A Five-Part Telescope Primer
For starters: Let's say you're interested in buying a telescope, or at least flirting with the idea. Your dilemma, however, is that you know a lot less about telescopes than Galileo. Before buying anything -- breathe in, breathe out and ask yourself these five common-sense questions.
The jargon: Buying a telescope may seem scary. Don't think of yourself as being lost in a bramble of jargon. Instead, imagine you're cutting a path through a jungle with a machete (i.e., your knowledge acquired here). In the clearing ahead, lies your quarry -- a really cool telescope.
Shiny mirrors, pretty lenses: To paraphrase George Orwell, all telescopes are equal, but some telescopes are more equal than others. Basically, it all boils down to what you do with glass. The telescope that's right for you, like so many of life's choices, is the compromise you can live with.
The toys: Perhaps you don't need a photometer or spectroscope yet, but honestly -- don't you want a few accessories? Here is an introduction to the must-haves, the sooner-or-laters and the (guilty-pleasure) luxury items.
Ready to buy: For those ready to buy, here is a list of telescope dealers and manufacturers around the United States. Many have active websites.
How to Buy a Telescope, Without Regrets
By Jeff Kanipe
Special to SPACE.com
posted: 04:05 pm ET
19 June 2000
If you've clicked your way to this article, I assume you are either interested in buying a telescope or are at least flirting with the idea.
Your dilemma, however, is you don't know a lot about telescopes in general, particularly what makes one a quality instrument and another not worth hanging your hat on.
Moreover, there appear to be a staggering number of models on the market at wildly different prices. How can you know if the telescope you might buy is worth the money you are forking over?
Before buying anything -- breathe in, breathe out and ask yourself five common-sense questions.
1) Why do you want to buy a telescope?
Do you want to provide a meaningful learning experience for a child, or do you want to test the waters to see if little Jimmy or Becky will follow through with their interest? Or is this the first step toward your own personal pursuit in astronomy? Do you envision sweeping up a panoply of galaxies and star clusters, discovering a comet or looking down into the craggy craters of the moon? Perhaps you just want to have a telescope on hand for the next lunar eclipse or bright comet, or to take with you on camping trips.
In short, if your purpose is event-driven or casual observing, shoot for a simple, lightweight, low-cost telescope with a wide field of view and good optics. Ditto if you're buying a telescope for a child whose interest might be ephemeral. You might even consider binoculars.
If your or the child's interest is unquestionably spirited and steadfast, then you will want a telescope that will nurture rather than limit a developing interest. In that case, lay out a few more bucks for a larger, more well equipped model that has, it should go without saying, good optics. You won't regret it.
2) How much do you want to spend?
The answer to this question usually cuts through the fog of indecision. Good beginners' telescopes run anywhere between $250 and $600. More money buys more bells and whistles, but not necessarily more enjoyment. If you spend a lot of money on a complex telescope you don't use very often, how much fun can that be? You want a telescope that you will use and, perhaps, grow into -- not one that limits you the moment you take it out of the box or that you later rue for having spent too much money.
3) What do you want to do with your telescope?
To be honest, it doesn't matter what you plan to do with your telescope, just as long as you want to look at celestial objects with it. If you simply want a telescope that "looks good," I suggest you purchase one of those antique brass remakes sold in tony furniture shops. Their optical quality is terrible, but they look smashing when posed beside an ornamental birdcage or an early 20th-century French lithograph.
Seriously, whatever you want to observe -- planets, stars or galaxies -- you need to consider the diameter of the telescope's main light-gathering lens or mirror, called the objective. We'll explore this in more detail in the next section, but for now, suffice it to say that a telescope's "power" is squarely vested in its aperture, not its magnification capability. Telescopes are often described as "light buckets." The larger the light bucket's aperture, the more light it gathers; the more light it gathers, the more detail you can see, no matter what you're looking at. Hence, always buy as much aperture as you can afford. Like real estate, the more you have the greater your return.
4) How dark are your skies?
In other words, where will you be observing the most: from the city, suburbs or country? If you are fortunate enough to live away from city lights then put as much money as you can into aperture. Costly accessories -- high-dollar eyepieces, a computerized drive system and an electric focuser -- can wait. Get out there and observe.
If you plan to do most of your observing from the city or suburbs, don't buy a big light bucket because all you'll see with it is more light pollution. Of course, there's nothing to prevent you from driving out to the country to observe, just make sure the telescope you buy fits in your car -- mount and all -- with room for yourself as well.
5) How serious are you about astronomy?
A difficult question to answer but give it a try. Can you recognize some of the major constellations and brighter stars? Do you know where the planets are in tonight's sky? Do you keep up with the latest astronomy discoveries? If a moderately bright comet could be seen in the predawn sky, would you drag yourself out of bed to see it? Do you own more than one astronomy book or software program, or subscribe to a popular astronomy magazine like Sky & Telescope? Is your wall plastered with astronomical art or photos? Do you have two or three or more internet bookmarks that take you to astronomy-related websites like SPACE.com?
I'm no psychologist, but it stands to reason that the more affirmative your answers are to these questions the more active your interest. That doesn't mean a telescope will necessarily enhance your enjoyment. Indeed, many people, myself included, enjoy scanning the night sky with the unaided eye and binoculars. But I suspect if you had the right telescope at your disposal, you'd probably get a lot of pleasure out of it. How much, of course, depends on you and how involved you want to become.
As you probably already know, there are all kinds of telescopes out there. In the next section, I'll cover basic telescope design, describe the various types of instruments available, and lay out their advantages and disadvantages. If you really want to buy a telescope that you won't later regret, press on to a quick lesson on light-gathering.
What's your telescope type?
Casual: buy a low-cost, lightweight scope or binoculars;
Show-off: score an antique store telescope;
Urban: opt for a moderate aperture scope;
Rural: reach for as much aperture as you can afford;
Committed: ditto.
Telescopes: Unpacking the Jargon
By Jeff Kanipe
Special to SPACE.com
posted: 04:05 pm ET
19 June 2000
Buying a telescope doesn't have to be scary. It should be fun, despite all those technical terms.
Don't think of yourself as being lost in a bramble of jargon. Instead, imagine you're cutting a path through a jungle with a machete (your newfound knowledge) where, in the clearing ahead, lies your quarry: a really cool telescope.
Aperture versus magnification
Even in these enlightened times, you can still find ads proclaiming telescopes that can magnify celestial objects "a million times" or some such ridiculous value. Swell. But does that mean it's any good? Nope. In fact, it probably means exactly the opposite.
Let's get this straight from the get-go. A telescope's main function is to gather light. If you don't gather enough light, you won't see anything, no matter how much magnification you throw at it. The aperture, or opening through which skylight passes, is what matters.
Every telescope has either a primary lens or mirror that is used for collecting light. This is called the telescope's "objective" -- and the width of that objective's aperture is key. In the world of telescopes, size -- or at least proportion -- matters, because a telescope's light-gathering power is proportional to the objective's surface area, not its diameter.
So an 8-inch (20-centimeter) telescope has four times the light grasp of 4-inch (10-centimeter) telescope -- not two times. Putting it in practical terms, an 8-inch, given a dark, clear sky and good seeing conditions, should be able to detect stars as faint as magnitude 14 or better.
This is more than 380,000 times fainter than the faintest star seen by the naked eye, which is usually considered to be magnitude 6. A 4-inch telescope should be able to detect stars as faint as 12.5; a 3-inch, about 12.2. Even a small telescope allows the eye to see millions of stars that the naked eye cannot.
Resolution = detail = information
More light gathering area translates into a more detailed image, that is more information. That concept is referred to as "resolution" or resolving power. All things being equal, an 8-inch telescope should have twice the resolving capability of a 4-inch.
Now, bear with me while we let out a little more air from this term. A telescope's resolution is measured by how well it can separate two distant objects that are very close to each other. It's purely a theoretical value and also depends quite a bit on the quality of the optics, but it gives you a "ballpark" feel for how well your telescope should perform optically.
Keeping with our 8-inch and 4-inch comparison, a 4-inch telescope can theoretically resolve two stars that are 1.2 seconds of arc apart. One second of arc is the diameter of a quarter seen a little over 3 miles (4.8 kilometers) away
An 8-inch telescope can resolve an even closer pair -- 0.6 seconds of arc apart. I say "theoretically" because when you take into account Earth's dense, turbulent atmosphere, true resolution is almost always less than this.
