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Astronomy Glossary

APPARENT FIELD OF VIEW: The size in degrees of the field as seen through the eyepiece of your telescope.
APERTURE: The diameter of the main light gathering element in a telescope.
ASTERISM: A grouping of stars resembling a familiar shape.
ASTIGMATISM: A defect in optics in which a lens or mirror has two different amounts of curvature at 90 degrees to each other.
CATADIOPTRIC TELESCOPE: A telescope using both mirrors and lenses to focus light. The most popular of these is the Schmidt-Cassegrain telescope.
CELESTIAL SPHERE: The imaginary sphere on which the the night sky is seen to be projected on.
CHROMATIC ABERRATION: The discoloration of a bright object caused by dispersion in a refracting telescope's objective lens(es). This dispersion causes different colors of light to come to a focus at different distances.
CIRCUMPOLAR: Star or objects that are close enough to the pole so that they never set from your particular latitude.
COLLIMATION: The systematic aligning of optical elements in a telescope to emsure the brightest and sharpest views possible.
COMA: A defect causing stars to be deformed into a fanlike or comet shape. This is typically caused by misaligned optics.
DECLINATION: The celestial equivalent of latitude. Declination is measured from the celestial equator to the celestial pole, spanning 90 degrees.
DIFFRACTION-LIMITED: A term used by telescope manufacturers to represent the 1/4-wavelength Rayleigh limit of optics. This represents an error where the amount of detail seen is limited only by the wave nature of light. Errors greater than this result in optics that degrade sharply.
DOUBLE-STAR: Two stars close together so that they appear as one at lower magnifications. Many of these stars are actually associated with each other gravitationally forming a binary system.
ECLIPTIC: The path in the sky the sun follows.
EXIT PUPIL: The diameter of the cone of light exiting the eyepiece. This can be calculated by dividing the diameter of the objective by the magnification used.
FOCAL LENGTH: The distance from the objective lens or mirror to the point where the light converges, the focal point. This distance is usually measures in millimeters.
FOCAL RATIO: The ratio of the focal length divided by the diameter of the primary mirror or lens.
GLOBULAR CLUSTER: An enourmous grouping of 25,000 to upwards of over 100,000 old stars. Globular clusters are gravitationally held together and orbit the outer reaches of most, if not all, galaxies.
LIMITING MAGNITUDE: The magnitude of the dimmest object which can be seen using a telescope of a given aperture or with your naked eye.
MAGNIFICATION: The amount of increase in apparent size of an object. This quantity can be derived by dividing the telescope focal length by the eyepiece focal length.
MAGNITUDE: A scale used to compare the brightness of objects in the sky. One magnitude is a difference of 2.512 times in brightness.
MESSIER OBJECT: Any of the 110 objects catalogued by French astronomer Charles Messier in the late 18th century. Messier was a comet hunter and catalogued these as objects to "avoid," or objects that could be easily mistaken as a comet. They include many of the best nebulae, star clusters, and galaxies that are visible from mid-northern latitudes.
NEBULA: A large cloud of gas and dust in space. Most are dark and obscure the light from background stars. Others glow brilliantly from the energy of hot star within them.
OPEN CLUSTER: A loose grouping of relativly young star grvitationally bound together. Most stars in open clusters formed as a group inside large nebulae.
OPPOSITION: The point in time when a planetary body is at the same heliocentric longitude as the Earth. Bodies at opposition rise at sunset.
PLANETARY NEBULA: The last gasp of a dying star, a planetary nebula consists of the outer layers of a star. These layers glow brilliantly from the hot ultraviolet radiation from the stellar core in the middle.
REFLECTING TELESCOPE: A telescope which uses mirrors to gather and focus light.
REFRACTING TELESCOPE: A telescope which uses lenses to gather and focus light. >RIGHT ASCENSION: The astronomical equivalent of longitude. It starts where the ecliptic intersects the celestial equator in Pisces and moves through 24 hours. >SEEING: A rating of how steady the atmosphere is at your observing site. Most astronomers use a scale of 1-10 or 1-5. Cold clear nights typically have the worst seeing. The better the seeing, the better high power views of the planets and double stars.
SPHERICAL ABERRATION: The departure of a wavefront of foucused light from a spherical shape.
TRANSPARENCY: A rating of how clear the night sky is. This is usually given by the magnitude of the faintest star easily visible to the naked eye. Cold clear nights have some the best transparency.
TRUE FIELD OF VIEW: The size of the actual area of the sky you are viewing measured in degrees. This quantity can be approzimated by dividing the apparant field of the eyepiece by the magnification.
ZENITH: The point in the celestial hemisphere that is directly above you or your observing location.

