Telescope Basics

By Chuck Hawks

This article is intended to help the person who needs some basic information about astronomical telescopes, especially someone who is considering acquiring one for visual astronomy. I don't do astro-photography; as a retired photographer, I originally got into telescopes to get away from taking photos! The companion article "Definitions for the Amateur Astronomer" (see the Astronomy and Photography index page) covers many of the terms commonly associated with astronomical telescopes ("focal length," "focal ratio" and so forth), so I will not repeat those definitions here. Refer to that article if you need help understanding the meaning of a word.

Any telescope has a prime focal length, an objective lens or primary mirror (this determines the aperture), ocular (eyepiece), focal ratio (f/stop), exit pupil and so forth. Unlike binoculars and spotting scopes, astronomical telescopes are not designed to give a correctly oriented, right side up view. There is no upside down in space, so there is no need to compromise the optical performance of an astro telescope with extra lenses or prisms to give a correctly oriented view. The design of astronomical telescopes (the basic optical tube) is about raw optical performance.

At various places in this article I have mentioned specific brand names and/or models and in some cases included a price. Please note that I have absolutely no commercial (or any other) ties to any of the companies or telescope manufacturers mentioned in this article. When a product is mentioned by name, it is simply because I have found it to be up to the task. All prices are in 2009 dollars.

Common types of telescopes

Most amateur telescopes fall into one of three general categories: refractor, reflector and catadioptric. Refractors are simply big lenses mounted in a long tube. Reflectors are usually of either the Newtonian or Cassegrain type, while catadioptric scopes normally use either a Schmidt or Maksutov design corrector lens in conjunction with a Cassegrain mirror system.

Different telescope designs perform differently and have different strengths and weaknesses. There is no perfect telescope design. The best way to get a feel for the different types of telescopes is to use them. Attend local star parties or astronomy club events before you buy a scope and look at a variety of objects through different types of telescopes of equal quality that are in the aperture range you are considering. (Note: that "equal quality" part is an important qualifier.) Most experienced telescope owners are happy to show off their prized instruments and answer questions about them.

Understand that, depending on the direction your astronomy hobby takes, your first telescope will probably not be your last. (On the other hand, it is not necessarily wasted money--see the section on "Quick look scopes" later in this article.) For example, seduced by its favorable price/aperture ratio, my first telescope was a 6" Newtonian reflector, which worked great, but required a lot of care and maintenance. I then switched to easier to care for catadioptric scopes and owned a series of Schmidt-Cassegrain and Maksutov-Cassegrain instruments, all of which I found very satisfactory. My last two telescopes have been apochromatic refractors, smaller and more expensive per inch of clear aperture than the others, but capable of delivering the most striking, high contrast views of the objects within their light grasp. You could say that my telescope wants and needs have matured, or you could say that I have gotten progressively lazier as time passed. Both may be true.

Refractors are what most people think of when they hear the word "telescope," a long tube with glass lenses in the front end. They are essentially similar to a big camera lens, but use fewer lens elements. Refractors are not easily damaged or knocked out of alignment (collimation) and thus are ideal for transporting to remote dark sky sites. High quality refractors are noted for their high contrast, excellent imaging capability and relative durability. They may produce views of bright, highly detailed objects (the moon and planets, for example) that are noticeably better than than equivalent mirror telescopes.

Properly designed and manufactured refractors, since they lack the secondary obstruction found in all mirror scopes, deliver higher contrast and more detailed views per inch of clear aperture. As a general guide, a good 4" ED or APO refractor performs as well as a 5" clear aperture mirror telescope; a 5" refractor performs as well as a 6" reflector and a 6" refractor performs almost as well as an 8" mirror telescope (probably about like a 7.5" mirror scope). In other words, in terms of its visual performance, a refractor performs essentially like a mirror telescope (Cassegrain, Newtonian or catadioptric) one size larger.

SV105-3 Telescope
Deluxe carbon fiber SVR105-3 Raptor refractor. Illustration courtesy of Stellarvue.

Large refractors are noted for their physical length (the light path is straight through, not folded upon itself) and high cost. It simply costs a lot more to grind and polish two sides of a lens than one side of a mirror and optical glass is more expensive than Pyrex mirror blanks. Therefore, refractors are relatively expensive for their light grasp.

The primary optical drawbacks to refractors are refraction dispersion and lateral color error (chromatic aberration). Not all of the light that enters a lens makes it out the other side; some bounces off or is otherwise dispersed. Since the three primary colors of light (red, green and blue) are slightly different wavelengths, no single lens--or pair of lenses--can focus all three colors to the same point. Most reasonably priced refractors use a two lens (achromatic) objective, which is able to focus red and blue light to the same point, but not green light, creating a visible, although usually minor, fringe of color around bright objects in the night sky. Longer focal length ameliorates this problem and shorter focal length accentuates it, so achromats tend to be long telescopes. A long focal length achromat can be an excellent planetary scope and they are often favored by those interested in detailed planetary studies.

An achromatic refractor can be a great first telescope, if you spend enough money to get a good one. In the 1990's, Celestron offered their FirstScope 80, an 80mm refractor (made by Vixen Optical) on their Heavy-Duty Alt-azimuth Tripod. It seemed like an ideal first telescope to me. Sadly, it is no longer in the Celestron catalog. However, the Vixen A80MF, an 80mm, f/11.4 refractor on their Porta Mount is very similar and can be had online for about $500.