About the best you can hope for under ideal seeing conditions is 1 second of arc, but that's pretty good.
Focal length and focal ratio
A telescope's focal length is the distance light travels from a telescope's lens or mirror to the point inside the telescope where it is focused.
Focal lengths for commercial telescopes vary from 15.8 inches to 118 inches (400 millimeters to 3000 millimeters). The longer the focal length, the larger the image at the focal point. Think of it like the distance between a slide projector and the screen. Move the screen and slide projector further apart, and the image gets larger and dimmer.
"Focal ratio" is the ratio of the instrument's focal length to its aperture. It's found by dividing focal length by objective diameter. A telescope with a mirror of 8 inches across and a focal length of 48 inches has a focal ratio of f/6. (Notice that you can also find a telescope's focal length by multiplying focal ratio by aperture.)
As implied in our slide-projector analogy above, though a long focal-length telescope produces a large image at focus, it will also be fainter because the long focal path spreads out the light more.
Long focal lengths are considered to be in the f/9 or greater range. A telescope of a given diameter coupled with a short focal length, say a 3.5-inch (8.9-centimeter) f/5.6 (focal length 19.6 inches, or 49.8 centimeters), produces bright images but wide fields.
This is fine for observing large deep-sky objects and star fields, but if you also want to observe planets and double stars, you're going to want a slightly longer focal length.
Refractors, reflectors and catadioptrics
Although a telescope is designed to gather light, how it accomplishes that task is a key factor in its design. It may deliver light in one of three ways: by bending light through a lens, reflecting it from a mirror or via a combination of both lenses and mirrors.
The lens-type is called a refractor. The mirror type is called a reflector. Telescopes that utilize both mirrors and lenses are called catadioptrics. Whichever type you prefer, they are all simply variations on a theme.
Although the classic design of the refractor has undergone significant changes since Galileo's time (thank goodness), the principle is still the same.
A main lens composed of two or more different pieces of optically figured glass brings light to a focus at the opposite end of the tube. Refractors have the advantage of rendering sharp high-contrast images, large image scales (due to higher focal ratios) and excellent resolution.
Reflectors come in various designs, but we'll stick with the simplest, which is the Newtonian. Since its invention by Sir Isaac in 1668, the reflector has been very popular with amateur astronomers. It consists of a concave mirror positioned at the bottom of the tube that reflects and focuses starlight to a point just inside the tube's entrance. A flat secondary mirror positioned there redirects the light out the side of the tube and into a lens.
Newtonian reflectors provide accurate color rendition of celestial objects and are less expensive by the inch than refractors. You could purchase an 8-inch reflector for the cost of a modest 4-inch refractor.
Catadioptric telescopes employ the features of both refractors and reflectors. One of the most popular models today is the Schmidt-Cassegrain telescope, or SCT.
The SCT employs a spherical primary mirror at one end of the tube and a correcting lens at the other. The secondary mirror is mounted directly on to the correcting lens (or plate). This, in turn, redirects the light back down the tube and through a hole in the center of the main mirror, where the eyepiece is placed.
"Folding" the light path allows a manufacturer to produce a telescope with a focal length that is twice the length of the tube. Thus, SCTs are lightweight and portable, and produce excellent images.
In a world of options, you, unfortunately, only have one choice. Which will it be? Refractor, reflector or SCT? Move on to the next section for the gory details.
Astronomy Argot -- A Cheat Sheet
Objective: a telescope's main light-gathering lens or mirror;
Apparent magnitude: a logarithmic measurement of the visual brightness of starsand other celestial objects. The brighter the object, the smaller the value. The brightest stars are of magnitude 1 or less (having 0 or negative values). The faintest star that can be seen by the naked eye is about magnitude 6.
Limiting magnitude: the faintest visual magnitude that may be discerned in a telescope or by the naked eye.
Aperture: the diameter of a telescope's lens or mirror.
Resolution: a telescope's ability to reveal fine detail, or a measure (in arcseconds) of that detail;
Arcsecond: a fractional angular measurement equivalent to 1/60th that of an arcminute or 1/3600 that of a degree. The full moon is 1800 arcseconds (or about 30arcminutes) in apparent diameter.
Focal length: the distance light travels from a telescope's lens or mirror to the point inside the scope where it is focused;
Focal ratio: the ratio of focal length to aperture;
Refractor: a telescope that gathers and focuses light using a lens;
Reflector: a telescope that gathers and focuses light using a mirror;
Catadioptrics: telescopes that rely on both refraction and reflection to gatherand focus light.
Mirrors and Lenses: The Fine Print Behind Telescopes
By Jeff Kanipe
Special to SPACE.com
posted: 04:40 pm ET
19 June 2000
To paraphrase George Orwell, all telescopes are equal, but some telescopes are more equal than others.
Your final decision, like so many of life's choices, is the compromise you can live with.
What you don't want to compromise on, however, is the enthusiasm that drove you to buy a telescope in the first place.
Choose wisely, and you will spend many hours of enjoyment with your new instrument. Choose poorly and, at best, you will end up with a unique and expensive hat rack.
Refractors: it's about glass -- and cash
Refractors have two main disadvantages. The inexpensive ones of the 2.4-inch (60-millimeter) caliber, have less-than-acceptable optics and low light-grasp, while the expensive 3.5-inch (90-millimeter) and greater instruments have near-perfect optics and better light grasp.
That's the sad truth. Money buys you quality in almost all aspects of optics, but particularly in the refractor domain. This situation arose largely because of manufacturers' efforts to eliminate another refractor disadvantage known as chromatic aberration.
The first refractor telescopes consisted of a single objective lens at the front of the tube. A single lens focuses different colors of light at slightly different points, so a star's image sports a fuzzy color fringe around it. Around the middle of the 18th century, telescope makers found they could relieve chromatic aberration by using two lens elements fashioned from different kinds of glass. They called this an achromatic lens.
Today, some telescope makers design three- or four-element lenses to bring the different wavelengths of light to concurrent focus. These are called apochromat refractors. Another option is to make one of the two-element lenses either from a material called calcium fluoride or fluoro-phosphate crystal. These latter telescopes are called, respectively, fluorite or extra-low dispersion (ED) apochromats.
As you might expect, the more elements or specialty glass you use -- not to mention the larger the aperture -- the more the cost. For a 3- to 3.5-inch (80- to 90-millimeter) telescope, the price is not too bad. But employ more glass elements or specialty materials to fashion the lenses and you have a telescope with near-perfect optics -- and worth a small fortune.
Consider that if two elements are used, four lens surfaces must be accurately figured. If you use three elements, that jumps to six, and so on. As for calcium fluoride and fluoro-phosphate, they are more difficult to work with than glass and consequently are more expensive. They can raise the cost of a telescope by as much as $1,000.
Still, you have to balance this with the fact that refractors deliver crisp, high-contrast images at high magnifications, which make them perfect for planetary, lunar and double-star observing. Moreover, today's achromatic refractors render almost color-free images at nearly half the price of the apochromats, fluorite and ED apochromats. Unfortunately, you won't find an affordable refractor in objective diameters over 4 inches (101 millimeters). For a typical 5-inch (127-millimeter) refractor with mount and basic accessories, expect to pay between $3,000 and $4,000.
Reflectors: a good deal for the money
So maybe the spouse won't let you spend that kind of money, or perhaps little Jimmy doesn't really need a 5-inch refractor to play with. What are your other options?
Despite many advances in telescopes, particularly accessories, which we'll get into in the next section, I think the best all-around telescope for beginners is the Newtonian reflector.
Reflectors are the least costly per inch of aperture to manufacture (only one optical surface to figure), they are adaptable to various designs and uses and, because light does not have to pass through glass, they provide color-free images.
Alas, the reflector's greatest strength is also its greatest weakness. The primary mirror is ground to a concave figure known as a paraboloid. The outer zones of such a mirror have a slightly longer focal length than the inner zones, particularly for telescopes with short focal lengths (their mirrors must be ground to a deeper concave figure, which only exacerbates the problem).
Hence, star images near the edge of the field of view look as if they have short cometary tails or "wings." This aberration is called coma.
Reputable telescope manufactures do their best to minimize coma, but it's an inherent design flaw that cannot be completely eliminated. Most commercial telescopes 10 inches (25.5 centimeters) and larger are made with short focal ratios -- f/4.5. They gather a lot of light and provide stunning views of deep-sky objects, but you have to accept a little coma at the edge of the field. Still, as long as the image in the center is sharp, who cares about the edge?