Tools and Toys of the Amateur Astronomer

There is only one things amateur astronomers love more than gadgets and that's showing them off. The number of different tools we use just to look at the sky is mind boggling. I'll try as best as I can to cover as much as I can, as clearly as I can.

Telescope Types

Many different optical telescopes have been created. There are three main types, refractors, those using lenses, reflectors, those using mirros, and catadioptrics, using both mirrors and lenses. Each type is broken up into different designs, each of which is used to solve inherent problems. I'd like to go through them all, however, that's complicated and boring to many. Let's begin.


Refracting telescopes, what most people picture when they think of a telescope, were first invented in the early 17th century by the Dutch, and first pointed towards the heavens by Galileo soon after. The refractor design basically uses a set of convex lenses to bend light to a point and magnify it's brightness and scale. Producing good-quality lenses is extremely difficult. Two surfaces must be ground and polished to an exact shape using glass with absolutely no internal flaws. A lens can only be supported on it's edges so large, heavy ones are easily distorted by their own weight, ruining images. Another flaw of lenses has to due with the wave nature of light. Shorter wavelength light (like blue light) is refracted more than longer wavelength (redder) light. This is called color dispersion, it's what makes a prizm break up light into a rainbow. Unless corrected, images will be discolored and distorted beyond recognition, especially on shorter focal length scopes. To help solve this problem, called chromatic aberration, a concave (negative) lens using glass of different optical characteristics is placed directly behind the first lens. This is called an achromatic design. This second lens brings the blue and red wavelengths of light closer together at the focal point, but only closer. The apochromatic refractor was created to better solve this problem. One or more additional lenses and/or lenses of low dispersion glasses are used to bring colors to a closer focus. Most do an excellent job of this, others not so good. This color usually only shows up on objects like brighter stars, the planets, and the moon and typically at higher power.

A well made refractor is a big investment though some good ones are available at lower prices. Their lack of a central obstruction (secondary mirror) allows them to produce the sharpest, most contrasty images you can find. Planetary views are usually unmatchable but if it's very faint deep-sky objects you want, a refractor is not ideal due to cost. This high cost per inch of aperture pushes them out of reach for most people.


The reflecting telescope was invented by Isaac Newton. The Newtonian reflector uses a concave mirror at the bottom of a tube to collect light and focus it to a point. A second mirror placed near the top of the tube at a 45-degree angle reflects it out the side. This design makes people unfamiliar to it scratch their heads, not knowing where to look. The main advantage of a reflector is the relative ease in their manufacture. The parabolic surface is much easier to make precisly, red light reflects at the exact same amount as blue light, a mirror can be supported on the back as well as the sides, and light does not have to travel through glass eliminating the need for perfectly clear homogeneous glass. The newest and largest telescope mirrors aren't even made of glass but opaque ceramics.

A Newtonian Reflector telescope is the least expensive design available. You can find a good quality scope which gathers ten times the light of a 4" Takahashi refractor for a good deal less money. There is a compromise optically though. The secondary mirror, being in the light path, blocks some of the light going to the primary mirror. If made too small in an attempt to minimize light loss, you may not reflect light from the outer edges of the primary out the side creating light loss at the field edges, vignetting. Light loss means less contrast and less sharpness. Every scope is a compromise.