A two-element objective can deliver near perfect focus of the primary colors of light if the second element is made from expensive extra-low dispersion (ED) glass or fluorite. These ED glass refractors cost about twice the price of a standard achromat. For instance, the Celestron Omni XLT 102 (achromatic) refractor carries a MSRP of about $544, while the MSRP of the similar Omni XLT 102ED is about $1000. However, the latter is a refined instrument and may prove to be all the telescope you ever need.

Apochromatic (APO) refractors solve the chromatic aberration problem by using a three-element objective (usually incorporating an extra-low dispersion glass element in the middle) and/or correcting lenses at the back of the main tube to bring all three primary colors of light to the same point of focus. True APO refractors deliver the most breathtaking views of any type of astronomical telescope, but they are also the most expensive, typically costing two to five times more than a two element ED refractor of the same size and many times more than an achromatic refractor or reflector of similar overall build quality. Of course, because they are inherently expensive telescopes, APO refractors tend to be very well made, often by small companies that specialize in the type and take great pride in their workmanship. Look at the APO refractors produced by small outfits such as Tele Vue, Stellarvue, TEC and Astro-Physics to see what I mean.

An apochromatic refractor provides the best quality view per inch of clear aperture, regardless of the subject (deep sky or planetary), but you have to pay the price. Their size and expense explains why refractors of any sort are seldom seen with objectives larger than 6", so serious deep sky observers favor other designs for their greater light grasp.

Reflectors use two mirrors instead of lenses. There is a (big) concave primary mirror that focuses the light and a small secondary mirror that redirects the light to the eyepiece. The two most common reflector designs are the Newtonian and the Cassegrain, both named for their inventors. The Newtonian is a relatively long telescope with the primary mirror at the bottom of the scope and the 45-degree angled secondary mirror and the eyepiece near the top of the tube. The Cassegrain is a short tube reflector design that folds the light on itself by placing the secondary mirror directly in front of the primary and reflecting the light from the primary through a hole in the center of the big mirror to an eyepiece at the back of the telescope.

Both designs work well and, for their light grasp, reflectors are relatively inexpensive to manufacture. That is why most telescopes with a clear aperture larger than 6" use some sort of mirror system and commercial reflectors are offered with clear apertures of 30" and more. ("Clear aperture" is the diameter of the front objective lens or primary mirror less any secondary obstruction.)

Celestron Omni XLT150
Celestron Omni XLT150, a 6" Newtonian. Illustration courtesy of Celestron International.

Since the light does not pass through the mirror, as it would a lens, there is no refraction dispersion and no lateral color error. This eliminates the biggest bane of refracting telescopes. On the other hand, the secondary mirror is necessarily an obstruction in front of the primary mirror and this reduces the contrast of the final image. It also reduces the actual light grasp of the telescope; the area of the secondary mirror must be subtracted from the area of the primary mirror when calculating the telescope's clear aperture and light grasp.

Another drawback is the aberration known as coma, which is the tendency for stars (or any point source of light) at the edge of the field of view to become little commas instead of pinpoints. Coma is as endemic to reflectors as color fringing is to refractors. Newtonian reflectors (the most common kind) use parabolic primary mirrors to minimize coma.

Reflectors are much more likely to go out of collimation than refractors or CAT's. Their secondary mirrors, usually suspended in front of the primary mirror by three thin rods called a "spider," are very subject to the shock and vibration that naturally occurs during transportation, not to mention simple temperature changes. Reflectors are constantly going out of adjustment and typically need to be collimated before every observing session, a real hassle.

Perhaps their most important drawback is that pure reflectors use open tubes (or, even worse, a truss system instead of a tube) and are thus very susceptible to the intrusion of dust, dirt and moisture. Telescope mirrors are front silvered and cannot be cleaned like a lens. If you try to do so, you will wipe off or severely scratch the silvery mirror coating, ruining the mirror. Usually the best policy is not to attempt to clean a telescope mirror; the reflective mirror surface should never be touched. Fortunately, reflectors work surprisingly well with dirty mirrors.

This susceptibility to dust and other crud, plus the difficulty of cleaning, is a real problem and the reason I long ago gave up on owning pure reflectors. I just could not stand watching the mirrors slowly accumulate dust.

Newtonian reflectors do, however, provide the most bang for the buck (in terms of light grasp) of the common astro telescope designs. This is the basis of their popularity with deep sky observers and that is where they excel. Because their design is simple, Newtonians are also favored by those willing to grind their own mirrors and build their own telescopes.

The entry-level astronomer who wants substantial light grasp can do well with a 5"-6" Newtonian. The Vixen R130Sf is a 5" (130mm) Newtonian on their Porta II alt-azimuth mount and can be had online for about $500. The Celestron Omni XLT 150 is a 6" Newtonian on Celestron's excellent CG-4 equatorial mount; it retails online for about $440 and that is a heck of a buy.

Catadioptric (CAT) telescopes combine a lens (called a "corrector" and usually placed at the front of the tube) with two mirrors. The most popular types are the Schmidt-Cassegrain (SCT) and the Maksutov-Cassegrain (Mak).