The most popular reflector for beginners is either a 6- or 8-inch (15- or 20-centimeter) Newtonian utilizing a Dobsonian mount design. ("Dobsonian" refers to telescope maker John Dobson who, in the 1970s, came up with a simple design whereby the tube moves up and down in altitude and pivots around a central axis in azimuth, much like an artillery mount.) A good 6-inch "Dob" goes anywhere from $250 to $400. Imagine how much a refractor of similar size would cost?
There is one final hitch with owning a reflector. The mirrors, particularly the primary mirror, often come out of alignment, particularly if the telescope is regularly portaged from one place to another. The user must then "collimate" the optical components so they square up. The process is not complicated, but some people prefer not to be bothered. Given the optical advantages, performance and affordability of reflectors, collimation is a minor inconvenience.
Catadioptrics: a compromise
Never mind the name. All you need to know is that the term refers to an optical system involving both the refraction and reflection of light. The most popular member of the catadioptric family is the Schmidt-Cassegrain telescope, or SCT.
SCTs are portable, compact and not as pricey as refractors (though they are still pricier than reflectors). More importantly, the combination of mirror and front corrector lens ensures colorless rendition of objects and sharp focus across the field of view. Because the light path of an SCT is "folded" inside the tube, you end up with a long focal-length telescope that is only twice as long as it is wide. This, in turn, yields large image scales, which are best for viewing the moon, planets and double stars, as well as small, bright deep-sky objects.
Devotees argue that SCTs really shine as astrophotographic telescopes. Indeed, they are easily adaptable for photography and digital imaging. Eyepiece placement at the rear of the telescope allows you to easily attach cameras and accessories, and their fork mounts are especially smooth when automatically tracking celestial objects. In fact, many models are now equipped with computer-driven mounts that do everything except set the telescope up for you. (More on that in the next section.)
But SCTs also have their limitations. For one thing, in sizes over 8-inches, they are expensive -- and adding in the necessary accessories drives up the price even more. If you want to get into serious deep-sky observing, you'll need at least an 11-inch (28 centimeter). Your basic cost will be around $3,500.
By design, SCT telescopes also have large secondary mirrors, which are usually about one-third the aperture's diameter. This results in scattered light and a slight loss of contrast. Their exposed corrector lenses are especially susceptible to dew (unless shielded), and the secondary mirrors often need re-collimating. Finally, at focal lengths of f/10 or greater, even a low-power eyepiece produces a limited field of view. If you want to look at or photograph star fields and large deep-sky objects, you will need to purchase yet another accessory -- the focal reducer. These can convert an f/10 system to f/6.
SCTs offer the advantages of both refractors and reflectors, while creating a new set of disadvantages that you either may or may not be able to live with. For astrophotography and electronic imaging, however, SCTs reign supreme.
Also in the catadioptric class are "hybrids," like Newtonian reflectors fitted with special corrector lenses. These are called Maksutov-Newtonians.
A similar arrangement with Cassegrain telescopes yields a Maksutov-Cassegrain. The type of corrector lens used on these instruments is easier to manufacture than the SCT and yet they provide coma- and color-free images that are sharp across the field of view. Expect to pay somewhere between what an SCT and a large Newtonian costs, but also expect near-perfect optics for your money. (Okay, I admit it. I own a Mak-Newt and love it.)
All telescopes have the same mission -- to gather and magnify light from distant objects, but they do it in different ways. And while no telescope is perfect in design or manufacture, there is a very likely a telescope out there that is perfect for you.
Our next stop is the world of telescope accessories. Let the fun begin!
Telescope Gadgets: Wants vs. Needs
By Jeff Kanipe
Special to SPACE.com
posted: 04:04 pm ET
19 June 2000
If you're just beginning your astronomical ventures, I'm going to assume you don't need a photometer or spectroscope for your telescope yet.
Instead, I'm going to cover three levels of accessories: the "must-haves," or the ones you really can't do without; the "sooner-or-laters," or those that you may one day desire and the "luxuries" which, I think, speaks for itself.
There are five broad categories of accessories: mounts, drive systems, computer-driven mounts, filters and eyepieces.
Altazimuth versus equatorial mounts
A mount really isn't an accessory, per se. It's a necessity unless you enjoy shouldering your telescope like a bazooka.
But right up front, you should be aware that no matter how good your optics are, if the tube shimmies with the slightest breeze or touch, or picks up nearby footfalls, your instrument and your enjoyment will be severely compromised.
So no matter which of the following mounts you decide upon, it should, above all, be sturdy. That's a "must-have."
The decision of whether to go with an altazimuth or equatorial mount really depends on what you want to do with your telescope.
Altazimuth mounts are easy to operate and less costly than equatorial mounts, and also are great for kids.
They allow the telescope to be moved in two intuitive directions: altitude (vertically with respect to the horizon) and in azimuth (360 degrees of horizontal motion). You don't align it with the celestial pole (as you must do with equatorial mounts), nor does it require much, if anything, in the way of balancing. You simply put the telescope on the mount and start observing.
Equatorial mounts are more involved. They too consist of two axes, but each is oriented to the celestial sphere -- the imaginary sphere surrounding the Earth and representing the night sky. One axis, called the polar axis, is locked on to the north (or south) celestial pole and is thus parallel to Earth's axis). The other, called the declination axis, is at right angles to the polar axis and pivots around it. (Quick astronomy lesson: Analogous to terrestrial latitude, declination refers to the position of a celestial body, in degrees, north or south of the celestial equator.)
What about those groovy U-shaped fork mounts? (I'm glad you asked.) The classic fork mount that you see holding those glossy Schmitt-Cassegrain telescopes (the folded optical design mentioned in past story) actually has a dual identity. It qualifies as an altazimuth mount when its axes are oriented toward the horizon (altitude and azimuth motion). But angle one Axis -- the one located in the valley of the U -- toward the celestial pole and voilà, you have an equatorial mount.
The advantage in having an equatorial mount is that you can attach a motor, or clock drive (see below), to the polar axis and automatically track the stars as they drift across the sky. If you want to get into long-exposure astrophotography, an equatorial mount is a "must have."
Both altazimuth and equatorial mounts may also be fitted with "slow-motion" controls. These simply allow the user to move the telescope incrementally in either axis by manually turning a knob or cable. These come in handy when you want to make small adjustments to positioning an object in the field of view.
Drive systems
Telescope manufacturers sell equatorial mounts with and without drive systems, although the more expensive models usually come equipped with one.
Is a clock drive a "must-have?" Only if: a) you find it bothersome to continually nudge the telescope to keep a star or planet from drifting out of the field of view or b) you plan to use your instrument for astrophotography. It's not an absolute necessity otherwise. So I put a clock drive into the "sooner-or-later" bin for those owning equatorial mounts.
In addition to a clock drive, you can also purchase a guiding system. This allows you to make incremental adjustments to the tracking speed in the polar axis (a single-axis guider) or both the polar and declination axes (a dual-axis guider). This may be more of a "later-than-sooner" item, since it mainly benefits astrophotographers.
Computer-driven mounts
I must admit that I am somewhat of a Luddite when it comes to embracing the computer automation of recreational telescopes. Still, there's no denying that a lot of amateur astronomers are excited by this latest techno-innovation and they can't all be wrong -- can they? Nevertheless, I put this into the "luxury" category.
Advertising jargon for computer drives refers to them as "go-to" or "auto-align" systems. Essentially, an on-board computer points your telescope for you. All you have to do is decide which object to look at. And you have plenty of choices too, if you consult widely available databases of coordinates for tens of thousands of galaxies, star clusters, nebulae, bright asteroids and periodic comets, not to forget double, multiple and variable stars.
They'll even find the planets for you, in case you can't recognize them in the sky.
Set up is fairly straightforward. Usually you align the telescope on two stars that the computer recognizes. (One model even finds the two alignment stars for you!) After that, you're free to roam the universe.
Another attractive feature of these robotic drives is the ability to control your telescope from your personal computer. For example, using a compatible astronomy software program, you can find an object on the screen that you want to observe, select it and the telescope will find it for you.
The price for these robotic telescopes is a lot less than you might think, but they don't come in a wide variety of sizes. The two major North American telescope manufacturers, Meade and Celestron, offer models with apertures from 4 to 8 inches (10 to 20 centimeters). No doubt larger sizes are in the offing, but they will be correspondingly more expensive. Still, the attraction of having your telescope do everything but your income tax may more than compensate for lack of aperture.
