3.5" Duplex, Broadband coatings, with factory installed Powerguide II drive/drive corrector

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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 broadband optical coatings and cordless Powerguide II 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, 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.

The normally-optional Powerguide II DC drive system/drive corrector that is factory-installed into this Questar Duplex eliminates the need to stay near an AC power outlet to operate the scope’s motor for astronomical observing. The Powerguide II replaces the standard Questar 110 volt AC drive with a DC servo quartz drive to power 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 II will smoothly track 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 II 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 II, 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. 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 or terrestrial objects in the low light of early morning or twilight.

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 #QD3), 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.

A thermally stable Zerodur ceramic mirror is available as an option to the Questar’s standard Pyrex mirror for 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 Fahrenheit 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, 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, 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.

Highest Useful Magnification:
This is the highest visual power a telescope can achieve before the image becomes too dim for useful observing (generally at about 50x to 60x per inch of telescope aperture). However, this power is very often unreachable due to turbulence in our atmosphere that makes the image too blurry and unstable to see any detail.

On nights of less-than-perfect seeing, medium to low power planetary, binary star, and globular cluster observing (at 25x to 30x per inch of aperture or less) is usually more enjoyable than fruitlessly attempting to push a telescope's magnification to its theoretical limits. Very high powers are generally best reserved for planetary observations and binary star splitting.

Small aperture telescopes can usually use more power per inch of aperture on any given night than larger telescopes, as they look through a smaller column of air and see less of the turbulence in our atmosphere. While some observers use up to 100x per inch of refractor aperture on Mars and Jupiter, the actual number of minutes they spend observing at such powers is small in relation to the number of hours they spend waiting for the atmosphere to stabilize enough for them to use such very high powers.
Visual Limiting Magnitude:
This is the magnitude (or brightness) of the faintest star that can be seen with a telescope. The larger the number, the fainter the star that can be seen. An approximate formula for determining the visual limiting magnitude of a telescope is 7.5 + 5 log aperture (in cm).

This is the formula that we use with all of the telescopes we carry, so that our published specs will be consistent from aperture to aperture, from manufacturer to manufacturer. Some telescope makers may use other unspecified methods to determine the limiting magnitude, so their published figures may differ from ours.

Keep in mind that this formula does not take into account light loss within the scope, seeing conditions, the observer’s age (visual performance decreases as we get older), the telescope’s age (the reflectivity of telescope mirrors decreases as they get older), etc. The limiting magnitudes specified by manufacturers for their telescopes assume very dark skies, trained observers, and excellent atmospheric transparency – and are therefore rarely obtainable under average observing conditions. The photographic limiting magnitude is always greater than the visual (typically by two magnitudes).

Focal Length:
This is the length of the effective optical path of a telescopeor eyepiece (the distance from the main mirror or lens where the lightis gathered to the point where the prime focus image is formed). Focallength is typically expressed in millimeters.

The longer the focallength, the higher the magnification and the narrower the field of viewwith any given eyepiece. The shorter the focal length, the lower themagnification and the wider the field of view with the same eyepiece.

Focal Ratio:
This is the ‘speed’ of a telescope’s optics, found by dividing the focal length by the aperture. The smaller the f/number, the lower the magnification, the wider the field, and the brighter the image with any given eyepiece or camera.

Fast f/4 to f/5 focal ratios are generally best for lower power wide field observing and deep space photography. Slow f/11 to f/15 focal ratios are usually better suited to higher power lunar, planetary, and binary star observing and high power photography. Medium f/6 to f/10 focal ratios work well with either.

An f/5 system can photograph a nebula or other faint extended deep space object in one-fourth the time of an f/10 system, but the image will be only one-half as large. Point sources, such as stars, are recorded based on the aperture, however, rather than the focal ratio – so that the larger the aperture, the fainter the star you can see or photograph, no matter what the focal ratio.