Catadioptrics, the most popular being the Schmidt-Cassegrain, use a combination of lenses and mirrors to bring light to a focus. In a Schmidt-Cassegrain, a fast (~f/2) concave primary mirror on the bottom of the tube reflects up to a convex secondary placed at the top of the tube. The secondary reflects the light back down to the eyepiece through a hold in the primary. The lens used is a slightly curved piece of glass called a corrector plate over the top opening to eliminate optical defects in the primary. This glass must be figured precisely to form a specific curve making the design more difficult and more expensive to produce. This design is extremely compact for its aperture. The secondary mirror is actually a hyperbolic magnifying mirror used to increase the effective focal length of the primary usually to around f/10 making them extremely compact. An SCT has a much narrower field of view and a larger loss of contrast due to toe larger secondary mirror.

Another popular type of catadioptric is the Maksutov-Cassegrain using a heavy miniscus-shaped corrector plate with an aluminum-coated portion inside as the secondary. The Mak-Cass is somewhat heavier than the SCT and usually has a longer focal ratio, around f/13. The Mak-Cass costs a bit more than the Schmidt-Cass but the quality is quite good. As with an SCT, they have a narrow field and due to their large central obstruction, may not be the best for picking out planetary detail.

There is also a Maksutov-Newtonian using the same type of corrector plate as the Mak-Cass, but with the optical setup of a Newtonian. A well-made Mak-Newt can have near refractor-like sharpness and contrast and produce no diffraction spikes on brighter objects like on classical Newtonians. Mak-Newts are around $1000 for a 6-inch, almost 3-times that of a regular Newtonian of the same aperture.

The Magnification Myth

First and foremost before you go shopping, any telescope with packaging that mentions magnifying power (something like "650X" or "650 power") isn't worth its weight in raw sewage. Most objects in the sky need relatively little magnification to see clearly. Raising the magnification makes images dimmer and fuzzier.

I commonly get questions from visitors at star parties like, "How far can you see?" A scope is not rated on how far it can see, but how MUCH it can see. The maximum magnification for any telescope is at best around 60X per inch of aperture. Atmospheric conditions can blur images even more, limiting the magnification to an even lower value. Increasing magnification just magnifies the blur. Final thought, any 60mm scope advertized as having a magnification of 650x is garbage no matter what magnification you use it at.


No optical design is better than another in the long run. Some perfom better on certain objects in certain situations, some are simpler to use, and some are easier to transport than others. Again, think about what you really need to do, what you really want to do, and where you intend to do it. Refractors and SCT's are typically more portable, but may be more complex and more expensive. Newtonians are the least expensive design but may be too large for your car, or even your spouse. More aperture is better, but not always, depending on your location. As far as skill level, each optical design is basically as easy to use as the next one. Catadioptrics may be a complicated design, but the manufacturer takes care of the difficult parts for you. A Newtonian requires optical alignment, or collimation, typically before each use to maximize performance (all scopes require collimation, Newtonians tend to lose alignment easiest depending on design). This is usually quite easy as long as the mirror cell is made for easy adjustment. Most of them are, some, surprisingly, are not. What is it you'll be concentrating your observing time on? If it's high-power planetary views, refractors are ideal. If you plan to view faint deep-sky objects from a dark-sky site, a medium to larger size Newtonian is basically the only choice.


A scope must be mounted on something ridged. You can't hold a telescope and expect to be able to hold it steady. The mount used for a telescope must be a ridged support which allows smooth movement and all axes. There are two types of mounts, alt-azimuth and equatorial.


Alt-Az mount, short for altitude-azimuth, is the simplest of all the mounts. The azimuth axis moves side-to-side, while the altitude axis moves up and down. The most famous of these mounts is the Dobsonian mount. They're extremely simple in design, and are very, very rigid. Because of their simplicity, it's somewhat difficult to make a Dobsonian which is truely bad. The most difficult part in their construction is smooth movement. A Dobsonian mount is stabilized by gravity. The point of rotation in altitude must be precisely at its center of gravity. Teflon bearings are used to provide just enough fristion to prevent unwanted movement, but not so much as to prevent slight adjustments. Alt-azimuth mounts are less than ideal for high power views as their axes are not lined up with Earth's rotational axis making movement by hand or the attachment of a motor drive more complicated. Their cost and simplicity in basic use and setup is ideal for beginners or those who like to setup quickly. They are also ideal for deep-sky viewing allowing more money to be spent on bigger, better optics.