Celestron CPC800
8" Celestron CPC800 SCT. Illustration courtesy of Celestron International.

The Schmidt corrector lens is thin and lightweight, but it is an aspheric lens (not ground as a segment of a circle). This is effective, but expensive and difficult to manufacture. The secondary mirror is housed in an adjustable cell mounted in the center of the Schmidt corrector, eliminating the flimsy spider that is used in conventional Cassegrain telescopes.

The Maksutov corrector is a much thicker and consequently heavier lens. The front and back lens surfaces are both ground as segments of a circle, but with different amounts of curve (radiuses). The front surface of a Mak corrector is concave, while the rear surface is convex. The secondary mirror is a silvered spot on the back of the Mak corrector, which is inherently the proper curve, making this possible. No adjustment of the secondary mirror is possible or necessary, adding to the scopes durability. Maksutov correctors work great, but due to their thickness they add substantially to the weight of the optical tube. For that reason, Mak telescopes are seldom seen with diameters in excess of 7".

Either design allows the use of a spherical primary mirror, instead of a more complex parabolic primary mirror, since the purpose of the corrector is to correct for coma. CAT's are short in overall length for any given focal length, which makes them handy to transport. They are also less subject to being knocked out of collimation than pure reflectors. The corrector lens not only holds the secondary mirror in place, it seals the optical tube, making CAT's much less subject to the intrusion of dust and foreign matter than open tube reflectors. As with any mirror telescope, the secondary obstruction (usually around 10-15% by area or 25-35% by diameter) must be subtracted from the area of the primary mirror when calculating clear aperture and light grasp.

Naturally, CAT's are more expensive than pure reflectors, but not as expensive as refractors of the same quality and clear aperture. CAT's are commonly offered in sizes ranging from about 3.5" (90mm) to 16" in clear aperture. Most CAT's have enough light grasp for viewing deep sky objects and sufficient focal length for detailed planetary views, making them an excellent general purpose telescope design. They are more easily transported than other designs of equal aperture and their combination of light grasp and short overall length provides the best views for any given level of portability.

For a first telescope, the Orion VersaGo 102 is a 4" Mak on a simple alt-azimuth mount with Teflon bearings that retails for about $550. A serious entry-level astronomer interested in a Schmidt-Cassegrain scope should checkout the Celestron Omni XLT 127. This is a 5" SCT on Celestron's CG-4 equatorial mount and it can be purchased online for about $600. It could be a lifetime investment.

Clear aperture (light grasp)

Clear aperture is the diameter of the objective lens in a refractor or, in the case of a reflector, the diameter of the primary mirror minus the secondary mirror obstruction. Clear aperture determines a telescope's light grasp, or how much light it collects.

Astronomical telescopes are categorized primarily by the diameter of their clear aperture, expressed in millimeters or inches. When astronomers talk about a 4" scope, they mean a telescope with a 4" clear aperture. Factors such as optical design, focal length and physical size are important, but it is clear aperture that defines the "size" of a telescope.

Entry level refractors usually come with objectives between 60mm (2.4") and 80mm (3.2") and the smallest reflectors usually have primary mirrors no smaller than 80mm in diameter. Objectives smaller than 60mm don't have the light grasp necessary for seeing even relatively easy deep sky objects. The theoretical limiting stellar magnitude of an optically perfect 60mm scope is 11.4 and the theoretical resolution (Dawes limit) is 1.93 arcsec. Needless to say, few inexpensive telescopes are anywhere near optically perfect and seeing conditions are seldom optimum, so expect actual performance in the field to be considerably less than those figures indicate, maybe as little as half. Most useful refractors have a clear aperture of 75mm (3") or more and serious reflectors or CAT's start at about 90mm (3.5"). I would not recommend telescopes with less light grasp, even for beginners or children.

Telescope light grasp is often compared to the light grasp (clear aperture) of the human eye, which is about 7mm when the eye is fully dark-adapted with the pupil dilated. For example, a scope with a 60mm clear aperture has a light grasp of about 73x; that is, it gathers about 73 times as much light as a dark adapted human eye. Here is the light grasp of some other common clear apertures: 3.2" = 131x, 4" = 212x, 4.5" = 265x, 5" = 319x, 6" = 459x, 8" = 843x.

In the most general terms and without reference to design, astronomical telescopes with clear apertures of about 4" or less are considered "small." Medium size scopes might run from about 5" to 9". Large scopes would be anything that gets into double-digit inches of clear aperture. Of course, a 4.5" (115mm) APO refractor is a good-sized scope of its type, while a 4.5" Newtonian reflector is considered small. (Remember, I said "general terms"!)

The generalization is that clear aperture, or light grasp, is what counts most in determining how much you can see in the heavens. ("Aperture is everything" or "Aperture rules.") However, this is only true if all other things, especially the design and quality of the optics, are equal. When they are equal, a good big scope beats a good little scope every time.

Optical quality

It is optical performance, above all else, that is essential to any worthwhile telescope, regardless of its design. An astronomical telescope needs very high resolution. Remember that we are often observing objects that are literally invisible to the unaided human eye, so second-rate optics is not going to do the job.

