NetNews Usenet Archive 1992 #19

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Path: sparky!uunet!cis.ohio-state.edu!ucbvax!nisc.sri.com!csl!ronnie From: ronnie@cisco.com (Ronnie B. Kon) Newsgroups: sci.astro Subject: Purchasing Amateur Telescopes FAQ (part 1/2) Message-ID: <1992Sep2.040036.28083@csl.sri.com> Date: 2 Sep 92 04:00:36 GMT Sender: news@csl.sri.com (NEWS Administrator) Organization: cisco Systems, Menlo Park, California, USA Lines: 574 X-Disclaimer: The views expressed in this message are those of an individual at SRI International, and do not necessarily reflect those of the company. Nntp-Posting-Host: lager.cisco.com Purchasing Amateur Telescopes FAQ Part I Ronnie B. Kon ronnie@cisco.com Last Updated: August 27, 1992 Questions in this FAQ: In Part I: 1. What is the single most important thing I should know before buying a telescope? 2. What is the single best piece of advice to give to someone thinking about buying a first telescope? 3. What Does All the Jargon Mean? 4. What Are Some Good Introductions To Amateur Astronomy? 5. What Will I Be Able To See? In Part II: 6. Buying A Telescope 6.1 What Company Makes the Best Telescopes? 6.2 How do Meade and Celestron Name Their Telescopes? 6.3 Comparison of Schmidt-Cassegrains 6.4 What is the Best Telescope to Buy? 7. Where Do I Buy My Telescope? 7.1 What About Buying Used? 7.2 What About Building A Telescope? 8. What Accessories Will I Need? 9. Why Should I Start With Binoculars? 9.1 How Do I Hold Binoculars 10. What Books and Star Charts Are Recommended? 10.1 What About Computer Programs? 10.1.1 What Programs Can I Get For Free? Contributors to this posting include: Pierre Asselin Doug Caprette Mike Collins Kevin Deane Jay Freeman Chuck Grant Christopher Gunn Dyer Lytle Doug McDonald Andy Michael Dave Nash Bill Nelson Leigh Palmer Alan Peterman Tom Randolph David Smith Geoff Steer C. Taylor Sutherland Mario Wolczko Paul Zander 1. What is the single most important thing I should know before buy- ing a telescope? This is the single most important thing you should get out of this FAQ: DO NOT BUY YOUR TELESCOPE FROM A DEPARTMENT STORE. Ignore every- thing any literature tells you about magnification and such. Buy from a telescope store, where you will get a telescope that doesn't make as big claims, but will give you FAR better performance. The reason is that as far as telescopes go, how much you can magnify is a function of the amount of light the telescope receives, which is almost entirely determined by the telescope's aperture (the size of the lens or mirror that points at the sky). As far as magnification goes, you can expect 50x per inch of aperture on a normal night, up to 62.5x on an exceptionally clear night (this is the number Meade uses in calculating their magnifications). Department stores always show little 2 1/4 inch refractors for up to 300+ dollars and say that the refractor can get up to a whopping 600X or so. Strictly speaking, this is true. However, applying the 50x rule, it is easy to see that 125x would be pushing the optics, and that's assuming that they were high quality ones. With the quality of the parts they usually give you are lucky to get 100x with reasonable resolution. More details will follow. I put this up at the top so even if you read nothing else, you will read this. 2. What is the single best piece of advice to give to someone think- ing about buying a first telescope? Find a local astronomy club and attend a star party. Find ways to look through telescopes of different quality and prices so you can determine what you want to buy. This FAQ can give you information, but cannot possibly compare to actually going out and looking for yourself. Besides, it's the last chance you'll get to look at the sky for free. 3. What Does All the Jargon Mean? OK, by popular request, here is a glossary of common astronomy terms encountered in amateur astronomy. Words in bold face (or underlined) can be found defined under their own heading. altazimuth mount This is what you think of when you think of a tripod mount. It allows movement in two directions: parallel to the ground, and at right angles to the ground. It is very useful for terrestrial observations, as it is a very natural way of observing. It is significantly less useful for astronomical use, where an equa- torial mount is preferred. aperture The diameter of the objective. Barlow A Barlow lens is a device which has the effect of increasing the magnification. It does this by shortening the effective focal length of the eyepiece you are using. Thus a 2x Barlow will dou- ble the magnification, a 3x will triple it. Barlows used to have a bad reputation, stemming largely from rather poor quality ones being sold. Modern Barlows are high quality and a good choice for expanding your collection of eyepieces. You should keep the Barlow in mind when buying eyepieces--buying a 3mm, 6mm, 12mm, and a 24mm and a 2x Barlow is a very dumb idea. The only use you get from the Barlow is changing the 3mm to a 