The 2” Astro-Tech star diagonal all the premium features serious astronomers look for in a star diagonal: a one-piece body for maximum reliability and durability; 99% reflectivity dielectric mirror coatings for the brightest possible images and highest contrast; a 1/10th wave accuracy quartz mirror for maximum thermal stability; and standard equipment 1.25” and 2” compression ring eyepiece holders. Priced as much as $200 less than other similarly-equipped premium diagonals, this Astro-Tech is not only a superb performer, but a superb value.
The one-piece body, including the 2” barrel and 2” eyepiece holder, is machined out of a single solid block of aluminum. There is no separate 2” barrel and 2” eyepiece holder threaded into a mirror housing where they could potentially loosen with use, allowing them to turn unexpectedly under the weight of today’s heavy 2” eyepieces and dump your eyepiece on the ground.
To assure the highest possible light transmission and maximum brightness, this Astro-Tech diagonal’s first-surface mirror uses state-of-the-art vacuum-deposited 46-layer dielectric mirror coatings. These have a reflectivity of 99%. This is substantially higher than the 88% reflectivity of a conventional aluminum diagonal that loses its reflectivity at rate of 1% to 1.5% per year or the 96-97% reflectivity of an easily-tarnished silver diagonal.
The durable ion-deposited dielectric coatings are applied to a 10mm thick quartz substrate that has better thermal expansion characteristics than a conventional glass mirror. There’s no need to worry about the focus changing as the diagonal cools down to ambient temperatures.
The quartz mirror of this Astro-Tech diagonal is guaranteed to have a surface accuracy of 1/10th wave peak to valley or better after the dielectric coatings are applied, not before being coated, as is common with other diagonals. This guaranteed 1/10th wave surface accuracy provides higher image contrast and resolution during critical planetary, lunar, and binary star observations. In addition, dielectric coatings typically have a smoother surface with fewer micro-pits at the 1/100th to 1/200th wave level than aluminum coatings, leading to less light scatter when observing bright objects such as planets and consequently higher contrast.
The 14.4 ounce body is anodized an antireflective matte black inside and out. The 2” focuser-side barrel is internally antireflection threaded to further improve contrast. The barrel is also threaded to accept standard 2” filters. The eyepiece stop in the 2” eyepiece holder that prevents 2” eyepieces from contacting the mirror is also antireflection threaded.
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. A gently slanted groove cut into the outside of the focuser-side 2” barrel engages with the scope’s focuser drawtube thumbscrew or compression ring to act as a safety stop. This keeps the diagonal from falling should the scope’s thumbscrew loosen slightly from cold or use.
The standard equipment 1.25” eyepiece adapter (typically an extra-cost option with competitive one-piece diagonals) also uses a compression ring to hold eyepieces in place. Dust covers are provided for both the 1.25” adapter and the diagonal’s 2” barrel.
With all these advanced features in one diagonal – one-piece body, quartz mirror, dielectric coatings, compression ring eyepiece holders – this Astro-Tech 2” star diagonal is, quite simply, unique among premium-quality 2” star diagonals for its guaranteed high performance and the value of its exceptional price/performance ratio.
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