As we have seen in the section above, the common perception is that clear aperture, or light grasp, is what counts most in determining what you can see in the heavens. However, this is only true if all other things, especially the design and quality of the optics, are equal. If the optical quality is not the same and the smaller scope has less optical aberration plus better resolution and contrast, the smaller scope may well reveal more detail than the bigger scope. This is particularly true when viewing objects that have fine detail or require high resolution, such as the moon and planets, globular star clusters, splitting double stars and the like. In addition, the worse the light pollution at the viewing site, the more important optical quality becomes, while the importance of raw aperture decreases.

Clear aperture is very important, but quality is even more so. I have, with my own eyes, seen a sharp 90mm scope reveal more lunar detail than a 16" reflector. Obviously, that was not a good quality reflector!

That is why even a first telescope should be a brand name scope from a recognized astronomical telescope manufacturer/distributor, NOT a sporting optics brand like Bushnell or Tasco. While any small telescope is limited in the number of deep sky objects that it can reveal, at least entry-level telescopes from sources such as Celestron, Orion and Vixen (to name three possibilities) can usually do what the popular guides to the night sky suggest a small telescope should be able to do. (See the article "Rocky's Telescope List" for specific telescope recommendations; it can be found on the Astronomy and Photography index page.)

Department store telescopes

Practically all experienced amateur astronomers advise against purchasing department store telescopes. Fuzzy and poorly collimated optics, imprecise mounts that lack slow motion controls, shaky tripods, stiff or gritty focusers and inferior materials (cardboard and plastic do not belong in telescopes) are the order of the day. They are often of such inferior optical and mechanical quality as to be functionally useless. A telescope that is hard to use is extremely frustrating and, ultimately, does not get used. Thousands of these cheap department store telescopes are gathering dust in closets and garages and thousands of children, naturally curious about astronomy, have been "turned-off" because of a well-intentioned telescope gift that could not be made to work.

Many astronomical guides to the night sky will advise that a given object is either visible to the naked eye, suitable for binoculars, requires a small telescope (less than about 4" clear aperture), or requires a large telescope to be observed. Unfortunately, most of the telescopes sold in department stores are simply not capable of performing up to "small telescope" expectations. Brands such as Tasco, Jason, Bushnell and department store house brands are best avoided, as they are rarely satisfactory.

Another clue to inferiority is a telescope marketed by its theoretical maximum magnification ("350x" or some such). That sort of promotion is intended to snare the ignorant. Astronomical telescopes are not rated by magnification, since that changes with the eyepiece used. They are rated by their clear aperture. A typical entry-level refracting telescope is not, for example, "375 power." (You can pick any number you find impressive, that is what the manufacturer does!). It has a specific clear aperture (for instance, 60mm) and a specific focal length (say, 700mm). Do not buy any telescope that is not marketed by its clear aperture and focal length.

Further, any astronomical telescope today should be supplied with, or designed to accept, 1.25" eyepieces. Contemporary telescopes supplied with 0.96" eyepieces should be avoided, for they are almost certainly not serious instruments. If eyepieces are supplied with the telescope, they should be of good quality. Today, that usually means four-element Plossl or orthoscopic eyepieces. A telescope that is supplied with inferior two or three element oculars is probably an inferior telescope; don't buy it.

A decent first telescope (new) with a mount/tripod should set you back about $500. If a telescope is markedly lower in price, it probably is not of adequate quality.

Seeing conditions

Light pollution and turbulent air currents severely degrade what you can see through any telescope, so the conditions under which you anticipate doing most of your observing play a role in selecting an appropriate scope. Telescopes with large clear apertures designed for viewing faint, deep sky objects are rendered useless if the sky glow in an urban area is brighter than the objects in question, and it usually is. Since only the moon, brightest stars and brightest planets (Mercury, Venus, Mars, Jupiter and Saturn) can be seen in such conditions and they can be viewed through a small aperture telescope about as well as through a big one, it makes no sense to set-up a big scope, or even own one. A relatively small aperture (80-100mm), long focal length telescope, such as an achromatic refractor, would be appropriate.

The darker the sky and the better the seeing conditions, the more useful a telescope with a large clear aperture becomes. If you live in the mountains, well away from any towns, you would be justified in buying the most light grasp you can afford. Dedicated amateur astronomers who live in such places sometimes buy very large 14"-16" telescopes and mount them under domes on permanent piers, creating their personal observatory.

If you normally travel to such places to do your observing, you logically have more latitude in telescope selection than the person confined to viewing from an urban backyard. A 4"-6" aperture refractor, 5"-9" CAT and 6" or larger Newtonian reflector would all be appropriate choices for transporting to dark sky observing sites.

Mounts, tripods and piers

An astro telescope mount can be either the alt-azimuth or the equatorial type and is differentiated from the tripod or pier that holds it up, in much the same way that a professional photographic tripod is purchased separately from the tripod head. However, the mount and supporting tripod must work as a unit and can be considered a "mounting system." They must not only hold the telescope securely and allow it to traverse smoothly across the sky, they must be rigid and massive to resist the inevitable movement, jiggle and quiver that make observing difficult or impossible. Vibrations must dampen-out quickly. If they don't, even focusing a telescope operating at high magnification is difficult. The finest telescope in the world is nearly useless on an inadequate mount.

