AT1D 1.25" 99% Reflectivity Dielectric Mirror Diagonal

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Requires Shockwave

Long-life 99% reflectivity dielectric first surface mirror coatings
1/10th wave BK7 optical glass substrate for high contrast/sharp details
Heavy machined metal body for precise collimation and long life
1.25” barrel has internal antireflection threading and fits any 1.25” focuser
1.25” compression ring eyepiece holder won’t scratch eyepiece barrels
Two-year warranty

This 1.25” Astro-Tech diagonal has state-of-the-art 46-layer dielectric mirror coatings for a full 99% reflectivity. In addition to higher reflectivity than the 88% of a conventional aluminum diagonal or the 96-97% of an easily-tarnished silver diagonal, the durable ion-deposited dielectric coatings on the mirror’s finely-polished 1/10th wave BK7 optical glass substrate also provide higher contrast and resolution during critical planetary, lunar, and binary star observations.

The mirror housing is precision-machined from solid aluminum, rather than simply flimsy plastic or a sheet metal stamping, as you’ll find with ordinary diagonals. A rigid die-cast metal U-shaped backplate with stainless steel hardware holds the mirror in perfect optical alignment in the machined body. The solidity of the construction is mirrored in the diagonal’s substantial 7 ounce weight. In addition to being anodized an antireflective matte black, the interior of the body is lined with light absorbing materials. The focuser-side barrel is also anodized antireflective matte black, and is internally antireflection threaded for even higher contrast. The barrel is threaded to accept standard 1.25” filters. The stop at the bottom of the 1.25” eyepiece holder that prevents eyepieces from hitting the first surface mirror may slightly vignette the image of a very wide field 40mm eyepiece. Shorter focal length eyepieces are generally unaffected.

The 1.25” eyepiece holder uses a non-marring soft brass compression ring to clamp eyepieces in place, rather than a thumbscrew that can scratch an eyepiece barrel by tightening down on it directly. The knurled and chromed thumbscrew that operates the compression ring is a full 1/2” in diameter, twice that of many competitive diagonals. This provides a sure grip in cold weather when wearing gloves or mittens.

A slanted groove cut into the outside of the focuser-side barrel engages with the telescope’s visual back or eyepiece holder thumbscrew to act as a safety stop. This keeps the diagonal from falling should the scope’s thumbscrew loosen slightly from cold or use. Dust covers are provided for both sides of the diagonal.

This premium 1.25” diagonal works with virtually any Schmidt-Cassegrain, Maksutov-Cassegrain, or refractor that uses a 1.25” star diagonal. The Celestron NexStar 8i Special Edition is the only known exception. Due to the short fork arm of this scope and the length of its 1.25” visual back, this diagonal will not clear the drive base when the NexStar 8i is pointed at the zenith. For all other scopes, the Astro-Tech diagonal provides a visible improvement. It gives you all the visual performance your scope is capable of delivering, at a price that’s very affordable.