< Return to product view

Optical features of this Astrograph . . .

• Ritchey-Chrétien optical design: This Astro-Tech astrograph is a true Ritchey-Chrétien (RC) reflector optical system. Unlike a Maksutov-Cassegrain or Schmidt-Cassegrain catadioptric scope (that uses simple spherical mirrors and corrector lenses), or Newtonian reflectors (that use a coma-producing parabolic primary mirror), this Astro-Tech RC is a Cassegrain-type two-mirror optical system that uses a concave hyperbolic primary and a convex hyperbolic secondary mirror to form its images. These sophisticated and difficult-to-make mirrors combine to produce images at the Cassegrain focus at the rear of this Astro-Tech scope that are free from coma and spherical aberration, with a smaller spot size, over a much wider field than conventional Newtonians or catadioptrics. The images are likewise free from the chromatic aberration found in refractors and some catadioptrics.
      Because of this wide coma-free field, small spot size, and relatively fast focal ratio, the Ritchey-Chrétien design is particularly well suited to wide field astrophotography, rather than visual observing. For imaging, the RC is the optical system of choice for most of the major professional observatory imaging telescopes built in the last half-century. For example, the Hubble Space Telescope, the twin 10-meter Keck telescopes in Hawaii, and the four 8.2 meter telescopes of the Very Large Telescope array in Chile are all Ritchey-Chrétiens. For serious amateur astronomers and astrophotographers without NASA’s optical budget, an Astro-Tech RC is likewise the imaging system of choice.

• Fully multicoated quartz and BK7 mirrors: The primary mirror of the 6” Astro-Tech is first-quality BK7 optical glass, while the 8” and larger Astro-Tech RCs use primary mirrors of low thermal expansion quartz for maximum focus stability during long exposure imaging sessions. Both 6” R-C mirrors are vacuum-coated with enhanced aluminum for high reflectivity and overcoated with a durable layer of silicon monoxide (quartz) for long life. The 8” and larger mirrors are dielectric coated for long life and reflectivity approaching 99%+.

• Computer designed and fabricated optics: To keep the cost of each Astro-Tech RC so reasonable when compared to competitive RC scopes, the computer-optimized Astro-Tech hyperboloid mirrors are automatically ground and finished to very high tolerances using custom-made computerized mirror grinding machines. This precision computer control guarantees an exact repeatability of figure from mirror to mirror that is difficult to achieve using more costly conventional hand figuring. After grinding and polishing, each mirror is individually tested using a Zygo interferometer to assure that it meets or exceeds its specified surface accuracy.

• Frill-free design: To further keep its cost reasonable, an Astro-Tech RC does away with most of the bells and whistles found on competitive scopes that add little to their performance (but much to their cost). For example, Astro-Tech front and rear cells are first die-cast, then CNC machine-finished, rather than completely CNC machined from raw stock at considerably greater expense but no significant improvement in performance as is the case with other RCs. Glare stops in the optical tube are a molded insert, rather than machined aluminum, resulting in a significant savings in cost at no appreciable difference in performance. The Astro-Tech scopes use an external manual dual-speed Crayford focuser, rather than the considerably more complicated and much more costly motorized movable secondary mirror system that other manufacturers use for focusing. The result of the Astro-Tech no-frills approach is genuine Ritchey-Chrétien wide-field performance at a fraction the cost of other commercial RC systems. While the mechanical bells and whistles may be limited in an Astro-Tech RC, an Astro-Tech scope still has the high precision flat field/coma-free true Ritchey-Chrétien optics that are the most important reason for buying an RC scope.

• Fixed primary mirror with computer optimized primary and secondary baffling: Unlike traditional Cassegrain designs that move the primary mirror fore and aft along the central baffle tube in order to achieve focus (which can lead to image shift as the mirror position is adjusted) each Astro-Tech RC primary mirror is fixed. The Astro-Tech is focused externally by means of a dual-speed 2” Crayford-style focuser on the rear cell, thereby eliminating a Cassegrain’s moving mirror image shift during focusing. Molded field stops are installed along the interior of the optical tube to effectively prevent stray off-axis light from reaching the image plane, resulting in improved contrast. In addition multiple glare-stop microbaffles on the inner surfaces of the primary mirror baffle tube and the secondary mirror light shield further prevent off-axis light from reaching the image plane, resulting in still further improved contrast.

• Collimatable secondary mirror: Since the primary mirror of an Astro-Tech RC is fixed in position, only the secondary mirror can (or needs to) be collimated. This makes it easy to keep the Astro-Tech RC optics aligned for peak performance. Collimation adjustments to the secondary mirror are made by adjusting the three collimating screws in the back of the secondary mirror holder.

• Cooling fan: The open tube RC design allows for fast cool-down of the primary and secondary mirrors. Built-in fans on the rear cell of the 10” and larger scopes increases the air-flow around the optics to achieve still quicker “cool down” times of the larger primary mirrors. The 6” and 8” scopes do not have primary mirror cooling fans, as their mirrors are small enough to cool down quickly without any external aid.

< Return to product view