A good rule of thumb is that if a mount/tripod looks adequate for a given scope, it is too small. The mounting system should physically overwhelm the telescope to be effective. For this reason, the mount should cost about as much as the telescope. If you pay $500 for an optical tube, you should also be paying about $500 for the mounting system. Stated another way, in a $1000 telescope package (telescope plus mounting system), about half of the total price should be in the mounting system. The lesson to be learned is that the mounting system is as important as the telescope itself.

Alt-azimuth (AZ) mounts track like a video camera tripod head; that is, they traverse from side to size (azimuth) and up and down (altitude). They are easy to understand and easy to aim at a target, whether terrestrial or astronomical. The better versions usually have slow motion controls, which are geared or worm drive controls that make small aiming adjustments easy. The observer who is willing to spend a little time learning his or her way around the night sky and has leaned to "star hop" to locate an object can do very well indeed with a good alt-az mount.

Celestron Heavy Duty Alt-Azimuth Tripod
Celestron Heavy Duty Alt-Azimuth Tripod. Illustration courtesy of Celestron International.

An example of an excellent alt-azimuth mount is the Stellarvue MG, designed for refractors with up to 4.5" objectives. The MG mount typically uses a heavy-duty aluminum or (optional) wooden surveyor's tripod as its supporting system. Even larger and more capable is Stellarvue's M7 heavy duty AZ mount, which is intended for use with Stellarvue's TLS-6 Pier Tripod and will support 5" (or larger) refractors weighing up to 35 pounds.

A variation on the typical AZ mount is the Dobsonian (Dob). This is, in essence, a turntable that sits on the ground onto which is mounted a stubby fork that supports the telescope. Dobsonian mounts are usually built of wood. They are used primarily with large aperture, short focal length Newtonian scopes and lack both a motor drive and slow motion controls. The scope is aimed by simply horsing it around to point in the desired direction. The Dob is a pretty crude mount, but it has been refined to the point where it can work satisfactorily if used in conjunction with a short focal length scope having a very wide field of view.

The drawback to any alt-azimuth mount is that, because of the earth's rotation and 20-degree tilt on its axis, everything moves across the night sky in an arc. Thus, for an alt-az mount to stay aligned on any astronomical object it must continually be moved in two directions. This can make it difficult for more than one person to use the scope, as the subject has often disappeared from the field of view (in an unknown direction) by the time the next person gets his or her eye to the ocular.

Equatorial (EQ) mounts solve these problems. Equatorial mounts move in declination (approximately up and down) and right ascension (tracing an arc across the sky).They are designed to track through an arc that, if the mount is properly aligned and adjusted, matches the apparent movement of astronomical objects in the night sky. Once aimed at an object, an equatorial mount need be adjusted only in right ascension. If the mount is attached to a motor drive, it will automatically keep the object in the field of view, since such motors are designed to turn the mount at the same rate the earth turns, canceling out the apparent motion. A motorized EQ mount is really the way to go if two or more people intend to use the same telescope.

One of the two common types of equatorial mounts is the wedge and swing through fork. It is generally associated with CAT's, because this type of mount works best with a short tube telescope. The base of the massive fork that holds the telescope usually contains a motor drive. Some motorized fork mounts include slow motion controls and some do not. Those that do are preferred, although their slo-mo controls are seldom as smooth and precise as a good AZ or German EQ mount.

Celestron CG-5 EQ mount
Celestron CG-5 motorized German EQ mount, incorporating NexStar go-to technology,
on stainless steel tripod. Illustration courtesy of Celestron International.

Longer scopes, such as refractors and Newtonian reflectors, are usually found on German equatorial mounts. These counter-weighted mounts are heavy and look rather ungainly, but they work exceedingly well once their operation is understood. German equatorial mounts can be motorized and usually incorporate excellent slow motion controls. The Vixen GP2 or GPD2, Celestron CG-5 or CGEM and Losmandy GM 8 or G-11 seem to be among the most popular German equatorial mounts and they are adapted to many good astronomical telescopes of various brands.

Supporting the weight of the telescope and mount is usually a tripod or pier. A pier is basically a column with a telescope mount at the top. A permanent pier is typically bolted to a cement foundation in the ground. Portable piers are usually held upright by three horizontally spread legs and this is probably the most stable type of field mount.

Stellarvue M7 mount with pier/tripod.
Stellarvue M7 mount with TSL6 pier/tripod. Illustration courtesy of Stellarvue.

Unfortunately, a pier is heavier and more hassle to transport than a heavy-duty tripod of similar height, so tripods are far more common and work well if they are sufficiently large and robust. Celestron's manual CG-4 German equatorial mount comes with a stainless steel tripod having extendable, two section legs that are 1.75" in diameter. The computerized and slightly heavier CG-5 German equatorial mount is supplied with a similar steel tripod boasting 2" diameter legs. These are good mounting systems.

Computerized "go-to" motorized mounts can be either the alt-azimuth or the equatorial type, but include navigation software and a database of thousands of astronomical objects. Celestron describes their NexStar computer control technology as a, "40,000 object database with over 100 user-definable objects and expanded information on over 200 objects." Once the mount has been programmed for the viewing location and is properly and precisely aligned (not always as easy as the advertising suggests), you can use the supplied hand controller to enter the code number of the object you want to see from the computer's data base and the mount will automatically point the telescope in the correct direction. Some go-to mounts incorporate a GPS system that enables them to precisely locate their position on the earth, a considerable advantage when beginning the alignment process.

