Celestron Dielectric Star Diagonal 1.25 inch with Twist Lock 58 Layer Coating 99% Reflectivity Review (2026)

Coating and construction

Celestron specifies 58 layers of dielectric coating on this diagonal's BK7 glass mirror, rated at over 99 percent reflectivity, which places it among the brighter mirror diagonals available at the 1.25 inch size. The housing is machined black anodized aluminum, with the interior of the barrel threaded and painted flat black to suppress internal reflections that could otherwise reduce contrast, and the whole unit weighs about 7.5 ounces, light enough to avoid meaningfully affecting balance on most refractors and Schmidt Cassegrain telescopes. Celestron backs the diagonal with a 2 year warranty, which is a useful signal of confidence in the coating and housing holding up over normal use.

The twist lock eyepiece holder

Instead of a simple setscrew that presses directly against an inserted eyepiece barrel, this diagonal uses a twist lock mechanism that closes evenly around the full circumference of the barrel to hold it centered and secure. That even grip helps keep the eyepiece precisely aligned with the optical axis, which can make a small but real difference to image sharpness at the edge of the field compared with a setscrew that can push an eyepiece slightly off center, and it also avoids the small marks or dents that a metal setscrew can leave on a barrel over repeated use. The barrel still accepts standard 1.25 inch filters through its threaded nosepiece as well.

Telescope compatibility and size trade offs

At 1.25 inches, this diagonal fits the focusers on the great majority of refractor and Schmidt Cassegrain telescopes without an adapter, making it a broadly compatible choice for observers who have not invested in 2 inch eyepieces. The trade off is the same one shared by every 1.25 inch diagonal, namely that it cannot accept larger 2 inch eyepieces, which some observers eventually want for the widest possible true field of view at low power. Anyone who already owns or plans to buy 2 inch eyepieces should look at Celestron's larger 2 inch diagonal instead, since this unit's compact barrel size is a hard limit rather than something an adapter can work around in that direction.

Price relative to similar coatings

This diagonal sits at a higher price than many basic dielectric diagonals that advertise similar reflectivity numbers, and buyers comparing options purely on a quoted reflectivity percentage may wonder why. The difference generally comes down to build quality details that do not always show up in a spec sheet, such as the twist lock mechanism instead of a simple setscrew, the flat black interior baffling, and the backing of a established brand's 2 year warranty, all of which add cost over a bare bones diagonal. Observers who mainly care about the lowest possible price for a stated reflectivity figure may find cheaper alternatives, while those who value the mechanical extras and warranty may find the premium reasonable.

Who this diagonal suits

This is a good fit for owners of a refractor or Schmidt Cassegrain telescope with a 1.25 inch focuser who want a bright, well built diagonal with a secure, non marring eyepiece grip and do not mind paying somewhat more than the cheapest dielectric options on the market. The twist lock in particular will appeal to anyone who frequently swaps eyepieces during a session and wants a fast, even grip rather than fiddling with a setscrew each time. Buyers focused purely on minimizing cost, or who need 2 inch eyepiece compatibility, will likely be better served by a cheaper 1.25 inch diagonal or by Celestron's larger 2 inch model respectively.

Frequently asked questions