Questar 3.5" Duplex, Broadband coatings, Zerodur mirror, factory installed Powerguide III


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Among all the optically superb Questar telescopes, the best small telescopes in the world, there can be only one "best of the best of the best." This world-class broadband-coated/Zerodur mirror DC-powered Questar Duplex 3.5" Maksutov-Cassegrain is that scope . . .
Our Product #: QDBZPG
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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 in all climate extremes, as well as for unsurpassed star-gazing, the 3.5" Questar Duplex - with broadband optical coatings, a Zerodur mirror, and a DC-powered go anywhere in the world drive system - is ideal. A simple turn of a knurled knob under the scope separates the optical tube of a 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, a broadband-coated 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 broadband-coated Questar Duplex gives you unexcelled optics and portability for nature studies and high power/high contrast 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!

This broadband-coated Questar Duplex 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, for an additional improvement in light transmission of 10% per surface. The broadband coatings package gives you a full 22% overall gain in light transmission and contrast that's very useful for photography and low light terrestrial observing and for viewing faint deep space objects at night.

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). If prolonged exposure to salt air might be in your scope's future, consider the Questar Duplex with standard optical coatings (Questar model #QD3PG), rather than this broadband-coated version. You will lose some light transmission, but will gain a measure of optical coating longevity. Adding a few packets of desiccant (silica gel or similar, available at most camera stores) to the case of any spotting scope to absorb moisture when near large bodies of salt water would be a helpful preventative measure in any event.

The scope's thermally-stable Zerodur ceramic mirror eliminates the minor need to refocus the scope as it cools to ambient temperature in situations involving large temperature swings. With a conventional Pyrex mirror telescope, if the difference in temperature between indoors and outdoors is 30 degrees or more Fahrenheit when the scope 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, the Zerodur mirror in this scope eliminates even the need for minor refocusing when the optical tube is taken outside in cold temperatures for a quick terrestrial photography session, or the complete scope is taken out during a winter's night for some star-gazing, should this be a concern or an inconvenience.

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

Absolutely pinpoint resolution, total freedom from spurious color and distortion, total freedom from temperature-related focusing issues, with an image clarity and contrast in a class all its own, the multifunction ability to convert in seconds from an astronomical telescope to a terrestrial spotting scope and telephoto lens, AC cord-free battery-operated operation - this Questar Duplex with broadband coatings and a Zerodur mirror 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, this 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


Review by:
This scope has been around for years and there's a reason why. The optics are superb, the scope is stunning to look at and conveniently portable. While both expensive and perhaps not as large as some of the other product offerings in this price range, I've found through past ownership of larger scopes that I didn't have the quick portability and quality that this scope provides. This may be why so few show up on the used market. I know mine is a definite keeper. (Posted on 10/8/2019)
Review by:
This scope has been around for years and there's a reason why. The optics are superb, the scope is stunning to look at and conveniently portable. While both expensive and perhaps not as large as some of the other product offerings in this price range, I've found through past ownership of larger scopes that I didn't have the quick portability and quality that this scope provides. This may be why so few show up on the used market. I know mine is a definite keeper. (Posted on 10/8/2019)
Review by:
This scope has been around for years and there's a reason why. The optics are superb, the scope is stunning to look at and conveniently portable. While both expensive and perhaps not as large as some of the other product offerings in this price range, I've found through past ownership of larger scopes that I didn't have the quick portability and quality that this scope provides. This may be why so few show up on the used market. I know mine is a definite keeper. (Posted on 10/8/2019)
Review by:
Questar, we have all seen, or at least heard of this diminutive mak cassegrain telescope whos' name is whispered in reverence... While one may balk at the high price of admission and the seemingly minuscule 90mm aperture, once you hold one of these gems in your hands and marvel at the level of fit, finish and optical quality, can you truly appreciate what is Questar!

The duplex model with broadband coatings, zerodur primary and latest Powerguide III offers the user a grab and go package that will never fail to put a smile on your face. Light grasp and through put with the enhanced coatings will render beautiful color-free lunar , planetary and major Messier object views of jaw dropping quality. Warm rich tones, contrasty and ultra-high resolution await. , flat field and tack sharp. The included and perfectly matched Questar/Brandon eyepieces compliment the package seamlessly and with the built in 2x barlow power switch doubles your eyepiece selection at the twist of a knob. Granted, while only 90mm, this is the little engine that could and it belies it's size and exceeds the Dawes limits of performance. Add to the optical performance, the fact that you can be outside and viewing in a moments notice with this portable piece of optical art, what's not to love? I live in the tropics therefore cool down is a non-issue however, I have toted my Q's with me to snowy crags and peaks to enjoy high altitude stargazing on back country hikes, yes, small enough to tote in a back pack, and thermal equilibrium was reached in short order. Tracking and slewing with the powerguide is accurate, precise and silent, typical Questar. The duplex variant offers a removable OTA with a single thumbscrew thus separating tube from forks for use as a telephoto lens (adapters available additionally), or use as a spotting scope on a monopod for birding or other terrestrial pursuits.

I believe you too will find the Questar to be a worthy performer and more importantly, your most used grab and go telescope.

Finery on an epic scale, truly a jewel that will become a family heirloom that doesn't only get used on Thanksgiving and Easter... Clear skies (Posted on 11/9/2018)
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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)
  • VHR silver coated and overcoated Zerodur glass ceramic mirror
  • Broadband dielectric multicoated 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).