When it is working correctly, a go-to mount allows an inexperienced observer to see many interesting objects without having to go to the trouble of finding them in the night sky. The observer does not even have to know what he or she is looking at, as the system will tell them!

My experience with go-to mounts has not been very positive. Back when I was the Manager of France Photo in Eugene, Oregon, I once watched the Celestron factory representative spend an entire viewing session at a France Photo sponsored "Celestron by Night" star party trying to get an early go-to C8 mount to work, but he never did. Much later, I purchased a computerized Meade ETX-90 telescope, but I was never able to successfully align the mount. After wasting a couple of very frustrating viewing sessions trying to get the Meade AutoStar system to work, I gave up in disgust and sold the little Meade to my friend Jim for a very low price. He wasted a subsequent viewing session without successfully aligning the ETX-90 AutoStar system, either, and from then on the little Meade has been stored in his garage, collecting dust. (Once again confirming the adage that a telescope that is hard to use doesn't get used.)

Part of the fun of amateur astronomy is learning the night sky and finding "new" objects. When they work correctly, the computerized go-to mount systems rob the amateur astronomer of this enjoyment. When they don't work correctly, they are immensely frustrating. They also drain alkaline batteries at a prodigious rate and if their batteries fail during a viewing session you have to re-align the system from scratch after you replace the batteries. If you opt for a go-to mount, I strongly recommend a rechargeable Celestron Power Tank.

I would pay not to have a go-to telescope mount, but many users could not get by without them and they are very popular with entry-level telescope buyers. Celestron, for example, no longer sells a separate mount without an integrated go-to system. If you buy a go-to mount, get the very latest version, as they are constantly being improved and today's "hot set-up" will be obsolete tomorrow. (Sort of like buying a home computer.) As I write these words in October of 2009, the Celestron NexStar 4 SE (4" Mak for $500) and NexStar 5 SE (5" SCT for $700) look like the go-to scope/mount combinations of choice for the serious beginner.


We tend to live in cities and towns and the night sky is so polluted by ambient light from buildings, streetlights, etc. that dim astronomical objects are lost in the haze of light. Only the moon, the brightest stars and the brightest planets can be seen from most urban or suburban backyards. Therefore, most amateur astronomers must transport their telescopes to a dark sky site for serious observing and portability becomes a crucial requirement.

Just how much hassle are you willing to put up with to move and set-up your telescope? How big is your vehicle? These are key questions and the answers have a lot to do with determining what size telescope and mount you should purchase. (Remember, a telescope that is hard to use doesn't get used.) I have had at least some experience transporting and setting-up telescopes ranging from 60mm to 14" in clear aperture and I have personally owned a number of scopes ranging from 3.5" to 8" in clear aperture. I drive a passenger car, not a truck or station wagon, and I want my telescope, mounting system and associated accessories to fit in the trunk. I live in a singlewide manufactured home, so storage space is also at a premium.

After accumulating about 20 years of observing experience, I have concluded that my needs are best served by telescopes in the 3.5" to 5" (90mm-130mm) aperture range. These are generally small enough and light enough that I can load, transport and unload them without assistance. In addition, setting-up these smaller scopes is comparatively easy and set-up time is reasonably short. This lets me spend most of my time in the field observing, not assembling a telescope and mounting system.

Admittedly, I am lazy and rather conservative, so while a smaller scope works for me, you may prefer something larger. However, I think that most experienced telescope owners will agree that even SCT's (the most portable of telescopes) bigger than about 9" become a problem to transport and set-up. Actually, the mounting system primarily limits telescope portability. As the size and weight of the telescope it must support increases, so must the size and weight of the mounting system and the mounting system is necessarily bigger and heavier than the optical tube. In that regard, alt-azimuth mounts are usually more compact and easier to set-up than equatorial mounts. Keep all of this in mind before you order that C-14 dream scope on its computerized/motorized mounting system.

Travel scopes

The ultimate in portability (and in this case "portability" means carrying by hand) is the travel scope. Travel scopes must be airline portable and the cased optical tube assembly should be a carry on item. Not many traveling astronomers are willing to trust their expensive optics to the tender mercies of airline baggage handlers! Alt-azimuth mounts are lighter and simpler than EQ mounts and are the usual choice of travelers.

3.5"-4" CAT's and moderate aperture, short focal length refractors are the typical choices for travel scopes. Questar, Tele Vue and Stellarvue are three sources for top-drawer travel scopes. Many travel scopes can be ordered with fitted, carry-on cases. When I took a cruise to the Philippines to see a total eclipse of the sun, I carried a cased Celestron C-90 Mak spotting scope, which proved ideal.

If you are looking to purchase a truly inexpensive travel scope, check out the Celestron Travel Scope 70 (under $100). At the other end of the price spectrum would be a Tele Vue-85 scope with a Tele-Pod alt-az mount/tripod. That combo will set you back around $2240 at online discount prices. The C-90 Mak mentioned in the paragraph above carries a MSRP of $248, not including a mount/tripod.

Quick look scopes

Similar to the travel scope and the ultimate in convenience is the quick look scope and its mounting system, which can also be easily carried by one person. The difference is that the quick look scope usually stays at home, so it can have a longer optical tube and a taller mount/tripod than the travel scope, which by definition must be cased and airline portable. Often a quick look scope will be stored ready to go, standing in a corner and covered by a towel.