There is yet another device available that is neither a clock drive nor a computer drive. Basically, it's a manual sky-guiding unit containing a database of thousands of objects. But rather than find your target for you, it directs you to your quarry using arrows on its LED display.
It's a little more work, but it's less expensive than the automated drives. Moreover, a manual sky guider can be used on altazimuth mounts including those for Dobsonians. The price falls between the $500 and $600 range. Not a "must-have" necessarily; maybe a "could-have."
Filters
For the beginner, I recommend three must-have filters: a solar, a lunar and a light-pollution filter.
The only kind of solar filter you should ever use is the reflective type, made of either Mylar or coated glass, which fits snuggly over the aperture of the telescope. These filters allow only a fraction of sunlight to pass through the telescope, making it entirely safe to observe sunspots, convective cells and bright filaments along the solar limb.
Depending on aperture and type of material used, solar filters cost between $30 and $150. Also, be advised that Mylar filters render the sun in a light-blue color, while glass filters (more expensive) render the sun in its natural yellow-orange hue.
If you like to look at the moon, a lunar filter is a must. In almost any size telescope, the moon is so bright, particularly when its phase is greater than first quarter, that it dazzles the eye into "lunar-blindness." Fortunately, like being snow-blind, it's only temporary, but you can avoid this photophobic experience altogether with a simple, low-cost (less than $20) filter that screws on to the eyepiece.
Finally, if you want to observe deep-sky objects and you live in or near a city or metropolitan area you must have a light-pollution filter. LPFs block the kind of light that masks the sky -- specifically that produced by evil mercury-vapor and high-pressure-sodium street lights, as well as naturally occurring ionized oxygen sky glow -- while passing the desirable light from nebulae or galaxies.
These filters often produce amazing results. Deep-sky objects that you think you wouldn't be able to see pop out in high contrast and in detail. Not surprisingly, there are many types of LPFs, some specifically for nebulae, others for galaxies, and still others for comets. Prices range from $60 to more than $150.
Eyepieces
In some respects, choosing eyepieces for your telescope is a moot point, since most instruments come equipped with two token eyepieces anyway, but these are sometimes marginal in quality.
Like the lens or mirror of a telescope, eyepieces are categorized by focal length, which is given in millimeters (mm). The longer the focal length, the lower the magnification. So, for any given telescope, a 32mm eyepiece will provide a greater field of view and thus a lower magnification than a 25mm. (To derive magnification, simply divide the telescope's focal length by that of the eyepiece. For example, a 25mm eyepiece used on an instrument with a 900mm focal length produces a magnification of 36x.)
Eyepieces also come in three barrel sizes: 0.965-inch, 1.25-inch and 2-inch. The 0.965-inch (2.45 centimeter) eyepieces are usually supplied with low-cost import telescopes. The quality and selection for most of these eyepieces are poor and the only reason for keeping them around is because they make cheap telescopes even cheaper. Don't buy a telescope with eyepieces of this size.
The 1.25-inch (3.175-centimeter) eyepiece is the most common and is considered the American standard. But gaining in popularity today is the 2-inch (5.08-centimeter) eyepiece, which has been embraced by advanced amateur astronomers who desire crisp wide-field views with their large light-bucket telescopes.
There are countless variations on eyepiece design, with price ranges to match. If you're just starting out, don't buy the most expensive eyepiece, but don't buy the least expensive either. I recommend Plössl eyepieces because they provide sharp, high-contrast images and are good all-around eyepieces for everything from planets to galaxies. A step up in quality, and a significant step up in price, are the Nagler eyepieces, which provide pinpoint star images across a breathtakingly wide field of view. (When the time comes to buy one of these, I suggest the 9mm.)
Miscellaneous accessories
The following list is purely arbitrary and in no particular order. Although they are not "must-haves," they are useful options that you may want to consider.
A barlow lens, which will double or triple the magnification of any given eyepiece
Dew shields for Schmitt-Cassegrain telescopes
A lens-cleaning brush
A modest eyepiece case
And in the end…
My final piece of advice may be taken with the proverbial grain of salt, but here it is: If you've reviewed your choices and are still stymied as to which telescope to buy, I suggest being less analytical and more instinctive.
Recreational astronomy should be fun, and fun is something you can't quantify. So, after all is said and done ask yourself -- which telescope do you think you'd enjoy using the most; which telescope has the most potential for gratifying your interest; which one gives you that little tingle of inexpressible excitement?
Go with that one.
And for where to find "that one," check out our list of telescope makers and dealers.
This is not an exhaustive list of manufacturers and dealers, but includes many of those with active websites. Check the advertising pages of Sky & Telescope or Astronomy magazines for more listings, or use your favorite internet search engine.
TELESCOPE MANUFACTURERS
Astro-Physics, Inc.
Rockford, IL
www.astro-physics.com
-- 4-, 5-, and 6-inch refractors
Celestron International
Torrance, CA
www.celestron.com
-- SCTs, refractors, reflectors (both equatorial and Dobsonian), and the "NexStar" computer-driven telescope (5- and 8-inch models)
Coulter Optical
West Palm Beach, FL
www.murni.com
-- Dobsonians telescopes (8- to 13-inch models)
Discovery Telescopes
Oceanside, CA
www.discovery-telescopes.com
-- Truss-style 12-, 15-, and 17.5-inch Dobsonian telescopes
Edmund Scientific
Barrington, NJ
www.scientificsonline.com
The Astroscan 2001
Jim's Mobile, Inc.
Lakewood, CA
www.jimsmobile.com
-- 6-, 12.5- and 18-inch Newtonian reflectors
Meade Instruments Corporation
Irvine, CA
www.meade.com
-- SCTs, refractors, reflectors (both equatorial and Dobsonian) and the "Autostar" computer-driven telescope (3.5- and 5-inch models)
Obsession Telescopes
Lake Mills, WI
www.globaldialog.com/~obsessiontscp/obhp.html
-- Large to huge Dobsonian reflectors (up to 30 inches)
Starsplitter Telescopes
Thousand Oaks, CA
www.starsplitter.com
Stellarvue
Auburn, CA
www.stellarvue.com
-- 3-, 3.5- and 4-inch refractors; Nagler eyepieces
Celestron International
Torrance, CA
www.celestron.com
Meade Instruments Corporation
Irvine, CA
www.meade.com
Televue (mfr. Nagler eyepieces)
Suffern, NY
www.televue.com
TELESCOPE DEALERS
Most of the telescope manufacturer's listed above, as well as some not listed, distribute their telescopes to many of the following dealers.
Adorama
New York, NY
www.adoramacamera.com
Amateur Electronic Supply
Milwaukee, WI
www.aesham.com
Anacortes Telescope & Wild Bird
Anacortes, WA
www.buytelescopes.com
Andromeda Astronomy Mall
Grand Forks, ND
http://www.m31mall.com/
Astronomics
Norman, OK
www.astronomics.com
Astronomy Mall
www.astronomymall.org
Astronomy Shoppe
Phoenix, AZ
www.darkskyinc.com/xmain.html
Camera Corner
Burlington, NC
www.camcor.com
Focus Camera
Brooklyn, NY
www.focuscamera.com
Hardin Optical Company
Bandon, OR
www.hardin-optical.com
Khan Scope Centre
Toronto, Ontario, Canada
www.Khanscope.com
Lumicon
Livermore and San Francisco, CA
www.lumicon.com
F. C. Meichsner Co., Inc.
Boston, MA
www.meichsner.com
The Observatory
Dallas, TX
www.TheObservatoryInc.com
Oceanside Photo & Telescope
Oceanside, CA
www.optcorp.com
Orion Telescopes and Binoculars
Santa Cruz, CA
www.telescopes.com
Pocono Mountain Optics
Moscow, PA; Las Vegas, NV
www.pocmtnop.com
Rivers Camera Shop
Dover, NH
www.riverscamera.com
Scope City
Locations in California
www.scopecity.com
ScopeTronix
Cape Coral, FL
www.scopetronix.com
Shutan Camera & Video
Vernon Hills, IL
www.shutan.com
Skywatch Products
Fort Worth, TX
http://members.aol.com/skywatchpr/home/html
The Telescope Store
Keene, NH
www.telescopes-earth.com
Texas Nautical Repair
Houston, TX
www.lsstnr.com
The Sky Plus
Pittsburgh, PA
www.theskyplus.com
Wholesale Optics
New Milford, CT
www.astroptx.com
Woodland Hills Camera
Woodland Hills, CA
www.telescopes.net
Getting Started
Astronomy is a fascinating hobby for old and young alike. However, newcomers can sometimes be disappointed by what they see in a telescope, especially the cheap, 'bargain' telescopes often found in high street and department stores.
