Understanding Telescopes and Mountings. Telescopes. Newtonian reflectors

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1 Understanding Telescopes and Mountings Telescopes While there have been many variations in optical design since Galileo turned his miniscule refractor skyward, the typical observing field of today reflects a distillation of optical configurations that work best for the amateur astronomer. While most are optimized for general observing, some are preferred for certain specific amateur observing requirements. An investigation into the history of the telescope will turn up many other interesting scope design innovations, but for our purposes a summary of the most popular designs may be of value to you. A word of warning when scope shopping: Don t buy based on a bargain price, and don t consider purchasing a telescope with less than 80 millimeters of clear aperture. Anything less will be quickly outgrown as you gain observing experience. Instead, try a variety of telescopes under field conditions. Ask owners for their opinion. Be prepared for equal amounts of praise and condemnation of various designs and manufacturers, since we all have our personal preferences. Separating the wheat from the chaff can be trying at times, but this wisely spent time will make you a more informed telescope buyer. Make your first purchase based on the best optics you can afford, not size, appearance or accessories. Remember too, biggest may not be best for you. Newtonian reflectors The most common telescope design in use, the Newtonian reflector is composed of a concave primary mirror at the rear of the instrument that reflects incoming light back up the light path to a flat diagonal mirror, then out the side to an eyepiece. Since there is no refraction of light in this all-mirror system, only two reflections, the Newtonian reflector introduces no spurious color (chromatic aberration) to the final image. As focal lengths are reduced below F/8, however, other optical degradations to the final image may be present. Some minor optical alignment is generally required each time the scope is moved to a new viewing site. The simplicity of the Newtonian design has made it a traditional favorite among amateur astronomers. Inch for inch, these scopes offer the biggest bang for your buck.

2 A tribute to the simple efficiency of the Newtonian design is that, in 2004, famed Newcastle, Ca. comet hunter, Don Machholz, is said to have discovered his 10 th comet, C/2004 Q2, using his stalwart thirty year-old 6 F/8 Newtonian reflector. Achromatic refractors The venerable achromatic refractor incorporates one or more lenses in the front of a rigid tube to focus an image into an eyepiece mounted at the opposite end. Since light passing through glass refracts, or bends, in different amounts depending on the wavelength of the light, refractor manufacturers select various types of glass for the lens components in order to bring as much of the light to one point of focus as possible. A blue haze of chromatic aberration in varying levels is generally apparent on brighter objects. Quality refractors with focal ratios of F-12 to F-15 have a reputation for producing finely detailed high-contrast images of the moon and planets, due in part to the absence of the central obstruction found in Newtonian reflectors and compound (Catadioptric) telescopes. One advantage of the refractor is that, unless dropped or otherwise abused, it is not necessary to adjust optical alignment. In fact, most refractors have no provision for alignment by the owner. Another advantage is that the time required to reach temperature equilibrium, or cool-down, which can be a major factor with both Newtonian and Catadioptric designs, is negligible with refractors. Apochromatic refractors The apochromatic refractor, or APO, is a fairly recent, but significant refinement of the achromatic design. The use of exotic glass and lens design combinations result in sharper, aberration free images with little or no color introduced into the image. As a result, many quality APO products with focal lengths between F/5 to F/8 are on the market today, albeit at premium prices. The major downside of owning an APO refractor is cost. Where a decent, completely equipped Newtonian reflector can be purchased for approximately $50.00 per inch of aperture, an APO refractor optical tube alone can cost upwards of $ $1, per inch.

3 Some so-called semi-apo scope designs boast better color correction than achromatic scopes but at prices considerably less than premium APO telescopes. Catadioptric Designs The catadioptric telescope design incorporates a primary reflecting mirror, a transparent correcting plate, and a secondary mirror. The correcting plate is figured to improve imaging qualities from the short focus primary mirror. Catadioptric scopes are generally represented by Schmidt-Cassegrain and Maksutov designs. The Schmidt-Cassegrain (SCT) design incorporates a concave, perforated primary mirror at the rear of the scope tube, and a thin glass corrector plate with an attached convex secondary mirror in the front. As with a refractor, viewing is from the rear of the instrument. Commercially offered scope apertures range from 6 to over 14. In general, focal lengths of these scopes are about F-10, or 10 times their aperture. SCT s are popular instruments for medium to higher power viewing, aided by sufficient aperture to maintain image brightness. The enclosed optical system is substantially protected from the ravages of dust, misalignment, and other image degrading conditions, so this scope design is easy to set up and use. Celestron and Meade Instruments are the major producers of commercially available SCTs. The Maksutov design incorporates the same perforated primary mirror arrangement but uses a relatively thick meniscus lens up front for image correction. The curvature of the meniscus is such that if the instrument were pointed straight up, the lens would hold water (Not recommended!). The secondary mirror is generally a silvered spot centrally located on the rear surface of the meniscus lens, and, therefore, not subject to misalignment. Maksutovs generally have focal lengths somewhat longer than the traditional Schmidt design, with F-12 to F-15 units commonly seen. Due to the smaller secondary mirror size, well made Maks have a reputation for somewhat better contrast and definition than the Schmidt design.

