This 2” Astro-Tech first-surface mirror star diagonal for catadioptric telescopes has state-of-the-art 46-layer
dielectric mirror coatings for a full 99% reflectivity, higher 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 10mm thick 1/10th wave BK7 optical glass substrate also provide higher contrast and resolution during critical planetary, lunar, and binary star observations. It has a physically short nosepiece and SCT rear cell attachment ring so it can clear the drive base on your telescope. You don’t have to buy a special version of the diagonal or special adapters to be sure that it can swing through your 8” scope’s fork arms, as you do with some competitive diagonals. This Astro-Tech diagonal is, quite simply, unique among premium-quality 2” SCT star diagonals for its exceptional value.
The SCT rear cell attachment ring will thread directly onto Celestron SCT rear cells and onto the rear cells of those Meade SCTs that do not have the electric microfocuser as standard equipment, These include older 2080 and 2120 scopes; all LX3, LX5, LX6, and Premier scopes; all current and past LX90s; and all earlier LX200 scopes that were supplied without microfocuser. The diagonal will thread onto a microfocuser-equipped scope if you install the SCT rear cell ring that came with the focuser into the microfocuser’s 2” drawtube. If you want a diagonal that will slip directly into the microfocuser’s 2” drawtube, order diagonal #AT2D 2" 99% Reflectivity dielectric mirror diagonal for refractors instead.
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 U-shaped die-cast metal 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 17 ounce weight.
The interior of the body is lined with light absorbing materials, in addition to being anodized an antireflective matte black. The short nosepiece that holds the rear cell attachment ring is internally antireflection threaded for high contrast. The attachment ring fits both Celestron and Meade rear cells. It is knurled for a sure grip so it can be easily loosened, allowing you to rotate the diagonal to the most comfortable observing position.
Rather than using a metal thumbscrew that contacts eyepiece barrels directly and can scratch them, the diagonal’s 2” eyepiece holder uses a non-marring soft brass compression ring to firmly hold 2” eyepieces and the standard equipment 1.25” eyepiece adapter. The knurled chrome thumbscrew that operates the compression ring is a full 1/2” in diameter, twice that of competitive diagonals. This provides a sure grip in cold weather when wearing gloves or mittens.
The 1.25” eyepiece adapter also uses a compression ring to hold eyepieces in place. The 1.25” adapter has a slanted groove cut into the outside of its focuser-side 2” barrel. This engages with the diagonal’s compression ring to act as a safety stop to keep the adapter from accidentally slipping out of the 2” diagonal should the compression ring loosen during use. Dust covers are provided for the 1.25” adapter and the diagonal’s SCT attachment ring.
This diagonal is a visible improvement over the standard 1.25” prism diagonal provided with virtually every Schmidt-Cassegrain or Maksutov-Cassegrain telescope. It is an absolutely top-rank 2” 99% reflectivity dielectric star diagonal at only a fraction the price of competitive models with equivalent performance and build quality. The same premium performance, for as little as half the price. If there ever was a bargain in SCT astronomy, this is it.