There are those starting out in astronomy who are tempted to go out and buy the best telescope one can afford. This is rarely the wisest thing to do, and can lead to frustration, disappointment and a depletion of one's bank account. Expectations are high at first. People expect to see the planets, particularly Jupiter, Saturn and other celestial objects, in full colour just like those they see on tv or in magazines, taken by the big telescopes at Mount Palomar, Hawaii or the Hubble Space Telescope. This, is simply not true. It is better to defer buying any optical aids, whether binoculars or telescopes until you are at least able to find you way around the sky just using your eyes.
Here is a guide to getting started in astronomy, and buying your first telescope.
1: Join your local astronomy club / society. Go to the observing sessions and try out their telescopes, and those of the members, if you can. You will learn a lot, and find people who are willing to help and give advice. To find clubs, you can visit the websites of the national astronomy societies for your country. These can be found on my MegaLinks page. For the UK, you can visit my list of local clubs and societies. Failing this, just ask at your nearest camera / telescope / optical shops, museums, planetariums (or visit their websites), or even University Astronomy and Physics departments. Many magazines, such as Sky and Telescope, Astronomy and Astronomy Now give lists of local clubs and societies.
2: Familarise yourself with the night sky, learn the constellations, as these are used as signposts to help find your way around the night sky.You do not need to invest in a telescope, or binoculars to do this. All you need are your eyes, clear skies, and a guide, such as a basic star chart, a planisphere or a Celestron Sky Maps combination of seasonal star charts and glow-in-the-dark planisphere. These are published on The Salopian Web's Sky Charts page, in various monthly magazines such as Astronomy, Sky & Telescope and Astronomy Now, and some newspapers.
Planispheres can be bought from most reputable bookshops. They consist of two discs, one of which has the sky printed on on, and the other, placed on top of the sky disc, has an opening which shows the sky at a particular time and date. The two discs are rotated relative to each other for varying dates and times.
There are plenty of good guides, such as Patrick Moore's Observer's Year, or Ian Ridpath's Pocket Guide to the Stars & Planets and others like Star Hopping, The Messier Album and The Universe from Your Backyard. These publications will describe the stars, planets, and other objects, for you and help to guide you around the sky.
The Salopian Web, publishes a monthly diary of what's happening in the night in the current month, and a set of sky charts for northern and southern hemispheres.
3: When you're familiar with finding your way around the sky with the unaided eye, invest in a pair of binoculars before splashing out on a telescope. You may already have a suitable pair around the house that are perfectly adequate for stargazing. They should have a wide field of view, making it easier to hunt around the sky when looking for deep space objects such as the Andromeda Galaxy, our nearest galactic neighbour, the stunning Pleiades (sometimes known as the seven sisters), the Horsehead Nebula in the sword of Orion, the moons of Jupiter and many more. Don't forget our own Moon. There's a vast amount of detail to be seen on with a pair of binoculars.
Sky-hunting with binoculars is a low cost, rewarding way of getting started in astronomy, and learning to find your way around the sky, but what sort of binoculars should you use, what magnification? To find out, read on...
Choosing Binoculars
Binoculars are often discounted by beginners, yet they offer: a low cost start to the hobby, portability and a wide field of view. One of the disadvantages is that, being hand held, they can give unsteady images when used at high magnifications. This is due to hand shake, which can be overcome by using a tripod. Some recent binoculars have an image steadying mechanism to overcome hand shake (similar to that in some camcorders) to a certain extent, but this technology comes at a price.
The performance of binoculars is specified using a two-number system, such as 6×30 or 8×50. The first number represents the magnification, and the second the diameter of the objective lens in millimetres. These are usually spoken as "six by thirty" etc.The object lens is the one at the front of the binoculars. The larger the diameter, or aperture, the better the light gathering power of the binoculars. This is the most important property for observing the night sky. Beginners usually assume that the higher the magnification the better. In general it is often better to choose a pair of binoculars with a wide field of view, i.e. a lower magnification. Higher magnifications can resolve double stars, but narrows the field of view, making it difficult to find your way around the sky. Higher magnifications also magnify the atmospheric instability, which causes stars to twinkle, and, of course, it also magnifies hand shake, making stars appear to dance around in the field of view. For this reason, 10 times magnification is considered the maximum for hand-held binoculars.
The bigger the aperture of the objective lens the brighter the image. Most astronomical objects are hard to see not because they are small, or distant, and need more magnification, but because they are faint and need a larger aperture. A pair of 8×50s collects twice the amount of light as 8×35s, and hence everything appears to be about 0.7 magnitude brighter.
Suitable binoculars are 10 x 50; 15 x 70 and 20 x 100 giving magnifications of 10 times, 15 times and 20 times with lens diameters of 50mm; 70mm and 100mm ( 2inches; 2.75 inches and 4 inches) respectively. For best results, the ratio of aperture to magnification should not exceed 5, that is lens size (in millimetres) divided by the magnification should be no more than 5.
Choosing a Telescope
1: Avoid buying a low cost "beginners", or "starter" telescope from high street stores, tv shopping channels or department / chain stores. You will almost certainly be disappointed and discouraged, and will probably never get a better one. Most of these inexpensive telescopes have poor optics, with flimsy and difficult to use mounts giving blurred and shaky images.
Generally speaking, refractors of less than 3 inches (75mm) or reflectors less than 6 inches (150mm) are best avoided. A good telescope will give a lifetime's pleasure. As with binoculars, the most important factor is the light gathering power of the telescope.
2: When buying a telescope, you need to consider portability.
A 16" Dobsonian "light bucket" might be appealing if you're interested in deep space observing, but not if you live in a high rise apartment, or flat, and in a city. Carrying around 80 kilos (176 lbs) of a 1.7m (67") long Dobsonian from your apartment, or flat, to a suitable dark sky site can be daunting. So match your ambitions to the observing sites you can get to easily, and to your own physical strength and transportation limits. Don't overdo size, weight, or price. Choose a telescope you can carry and set up by yourself, in case a family member or friend can't or won't go with you.
3: What kind of telescope?
Many telescopes are capable, with varying degrees of success, of showing you virtually everything in the heavens. But no telescope does it all perfectly. Every telescope has excels in particular areas, and others where it's only adequate. Refractors, for example, are usually better at high power lunar and planetary observing than they are at finding faint nebulae and galaxies. Reflectors are just the reverse.
4: What magnification should you use?
Any telescope can magnify to any extent, however, the highest useful power of a telescope under ideal seeing conditions is only 50 to 60 times per unit of aperture. Under average seeing conditions, atmospheric turbulence limits the highest useful magnification to 25 to 30 times per unit of aperture.
So, how much power do you really need? High magnifications are OK when viewing the solar system, as there is plenty of light available, although you don't always need to use high magnification as there is plenty of lunar and planetary detail to see at 50 to 100 times.
Except for resolving close binary stars and globular clusters, very high magnifications are not usually needed outside the solar system either as stars always look like points of light, no matter what the magnification. Many planetary nebulae, like the Ring, Nebula, M57, look great at 100 times, but are too dim to see well if you increase the magnification. The Andromeda Galaxy is over 3 degrees across, or six times the diameter of the moon. You don't need high magnification to see something that big!
There are three principal telescope types. These are:
Refractors
These are based on a lens system. Refractors have long, relatively thin tubes with an objective lens at the front that collects and focuses the light. The earliest of this type of telescope was used by Galileo to study the four brightest moons of Jupiter.
They are now used for serious lunar, planetary, globular cluster, and binary star observing -- as well as for surprisingly good views of the brighter Messier, NGC, and IC objects -- many amateur astronomers prefer the crisp, high-contrast, diffraction-free images of a good refractor.
A useful rule of thumb in astronomy is that a good 3" to 4" refractor will usually out perform an average 6" to 8" reflector or catadioptric for seeing details on the Moon and planets, resolving binary stars, and clusters.