This is the ability of a telescope to separate closely-spaced binary stars into two distinct objects, measured in seconds of arc. One arc second equals 1/3600th of a degree and is about the width of a 25-cent coin at a distance of three miles! In essence, resolution is a measure of how much detail a telescope can reveal. The resolution values on our website are derived using the Dawes’ limit formula.

Dawes’ limit only applies to point sources of light (stars). Smaller separations can be resolved in extended objects, such as the planets. For example, Cassini’s Division in the rings of Saturn (0.5 arc seconds across), was discovered using a 2.5” telescope – which has a Dawes’ limit of 1.8 arc seconds!

The ability of a telescope to resolve to Dawes’ limit is usually much more affected by seeing conditions, by the difference in brightness between the binary star components, and by the observer’s visual acuity, than it is by the optical quality of the telescope.

1.3 arc seconds
This is the diameter of the light-gathering main mirror or objective lens of a telescope. In general, the larger the aperture, the better the resolution and the fainter the objects you can see.
The weight of this product.
8 lbs.
Heaviest Single Component:
The weight of the heaviest component in this package.
8 lbs.
Telescope Type:
The optical design of a telescope.  Telescope type is classified by three primary optical designs (refractor, reflector, or catadioptric), by sub-designs of these types, or by the task they perform.
Based on Astronomy magazine’s telescope "report cards", scopes of this size and type generally perform as follows . . .
Terrestrial Observation:
Observing terrestrial objects (nature studies, birding, etc.) is usually possible only with refractor and catadioptric telescopes, and convenient only when the scope is on an altazimuth mount or photo tripod. Most reflectors cannot be used for terrestrial observing. Scopes with apertures under 5" to 6" are generally most useful for terrestrial observing due to atmospheric conditions (heat waves and mirage, dust, haze, etc.) that degrade the image quality in larger scopes. 
Lunar Observation:
Visual observation of the Moon is possible with any telescope. Larger aperture scopes will provide more detail than smaller scopes, thereby getting a higher score in this category, but may require an eyepiece filter to cut down the greater glare from the Moon's sunlit surface so small details can be seen more easily. Lunar observing is more rewarding when the Moon is waxing or waning as the changing sun angle casts constantly varying shadows to reveal craters and surface features by the hundreds.  
Planetary Observation:
Binary and Star Cluster Observation:
Galaxy and Nebula Observation:
Terrestrial Photography:
Photographing terrestrial objects (wildlife, scenery, etc.) is usually possible only with refractor and catadioptric telescopes, and convenient only when the scope is on an altazimuth mount or photo tripod. Most reflectors cannot be used for terrestrial photography. Scopes with focal ratios of f/10 and faster and apertures under 5" to 6" are generally the most useful for terrestrial photography due to atmospheric conditions (heat waves and mirage, dust, haze, etc.) that degrade the image quality in larger scopes.
Lunar Photography:
Photography of the Moon is possible with virtually any telescope, using a 35mm camera, DSLR, or CCD-based webcam (planetary imager). While an equatorial mount with a motor drive is not strictly essential, as the exposure times will be very short, such a mount would be helpful to improve image sharpness, particularly with webcam-type cameras that take a series of exposures over time and stack them together. Reflectors may require a Barlow lens to let the camera reach focus. 
Planetary Photography:
Star Cluster / Nebula / Galaxy Photography:
10 years
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Visual Accessories
Eyepieces (3)
8mm Brandon
by Questar
12mm Brandon
by Questar
32mm Brandon
by Questar
  • 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 pyrex 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 (five years on coatings and two years on drive and focuser).
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Questar - 3.5" Duplex, Broadband Coatings, with factory installed Powerguide II drive/drive corrector

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Questar - 3.5" Duplex, Broadband Coatings, with factory installed Powerguide II drive/drive corrector
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Clear skies,

The world-class broadband-coated and exceptionally flexible Questar Duplex 3.5” Maksutov-Cassegrain becomes a world traveler with its factory-installed battery-operated Powerguide II drive system . . .

. . . our 34th year