If your first telescope is a small refractor or CAT (say, 90mm or less) on an alt-azimuth mount, at some point you will probably want to try a larger scope and, perhaps, an equatorial mount. If you took my advice and bought a quality scope, your small scope may not be rendered obsolete. It can serve as your quick look scope, something to grab when you glance out the window and the moon looks like it might fall in your lap, Jupiter beckons from on high, or Orion glistens on a clear winter night. You've seen these objects before and it is not worth setting up your big scope for a quick look, so that is where a small, hand portable telescope comes in. A true "grab and go" scope. (Astronomical binoculars are another possibility for this role; I use the Celestron 20x80.)

3.5"-4" CAT's are ideal quick look scopes when removed from their EQ forks and mounted on an AZ tripod. Almost as convenient are short focal length refractors. The Celestron Heavy-Duty Alt-azimuth tripod (under $100 online) works well with small telescopes. I have a friend with a Stellarvue SV90T APO triplet, a very nice scope indeed, who claims that it is the ultimate quick look scope when mounted on a Stellarvue M1 AZ mount. Certainly, it will outperform 99.9% of the world's quick look scopes! Another friend uses a Sightron 60mm spotting scope and a third has an earlier version of the Celestron Power Seeker 60mm AZ refractor. All of these are suitable quick look scopes and if you have a travel scope, it can serve as your quick look scope.

Eyepieces (oculars)

Celestron Telescope Eyepiece - Filter Accessory Kit
Celestron Eyepiece and Filter Kit. Illustration courtesy of Celestron International.

Unlike a spotting scope or a binocular, astronomical telescopes do not have built-in eyepieces (called oculars). The telescope focuses the image to a point in space called the focal point and in order to see the image an interchangeable magnifying eyepiece is used. Oculars have a focal length, just like the telescope itself. It is the focal length of the eyepiece, in conjunction with the focal length of the telescope, that determines the magnification of the total optical system. Calculating magnification is simple. Just divide the focal length of the telescope by the focal length of the eyepiece. For example, a 1000mm telescope and a 10mm ocular yield 100x magnification. The shorter the focal length of the ocular, the more magnification it provides.

Note that the telescope's clear aperture has no direct effect on determining magnification. This is how 60mm department store telescopes with, say, a 700mm focal length get away with advertising "350x magnification." With a 2mm eyepiece, that would be mathematically correct.

Unfortunately, if the telescope lacks adequate clear aperture to illuminate the highly magnified FOV, what you have is useless magnification and a fuzzy, worthless image without detail. A practical rule of thumb is that a good telescope used in excellent seeing conditions can support about 50 power-per-inch of clear aperture. Thus a 60mm (2.4") telescope has a theoretical magnification limit of 120x, not 350x or some other fanciful figure.

Even 50 power/inch is optimistic in the field, where observing conditions are seldom perfect. I find that something around 30-35 power/inch is a more realistic maximum magnification where I usually observe (western Oregon). For example, in my 4"/900mm focal length telescope, a 7mm ocular is the shortest I find useful (32 power/inch).

Telescopes also have a minimum practical magnification, which is based on the size of the exit pupil--the beam of light coming from the ocular to your eye. When the exit pupil exceeds 7mm in diameter, it is passing more light than your eye can accept. (Exit pupil is calculated by dividing the clear aperture in millimeters by the magnification of the system.) This seldom comes into play with astro telescopes, since 1.25" oculars with a focal length exceeding 40mm are rare. In my 4.5"/800mm telescope, a 40mm ocular (the longest focal length ocular I own) has a 5.75mm exit pupil. Using that ocular the telescope is operating at 20x magnification and only 4.4 power/inch.

Eyepieces are commonly available in three mounting barrel diameters (0.96", 1.25" and 2") and a wide range of focal lengths. 1.25" diameter oculars are by far the most common for use in amateur telescopes and that is the size on which I suggest standardizing. Almost all serious astro telescopes accept 1.25" eyepieces. Tele Vue Optics, Inc. offers one of the most extensive (and best) ocular lines. Their 1.25" oculars are available in focal lengths ranging from 2.5mm to 40mm.

There are also zoom (variable focal length) eyepieces. These allow a range of focal length settings, and therefore magnifications, from a single eyepiece. As I write these words, the most common zoom oculars have focal lengths of 7-21mm or 8-24mm; these are both 3:1 zoom ratios. Contrary to what beginning observers often assume, zoom eyepieces are not intended to replace fixed focal length eyepieces. Zooms are handy for determining the maximum magnification practical for viewing a given object in the prevailing seeing conditions. They also allow multiple users of the same telescope to set the field of view they find appropriate, very handy at star parties.

Most zoom oculars are visibly inferior to equivalent fixed focal length oculars, but a few of the latest generation zooms can provide very good views. The Tele Vue 8-24mm Click Stop Zoom is probably the most highly regarded of the zoom oculars. It provides fixed focal length ocular quality views, but like all good zooms, it is rather expensive. (I have reviewed this zoom and that article can be found on the Astronomy and Photography index page.) The Vixen LV 8-24mm zoom is also well regarded and slightly less expensive than the Tele Vue. The Meade Series 4000 8-24mm zoom is cheaper than either and performed well in a recent Astronomy and Photography review.

