch filter threads of its own, it suits observers who own eyepieces in both barrel sizes and want to avoid buying and keeping track of a second, separate diagonal. It is less suited to owners of small, light travel refractors, where the added weight of a full 2 inch diagonal can affect balance on a light mount.

Coating quality and optical performance

The reflective surface uses 58 layers of dielectric coatings, a construction that Celestron rates at over 99 percent reflectivity and diffraction limited optical quality, which in practical terms means less light is lost at the mirror than with basic aluminum or standard enhanced coatings found on cheaper diagonals. That higher reflectivity is most noticeable at higher magnifications and on faint deep sky targets, where every bit of transmitted light helps resolve fine detail or a dim companion star. The diagonal bends the light path 90 degrees for a more comfortable viewing angle, which matters most on telescopes pointed high in the sky where a straight through view would force an awkward neck position.

Build, twist lock and included adapter

The housing is black anodized aluminum with an orange anodized twist lock ring that stands out visually and secures 2 inch eyepieces firmly and concentrically in the light path, which helps keep heavier 2 inch eyepieces, including premium wide field designs, centered and secure rather than resting loosely in a compression ring. The included 1.25 inch step down adapter also carries its own 2 inch filter threads, so filters can be threaded onto either the 2 inch barrel or the adapter depending on which eyepiece size is in use. Two interchangeable barrels are included, a standard 2 inch barrel for refractors and a threaded barrel for Schmidt-Cassegrain telescopes, extending its compatibility across telescope types.

Weight and value considerations

Because this diagonal carries a full 2 inch prism or mirror assembly plus a twist lock mechanism and adapter, it is heavier than a 1.25 inch only diagonal, and mounting it on a small refractor with a lightweight alt-azimuth or photo tripod mount can shift the balance point enough to need retightening the tube rings or adding a counterweight. Its price sits above basic 2 inch mirror diagonals that use standard coatings rather than dielectric layers, which is the tradeoff for the brighter, higher contrast views the coating provides. Celestron backs the diagonal with a 2 year US warranty, which is a meaningful consideration for an accessory that will see years of use.

Verdict and how it compares

This diagonal earns a strong score as a premium, dual barrel accessory for telescopes with 2 inch focusers, and it ranks at the top of the star diagonal comparison it belongs to, ahead of several respected 1.25 inch and 2 inch dielectric diagonals from Orion and SVBONY at similar or lower prices. Buyers should weigh whether they actually own or plan to own 2 inch eyepieces, since the price premium and added weight over a 1.25 inch diagonal are only worthwhile if that wider format eyepiece will see regular use. For anyone building a serious refractor or SCT setup around 2 inch wide field eyepieces, it is a logical and durable long term accessory.

Frequently asked questions