CHOOSING A TELESCOPE
There was a time when the choice among telescopes was simple, even if the instruments themselves wereheavy and awkward to use. Today the conflicting claims of the manufacturers of so many different
telescopes make it difficult to discriminate among them. In this paper we discuss the chief types ofcommercially available telescopes and suggest why the Questar® is a unique solution to this opportunityof choice.

REFRACTING TELESCOPES
Refracting optical systems are either achromatic or apochromatic; the first brings two colors to acommon focal plane and the second brings three. The major weaknesses of refracting telescopes are theresult of their long destabilizing barrel length and their significant chromatic aberration. Both achromaticand apochromatic optical systems have a destructive effect on the ability to create images of high quality.
The classic solution is to increase the focal length in order to reduce secondary color. This is a poor
solution for the amateur astronomer; it increases the barrel length, which, in turn, demands a heavy andexpensive mounting. Therefore, it is difficult to transport and assemble. In the past few years advances
in materials technology with the development of low dispersion and fluorite glasses allow manufactures
to improve chromatic aberration to about one eighth (⅛th) the aberration of an achromat and, at the sametime, reduce the barrel length by about one half (½).
The materials used in a modern apochromat optical system are very fragile. Fluorite has acrystalline structure which is prone to fracture. Such material cannot be heated to the temperatures
required to deposit magnesium fluoride or dielectric coatings, and, instead, is "cold-coated" with a far lesspermanent result or even left uncoated as the interior surfaces often are. Fluorite is also hygroscopic(absorbs moisture) and the surface figure may gradually degrade. A microscope manufactured fromfluorite materials is always stored with a desiccant; this precaution is taken even with use under far lesshostile conditions than a personal telescope must withstand.

Three further points of interest; first, take a look through an apochromat and notice a glow aroundthe object you are observing. This is longitudinal color, which occurs on-axis and causes the effective
focal length of the telescope to change with the wavelength of light. Although these lenses focus three
wavelengths of light, the halo which you see is defocused light at wavelengths to which the eye may beless sensitive, but will still see; second, consider the negative effects of the long barrel of a refractor. Its
large overhang inevitably results in a less than stable optical system, susceptible to vibration transmitted
through the ground and by wind. Each time you touch the telescope, the recovery time is far greater thanthat of a short barrel instrument; third, as with any telescope the most unstable air you are required to lookthrough is in the telescope itself. A tube length of thirty-two inches (32") may cause four times theinternal turbulence of a Questar 3.5 with its eight-inch (8") barrel.
The argument used to counter the presence of these easily observed difficulties in a refractor isthe presumed optical effect of the central obscuration in a Catadioptric system. The milky appearance ofthe background sky sometime associated with Catadioptric telescopes is not the effect of the obscuration.
This milky appearance is the result of inadequate baffling and the dispersion of light within the glass ofthe correcting lens. If you compare the background sky through a Questar and through a good refractor
you will see that the background sky of both is black.
The central obscuration causes a light loss to the system of about 10%. This amounts to less than
two tenths (2/10ths ) of a stellar magnitude! Unless the sky transparency conditions are perfect, thedifference in overall image quality and contrast is impossible to see.
Even under hypothetically perfect seeing conditions, the loss of light energy needs to beconsidered relative to the secondary color in a refractor which will be present under all seeing conditions.
As an article in the March 1992 issue of SKY and TELESCOPE puts the matter, "The conventional
wisdom about refractors being better than reflectors has definitely been overstated. Perhaps this isbecause reflectors with marginal optics have been compared unfairly with well made refractors. "Frommy observations," says Douglas George, author, "the difference between an excellent refractor is verysubtle." One must consider this along with the fragility of the optics, the long barrel length, and thechromaticism inherent in the apochromatic design. At the same time, the major effect of the obscuration

is that, in fact, it increases the fine resolution of the optics, resulting (with a superbly manufactured
Catadioptric lens like the Questar) in the easy exceeding of Rayleigh's Criterion and the Dawes Limit for
the limiting resolution of a lens. (If the refractor you are comparing with a Catadioptric lens of the sameaperture is not significantly better than 1/4 wave total throughout, the loss of energy will be the same ormore, but without the increase in resolution).