An equatorial mount can simply be thought of as an alt-az mount with its azimuth axis pointed directly at one of the celestial poles. These axes are right-ascension, right to left, and declination, up and down. This allows you to easily follow an object's movement by adjust only the right-ascension axis. Pointing them at objects can be confusing for beginners. Unless you're at the North or South Poles, either axis can be up and down or right to left depending on where in the sky you're pointing it. Most EQ mounts have setting circles allowing objects to be located by their celestial coordinates with ease once aligned.

The two types of EQ mounts most amateurs will encounter are the German and the fork mount. A german mount is the one which make newcomers scratch their heads due to its awkward setup. Most EQ mounts must also be balanced which reduces stress on motors ar gears, but most come with counterweights making this much easier than balancing a Dobsonian mount. Equatorial mounts are the most expensive of the two types. Precisely made mounts that have smooth movement and rock-solid rigidity push $1500 all the way up to around $8000. Cheap ones are small and not very good for high-power use, jiggling at even the slightest touch. Equatorial mounts are also more bulky and must be assembled on site and manually aligned to the pole. Typical visual use only requires that it be pointed toward the pole while photography requires it to be dead on to prevent field rotation.


When deciding what mount you would like to get, think about some of the same things you would when deciding on optical design. Dobsonians are great for quick setup and straightfoward use, but can be frustrating at high power. Equatorials must be transported in pieces and assembled on site. Alt-Az mounts are relatively inexpensive while EQ mounts can be costly. If you want the ease of a Dobsonian mount and the tracking capabilities of an equatorial mount some Dobsonian scopes, those made with Sonotube, can either be adapted for us on larger EQ mounts, or computerized drive systems offered by some smaller companies can be attatched. These drive systems are quite expensive and may still use the stock mount, making them less than ideal for photography. Remember what you intend to do later on.

Some mounts made today come with automatic "go-to" motor drives and software. This system is built into the mount, but it typically comes together with a scope as a unit. Making it extremely easy to find nearly any, a go-to mount can be an invaluable tool for the beginner. The only thing you need to find on your own is two or three bright stars for alignment, but finding bright stars is easy, finding faint objects manually requires skill. Not everybody is so patient in the field. If you are one of these people try not to abuse the computerization early on. If you let the scope do everything, you may not learn many of the key elements of astronomy. You may also lose valuable observing time if something breaks or your battereis go dead. I usually take pride in being able to find objects faster than someone with many years more experience than me. If you have one, don't abuse it early on.


No matter how much you spend on your scope and mount, it's completely usless without eyepieces. A nice set of eyepieces allows you to choose the field of view and the magnification to suit your needs for any particular object. Some amateurs collect eyepieces like baseballs cards. They have two or three of every focal length and each different design. Many times this is necessary due to the different uses of eyepiece designs and the fact that some work better in different scopes than others.

Yes there are different design of eyepieces, from simple and inexpensive, to complex and high priced. Each design has it's own use. Let's discuss some of them.

Eyepiece Designs

The simplest modern descent design is the Kellner. Made with three elements it's very inexpensive and produces images good for beginner or those on a very tight budget. They suffer from a narrow field, small eye relief, and sometimes reduced sharpness. One the other hand image brightness is typically excellent.

The orthoscopic is probably my favorite design and was once considered "premium" though its widespread use today is minimal. These are four element designs costing a little more than Kellners but their sharpness and contrast is very difficult to match making them excellent choices for planetary and double-starviewing. Their narrow field and relatively short eye relief is what turns off 99% of people who look through them.

The Plossel is by far the most popular design available. You could easily survive with just these. Being a simple four-element design, good Plossels can be found at great prices, but great ones are a bit more expensive. Planetary views are sharp and contrasty as are its deep-sky views. They have a good apparent field, typically around 50 degrees, and nice eye relief one shorter focal lengths.

