Questar 3.5" Duplex, magnesium fluoride coatings, factory installed Powerguide III

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With the factory-installed addition of the normally-optional battery-operated Powerguide III drive system, the multi-purpose Questar Duplex 3.5" Maksutov-Cassegrain becomes a world traveler . . .
Our Product #: QDPG
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Product Description

For the serious amateur astronomer who would like to take Questar quality into the field for nature studies and photography, as well as for unsurpassed star-gazing and astronomical photography far from any AC power supply, the 3.5" Questar Duplex with its cordless Powerguide III DC-powered drive is ideal. You can use this superlative scope anywhere in the world without worrying about finding the right kind of AC power to operate its motor drive. In addition, a simple turn of a knurled knob under the scope separates the optical tube of this specially modified 3.5" Questar from its fork mount. It converts the astronomical telescope into a spotting scope and telephoto lens for terrestrial use, ready to be mounted in seconds on any sturdy camera tripod.

Because of its high magnification and its ability to focus as close as 8', something virtually no other spotting scope can do, this Duplex makes a superb long distance microscope for close-ups of subjects too small or too dangerous to examine face to face. At its standard 54x magnification, a subject 10 feet away appears the same size as it would if you could get your eye within a mere 2.2" of it, with a field only 1.8" wide filling your eyepiece! Imagine the close-up views you will get of hornets' nests, spiders lurking in dew-soaked webs, gape-mouthed baby birds being fed in nearby nests, and more!

The Questar Duplex gives you unexcelled optics and portability for nature studies and high power photography by day, plus an ultra-compact telescope that opens a universe of viewing pleasure by night. In terms of operational flexibility and visual quality, the Questar Duplex is the optical equivalent of a Swiss Army Knife made by Rolls-Royce.

At a distance of one mile, optical theory prescribes a resolution of 1/3rd of an inch for the 3.5" aperture Questar - but terrestrially this 3.5" Questar, under good seeing conditions, can routinely resolve much thinner bicycle spokes at one mile, and leaf stems at two miles! As a review in Audubon magazine noted, "Brighter than reality, the Questar has unbelievably sharp resolution and an extremely flat field. It shows every feather from a half-mile away!" (The exclamation point is theirs.) And optical theory says that a Questar should not be able to resolve lunar craterlets and rilles under 1.3 arc seconds across - yet there are countless Questar owners and Questar photos that prove optical theory wrong.

What is it about a Questar that lets it exceed the theoretical limits 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 a guaranteed total system performance of 1/8th wave or better at the eyepiece. 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's indistinguishable from perfect optics.

Most commercial telescopes claim to be "diffraction-limited" (which is generally assumed to mean 1/4th wave accuracy). However, those other manufacturers do not specify whether their performance is for individual components or for their 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 3.5" Questar can often outresolve an 8" Schmidt-Cassegrain 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 that all telescopes must look through, day and night. 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 image-blurring effect of these microcells is largely invisible as long as the bundle of light entering the telescope is smaller than the 4" diameter of the cells.

It was for just this reason that an aperture of 3.5" was chosen for the Questar. In average to mediocre seeing conditions, a 3.5" Questar will see through individual 4" microcells undisturbed, showing more detail than a larger scope that has to put up with the blurring of multiple turbulent cells.

Finally, there is the matter of contrast. The small secondary mirror of a Questar Maksutov scatters less light than a Schmidt-Cassegrain or reflector's larger secondary. The secondary of an 8" Schmidt can be a full 3.45" in diameter, for example - a light-scattering obstruction almost as large as the entire 3.5" Questar aperture! In addition, a Questar's central baffle tube is not merely black plastic or metal painted black to reduce reflections, as with lesser scopes. The Questar central baffle tube actually contains 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 terrestrial, planetary, and lunar details that a larger catadioptric scope or reflector can wash out in a haze of scattered light.

A large aperture scope does have greater light-gathering than a 3.5" Questar to capture additional faint deep space objects from a dark sky site. However, the higher contrast of a 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 Schmidt'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.

But Questar's quality does not stop with optical performance. What are costly options with other scopes - a glass solar filter, a premium Barlow lens, two premium eyepieces - are all standard with a 3.5" Questar. Also standard are amenities that are simply unavailable on other scopes - a glass solar filter for the finder, an accurate star chart on the self-storing dewcap that slides forward to reveal a useful map of the Moon on the optical tube itself, and a velvet-lined carrying case.

The 3.5" Questar Duplex is a complete telescope in an eight pound package. Remove it from its luggage-quality Naugahyde case (or optional leather case), attach its tabletop tripod legs, place it on a table, and you have all the user-friendly controls of a great observatory scope at your fingertips.

Unscrew the dust cap, and you can begin to appreciate the attention to detail lavished on a Questar - for the dust cap is not flimsy press-fit plastic, but solid machined aluminum that threads into the barrel to afford absolute protection to the optics.

Look into the Questar's premium 24mm Brandon eyepiece and you're looking into a 4x finder with an exceptionally wide 12° field. A finger touch on a convenient lever at the rear of the scope changes the finder into a 53x telescope for observing the Moon, nebulas, and star clusters. Touch a second lever and a built-in Dakin Barlow instantly increases that eyepiece power to 80x for closer observing. Exchange the 24mm eyepiece for the supplied 16mm Brandon eyepiece and you extend the power range still further, to 80x and 120x. And optional higher and lower power eyepieces are available, for magnifications as low as 40x and as high as 320x. For observing comfort, a rare thing with many scopes, the eyepiece tilts from side to side to the most convenient observing position.

