Kalamazoo Astronomical Society Looking Up Since 1936 Introduction to Amateur Astronomy Part 4: Telescope Tutorial www.kasonline.org Table of Contents... Starting Out Right .................................................................... 3 Telescope Terminology ........................................................... 3 Aperture .......................................................................... 3 Focal Length ................................................................... 4 Magnification .................................................................. 4 Focal Ratio ...................................................................... 5 Telescope Types ........................................................................ 5 Refractors ....................................................................... 5 Reflectors ....................................................................... 6 Schmidt-Cassegrain (Catadioptrics) ......................... 7 Telescope Mounts ..................................................................... 9 Altitude-Azimuth Mounts ............................................. 9 Equatorial Mounts ......................................................... 10 Eyepieces .................................................................................... 11 Huygens ....................... 11 Ramsden ......................... 11 Kellner ......................... 11 RKE ................................... 11 Orthoscopic ................. 12 Erfle .................................. 12 Plössl ........................... 12 Ultra-Wide ....................... 12 Apparent & Actual Field-of-View ............................... 13 Telescope Accessories ........................................................... 13 Barlow Lenses ............................................................... 13 Powermates ................................................................... 14 Finderscopes .................................................................. 14 One-Power Finders ....................................................... 14 Red Dot Finders ................................................... 14 Tel-Rad ................................................................ 15 Rigel QuikFinder .................................................. 15 Filters ............................................................................... 15 Colored Planetary Filters ................................... 15 Deep Sky or Nebula Filters ................................ 16 Solar Filters ........................................................ 16 Dew Prevention ......................................................................... 17 Anti-Dew Accessories .................................................. 17 Purchasing Advice ................................................................... 18 In Conclusion ............................................................................. 18 Colored Planetary Filter Breakdown ................................. 19 Some Recommended Observing Books ............................ 20 Starting Out Right larger a telescope’s aperture the more light it will collect and provide brighter images, and resolve fin- Anyone can become an amateur astronomer. There er detail. In general, you should buy the largest ap- are no requirements other than a passion for the erture telescope which fits your portability and night sky. You can pursue this passion with as much budget requirements. or lile effort as you desire. Many amateurs just en- joy looking up to the heavens on a clear night and For Example: A globular star cluster such as M13 is poinng out the bright stars and imaginary shapes nearly unresolved through a 4-inch aperture tele- of the constellaons. Others enjoy scanning the sky scope at a magnificaon of 150× but with an 8-inch with binoculars and nothing else. The goal for many aperture telescope at the same power, the star clus- in this wonderous hobby is to purchase a telescope ter is 16 mes more brilliant, stars are separated and take a closer look at the Moon, planets, and into disnct points and the cluster itself is resolved deep sky objects. to the core. The goal of the Introducon to Amateur Astronomy Aperture directly relates to light-gathering power, lecture series is to get you started off on the right the ability of a telescope to collect light. This can be foot. No one will punish you if you skip over the pre- expressed mathemacally by using the area of a cir- scribed steps to a telescope, but this oen leads to cle (using its diameter instead of radius): disappointment, frustraon, and wasted money. If you educate yourself in the basic principles of as- tronomy, become acquainted with the bright stars and major constellaons of each season, and learn to locate objects with binoculars then you’ll get much more joy out of your telescope - and, along To compare the relave light-gathering powers the way, you’ll know which one is best for you! (LGP) of two telescopes, the rao of their diameters can be calculated by: For Example: How much more light will a 10-meter telescope (one of the world’s largest) collect than a common amateur 10-inch (0.25m) telescope? more light! Telescope Terminology Angular resoluon (or resolving power) is the ability of a telescope to reveal fine detail. The angular reso- Aperture luon α, in arcseconds, equals 11.6 divided by the If any one thing affects the overall performance of a telescope diameter expressed in cenmeters. telescope the most, it is aperture: the diameter of the objecve lens or primary mirror specified in ei- ther inches, cenmeters (cm), or millimeters (mm). You know how professional observatories are al- ways aer bigger telescopes? That's because the For Example: What is the angular resoluon of a 10- Focal Length inch (25cm) telescope? A telescope’s focal length is the distance (usually expressed in millimeters), in an opcal system, from the lens (or primary mirror) to the point where the telescope is in focus (the focal point). Longer focal length telescopes generally have more power, larger images, and smaller the fields of view. A diffracon fringe (or airy disk) is a blurred fringe surrounding any image and caused by the wave For Example: A telescope with a focal length of properes of light. This is what limits the theorecal 2,000mm has twice the power and half the field of resoluon of a telescope. view of a 1,000mm telescope. Because of this, no image detail smaller than the fringe Magnification can be seen unless you use a No other characterisc of telescopes is so widely larger aperture telescope. known and yet so misunderstood by most people. When beginners ask, "What's the power of this tele- Of course, no telescope will scope?" we explain that the queson is like asking a ever be able to reach its the- car salesman, "What's the speed of this automo- orecal resoluon thanks to seeing ― a term used bile?". The car might, if pushed, do 90 or 120 mph, by astronomers to rate the steadiness or turbulence but does this really maer? of the atmosphere. When the atmosphere is un- steady, producing blurred images, the seeing is said The magnificaon of a telescopes is variable and is to be poor. A related term is transparency, which determined by the focal length of the telescope and refers to the clarity of the sky. The more transparent the focal length of the eyepiece: the sky, the more stars you can see. Transparency is generally best aer a surface cold front/low pressure system moves through the re- gion and surface high pressure builds in. Typically the air mass accompanying surface high pressure is cooler, drier, and more stable. For Example: A 10mm eyepiece is being used with a telescope 2,500mm in focal length. What is the However, many mes, especially during the fall, magnificaon? winter, and spring the upper air paern with this situaon may have the jet stream in close proximity (within 300 miles), thereby causing poor seeing con- dions due to turbulence in the upper atmosphere. In addion, the local effects of strong radiaonal cooling, especially an hour or two aer sunset, can As power increases, image brightness and sharpness contribute to poor seeing within the lower atmos- decrease rapidly. Half the power, four mes the phere despite very clear condions. brightness. Twice the power, one-fourth the bright- ness. This is a basic law of opcs. Depending on how much moisture is in the air, the formaon of dew, frost, and/or fog can also become With lower powers, you see brighter, sharper imag- a problem for observing. If early evening dew points es and cover a larger area. It is easier to find your are above the forecast lows for a given night, and target, vibraon is less troublesome, and things usu- clear skies and light winds favor decent radiaonal ally look more detailed. Experienced observers cooling, then dew, frost or fog will likely form. know this, and usually use the lowest power availa- ble most of the me. Higher powers are quite useful Telescope Types for viewing small and distant objects like planets and double stars. Most observers have a selecon of Refractors eyepieces to match the objects they are viewing. The familiar long tube, with lenses in front and the eyepiece in back, has in modern mes evolved into The general magnificaon limit is 50× per inch of two disnctly different kinds of telescopes, achro- aperture, or 2 mes the aperture in millimeters. For mac and apochromac. example, the maximum magnificaon of a 10-inch telescope is 500×. Please note that this is merely a guideline and assumes prisne condions.