This 2” Astro-Tech first-surface mirror star diagonal 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. This Astro-Tech diagonal is, quite simply, unique among premium-quality 2” star diagonals for its exceptional value.
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 2” 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 2” filters.
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 secure 2” eyepieces and the standard equipment 1.25” eyepiece adapter in place. The knurled and chromed 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. A slanted groove cut into the outside of the focuser-side 2” barrel engages with the scope’s focuser drawtube thumbscrew to act as a safety stop. This keeps the diagonal from falling should the scope’s thumbscrew loosen slightly from cold or use.
The 1.25” eyepiece adapter uses a compression ring to hold the eyepieces in place. The scope-side barrel of the 1.25” adapter is grooved to keep it from accidentally slipping out of the 2” diagonal. Dust covers are provided for the 1.25” adapter and the diagonal’s 2” barrel.
This diagonal is a visible improvement over the standard diagonal provided with virtually every refractor capable of using 2” optics. It can also be used with a Schmidt-Cassegrain or Maksutov-Cassegrain with a suitable adapter, such as the #7785C. It is an absolutely premium 2” 99% reflectivity dielectric star diagonal at a fraction the price of competitive models with equivalent performance and build quality. The same performance, for as little as half the price. If there ever was a bargain in astronomy, this is it.
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