Observe through a Questar, and you'll appreciate the attention to detail even more. The gearless 25:1 ratio slow motion controls operate with a smoothness and freedom from backlash unmatched by any other amateur telescope. The drive gear diameter is fully half the length of the telescope itself, for tracking precision that must be experienced to be believed. No tiny levers need be thrown to disengage the drive for manual operation, as a butter-smooth internal clutch made from micro-rolled discs of stainless steel lets you move the telescope at will. The large setting circles are not merely painted on, but are engraved and then paint-filled, to remain visible even after years of use. 'Jewel-like precision' may be an overworked term, but it's the only one that does justice to a Questar.

A Questar Powerguide III DC drive system/drive corrector is factory-installed into this Questar Duplex. It replaces the AC motor and 110V power requirement of the standard Duplex. The Powerguide III has a DC servo quartz drive that powers the scope's drive motor for up to 50 hours from a single 9 volt transistor radio battery. This frees you forever from the need to stay near an AC outlet to observe the skies. The Powerguide III smoothly tracks the Moon, planets, star clusters, galaxies, and a host of other deep space objects across the heavens for you, keeping them centered in the eyepiece all night long. During the day, the Questar will also track the Sun, allowing you to observe sunspot patterns. You'll do it in complete safety, as standard equipment glass solar filters provide complete protection against the Sun's fierce radiation for both your eye and your telescope.

Pushbuttons on the quartz-controlled Powerguide III hand control allow single axis guided astrophotography with 1.4x and 10x sidereal guiding rates in right ascension (dual axis with an optional declination motor). Other buttons control a built-in map light and the brightness of an optional illuminated reticle guiding eyepiece, and select either a lunar or sidereal drive rate. Another button selects northern or southern hemisphere operation, allowing you to use the Questar Duplex anywhere in the world without having to worry about finding the proper power frequency or voltage, or having the right kind of AC adapter. With the factory-installed Powerguide III, the Questar Duplex is truly a use-anywhere/use-anytime telescope!

High transmission magnesium fluoride coatings for high light transmission on both sides of the BK7 corrector lens are standard on a Questar Duplex. Having higher transmission broadband dielectric multicoatings added to the corrector lens during manufacture (which also includes a change of mirror coatings from aluminum to silver) is an option worth considering. This option gives you a total of 22% greater light transmission for deep space viewing and low light terrestrial observing, and higher lunar, planetary, and photographic contrast. (Broadband coatings are not recommended if you live by the seaside, as the silver mirror coatings can be adversely affected by constant exposure to salt air.)

A thermally-stable Zerodur ceramic mirror is also available as a factory-installed option to replace this Questar Duplex's Pyrex mirror to reduce the need for minor refocusing during large temperature swings. As with any Pyrex mirror telescope, if the difference in temperature between indoors and outdoors is 30 degrees Fahrenheit or more when a Questar is taken outside, minor refocusing will be required as its mirror contracts while cooling down to the outdoor air temperature. Although the 3.5" mirror of the Questar cools down much more rapidly than a larger 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 the need for refocusing, should this be a concern.

This Questar is protected by a ten-year Questar warranty (two-year warranty on the focuser mechanism).

Absolutely pinpoint resolution, total freedom from spurious color and distortion, with an image clarity and contrast in a class all its own, plus the multifunction ability to convert in seconds from an astronomical telescope to a terrestrial spotting scope and telephoto lens - this DC-powered Questar Duplex is not only the "Rolls-Royce" of telescopes, it is the "Swiss Army Knife" of telescopes. If you want the best and most flexible small telescope in the world, the Questar Duplex is it. Period.

Tech Details

Aperture 3.5"
Binary and Star Cluster Observation Good
Focal Length 1300mm
Focal Ratio f/14.6
Galaxy and Nebula Observation Fair
Heaviest Single Component 8 lbs.
Highest Useful Magnification 162x
Lunar Observation Great
Weight 8 lbs.
Planetary Observation Good
Resolution 1.3 arc seconds
Telescope Type Maksutov-Cassegrain
Visual Limiting Magnitude 12.2
Warranty 10 years


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Supplied Accessories

  • 1.25" 16mm and 24mm eyepieces (54x to 120x, depending on eyepiece and whether built-in 1.5x Barlow is used)
  • Built-in 4x finder (6x when 16mm eyepiece is used)
  • Aluminized and overcoated pyrex mirror
  • Magnesium fluoride coated optics
  • Built-in star diagonal
  • Dual arm fork mount
  • Quick disconnect optical tube with built-in photo tripod adapter for terrestrial use
  • Setting circles (r. a. circle power driven)
  • Sidereal rate AC motor drive (2.7 watts)
  • Manual slow motion controls
  • Thread-in dust cap
  • 1.5" aperture off-axis solar filter
  • Solar filter for finder
  • Tabletop tripod legs
  • Locking hard case
  • Self-storing dewcap with embossed star chart
  • Embossed Moon chart on scope barrel
  • Ten-year warranty (two years on drive and focuser).