1.5mm (which is probably going to give you higher than usable magnification any- way). On the other hand, a 6mm, 9mm, 15mm and 24mm would be com- plemented very well by a 2x Barlow. catadioptric Any of a number of compromise telescope designs, using both a lens and mirrors. Examples are the Schmidt-Cassegrain and Maksutov-Cassegrain. These offer large apertures without the coma problems of a Newtonian. The light path is folded twice, which makes the telescope very compact. These are pretty expen- sive. Pictures can be seen in the ads in any issue of a popular astronomy magazine: the Meade 2080 and the Celestron C-8 are examples of Schmidt-Cassegrain; the Celestron C-90 and Questar are examples of Maksutov-Cassegrain. chromatic aberration In refractor telescopes, which use lenses to bend the light, dif- ferent wavelengths of light bend at different angles. This means that the stars you see will usually have a blue/violet ring around them, as this light is bent more than the rest of the spectrum. It is not present at all in reflectors, nor to any significant degree in catadioptrics. Different glasses and cry- stals (notably fluorite) are sometimes used to compensate for the aberration. Such telescopes are termed "achromat," or "apo- chromat" if the correction is is nearly perfect. collimation This refers to how correctly the optics are pointing towards each other. If a telescope is out of collimation, you will not get as clear an image as you should. coma This refers to the blurring of objects at the edge of the field of view, most common in short focal ratio Newtonian telescopes (at f/10 and longer, Newtonians are very well corrected for coma). declination All astronomical objects are located via a pair of coordinates: Right Ascension and Declination. These are easily visualized by imagining that the Earth is in the center of a hollow celestial sphere, which has all the stars, nebulae and galaxies painted on the shell, and the sun, moon and planets move around the shell's inner surface. We can then project the Earth's lines of Latitude and Longitude onto the sphere, and have a unique location for each object (obviously, these will change rapidly for quick mov- ing objects, very slowly for slower objects. See also "preces- sion"). The Declination is the celestial latitude, the Right Ascension is the celestial longitude. Dobsonian Named for John Dobson of The San Francisco Sidewalk Astronomers, this is a design which allows for very large apertures at very affordable prices. The trade-off is that they are mounted on altazimuth mounts, instead of equatorial ones, which makes them essentially useless for astrophotography, but an inexpensive alternative if you only plan to do visual work. These are light buckets. If you are planning to build your own telescope, you might want to consider a Dobsonian. double star A double star is a pair of stars which appear to be very close together. There are two types of double stars: binary stars, where the two stars are actually a part of a system and orbit each other; and optical doubles, where the two stars are not gravitationally bound at all, they just happen to lie on the same line of sight from the Earth. ephemeris Plural, ephemerides. A table of the location of a celestial object at regular intervals in time. equatorial An equatorial mount is set to the current latitude, and is polar aligned (pointed at the north pole) and then moves only in Right Ascension and in Declination. This take a while to get used to, but offers the wonderful side effect of being able to track the astronomical objects you are looking at as they move across the sky (which is very visible motion at telescopic magnifications) by moving in only one direction (Right Ascension). Most equa- torial mounts come with motor drives that take care of this for you. exit pupil This refers to how wide the beam of light exiting the eyepiece is, and is equal to the aperture divided by the magnification. If it is bigger than 7mm (the size of your pupil in the dark) you will not be taking in all the light available--effectively, you will be using a smaller aperture telescope than you have. eyepiece This is the thing you actually look into. Almost all telescopes separate the Optical Tube (the telescope proper) from the eye- piece. Essentially, the telescope makes a really tiny image of what it's pointed at. The eyepiece acts as a magnifying glass to allow you to see the image bigger than it would otherwise be. The magnification is the focal length of the telescope divided by the focal length of the eyepiece. Eyepieces are described by the diameter of the barrel, always expressed in inches (.965", 1.25" and 2" are the sizes in common use) and the focal length always expressed in millimeters (4mm - 40mm is the usual range). Short focal length eyepieces are also termed high power, long focal length are low power. Also significant with eyepieces is the apparent field of view (expressed in degrees) and eye relief (expressed in millimeters). The