Refractors do not have a secondary mirrors or multiple optical paths to introduce light scattering, diffraction and internal reflections. These brighten the sky background and reduce contrast.
Refractors also have the highest light transmission, ie the percentage of the light gathered by the scope that actually reaches your eye and can transmit 90% or more of the light they collect, compared with the 77% to 80% transmission of reflectors and 64% to 75% typical of catadioptrics.
The light transmission of a refractor rarely deteriorates with age, as do reflectors and catadioptrics, since their mirrors slowly oxidise and lose reflectivity over a period of time.
With good seeing conditions, refractors can reveal subtle lunar and planetary detail with a better contrast range, than is possible with larger reflectors and catadioptrics.
Diffraction spikes on a reflector's star images, caused by its secondary mirror spider vanes, are absent in an unobstructed refractor. With no diffraction spikes to hide faint binary star components or smear globular clusters, refractors can resolve closely spaced binary stars more easily than a typical reflector.
Since the Moon and planets are all brightly lit by the Sun, high light gathering power is not as important as high magnification within the solar system. The relatively small aperture of a refractor is therefore often an advantage for this kind of observing, as is the high magnification capability due to its long focal length.
For lunar, planetary, binary and star cluster observing, a refractor may be perfectly adequate. Due to the limited light gathering of refractors, long exposure photographs of deep space objects such as nebulae and galaxies are rarely taken with refractors.
Shortcomings.
Refractors suffer from chromatic aberration, or false colours. This is an optical defect that produces a faint coloured 'halo' around bright stars, the limb of the Moon, and the planets. Chromatic aberration becomes more visible as the aperture increases and the focal ratio decreases, although modern optical systems can minimize the problem in two element achromatic refractors and virtually eliminate it in three to four lens apochromatic systems.
While they are light weight in smaller sizes, refractors become bulkier and considerably more expensive than reflectors or catadioptrics for apertures approaching 100mm (4"). A 100mm (4") refractor typically costs and weighs 2 to 6 times as much as a 115mm (4.5") reflector or 90mm (3.5") Maksutov-Cassegrain.
If high light grasp is not essential, ie for observing faint galaxies, where a larger reflector would have the edge, the clarity, contrast, and good image quality of a refractor is worth considering.
Newtonian Reflectors
These are based on mirror systems, the most popular of which is the Newtonian invented by Sir Isaac Newton in 1668. Newtonian reflectors offer more performance for your money than any other telescope type. Reflectors have no lens at the front. Instead, the light is collected by a parabolic mirror at the bottom of the telescope tube, which reflects light onto a secondary mirror near the top of the tube. This secondary mirror, often called the flat, is used to reflect the light from the main mirror into the eyepiece at the side of the tube.
The reflector's large light-gathering area and relatively short focal length can provide bright images of deep space objects that are too faint for any small aperture refractor to see. And the reflector's large aperture can resolve details within those objects with a precision no small scope can match.
The penalty you pay for this performance is typically one of large size and weight, although not one of high cost, as reflectors cost the least per millimetre (inch) of aperture of any telescope type.
The reason is because a Newtonian reflector has only one mirror to grind and polish to a precise curve (with an accuracy of around 100 nanometres [100 x 10-9m ]( 4 millionths of an inch) or better.). A refractor, on the other hand, has two to four lenses ( not including the eyepiece ), with four to eight precision surfaces to shape. And those lenses might have to be expensive and esoteric formulations in order to provide satisfactory images. Similarly, a catadioptric scope has three or four curved optical elements to shape to a high degree of accuracy.
All that extra mirror, lens grinding and costly optical glasses in refractors and catadioptrics isn't cheap. It makes the one parabolic-mirror and one flat mirror optics of the Newtonian reflector the least inexpensive to make, hence its lowest cost.
For the same money you get more aperture with a reflector than with any other type of telescope. All other things being equal, the bigger the aperture, the better the performance. An 8" reflector typically costs 50% less than a 4" refractor, and little more than 4" catadioptric, but will have four times the light grasp of either.
For purely visual deep space observing, Dobsonian reflectors are very cost effective. These are Newtonian types with a simplified mount. With huge mirrors (up to 16" in diameter) to gather light, and inexpensive wood mounts, these new Newtonians have brought about the age of the "light bucket" in amateur astronomy. The deep space observer on a budget has never had it so good.
The astrophotographer will also find that a large aperture equatorial mount reflector is excellent for recording deep space objects in detail, as well as visual observing. (Photography is not possible with Dobsonian scopes, unless they have computer controlled drives.)
Shortcomings.
There are five:- diffraction, coma, size, weight, and added maintenance.
Light diffracted, or scattered, by a reflector's diagonal mirror reduces image contrast in lunar and planetary observing, masking subtle surface details. In addition, diffraction spikes on star images, due to the spider vanes that hold the diagonal mirror, can hide faint binary star components and smear globular cluster detail.
Because of the parabolic shape of their primary mirrors, all reflectors have coma -- an optical defect in which stars appear triangular or wedge-shaped at the edge of the field. The faster the focal ratio, the smaller the coma-free field. This can be annoying in photos, where the entire field is available for leisurely inspection. It is usually unobjectionable visually, however, since objects of interest are normally kept in the center of the field, where eyes and eyepieces are sharpest and coma is not a factor.
Since an 8" reflector can weigh 50% more than an 8" catadioptric, and its 48" long optical tube is not the easiest thing in the world to manage in an apartment elevator, a large reflector usually requires the elbow room afforded by a suburban environment. Also, since city light pollution compromises deep space performance by washing out faint nebulae and galaxies, dark sky observing sites are always recommended with large reflectors.
In addition, unlike refractors or catadioptric telescopes, a reflector can require frequent re-collimation or alignment of its optics, and its exposed mirrors mean that periodic cleaning will also be required. However, this maintenance typically averages only a few additional minutes of work per observing session.
These drawbacks aside, for serious visual observing of faint galaxies and nebulae, as well as for more-than-acceptable lunar and planetary observing, you'd be hard-pressed to equal, much less surpass, the price-to-performance ratio of a Newtonian reflector. It's been a best-seller for over 300 years -- and sheer value for the money is why.
Catadioptric Reflectors
These are modern compound, or lens/mirror combination telescopes and combine many of the best features of refractors and reflectors into one package, with few of their drawbacks. Catadioptrics include Schmidts, Maksutovs and Schmidt-Cassegrains.
They allow the performance of a large aperture, long focal length scope to be folded into a reasonably lightweight and transportable package which can be very useful if the telescope must be transported to dark sky sites.
Because of their optical design, catadioptrics are almost completely free of the coma found in reflectors and the chromatic
aberration found in refractors. Stars are point-like and coma free across the visual field of a catadioptric telescope, and there is no trace of coloured fringes around bright stars and planets to mask faint detail and colours. Some curvature of field is often visible in catadioptrics, particularly in fast focal ratio models, but it usually shows more at the edges of wide field photo's than in visual observing.
The typical catadioptric fork mount cradles the telescope's short optical tube securely on two sides, reducing image-degrading vibration to a minimum. A catadioptric's setup and takedown time is short, due to its lighter weight, and compact size.
Shortcomings.
An 8" Schmidt-Cassegrain costs 50% to 300% more than an 8" reflector (although about the same as many 4" refractors).
Schmidt-Cassegrain catadioptrics do not have as wide a contrast range on the moon and planets as a refractor or most f/6 to f/8 reflectors, because of the extra light scattered by its more complex optics and folded light path, nor will it be able to resolve close binary stars as easily. However, a Schmidt-Cassegrain will usually outperform a fast (f/4.5) focal ratio reflector of similar aperture on the planets and binary stars due to the absence of secondary mirror spider vanes, which usually cause diffraction effects in Newtonian types.
A Maksutov-Cassegrain has better contrast than a Schmidt-Cassegrain, often equalling that of a refractor of similar aperture, because it has a smaller secondary mirror obstruction.
Although their large apertures allow detailed deep space observing, catadioptrics generally do not have as bright an image as other telescope types of similar aperture and power. A catadioptric can have a light transmission of 64% before secondary obstruction light losses are allowed for ( 70% to 75% with multicoated optics ), compared to 90% of a similar aperture multicoated refractor and the 77% to 80% of a reflector.
Despite these shortcomings, if it fits your budget and you need a portability, then a good catadioptric is a wise investment.
A
Achromat / Achromatic.
Usually, a two element telescope lens, or eyepiece, designed to reduce the false colour effects caused by chromatic aberration.
