RASC Choosing and Using a Telescope for Astronomy

of binoculars accompanied by a good rainbow, obscuring fine detail. The objective astronomy guide like the Royal Astronomical lens in achromat refractors is commonly made Society of Canada’s Observer’s Handbook or of two or more lenses of different glass types to Beginner’s Observing Guide is all you need to begin help correct this problem, Choosing understanding the night sky. known as chromatic aberration. This most and Using a TELESCOPE READINESS TEST commonly manifests itself as faint fringes of colour “Space is mostly empty and you can find a around objects like the whole lot of nothing with a telescope” --- Moon or Jupiter, but it is Richard Weis, former President of rarely fully cured except in RASC-Calgary Centre. Telescope very expensive If you can do the following, then you are ready apochromat refractors. for Astronomy to make full and rewarding use of a telescope: Advantages: Refractors, by default, have a Find and name four circumpolar totally clear aperture. No central obstruction constellations causes light to be scattered from brighter to For many people, the words ‘astronomy’ and Find and name three constellations seen in darker areas. Thus, contrast is good in ‘telescope’ are inseparable. Many newcomers to the southern sky in each of the four seasons refractors. Refractors are often chosen as the the hobby of astronomy mistakenly believe that Find and name - (a) 4 double stars (b) 4 premier instruments for planetary and double- it is important to get a telescope as quickly as variable stars (c) 5 star clusters (d) 5 nebulae star observing. Refractors are also low in possible. Because of the large variety in or galaxies maintenance. telescope designs, size, and quality, choosing a Have you seen all of the above objects with Disadvantages: refractors are very massive and first telescope can be a daunting task that has either the unaided eye or with binoculars? expensive in larger sizes! Residual uncorrected large ramifications for the budding amateur Can you find all of the above objects by colour haloes form around bright objects unless astronomer. Choose a telescope that is too large yourself (using binoculars if necessary)? apochromatic lens are used. or complex to use and it may never come out of Are you familiar with at least one star atlas? storage for a quick viewing session. Pick one Are you sure that you want to spend time Reflector that’s too small, of poor optical or questionable learning how to operate a telescope so that mechanical quality, and it may kill the passion you can explore the universe on your own? Reflectors use for visual astronomy altogether. mirrors, coated on TELESCOPE DESIGNS the front surface BINOCULARS with aluminum, to Refractor collect and focus As outlined in our companion brochure, The refractor is what most people imagine a “Choosing and Using Binoculars for light. Better systems telescope should look like. This design uses use Pyrex mirrors; Astronomy,” the ideal first telescope for lenses to bend, (refract) light to gather and focus astronomy is actually binoculars. They give a cheaper telescopes light. The large lens on the skyward end is called have plate glass wide field of view and a bright, right-side-up the objective and at the other end is the image of many interesting celestial objects. mirrors. In a eyepiece. A lens will not only bend the light, it Newtonian reflector Binoculars are relatively inexpensive and will unfortunately spread it out into a small versatile enough for other applications. A pair light enters the tube, strikes the large primary parabolic mirror at the bottom of the tube, is reflector, a secondary mirror bounces light azimuth, the compass heading. A telescope on reflected to the flat secondary mirror near the through a hole in the this type of mount will move up and down, left top of the tube, and then is reflected sideways primary mirror to the and right, but will not readily compensate for into the eyepiece. eyepiece. The the motion of the Earth. Maksutov telescope The Cassegrain reflector also has a parabolic uses a thick spherical Dobsonian mounts are a version of the alt- primary mirror. Light is reflected off a corrector lens while azimuth mount that is commonly made of hyperboloidal secondary mirror through a the Schmidt corrector wood. This simple design is very rigid and is central hole in the primary mirror, allowing the plate has subtle ideal for a Newtonian telescope tube. eyepiece, or a camera, to be mounted at the curves. Newtonian Equatorial back end of the tube. versions of both telescopes also exist. If Earth did not move, an alt-azimuth mount Advantages: Reflecting telescopes suffer no would be all that any of us would ever need. An Advantages: Most compact of the three designs; chromatic aberration. Mirrors have only one equatorial mount readily compensates for