apparent field refers to how big the circle of space you see in an eyepiece appears. Bigger is better. Eye relief is a meas- ure of how far from the eyepiece you can have your eye and still see. If you wear glasses to correct astigmatism, you will need fairly long eye relief (the focus knob will correct for almost all vision problems except astigmatism). There are several types of eyepiece designs. The most popular are Kellner (inexpensive, most popular for cheap telescopes, short eye relief and narrow fields of view. Good to avoid if you can afford better); Orthoscopic (good price/performance comprom- ise); Erfle (wide field of view, expensive); Plossl (perhaps the best all-around eyepiece. Some moderately expensive versions available); and Ultra Wide (very expensive, almost double the number of lenses as other designs makes for more light loss in the eyepiece, large exit pupils. Can cost more than a small telescope. Not a good place to spend your money when you are just starting out). You really don't want to buy many .965" eyepieces--they are gen- erally not as well made as the 1.25" ones, and if you get a bigger telescope it will probably not accept your .965" eye- pieces. You can buy an adapter to let you use 1.25" in your .965" focuser. This is probably worth the money. f/10, f/6,3 See Focal Ratio finder scope The finder scope is a low power telescope attached to the tele- scope you are using. Because most telescopes show such a small portion of the sky, it is virtually impossible to locate anything just by looking through them. So you look through the finder scope to center the object you want (the finder has crosshairs) and then you can use your real telescope on it. Note that you can ignore all the claims about big finder scopes. You almost certainly don't care. All you need is to be able to point your main telescope at something in the sky. Finder scope size only matters when you are starhopping through fairly dim stars (where the larger aperture allows you to see dimmer stars). This will not be an issue for you for quite a while (if ever). Many people use a Telrad sight, which is simply a red LED you can sight on-- you get absolutely no more aperture than your naked eye. The finder scopes are usually advertised as 8x50 (or such). The eight refers to the magnification, the 50 to the aperture in millimeters--just like binoculars. focal length This is the length of the light path, from the objective to the focal plane. The magnification is the focal length of the tele- scope divided by the focal length of the eyepiece. See also focal ratio. focal plane The plane that the telescope (or eyepiece) focuses on. When you turn the focus knob on the telescope, you are moving the eyepiece back and forth until you make the two focal planes coincide. focal ratio Also referred to as the "speed" of the telescope, is the ratio of focal length to aperture, and is always expressed as an f/ number. Thus an 8" telescope with a 2000mm focal length is f/10 (because 8" is 200mm, and 2000 / 200 = 10). An f/10 telescope is "slower" than an f/4. Fast telescopes give wider, brighter images with a given eyepiece than slower ones (but note that at a given magnification, the images are--assuming identical optics--exactly the same: what you see through a f/6.3 telescope with a 12mm eyepiece is identical in width and brightness to what you would see through a f/10 telescope with a 19mm eyepiece). In general, the slower the telescope the more forgiving it is of optical errors in the objective and eyepiece. A telescope of f/10 is fairly forgiving, f/6.3 much less so. focuser This is the thing that holds the eyepiece. It moves in and out so you can focus the telescope. It is always included with the telescope when you buy one. The size, almost always .965", 1.25" or 2" refers to the barrel diameter of the eyepieces it accepts. fork mount A fork mount is a type of mount where the telescope is held by two arms, and swings between them. A fork mount can be either alt-azimuth or equatorial (through the use of a wedge). Fork mounts are most commonly used with Schmidt-Cassegrain telescopes, and are almost always equatorial). German Equatorial Mount The first equatorial mount devised and still the most common for small to moderate sized reflectors and refractors. Unlike the equatorial fork, the german equatorial is suitable for telescopes with either short or long tubes (although, if poorly designed, a long tube may strike the tripod, preventing viewing at the zen- ith). They usually are designed with movable counterweights, which make them easy to balance, but heavy and bulky. The tube of the telescope is joined to a shaft (the Declination shaft or axis) which rotates in a housing that in turn is joined at right angles to another shaft (The polar axis). The polar axis is pointed at the celestial pole (just like any other equa- torial mount). A counterweight, which is required for balance, is placed on the other end of the decination shaft. Tracking an object past