Questar 7" Astro Maksutov-Cassegrain, broadband coatings, Zerodur mirror

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If there are telescopes in heaven, chances are good they use Questar Seven Astro optical tubes with broadband optical coatings and a thermally-stable Zerodur ceramic mirror . . .
Our Product #: Q7ABZ
Questar Product #: 20311

Product Description

Observe through a Questar Seven Astro with broadband optical coatings and a thermally-stable Zerodur mirror, and you'll see detail and clarity in the skies that you have never seen through a lesser scope, or even through many larger scopes. I have compared my own Questar Seven to a 14" Schmidt-Cassegrain on deep space objects such as the Sombrero galaxy, the Saturn Nebula, and Omega Centauri. While the 14" scope was brighter on the fainter objects, the superior contrast and resolution of the Questar Seven was easily visible . . . even to the owner of the 14" scope. The Crêpe Ring and Encke's Division in the rings of Saturn are routine with a Seven in good seeing, as are myriad whorls and festoons in Jupiter's belts and hundreds of sub-kilometer craters on the lunar surface.

What is it about a Questar that lets it outperform larger scopes on a day in/day out basis?

Simply this: a fanatical devotion to hand-crafted accuracy.

Each optical element in a Questar typically tests out at a truly outstanding 1/50th wave accuracy (shaped to within four ten-millionths of an inch of perfection!) This produces guaranteed total system performance at the Cassegrain focus of 1/8th wave or better. That's twice the accuracy needed to meet Lord Rayleigh's Criterion, which specifies the level of optical excellence required to yield visual performance that is indistinguishable from perfect optics.

Most commercial telescopes claim to be "diffraction-limited" (which is generally assumed to mean 1/4th wave accuracy). However, they do not specify whether that is for individual components or the system as a whole. In either case, it's a far cry from the 1/8th wave total system accuracy at the eyepiece of a Questar.

This significant difference in total system accuracy is one reason why a 7" Questar can routinely outresolve a 14" Schmidt-Cassegrain (or virtually any other larger aperture catadioptric or reflector scope) on globular clusters, binary stars, and lunar and planetary details - with the Questar invariably exceeding Dawes' limit for the best resolution available from an optical system of its aperture.

A second reason is the turbulent Earth atmosphere all telescopes must look through. In essence, when observing, you are usually looking through bubbles of disturbed air - microcells typically 4" in diameter in the layer of the atmosphere nearest the surface of the Earth. The larger the aperture of the scope, the greater the image-blurring effect of these microcells, as the larger scopes simply have to look through more of these turbulent cells than a smaller scope.

Finally, there is the matter of contrast. The small secondary mirror of a Questar Seven Maksutov (only 1.87" in diameter) scatters far less light than a 14" Schmidt-Cassegrain's much larger 4.5" secondary. The same holds true with any other larger reflector or catadioptric telescope you care to name. They all have larger light-scattering secondary or diagonal mirror obstructions than a 7" Questar Astro. In addition, a Questar's central baffle tube is not merely black plastic or painted black to reduce reflections, as with lesser scopes, but is a centerless ground stainless steel tube, anodized matte black and containing a wire helix with 19 internal knife-edge baffles to eliminate low-angle reflections that no paint alone can stop. The result is that a Questar scatters less light from the bright areas of an image into the dark - crisply defining high contrast planetary and lunar details that a larger scope can wash out in a haze of scattered light.

A large aperture scope does have greater light-gathering than a 7" Questar Astro to capture additional faint deep space objects from a dark sky site. However, the higher contrast of the Questar lets the multitude of galaxies and nebulas within its grasp stand out more distinctly against a darker sky background, particularly from light-polluted suburban or city sites where a larger scope's greater light-gathering capacity submerges subtle low-contrast deep space details in a fog of city light.

As a Rolls-Royce is to automobiles, so is a Questar to telescopes - the very finest hand-crafted optical performance that money can buy.

The Questar Astro is precision-fabricated of lightened aluminum and titanium components, black and blue anodized for long life. It has a 1" diameter knurled anodized aluminum focusing knob on the rear cell. The knob focuses the scope by turning a spring-loaded 32-pitch stainless steel focus rod that acts directly on the mirror thimble to move the primary mirror within the optical tube. A precision linear rotor bearing, integrated with the mirror mounting, is matched to central tube thimble to minimize mirror shift when focusing.