Airy Disk
This is the central spot of the image of a star, surrounded by diffraction rings, formed when viewed in a telescope. Because of the large distances of stars, it is impossible to resolve a star into a disk, no matter what the magnification. This is due to diffraction. The disk formed by the diffraction of stellar images is known as an Airy Disk, after the English Astronomer Royal, George Airy.
Altazimuth Mount.
This telescope mount allows movement in two directions: azimuth ( horizontally ), and elevation ( vertically ). This is a common mounting, though tracking objects is more difficult, as it requires moving the telescope in both axes, which can be difficult. Dobsonian Telescopes are mounted this way.
Aperture
This is the diameter of the objective lens, or primary mirror usually expressed in millimetres, or sometimes in inches.. The larger the aperture, the greater the light gathering ability of the telescope.
Apochromat / Apochromatic.
Usually, a three element telescope lens, or eyepiece, designed to provide a high degree of correction for false colours caused by chromatic aberration.
Astigmatism.
This is a defect in a lens, or mirror, which causes light, which is off the central axis of the lens or mirror, to form an ellipse, or straight line, instead of being brought to a point focus. A lens designed to avoid such defects is known as an Anastigmat.
B
Barlow Lens.
An extra lens, used in conjunction with a telescope's eyepiece to increase the magnification, usually by a factor of two or three times, by increasing the effective focal length of the telescope. Named after the English physicist Peter Barlow.
C
Cassegrain.
A reflecting telescope comprising a primary mirror with a central hole through which the light from the primary mirror is reflected to an eyepiece at the focus, the cassegrain focus, beyond the primary mirror. The design is often used in compact and portable telescopes.
Catadioptric.
A series of telescopes comprising a mirror with a full aperture lens ahead of it, which is used to correct for spherical aberration in the primary mirror.
Chromatic Aberration
This is the name given to false colour fringes, or coloured halos, around objects, and is caused by unequal refraction of light of different wavelengths being brought to a focus at different distances from the lens. This is a problem commonly associated with refracting telescopes. The use of achromatic or apochromatic lenses greatly alleviates the problem, but does not completely eliminate it.
Collimation.
The process of aligning the optical system of a telescope so that the light gathered is brought to a focus at the correct position.
Coma.
A defect in an optical system which gives rise to a blurred, pear shaped, comet-like image at the edge of the field of view.
Contrast.
Good image contrast is desirable for viewing low contrast objects such as the lunar surface and planets. Newtonian and catadioptric telescopes have secondary (or diagonal) mirrors that obstruct a small percentage of light from the primary mirror. Light scattering and diffraction from such obstructions can cause a reduction in image contrast. It is commonly believed that image contrast is severely reduced with Newtonians or catadioptrics because of this obstruction, but this is not the case. This would only be true if more than 25% of the primary mirror's surface area was obstructed by the secondary.
Seeing conditions (or air turbulence) is the single most important factor that affects image contrast when seeking planetary detail through a telescope. Instrument problems that can also adversely affect contrast in order of decreasing importance are: collimation, baffling and a small increase in central obstruction. Note that the increase in central obstruction is rated as the lowest contributor adversely affecting contrast.
Coudé.
This is an set of auxiliary mirrors so arranged as to always direct light to a fixed position, usually the polar axis, irrespective of the actual position of the telescope. It is used with instruments, such as spectrographs, which are too large to mount on the telescope itself.
D
Declination.
A system for measuring the altitude of a celestial object, expressed as degrees north, or south, of the celestial equator. Angles are positive if a point is North of the celestial equator, and negative if South. It is used, in conjunction with Right Ascension, to locate celestial objects.
Diffraction.
The spreading out of light as it passes over a sharp edge, or through a narrow (in terms of wavelength) slit. The obstructions in the tube of a telescope can give rise to such effects, thereby reducing image contrast and giving rise to rings and 'spikes' around stellar images, which should be purely point-like.
Drive.
The means of automatically compensating a telescope for the Earth's rotation so that it always points to the same place in the sky.
Dobsonian Telescope.
Named after the American astronomer, John Dobson, this is a Newtonian telescope mounted onto a form of altazimuth mount such that the mount is at the mirror end of the telescope. This design allows large aperture telescopes to be made for relatively low cost, when compared to other mounts.
Doublet.
A lens system comprising two elements, used to reduce chromatic aberration.
E
Equatorial mount.
A method of mounting a telescope whereby one axis is parallel to the Earth''s axis, and points to the celestial north pole. This is known as the polar axis. The other axis is mounted at right angles to the polar axis, and is therefore parallel to the plane of the Earth's equator. This is known as the declination axis. Such a mounting system allows an object to be kept in the centre of field of view by moving the polar axis only. If the polar axis is driven at the sidereal rate, it will compensate for the rotation of the Earth, and keep the object centred in the eyepiece.
Exit pupil.
The width of the beam of light exiting the eyepiece. It is equal to the aperture divided by the magnification. If it is larger than the size of your pupil in the dark, around 5 or 6mm, you will not be using all the available light effectively.
Eyepiece.
Sometimes known as an ocular, the eyepiece is a system of lenses closest to the eye. Its purpose is to magnify the image at the focus of the telescope. The magnification of an eyepiece can be found by dividing its focal length into that of the telescope, provided the units of measurement are the same in each case.
There are several types of eyepiece designs. The most popular are:
Kellner: These are low cost and the most popular for cheap telescopes. They have short eye relief and narrow fields of view. Best avoided if you can afford better ones.
Orthoscopic: These have good all round price / performance.
Erfle: These have a wide field of view, and are expensive.
Plossl: These are a good compromise and, perhaps, offer the best all-around price / performance.
F
Finder.
A small telescope, with a wide field of view, mounted on the main telescope tube to enable an observer to easily locate celestial objects, and place them within the field of view of the main telescope.
Focal Length.
The distance between the objective lens, or primary mirror, and its focus, or focal plane.
Focal Ratio.
Focal ratio, or f number, is the focal length of a lens, or mirror, divided by its diameter. A focal ratio of 8 would be written as f/8.
An f/8 telescope is "slower" than an f/4. Fast telescopes give wider, brighter images with a given eyepiece than slower ones. In general, the slower the telescope, the more forgiving it is of defects in the objective / mirror and eyepiece.
Focusser.
The mechanism which holds the eyepiece and allows adjustment for focussing the image.
G
German Mount.
A variant of the equatorial mount.
Graticule.
Reference marks, or measuring scale, placed at the focal plane of a telescope to aid object centring, or to make measurements.
H; I; J; K;
L
Lens.
An optical component of telescopes, binoculars and cameras which is made of a transparent material, and is so shaped as to bend light in such a way as to aid the formation of an image.
Light Bucket
A slang term given to large aperture telescopes such as dobsonians..
Limiting Magnitude.
The faintest object that can just be detected by a telescope.
M
N
Nagier Eyepiece.
An eyepiece having a very wide field of view, typically greater than 80 degrees. Particularly suitable for comet hunting
Newtonian.
Usually refers to a type of reflecting telescope based on a parabolic mirror. First invented by Sir Isaac Newton in 1668. Newtonian telescopes are the most common types used for amateur astronomy.
O
Objective.
The main light gathering element in a telescope or binoculars. In a refractor, this is the large lens at the front, sometimes called an object glass. In a reflector, it is a mirror.
P
Primary Mirror.
The principal light gathering mirror in a reflecting telescope..
Q
R
Reflector.
A telescope in which the main light gathering element is a mirror. The most common type for amateur astronomy is the Newtonian, but there are also Cassegrain, Schmidt and Maksutov types found in common use.
Refractor.
A telescope in which the main light gathering element is a lens, known as the objective, or object lens.
S
Schmidt.
A wide field reflecting telescope mainly used for photographic sky surveys.
T; U; V
W
Wedge.
A device used to attach a fork mounted telescope to a tripod.
X; Y; Z
Sky & Telescope
Choosing Your First Telescope
By Joshua Roth
So you've decided to take the plunge and get yourself a telescope. Congratulations! That alone is a big step. But what comes next? Not an impulsive shopping spree at the nearest mall! Buying a telescope is very different than buying a television, and department-store salespeople are rarely familiar with the needs of amateur astronomers.