the zenith requires that the telescope be turned (both Right Ascension and Declination rotated through 180 degrees), which reverses the field of view. Not so much a prob- lem for visual astronomy, but a limitation on astrophotography. light bucket A common slang term for a large aperture. The cure for "Aperture Fever." Well, not really. After a month with the Keck tele- scope, any amateur worth her salt would be bitching about how much more she could see if only she could double the aperture. Maksutov-Cassegrain See catadioptric. meridian An imaginary north/south line passing through the zenith. nebula Plural nebulae. An unfortunate term which basically means "some- thing up there which isn't a star or a planet." Until the early years of this century nobody knew what the diffuse light sources in the sky were, so they were all termed nebulae, from the Latin for mist. They encompass galaxies, supernova remnants and space dust. "Emission nebulae" are nebulae that emit light, and thus you can see, "dark nebulae" are things which don't emit light, and hence can only be seen as a dark foreground against a brighter light. The Horsehead Nebula is probably the most famous dark nebula. Note that many astronomers will grimace loudly if you refer to galaxies as nebulae. Newtonian See reflector. nutation Like precession, this is a term you really don't want to know about. It turns out that not only does the sun make the Earth wobble in her orbit (see precession), the moon does as well. This wobble (about 10 arc seconds with a period of about 18 years) is termed nutation. You will never have to worry about it. objective This is the thing that gathers light from the sky and folds the light into a cone. In a refractor it is the big lens that points at the sky, in a reflector it is the big mirror at the bottom of the tube. The job of the objective is to create a light cone which comes into tight focus at a single focal point. optical tube This is the telescope proper. It is the tube which holds the objective. The rest of the stuff is accessories, such as the mount, tripod, eyepieces, etc. When reading ads, note that some- times optical tubes are sold by themselves. You will need to go out and buy (or build) a mount for them before you can use them. precession This is a motion which you don't need to think about. It is the drifting of the north pole, and all other celestial coordinates with it, in a rough circle. This occurs over so long a timespan (it is a 26,000 year cycle) that it will not affect you. It is, however, one of the main reasons (proper motion being the other) that star atlases are all prepared as of a specific date (Jan 1, 2000 is the current standard, the previous was Jan 1, 1950. Except they are really Dec 31 of the year before--which makes a huge difference, as you can well imagine). proper motion Everything in the Universe is moving, relative to the Earth. It is convenient to divide this motion into two vectors, the motion directly toward the Earth (termed radial motion), and the motion perpendicular to the former, termed proper motion. Motion directly towards or away from the Earth has no effect on the body's position as seen from Earth. Proper motion, however, does. This is one reason (along with precession) that star charts are prepared as of a specific date. reflector A reflector is any telescope which uses a mirror as its objec- tive. The most common type is the Newtonian reflector, which has a mirror at the bottom of a tube, which focuses the light into a cone which is deflected by a flat "secondary" mirror (which is mounted near the top of the tube in something called a "spider") out a hole in the side. This is where you put the eyepiece. The advantages of the Newtonian design are numerous: there is only one optical surface on a mirror, as opposed to two on a lens, so it is cheaper to make; part of the light path is at right angles to the length of the tube, so it can be somewhat shorter than a similar refractor; you can get it in much larger apertures than a refractor, and there is no chromatic aberration . The disadvan- tages are that you might get coma away from the center of the image (in short focal ratio telescopes), and you don't get as good resolution as with a refractor of equal aperture (all other factors being similar). refractor This is what you usually think of as a telescope--it has a lens at one end, and you look straight through the other. This is sometimes referred to as a "Galilean" telescope, as it is of the same design that Galileo used (although strictly speaking, a Galilean telescope is a specific kind of refractor--one with a simple double-convex objective lens and a simple double-concave eye lens. This will not be on the quiz, so you needn't memorize it). See reflector for a comparison of the two designs. right ascension See declination. Schmidt-Cassegrain See catadioptric. spherical aberration A problem where a spherical lens or mirror in a telescope is not shaped correctly, so the light from the