The Seven Astro uses a 7" diameter meniscus corrector lens of Grade A BK7 optical glass, with magnesium fluoride anti-reflection coatings for high light transmission and minimal reflected light loss. The 7.6" diameter primary mirror is Pyrex, aluminized, with a silicon monoxide (quartz) overcoating for long life.

The Questar Astro comes with a 2" mirror star diagonal (with a 1.25" eyepiece adapter for Questar Brandon eyepieces) that threads onto the rear cell. A swivel coupling allows the eyepiece/diagonal combination to be tilted from side to side to the most comfortable observing position. A 1.25" Questar Brandon eyepiece is supplied with the scope to get you started observing. It provides a magnification of 100x and a 0.53° field of view that's a little larger in diameter than the full Moon. A dust cap and thread-on lens shade are standard, as is a hard carrying case. The Questar Seven Astro optical tube uses the well-known TeleVue Starbeam red dot unity-power illuminated finder that makes pointing a telescope as easy as pointing your finger.

The Questar Astro weighs 19 pounds (with diagonal and eyepiece in place, 21 pounds if you add the thread-on dew shield). It measures 17.6" long (21.9" with the star diagonal in place). The low profile mounting adapter block on the underside of the Astro has both 1/4"-20 and 3/8"-16 mounting holes to allow to be installed on the Questar Equatorial Fork Mount (our part #20004) and Questar Large Astro Pier (our part #29338) for astronomical use. It is well-balanced and relatively light, allowing it to be mounted on any suitably sturdy tripod for terrestrial operation. It can also be adapted to your own equatorial mount with minimal effort.

This broadband coated Questar Seven Astro includes ultra-high transmission/low reflectivity broadband dielectric multicoatings on both sides of its objective lens for a light loss of less than 1/10th of 1% per surface for the brightest possible images. This compares with a light loss of 1% per surface with standard magnesium fluoride antireflection coatings. This multicoatings package also includes high reflectivity silver mirror coatings with a protective overcoating of thorium fluoride instead of standard aluminum coatings with a silicon monoxide overcoat. The broadband coatings package gives you a full 22% overall gain in light transmission and contrast that's very useful for photography and when viewing faint deep space objects.

This broadband coatings package is not recommended if you live full time on ocean-front property, or spend much of the year at the seaside. Constant exposure to salt air can adversely affect the silver mirror coatings. Occasional visits to the shore are not a problem, only extended stays (particularly if the scope is not packed away in its case when not in use). Adding a few packets of desiccant (silica gel or similar, available at most camera stores) to the case to absorb moisture when near large bodies of salt water would be a helpful preventative measure in any event.

This Questar Astro's thermally stable Zerodur ceramic mirror eliminates the need to refocus the scope as it cools to ambient temperature in situations involving large temperature swings. As with any Pyrex mirror telescope, if the difference in temperature between indoors and outdoors is 30 degrees or more when a Questar is taken outside, some refocusing will be required as its mirror contracts while cooling down to the outdoor air temperature. This is particularly true with a relatively large aperture scope like this Seven Astro, which has an oversize 7.6" primary mirror. Some people find the need for even an occasional refocusing to be annoying. Since a Zerodur mirror exhibits virtually no expansion or contraction as temperatures change, this option eliminates this need for refocusing. This option is well worth considering due to the large thermal mass of the 7" mirror and its consequently longer cool-down time than a smaller scope. I have both broadband coatings and a Zerodur mirror on my Questar Seven and can recommend them without reservation.

The Questar Seven Astro comes in a high impact ABS plastic sealed carrying case. A basic 35mm camera coupling set is also included (needs a Questar T-ring to connect to your camera).

This Questar is protected by a ten-year Questar warranty (two-year warranty on the focuser mechanism and five years on the broadband coatings).

Absolutely pinpoint resolution, total freedom from spurious color and distortion, with an image clarity and contrast in a class all its own - a Questar is truly the Rolls-Royce of telescopes. For those individuals who appreciate the very finest that life has to offer, a Questar Seven Astro with broadband optical coatings and a Zerodur mirror will prove to be an absolutely eye-opening revelation. It may be as close to astronomical heaven as you can get on this good Earth.

Tech Details

Aperture 7"
Binary and Star Cluster Observation Very Good
Focal Length 2400mm
Focal Ratio f/13.4
Galaxy and Nebula Observation Good
Heaviest Single Component 17 lbs.
Highest Useful Magnification 362x
Lunar Observation Great
Weight 19 lbs.
Planetary Observation Good
Resolution 0.65 arc seconds
Visual Limiting Magnitude 13.8
Warranty 10 years


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