Wide-field eyepieces are considered the best, and are easily the most expensive. They can have anywhere from 5 to 8 elements producing the widest, flattest field possible. This complexity raises their price greatly. They range 2 to 5 times the price per millimeter of focal length than Plossels. Good wide-field eyepieces produce bright contrasty images and are well corrected for coma, edge-sharpness, astigmatism, and field curvature. Longer focal length models must be made with 2" barrels requiring 2" focusers. Wide field eyepieces are for just that, wide fields. Planets suffer because of the number of elements which may block or scatter a small fraction of light, reducing contrast and sharpness. Most were created after the "Dobsonian Revolution" and are made to handle the steep light cones of fast Newtonians.


Plossels are a must in my opinion. A good set is all you need to view everything from the Orion to Saturn's rings. As for brand, it's completely up to you. I'm not a TeleVue snob. I know there are well made, inexpensive eyepieces available. Most beginners won't even be able to tell the difference. Don't go too cheap though, you may regret it. Each eyepiece type, like each telescope type, has its own uses. Some eyepieces perform their best in large, fast scopes. Most companies try to make all-around designs, but they cost money, and usually don't succeed. Do you where glasses like me? You may have trouble looking through high-power eyepieces with small eye relief. Sure you can take your glasses off, but that can get bothersome, requiring refocusing. TeleVue and Vixen make models of very long eye relief for those of us, but they are a little expensive. My ideal eyepiece collection would contain a complete set of Plossels, a few Orthos, a Radian or two, and four or five wide field eyepieces. If you get re ally serious with astronomy, plan to spend about as much on eyepieces as you did for your scope. Some people spend thousands. If you want to know my personal favorite it's the Zeiss/Doctor 12.5mm. If I could get my hands on one of these somewhat rare but wonderful pieces of glass I could die a happy man.


Finding objects without some kind of aid will drive anyone crazy. Let's discuss some of the choices out there.


The typical finder is a small refracting telescope attached to the optical tube and aligned to point exactly where the main scope is pointed. This scope magnifies the image in both scale and brightness making it easier to find faint object far from bright stars. One problem for beginners is that the image is reversed making left right, and up down, different from the view on star charts. This isn't very hard to adapt to with a little time, but I still scratch my head. Corrected finderscopes are available, for a price.

One-Power Finders

The one-power finder solves the problem of image reversal, but they can make it more difficult to find objects without bright stars nearby. The first and most popular one-power finder is the Telrad. This works exactly like the heads-up-display in military aircraft. A small bullseye is reflected off a piece of glass in the path presenting a transparent target. This is an ingenious design copied on some way by other companies. They are very popular and are use by a vast majority of amateurs.

I use both types. I have a 6x30 finder scope with a Rigel finder on the other side. Which one I use depends on the location and type of object. Bright stars and planets along with brighter deep-sky objects are found with the Rigel finder. Everything else is bagged with the finderscope. Try both on your own or at a star party and decide for yourself which one is easier.


Binoculars are an indespensible tool of nearly all amateur astonomers. Most telescopes don't provide a nice low-power wide field view good enough for exploring the large Milky Way star clouds or even M31, the Andromeda Galaxy. A good pair of binoculars can be used instead of a telescope. If you cannot afford a good quality telescope, I urge you to get a pair of binocs. Ah hell, get a pair of binoculars anyways, it should be the law. It's one of the greatest ways to learn the night sky up close while still exploring brighter nebulae and star clusters. Binoculars are also a great supplement to a telescope. When observing at Mount Pinos, I almost always have mine hanging around my neck. Since the limiting magnitude of both my binocs and my main star chart is around 9-10, I use them to help me aim my telescope.


A Barlow lens contains a negative lens element which, when placed in front of the eyepiece in the optical path, increases the effective focal length of the telescope. This in turn increases the magnification when used with any given eyepiece. This is helpful for two main reasons. Eyepieces can be expensive, but a barlow can re relatively cheap, though some high quality ones are just as much as a premium eyepiece. Purchasing a barlow quite literally doubles the number of the eyepiece in your collection. If you have a 25mm and a 10mm, a 2x barlow now gives you the equivalent of 5mm and 12.5mm eyepieces. Shorter focal length eyepieces are notorious for their lack of eye relief. A Barlow can give you 12.5mm magnification with the eye relief of a 25mm. In fact, many manufacturers actually use a small barlow inside their short focal length eyepieces to keep the long eye relief. A decent Barlow will cost somewhere around $35-$50. A high quality Barlow is anywhere from $70-$120. A premium Barlow will set you back a couple hundred.

