Glossary of Commonly Encountered Terms in Amateur Astronomy

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OK, by popular request, here is a glossary of common astronomy terms encountered in amateur astronomy.
Term: Summary:
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 (azimuth), and at right angles to the ground (altitude). It is very useful for terrestrial observations, as it is a very natural way of observing. (Note: Dobsonian Telescopes are mounted this way)
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 lengthening the effective focal length of the telescope you are using. Thus a 2x Barlow will double 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 anyway). On the other hand, a 6mm, 9mm, 15mm and 24mm would be complemented very well by a 2x Barlow.
Editorial note: Barlows have a tendency to work better on telescopes of high focal ratio, say f/6 or above. Some inexpensive Barlows will give very poor results on the f/4 to f/5 Newtonians popular nowadays, though you can buy Barlow lenses optimized for such scopes.
catadioptric Any of a number of compromise telescope designs, using both a lens and mirrors. Examples are the Schmidt-Cassegrain and Maksutov-Cassegrain. Because the light path is folded twice, the telescope is very compact. These are pretty expensive. 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.
Editorial Note: Model numbers change quickly in amateur telescopes, just as in anything else. As of early 1998 better examples of these telescopes might be the Celestron Ultima and Meade LX200 for Schmidt-Cassegrains, and the Questar and Meade ETX for Maksutov-Cassegrains.
chromatic aberration In refractor telescopes, which use lenses to bend the light, different 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 crystals (notably fluorite) are sometimes used to compensate for the aberration. Such telescopes are termed "achromat," or "apochromat" if the correction is is nearly perfect.
Editorial Note: A simple (non-achromatic) lens brings all wavelengths of light to different foci. Strictly speaking an achromatic lens or "achromat" brings two wavelengths of light to a common focus (reducing chromatic aberration greatly); apochromats bring three wavelengths to a common focus. Complete absence of chromatic aberration is impossible in a refractor, but it is negligible in most apochromats and in small (< ~4 inches or 100 mm) achromats.
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. Refractors generally haved fixed optics, so you don't have to collimate them. Reflectors and catadioptrics usually have screws that you turn to collimate. (This only takes a few minutes to do--it is dead easy).
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).
Dobsonian Named for John Dobson of The San Francisco Sidewalk Astronomers (who prefers to call these "Sidewalk Telescopes"),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.

Note: This design is now the #1 design seen at many star parties.

equatorial An equatorial mount is set to the current latitude, and is polar aligned (pointed at the North Pole in the Northern Hemisphere, the South Pole in the Southern Hemisphere) 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 equatorial 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 the size of your pupil in the dark (7mm when you are young, 5 or 6mm when you are over 40, as a general rule) 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 eyepiece. 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 measure 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 compromise)
  • Erfle (wide field of view, expensive)
  • Plossl (perhaps the best all-around eyepiece. Some moderately expensive versions available)
  • 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).

Editorial Note: There are lots of variants on these basic themes. Televue's "Nagler" and "Panoptic" eyepieces are a kind of ultra-wide-view. Meade's "Modified Achromats" and Edmund Scientific's "RKE" eyepieces are similar to Kellners overall.

You really don't want to buy many .965" eyepieces--they are generally not as well made as the 1.25" ones, and if you get a bigger telescope it will probably not accept your .965" eyepieces. 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 telescope 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.
Editorial Note: Although the above suggests that finder scope size is not terribly important, very small finders (less than 30mm in aperture) tend to be poorly built.
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 telescope 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.
Editorial Note: f/ratio also affects how forgiving the telescope is towards eyepiece design. Many simple, inexpensive designs like Kellners (see the Eyepiece entry) work well on telescopes in the f/12 to f/15 range, but very poorly at f/5.

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 zenith). 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 equatorial 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 problem for visual astronomy, but a limitation on astrophotography.

light bucket A common slang term for a large aperture. The cure for "Aperture Fever."
Maksutov-Cassegrain See catadioptric.
meridian An imaginary north/south line passing through the zenith.
Newtonian See reflector.
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 are accessories, such as the mount, tripod, and eyepieces. 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.
reflector A reflector is any telescope which uses a mirror as its objective. 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 .
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.
Schmidt-Cassegrain See catadioptric.
spherical aberration A problem where a 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.
Editorial Note: Not always. Spherical aberration is one of the commoner flaws in relatively inexpensive reflectors of otherwise competent design.
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.
wedge This is the thing that a fork-mounted Schmidt-Cassegrain telescope 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 telescopes come with drives which compensate for the mechanical defects.
zenith The sky directly overhead. An object "transits" when its line of right ascension crosses the zenith.
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Last Modification: December 16, 2000