center is focused at a different location than the light from the edges. You should never have to worry about this. This only shows up in really cheap telescopes, like the Hubble Space Paperweight. spotting scope A small telescope, always a refractor or catadioptric, generally used for terrestrial viewing. Of limited utility for astronomy, though many are marketed as such. Probably the wrong choice unless you want to use it also for birdwatching, or as a powerful telephoto lens on a SLR camera. The Starry Messenger The Starry Messenger (or TSM) is a classified advertising monthly for astronomy stuff. Lots of ads, so a must-have for anyone con- sidering buying used. It has started charging the sellers a 4% commission, though, which has prompted a new paper, "The Cosmic Exchange," to begin publishing without charging the commission. I expect only one of them to last. Personally, I'm betting on TSM, but it's hard to say. wedge This is the thing that a fork-mounted Schmidt-Cassegrain tele- scope will attach to, to connect it to the tripod. You want it to be sturdy. worm drive This is the sort of drive most telescopes come with, if they come with a drive. It is a very accurate and smooth drive. However, due to imperfections in the manufacturing process, there will be periodic errors that occur at the same point in every worm cycle (usually about 8 minutes). To deal with this, higher end tele- scopes come with drives which compensate for the mechanical defects. Celestron's is the PEC (Periodic Error Correction), Meade's is the Smart Drive. zenith The sky directly overhead. An object "transits" when its line of right ascension crosses the zenith. 4. What Are Some Good Introductions To Amateur Astronomy? In the United States, there are two popular astronomy magazines: Sky and Telescope (S&T), and Astronomy. Of the two, S&T is more techni- cal, while Astronomy has more things like "artist's conception of Jupiter-rise on Ganymede" which are very pretty. I consider S&T a necessity, but getting both is not a bad idea. In the U.K., there is a magazine called Astronomy Now which you prob- ably want to subscribe to. It is rather like Astronomy in style, but slightly less bulky. Also, fewer pretty pictures. There is also a magazine called Popular Astronomy (which is not sold on newstands--you have to join the Junior Astronomical Society). There are many good introductory books. I can recommend The Light- Hearted Astronomer by Ken Fulton as being an excellent introduction for the complete neophyte. The writing style is a little irritating, but it is full of practical information. It is more about observing than astronomy, though. It has advice like "if you are in bear coun- try, make a lot of noise so the bears don't bother you." P. Clay Sherrod's A Complete Guide to Amateur Astronomy, available through Sky Publishing Company, is a more technical introduction. Sidgewick's books are absolutely excellent books, probably the very best ever written on amateur astronomy. They are also probably over a beginner's head. Holding off on these for a while would not be a bad idea. Also see below, the section on Books and Starcharts. 5. What Will I Be Able To See? The best way to find out is to go observing with someone. Look for a local astronomy club (S&T lists them periodically). This is also a very good way to get a good price on a used telescope of proven qual- ity. In general, you will be able to see all planets except Pluto as disks. You will be able to see the bands on Jupiter and the rings around Saturn. You may be able to see the ice caps on Mars (although Mars is probably the most disappointing object in the Solar System). Venus and Mercury will show phases but not much else. You will be able to see four of Jupiter's moons as points. Ditto Saturn's moon Titan. You will be able to see comets. Do not expect your images to be anywhere as nice as the ones you see from the Voyager spacecraft. If a $2000 telescope could get these, nobody would ever have spent the billions to send a spacecraft out there. As far as "deep sky" objects, you will be able to see all the Messier objects in most any modern telescope. Galaxies will tend to look like bright blobs. Look a while longer and you may find some spiral arms or dust lanes (assuming it has them). Galaxies look nothing like their pictures--you will not see the arms anywhere near as clearly. You will also find that the colors you see are considerably more muted than the pictures you see. This is because our retinas work by having two different types of light sensitive organs, rods and cones. Rods are very sensitive to dim light, but relatively useless for color vision. Cones are the opposite. Thus when looking through a tele- scope you are using your rods, and you aren't seeing a lot of color. -- ------------------------------------------------------------------------------- Ronnie B. Kon | "Boy, they were big on crematoriums, weren't they?" ronnie@cisco.com | (415) 688-4592 | -- George Bush at Auschwitz, Sept 1989