Light Pollution Filters

Thanks to the wave-nature of light, we can selectively eliminate most of the light pollution that drives us so crazy. These highly specialized filters you screw into the end of eyepieces block out the pesky light emmitted by sodium and mercury vapor street lamps yet allow the light from planetary and emmission nebulae, hydrogen-alpha and beta and oxygen-III, to pass through. This sounds like science fiction but THEY WORK. They don't just have to be used in the city either. They can dramatically increase the contrast at even the darkest sites. With some objects, it may be the only way.

There are two types of light pollution filters, broadband and narrowband. The broad band lets in all of the above mentioned wavelengths, plus a few extra. Narrowband only let one or two through, nothing more. This makes them more expensive than broadband, but more useful on specific objects. An OIII filter is great for planetaries, while the Lumicon H-beta is great for lower energy emmissions like the horsehead. These filters are sometimes called "nebular" filters referring to the fact that they are best used on nebulae. Galaxies emit a broad range of light. An LPR filter will simply dim the light from the stars in the galaxy reducing its brightness. I definitely recommend investing in one of these.

Color Filters

Color filters attach to eyepieces in the same way as LPR filters but they're basically colored optical glass. These are used mainly for planetary viewing to increase contrast in specific structures like the belts on Jupiter, or the Martian polar caps and features. They're pretty inexpensive and if you do a lot of planetary viewing, a must.

Solar Filters

Looking at the sun through an unfiltered scope for even a split second can and will cause permanent eye damage. Solar filters like block out 100% of UV, 100% of infrared, and 99.999% of visual light. They offer excellent views of sunspots and solar granulation, and even a very rare transit of a planet across the sun's surface. They're cost less than $80 for an averaged size scope or you can buy some of the material yourself for around $20-$30 and make the mounting yourself. WARNING: Many cheapo scopes come with a "solar filter" that screws into the eyepiece barrel. NEVER, EVER use these to look at the sun under ANY circumstances. They don't block the dangerous UV and IR light effectively and because they're close to the focus of the telescope objective, they absorb huge amounts of heat and typically melt or shatter in a matter of minutes. If this happens while you're looking through the eyepiece you can get permanent eye damage.

Portable Power

Dark-sky sites typically have no easily accessible electrical outlets, that's what makes them dark of course. Alkaline batteries are somewhat expensive and can easily go dead in cold weather. Unless you have a few miles of extension cord lying around the house, you're going to need some kind of portable battery power to run your scope's drive, your laptop computer, etc. Using your car battery can be dangerous. A car battery isn't made for powering something for long periods. A car battery is made to give a lot of juice in a short period of time to start your car. If you plug into your cigarette lighter and you use up too much battery power, now what are you going to do. Sure some fellow astronomer may give you a jump, if there's cables handy, but what if you're alone? The best choice is to purchase an inexpensive marine deep-cycle battery from an auto parts store or Walmart (they have everything) and plug into it with alligator clips. Another option is in portable emergency battery packs used for jump starting cars or packs offered by companies that sell astronomy products. These are great but are a little expensive. The last option is to make a battery pack yourself like I did (tips and tricks). It may not look as pretty as a commercial pack, but it's far cheaper, more powerful, and much neater than a milk crate from the supermarket.


I don't care what anyone says but you can never have enough red flashlights. It takes between 20 and 30 minutes for your eyes to dark-adapt. One little speck of white light, there goes that 14th mag. galaxy you were trying to find. Regular flashlights using an incandescent bulb work good but require more battery power. Red LED (light emmitting diode) flashlights require 1/4 the juice and are a better shade of red but are also more expensive. Want to save money, make them yourself. It's actually very easy and a third the cost, as long as you know how to use a soldering iron. I explain the process in my tips and tricks page. It's also good to have a few extra in case a fellow astronomer forgot his or your batteries go dead.

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