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More Information  · Images of Some Features Price $569.95 Shipping $29.95 Highest Useful Magnification 300x Focal Length 1800mm Focal Ratio f/12 Resolution 0.77 arc seconds Visual Limiting Magnitude 13.4 Aperture 6" Weight 14.2 lbs. Heaviest Single Component 13.4 lbs. Warranty 2 years

The 6" Astro-Tech AT6M Rumak-type Maksutov-Cassegrain is a superb optical system for the serious amateur astronomer who wants apochromatic refractor-type performance on solar system objects and the brighter deep space objects – but who doesn’t have the budget for a big apo. Its multicoated optics compare quite favorably with the image quality of 5" and larger apochromatic refractors that are five and six times its very reasonable price.

Maksutov-Cassegrains are known for their image quality, and the Astro-Tech AT6M is no exception. Like all Maksutovs, the AT6M optics use all spherical surfaces, intrinsically easier to polish to a higher degree of surface accuracy than the often complex curves required by other catadioptric telescope designs. This higher surface accuracy results in sharply-defined images, as a higher percentage of each star’s light is concentrated in the Airy disk, with less being scattered into the diffraction rings. The relatively long focal ratio essentially eliminates any possible chromatic aberration from the meniscus corrector lens, without having to resort to expensive ED glass elements, as is the case with apochromatic refractors.

Maksutov-Cassegrains fall into two general types: the Rumak design, where the secondary mirror is a separate optical element affixed to the rear of the Maksutov corrector; and the Gregory design, where the secondary mirror is simply an aluminized spot on the rear of the Maksutov corrector lens itself. The Rumak design offers the greatest possibilities for optimizing the scope’s performance, as it gives the designer a fourth optical surface to work with (front surface of corrector, rear surface, primary mirror, and Rumak secondary mirror). The Gregory design (where the secondary mirror is simply an aluminized spot on the back of the corrector) can be very effective, but only if the secondary mirror is optimized by being ground and polished to a different radius of curvature than the rear surface of the corrector lens. Very high-end Questar telescopes are this type of Gregory Maksutov design, with a secondary mirror spot on the rear of the corrector that is ground and asphericized to a separate unique curvature. Questar prices (starting at $3000 for a 90mm optical tube) reflect the cost of this approach. Most inexpensive Maksutovs use the Gregory design and do not grind the secondary mirror spot to a separate curvature. They frequently suffer from astigmatism and coma as a result.

The Astro-Tech AT6M is a Rumak-design Maksutov, using a BK-7 optical glass corrector lens and BK-7 primary mirror, with a separate BK-7 secondary mirror mounted on the back of the Maksutov corrector lens. It affords all of the image-optimizing benefits of the Rumak design, but at the low price of an inexpensive Gregory-type optical system. The optics are multicoated and fully-baffled for high light transmission and high contrast.

The 150mm diameter AT6M Maksutov-Cassegrain optics gather better than 40% more light than a 125mm (5") Mak, with 20% higher resolution, and more than two and three-quarter times as much light as a 90mm (3.5") Mak, with two-thirds higher resolution than the 90mm. Whether inside the solar system or out, this 6" Astro-Tech Rumak-type Maksutov clearly outperforms a 3.5" or 5" Mak and many 5" apo refractors. It resolves finer detail in lunar and planetary subjects than the smaller scopes, split more closely-spaced binary star pairs, and permits exploration of a wider array of faint deep-sky objects.

The optical tube of the Astro-Tech AT6M measures only 19.5" in length, 21.5" with its 2" accessory holder installed. The tube weighs a very reasonable 14.2 pounds with its 2" accessory holder and 8 x 50mm finderscope installed. That makes it quite light for a 6" scope, allowing safe and convenient use on a wide variety of mounts using the scope’s standard equipment Vixen-style dovetail rail.

Features of the Astro-Tech AT6M Maksutov-Cassegrain optical tube . . .

• Maksutov-Cassegrain optics: Hand-figured 150mm (6") aperture, 1800mm focal length, f/12 focal ratio, Rumak-type optics. BK-7 optical glass corrector lens and mirrors.

• Multicoated and fully-baffled optics: The primary mirror and separate secondary mirror on the rear of the meniscus corrector lens are coated with highly reflective enhanced aluminum, overcoated with quartz for long life. There are antireflection multicoatings on both sides of the Maksutov corrector lens. The secondary mirror is provided with a light baffle to shield it from ambient light for higher contrast. The primary mirror rides on a central baffle tube that very effectively prevents ambient light from reaching the image plane.

• Cooldown time: Like all closed Maksutov-Cassegrain systems, the thick meniscus lens and primary mirror of the AT6M will take some time to cool down to ambient temperature when taken outdoors on a cold winter evening. The cooldown time can be shortened by removing the three plastic plugs in the rear cell of the scope to expose the rear of the mirror cell and placing the scope, meniscus end down, in an unheated garage or protected outdoor space for an hour or so before observing. This will allow the warm air inside the tube to rise and escape out the three one inch apertures in the rear cell. These apertures also expose the factory collimation screws for the primary mirror cell, which should not be adjusted in the field. Attempting to adjust the collimation screws, thereby breaking the clear locking seal on each screw, will void the scope warranty.

• Dust covers: A slip-on plastic dust cover protects the meniscus lens. A plastic plug in the 1.25" accessory holder slipped into the 2" holder on the rear cell seals the Cassegrain focus.

• Focuser: A large ribbed focusing knob on the rear cell moves the primary mirror back and forth along the central baffle tube to focus the AT6M. Focusing is slow, but very precise, taking 28 turns to move from lock to lock. Once focused on a celestial object however, changing the focus to another sky object will not take much time or effort, however. Only a turn or two of the focusing knob will suffice to move the sharp focus from the terminator of the Moon to a star cluster thousands of light years away, for example.

• Finderscope: The supplied 8 x 50mm straight-through finderscope comes with a quick-release bracket. The dual-ring bracket has six collimating screws for precise alignment with the main scope optics. The quick-release bracket slips into a dovetail mounting shoe at the rear of the scope during use.

• Four accessory holders: Two adapters are provided that thread onto the 2.3" rear cell thread of your AT6M. One is a 2" compression ring accessory holder that accepts 2" star diagonals, camera adapters, etc. The second is a ring that emulates the standard 2" rear cell thread used on Schmidt-Cassegrain telescopes. This lets you use any SCT thread-on accessories you might already have (star diagonals, camera adapters, etc.) In addition, two additional slip-fit accessory adapters are provided. One is a 1.25" compression ring adapter that slips into the 2" holder mentioned above. This lets you use optional 1.25" accessories (star diagonal, camera adapter, terrestrial image erecting diagonal, DSI-type camera, etc.) The second is a standard T-adapter that also slips into the 2" holder and lets you use a DSLR for prime focus imaging (full disk lunar or terrestrial, for example).

• Vixen-style dovetail: The AT6M has a Vixen-style dovetail rail for mounting the scope quickly and directly on an altazimuth or equatorial mount such as an Astro-Tech Voyager #ATV altazimuth, a Celestron CG-5 or Meade LXD-75 go-to equatorial, or a Vixen Great Polaris or Sphinx equatorial. The dovetail has a deep groove on one side to engage the mount dovetail’s locking screw. There are two safety screws installed at the ends of the groove so that the scope cannot slip out of the mount, even if the mount locking screw loosens during use.