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

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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.

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.
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.

0.65 arc seconds
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).

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.
19 lbs.
Heaviest Single Component:
The weight of the heaviest component in this package.
17 lbs.
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:
Very Good
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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Questar - 7" F/13.4 Astro Maksutov-Cassegrain, broadband coatings, Zerodur mirror

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Questar - 7" F/13.4 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 38th year