the less expensive than refractors; huge assortment surface of careful tolerances and thus are less motion of the Earth. It does this sinceone of its of after-market accessories; can be totally expensive to produce than refractor objectives, axes is parallel to Earth’s spin. There are two computer driven; very popular. which require four surfaces to be refined. main types of equatorial mounts; the German Equatorial, which places the telescope on one Reflectors are generally more portable than Disadvantages: More expensive than reflectors; end of a shaft and a counterweight on the other, refractors of similar aperture. Newtonian most dew-prone of the telescope types. designs are the most comfortable to use since and the fork mount, which looks like a tilted alt- the eyepiece is nearly always at a convenient MOUNTING SYSTEMS azimuth mount and has no counterweight. height. These mounts both come in two versions, An unstable mount will render the finest manual and motorized. If you have a choice, Disadvantages: All Newtonian reflectors suffer telescope unable to deliver quality images. If the select the motorized version. from coma, a defect that causes stars near the mount is undersized, wind — the bane of most Although most beginners chose this mount to edge of the field of view to look like a comet. large telescopes — will not be your only enemy. try astrophotography, avoid any thoughts of The smaller the focal ratio of the Newtonian astrophotography for now because the steep scope, the greater the coma. You will experience ‘bouncing’ images even when you are focusing. The test of a quality learning curve, complexity, and difficulty of this aspect of the hobby can dissuade you from Reflectors are quite sensitive to being bumped mount is its ‘damp-down factor.’ This is the more enjoyable parts of astronomy. or transported. A reflector that is not set up as a time it takes an image to stabilize (for the permanent instrument should be collimated vibrations to dampen) after the telescope is GoTo drives prior to each observing session. moved or refocused. Under no circumstances should the image take more than five seconds to A GoTo system combines a small computer Catadioptric or Compound Designs damp down. with a dual-axis motor driver. GoTo drives require you to initially centre one or two stars The Schmidt-Cassegrain and Maksutov- Alt-Azimuth for alignment. Most go-to telescopes Cassegrain are hybrids that have a mix of manufactured today have a large database of refractor and reflector elements in their design An alt-azimuth mount is the simplest type of objects from which you can select. Such systems to fold the optical path back onto itself, telescope mount. The word derives from can operate in either alt-azimuth or equatorial resulting in a compact tube. Like the Cassegrain altitude, the distance above the horizon and modes. TELESCOPE BASICS maximum useful magnification is usually given Accessories as 50x per inch or 2x per mm of aperture. Thus, In spite of the variety of designs, there are Because of the generally narrow fields of view, certain key functions all telescopes perform. a typical 60-mm refractor is, at best, really only a 120x telescope. So, if a low-quality, high-power most telescopes require some type of aiming Light-Gathering Power & Aperture device. Traditionally, this has been a finder Gathering light is the single most important eyepiece is included with the telescope, high (albeit empty) magnifications can be achieved. A scope, a small low-power wide-field telescope function a telescope performs since many mounted parallel with the main optical system. astronomical targets are faint. On top of this, 4-inch telescope can be used with eyepieces that provide up to about 200x, and so on. Choose one with good optics, and it will allow the act of magnifying an image spreads out the for pleasing views of star fields or large star light. Aperture is the size of the telescope’s Magnification can be changed by simply clusters that are too large for the main light-gathering element; whether it is an switching eyepieces in the telescope. To find out telescope.6x30 or 8x50 are good sizes to choose. objective lens, as in a refractor, or its main how much magnifying power an eyepiece gives, mirror, as in a reflector. The bigger the lens or divide the focal length of the telescope by that A red dot or ‘heads-up’ display creates the mirror, the more light the telescope will gather, of the eyepiece, using the same units, of course. illusion of a red reticle in the sky – showing you and the brighter the image will be. Telescope So, 1200mm/25mm = 48x. exactly where you are pointed. But it does not apertures are circles, and the areas of circles magnify, or show objects fainter than your increase with the square of the radius, so Focal Length unaided eye can see.

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