4 Major suppliers of Maksutov scopes include Meade, Orion, and the legendary 3.5 and 7 Questars. The Schmidt-Newtonian reflector is another compound optical design now being seen at star parties. These short focal length instruments combine the optical and mechanical simplicity of the Newtonian scope with improved image correction imparted by a frontmounted Schmidt corrector plate. Currently manufactured by Meade and a few Russian optical firms, these scopes provide low-to-medium power views of the sky with exceptional image correction not found in typical short focus Newtonians. Telescope Mounts A harsh fact of life for the amateur astronomer is that the best optical system in the world can be rendered ineffectual by a poorly designed and constructed mount. So-called department store telescopes are almost invariably cursed by a combination of low-grade eyepieces and shoddy mounts. This is definitely a case of Let the buyer beware. So what mounting is best? Read on and decide for yourself based on your specific needs. Whatever design you choose, select a well made and sturdy mount that will assure solid images. Four basic astro-mount designs are found on modern telescopes; the simple Alt-Azimuth, the English Fork, the German Equatorial, and a very popular and practical variation of the alt-azimuth, the Dobsonian mount. The basic Alt-azimuth Mount is commonly found on terrestrial scopes, for which it is a fine choice, and on many cheap astronomical scopes, for which it is not. With this design, the act of following celestial objects across the sky involves coordinating the movement of both the altitude (Alt) and the horizontal (Azimuth) axes at once. While this may be accomplished with some degree of success at low magnifications, the act becomes quite tedious as magnification increases. These mounts do not incorporate any accurate form of celestial drive mechanism so keeping objects in the field of view is a manual and constant chore.

5 Advocates of short focal length refractors sometimes use one of the higher quality alt-azimuth mounts on the market today for informal low power observing. I frequently use a TeleVue Gibraltar alt-azimuth mount during the summer months for low power scanning of the Milky Way and the star clouds of Sagittarius and Scutum. This mount is also ideal for scanning for comets. The English Fork Mount is essentially a beefed up, higher quality version of the basic alt-azimuth design. But when the Azimuth axis is tipped up to point toward the celestial pole, it becomes an equatorial platform able to track stars and planets in their celestial journeys by moving only one axis. Extremely effective versions of this design may be found on most popular Celestron and Meade telescopes. Available options include drive units on both axes and computer GoTo capability. Many scopes of this design, equipped with computerized drives, are capable of tracking objects in either the Alt-azimuth or equatorial modes, making them very popular in today s fast moving culture. Unless specially fitted with a field rotation device, the Alt-azimuth mode is not suited for long exposure photography. The German Equatorial Mount is an old design that, if made well, is capable of accommodating large, heavy instruments with excellent tracking accuracy. The design incorporates a shaft for each axis, one for the polar (R.A.) and one for the declination (Dec.) axis. In general, they require a counter weight to offset the mass of the telescope, and are bulkier than the fork mount. Practical examples of these mounts are manufactured by renowned firms such as Celestron, Meade, Orion, Losmandy, Vixen, and, for the ultimate in German equatorial mounts, Software Bisque s Paramount ME model. All the modern technical accessories, such as computer drives and GoTo capability, are available and sought-after options on these mounts. The popular Dobsonian Mount is actually a very sturdy and practical modern day improvement on the simple Alt-azimuth mount. Designed by famed monkturned amateur astronomer, John Dobson, this mount may be constructed in an evening by nearly anyone who has a Newtonian optical tube assembly needing a home. Look in any astronomy magazine and you will see scopes from 4 to 36 in aperture on Dobsonian mounts. They can even be equipped with computerized drives that allow for tracking. One caveat, however. This design is for visual use only. If you want to do long exposure astro-photography you will not be able to use this mount as-is due to field rotation as the object is tracked.