CATADIOPTRIC TELESCOPESThe Schmidt-Cassegrain telescope is a distinguished design with superb performance
characteristics; however, it is a very difficult design to manufacture. The correcting lens of the Schmidt isdesigned with a complex fourth order curve that must be painstakingly and very accurately ground.
Deviations from the prescribed curve cause a falloff in the wavefront accuracy of the optic with very littleroom for error. One of the highly significant breakthroughs in Maksutov's design was his achievement of
the correction for spherical aberration using a spherical curve, which is far easier to grind. The thinnessof the meniscus of the Schmidt, on the other hand, compounds the difficulty since grinding on one side of
the corrector warps the already completed figure on the other. It is no surprise that one very rarely sees
Schmidt's fabricated by amateur telescope makers.
Why can numerous manufacturers offer Schmidt telescopes at low prices? Ignoring the issue of
mechanical quality which becomes apparent when you compare Questar with any other commerciallyavailable telescope, the chief economy introduced results from approximating the fourth order curve ofthe true Schmidt by drawing the corrector with a vacuum, which, in effect, reduces the complex curve to asphere. This major compromise makes telescopes of reasonable quality available to many people. Theeconomy is further suspect, however, since the secondary mirror, which is a part of the curve of thecorrector in a Maksutov, is a separate component in the Schmidt, and must be ground and figuredseparately. The collimation of the telescope also requires the additional step of collimating the secondary,an additional cost to the manufacturer. (Incidentally, the presence of a separate secondary causes Schmidt
to go out of collimation, and require frequent re-collimation by the owner. The Questar is permanently
collimated). Considering the fact that a professional quality Schmidt is significantly more difficult tomanufacture than a Maksutov, and that a commercial grade Schmidt can still be purchased for less withmore than twice the aperture, there has clearly been a relaxation of the optical and mechanical tolerances.
Manufacturers of Schmidt have made claims such as "defraction-limited one-tenth (1/10th) wave
optics." This claim should not be made for system performance but for component performance.
Questar components test individually and typically at one-twentieth (1/20th) or better wave, in order toproduce one-eighth (1/8th) wave front system we strive to achieve for each telescope. A Schmidt
telescope with tenth (10th) wave optical components can produce a system performance of close to onehalf (1/2) or better wave.
Commercial manufacturers of Schmidt have marketed the great advantage of a larger aperture,and have gone so far as to suggest that a larger aperture in itself will produce better resolution thansmaller. That assumption is open to serious question. When you observe you are, in effect, looking
through cells in the atmosphere. Their turbulent effects are largely invisible as long as the cone of lightentering the telescope is smaller than the physical dimensions of the cell; the 3.5 inch aperture of theQuestar was determined with this fact firmly in mind. With mediocre viewing conditions, the larger
aperture can become a liability to good seeing; this is one reason most owners of the Questar Seven® alsoown a Questar 3.5® for their regular use. When you compare the larger aperture instruments offered bycommercial Schmidt manufacturers, the argument about resolution and aperture size is thoroughly
discredited. We do not make comparisons with specific brands of telescopes, but we encourage you tocompare performance. We regularly receive comparisons of the Questar 3.5 with eight-inch (8") aperture
Schmidt. Questar telescopes routinely outresolve them for all the reasons, which we have mentioned.
One way in which the larger aperture does outperform the smaller is in its ability to grasp light.
Yet, if you compare a larger Schmidt with the Questar 3.5, the image although brighter in the larger
telescope exhibits less contrast. This is the result of a poorly baffled and less precise optical system withadditional light scatter from the corrector lens. Light outside the focussed light cone of the image hasentered the telescope and washes out the light-to-dark transitions. When you observe a planet, the detailsof the subtle striations of color are lost, and when you observe an object against the background of deep
space the subtle features are reduced to a fog. The last word in this argument about aperture comes when

we compare the image of the Questar Seven with that of an eight inch (8") Schmidt. In spite of theslightly smaller aperture of the Questar, the eight inch (8") Schmidt does not remotely compare to theSeven Inch Questar in contrast, brightness and resolution.

Over and above these discussions of difference in design, there are numerous mechanical and design
features of the Questar, which make it more than just a Maksutov. They are the reason why Questar
telescopes go into outerspace, why they are used in every major industry and government agency. A
Questar telescope is on permanent display in the Eastern Museum, as well as the Museum of Science andIndustry. We hope that the information in this paper puts you in a better position to make a criticalevaluation of what is available to you and that most of all you make the ultimate test and compare aQuestar carefully with any other telescope in the market.

Questar Corporation
6204 InghamRoad
New Hope, PA 18938 USA
Telephone: 215-862-5277
Fax:215-862-0512

Email: questar@erols.com
http://www.QuestarCorporation.com