Meade Series 4000 8-24mm Zoom
Meade Series 4000 8-24mm Zoom ocular. Illustration courtesy of Optics Planet.

A characteristic of all zoom eyepieces is that their apparent field of view narrows as the focal length increases, the exact opposite of what you would expect and what you want. In other words, at low magnification, where you probably need the widest possible field of view, the zoom delivers its smallest field of view. For example, the aforementioned Tele Vue 8-24mm Click Stop Zoom has an apparent field of view of 55 degrees at 8mm, but only 40 degrees at 24mm. The eye relief also varies, but in the case of the Tele Vue zoom it is a reasonable 15-20mm, depending on focal length.

There are many optical designs used for eyepieces, most of which are named for their designer (Kellner, Erfle, Brandon, Plossl, Nagler, etc.). From a practical standpoint, any modern ocular with less than three lens elements is not worth owning. The common three element ocular designs are the Kellner and the modified achromat (MA), sold by Meade. These have a narrow apparent field of view and restricted eye relief, but provide a reasonably sharp center image; their sharpness noticeably decreases at the edges. These are not true apochromatic eyepiece designs, so they will cause some lateral color error (color fringing).

Four element oculars are the standard, usually of Plossl or orthoscopic design. These are capable of very good, apochromatic performance if properly made. Plossl oculars are the most popular type and they generally have about a 50-52 degree apparent field of view, very good sharpness from center to edge and high contrast. Their eye relief, excellent at long focal lengths, is limited at the shortest focal lengths.

Premium oculars with a wider field of view and/or extended eye relief usually have five to seven lens elements and cost more, but do not necessarily provide better contrast, sharper images or a flatter field than a Plossl or ortho. As with telescopes themselves, eyepiece performance is ultimately more dependent on the quality of manufacture than anything else and top quality commands top prices. All Plossl's, for instance, are not created equal. See my articles "Ocular Basics" and "Ocular Recommendations" (both are listed on the Astronomy and Photography index page) for more information about eyepieces.

Finder scopes and other accessories

After you have a scope, mounting system and eyepieces, what else do you need? A good finder scope would be close to the top of my list. A telescope is only as good as its aiming system. Most of the finders supplied with telescopes are the straight through, inverted image type that I find very difficult to use. Far better is a right angle, correct image finder, such as the 9x50 models sold by Orion and StellarVue.

Red dot finders without any magnification are probably the most popular type of finder today and they generally work well. Another option is a green light laser, which can be used to point at objects in the night sky. Jasper Laser and Celestron are sources for green light lasers and they offer mounting brackets that will fit most telescopes.

The best idea is to have a traditional right angle, correct image, magnifying finder scope and either a red-dot finder or green light laser mounted on your telescope. (I have all three on my Celestron 102ED refractor.) The red dot or laser can be used to get you into the right area and the magnifying finder can be used to refine your aim. It is also a big asset for star hopping.

Celestron 2
Celestron 2" XLT diagonal. Illustration courtesy of Celestron International.

A Star diagonal is necessary for comfortable viewing at high angles of elevation with either a refractor or a CAT. (Newtonian reflectors do not need star diagonals.) The diagonal is a prism or mirror in a holder that mounts to the back of a telescope and bends the light path 90-degrees. Mirror diagonals transmit almost 100% of the light that enters them and are therefore more efficient than prism diagonals. However, prism diagonals are self-collimating and more durable than mirror diagonals; they are preferred for long focal length telescopes. When using a 90-degree star diagonal of either type, the image you see is reversed (a mirror image). Like the oculars they are designed to accept, star diagonals come with .96", 1.25" and 2" mounting barrels. Star diagonals are supplied with most commercial telescopes. If your scope did not come with one, you will have to buy it.

Beyond a good finder scope and a star diagonal, the beginning amateur astronomer will need a sky map/planisphere and a red light flashlight. The first is how you learn what to look for and where to look for it in the sky, while the second lets you see what you are doing at night without totally destroying your night vision.

A moon or neutral density filter is a practical necessity, for everyone looks at the moon. The moon is in direct sunlight and it is so bright as to be nearly blinding, even when viewed through small telescopes. This small filter screws into the mounting barrel of your star diagonal or eyepiece. Think of the moon filter as sunglasses for your telescope.

If you wish to view the sun, which is the only star you can see close-up, a solar filter that covers the front of your telescope is absolutely necessary. The power of the sun is not to be trifled with. NEVER use an eyepiece filter to view the sun, the result can be permanent blindness. Be sure to cover the objective of your magnifying finder scope if you point your telescope at the sun, or the sun's heat will probably melt it.

Most other accessories, such as colored eyepiece filters, Barlow lenses, wide-field adapters, motor drives for German EQ mounts, polar alignment finder scopes, SCT counter weight sets and the like are not necessary for the novice astronomer and they are beyond the scope of this article. You will learn over time what additional accessories you actually need or want. There is a whole article about accessories ("Telescope Accessories") on the Astronomy and Photography index page that you may find helpful.

Back to Astronomy and Photography Online.

Copyright 2009, 2016 by Chuck Hawks. All rights reserved.