For starters, shun the flimsy, semi-toy, "600 power!" department-store scopes that may have caught your eye. The telescope you want has two essentials: high-quality optics and a steady, smoothly working mount. You may think you need a large instrument, but don't forget portability and convenience. Your first telescope shouldn't be so heavy that you can't tote it outdoors, set it up, and take it down reasonably easily.
Those are the basics. But to actually choose a telescope that meets your needs, you need to ask some questions — of yourself; of practicing amateur astronomers; and, finally, of the people who make and sell telescopes for a living.
A Telescope's Heart
All astronomical telescopes, large or small, share a common goal — to brighten and magnify your views of celestial bodies. Refractors, reflectors, and compound (catadioptric) telescopes do their jobs in different ways — each with its own benefits and drawbacks.
Yet many fundamentals apply to any telescope. Of primary importance is a telescope's aperture: the diameter of its light-gathering lens or mirror. (That lens or mirror is often referred to as a telescope's objective.)
Aperture makes a big difference in the level of detail you can see. A telescope that can only be pushed as high as 50x (50 times magnification) will reveal Jupiter's moons, Saturn's rings, and some degree of detail in the brightest star clusters, nebulae, and galaxies. But to discern Martian surface features or to see both members of a tight double star, you really would like to be able to use at least 150x. Depending on optical quality and observing conditions, you can expect to get anywhere from 20x (mediocre) to 50x (excellent) per inch of telescope aperture.
Aperture also enables you to see fainter objects. For example, several dozen galaxies beyond our own Milky Way can be discerned through my 4½-inch (105-mm) reflector. Some are more than 50 million light-years away. Not bad for a telescope I can tuck under my arm and carry on a plane! But with my 12½-inch Dobsonian, hundreds of galaxies are within reach.
If a telescope's aperture is arguably its most important "spec," its focal length comes in as a close second. Say you have two telescopes with the same aperture but different focal lengths. The one with the longer focus (and hence, a higher f/ratio) will generally lend itself better to high-magnification viewing. (The f/ratio is the focal length divided by the telescope aperture in the same units.) One reason: you can stick with longer-focus eyepieces, which are easier to use, especially for eyeglass wearers. Another reason: "fast" objectives (those with small f/ratios) tend to make fuzzier images, unless you've paid a premium for high-quality optics.
"So it seems clear: I should go after the largest, longest telescope I can afford." Maybe; maybe not! A long focal length is preferable if your primary targets are the Moon, the planets, or double stars. And a large objective is a necessity if you dream of viewing numerous distant galaxies. But if you want to take in large swaths of the Milky Way or sparkling showpieces like the Pleiades, a short, small scope is called for.
"Why's that?" Because a long focal length only lets you see a small patch of sky at one time. With standard eyepieces (those that have 1¼-inch-wide barrels), a focal length of 20 inches (500 mm) can provide a 3° field of view — enough to take in all of Orion's Sword. A focal length of 80 inches (2000 mm), by contrast, barely lets you encircle M42, the famous Orion Nebula in the Sword's center.
"What if I want to do a bit of everything?" Don't worry. There are plenty of acceptable compromises. Many astronomers take the 6-inch (152-mm) reflector to be an ideal "do-it-all" instrument. But keep in mind that even with that aperture, you still face a tradeoff between a wide field of view (f/5 or thereabouts) and high-power performance (optimal at f/8 and up). The long-focus unit will also be heavier and require a beefier mount.
A Telescope's Other Half
Just as a car's engine is useless without a chassis, an optical-tube assembly is only half a telescope. Even stargazing with binoculars only really works well if you're leaning on a windowsill or reclining in a lawn chair with arm supports. And binoculars typically magnify things only 7 to 10 times. By contrast, even the smallest telescopes typically work at 30x and up. Finding objects without a mount is next to impossible at such magnifications, as are steady views.
An equatorial mount allows motion in two directions: north-south and east-west. Because one axis is aligned with the Earth's, an equatorial mount allows you to track celestial targets with a single push or knob. Furthermore, many equatorial mounts have electric motors for hands-off tracking. This is especially useful for high-magnification viewing and for showing celestial objects to groups of people. It's also a prerequisite for long-exposure photography.
An altazimuth (altitude-azimuth) mount, by contrast, permits up-down (altitude) and right-left (azimuth) motions. A video tripod is a familiar example of an altazimuth mount (and indeed, a sturdy one suffices for a lightweight, low-power scope). Another now-universal variation is the Dobsonian mount, shown below.
Altazimuth mounts are generally lighter than their equatorial counterparts, in part because they don't require counterweights to balance the telescope. (I hasten to note, however, that the equatorial "fork" mounts supplied with many compound telescopes are relatively lightweight, too; the photo above shows one example.) Dobsonian mounts, in particular, can be extremely stable and economical.
But altazimuths do not readily lend themselves to motorized operation, and you have to move the telescope in two directions simultaneously to track celestial targets. While this becomes second nature to many observers, others find it maddening. (See the section below on "smart" telescopes for a novel, high-tech way around this problem.)
Your own personality should play a part in choosing a mount. Are you comfortable with instruments that require tools and a head for numbers to set up and use? Or are you looking for the astronomical equivalent of a point-and-shoot camera? Equatorial mounts generally require several minutes of assembly, careful balancing, and polar alignment, while some Dobsonians can be set up in the time it took to read this paragraph, and some "smart scopes" on "alt-az" mounts can be deployed nearly as rapidly.
Other Essentials
We've already covered a lot of ground, and hopefully the tech talk you may get from a salesperson or stargazer will make more sense now. But a few topics remain before we can set you loose on your hunt. Most of us picture the big things when we think of a telescope, and those stand out in catalogs and ads. But just as you can't drive a new car off the lot without the keys, there are a few little things you'll need to use a telescope to journey among the stars.
Eyepieces. By bringing light from distant objects to a focus, a telescope forms an image. Now you need a way to view that image. That's what eyepieces are for. Swapping eyepieces lets you change a telescope's magnifying power, which simply equals the objective's focal length divided by that of the eyepiece. Every telescope owner should have several.
Eyepieces come in a bewildering variety of designs with exotic names. Generally speaking, the more expensive an eyepiece, the more lens elements it has. Complex multi-element designs can give a wider field of view, and they also can compensate to a degree for the aberrations that plague "fast" (low f/ratio) objectives. By contrast, many amateurs find that simpler designs like Kellners, Plössls, and Orthoscopics suffice for use on "slow" (high f/ratio) telescopes like the once-universal 6-inch f/8 Newtonian reflector.
Most telescopes come supplied with one or two eyepieces. Ideally, you'd like to have a set that spans a range of magnifications. You can expect to spend anywhere from $25 to $250 on a good eyepiece. A Barlow lens is worth considering, too: it will double or triple each eyepiece's power, effectively doubling the size of your eyepiece collection.
One useful hint: try to avoid buying a telescope that uses eyepieces with stalks or barrels that are 0.96 inch (24 mm) wide. The better designs are generally not available in this size.
Finders. You've got a telescope mounted with an eyepiece in place. Now what? Naturally, you'll want to point it toward celestial targets! Sighting alongside the tube may enable you to find the Moon and a few bright stars or planets with a small, wide-field scope. But, just as a hunter won't get far without a gun sight, a telescope can't be put to good use without a finder of some kind.
Three types, shown here, are commonly available. A few wide-field scopes come with lensless peep sights that encircle a patch of sky without magnifying or brightening it. The next step up is the so-called "reflex" sight. This device projects a red dot or circle on your naked-eye view of the sky; to set your telescope on a desired star or planet, you simply line that object up with the red dot or circle. Note that few telescopes are supplied with a reflex sight, you generally have to buy one separately. Most commercially available telescopes are sold with a real finderscope, a small refractor that rides piggyback upon the main telescope. The finderscope's eyepiece has cross hairs that you set on your desired target as you look through the device.
A finderscope has several advantages. Because its objective is larger than your eye's pupil, it brightens stars (and the larger models can actually show you some star clusters and nebulae directly). And, when properly aligned, a finderscope allows you to point a telescope more precisely than do peep sights or reflex finders. This is especially important for long-focus telescopes that have narrow fields of view.
On the downside, most finderscopes turn the stars upside down, and many entry-level finders cannot be used by eyeglass wearers. In any case, you'll want to avoid (or replace) any finder that doesn't give sharp images.
Star Charts. Once you warm up a new car and hit the highway, you need a map to find your way. So it is with a new telescope as well. You may already own a planisphere, the rotating "star wheel" that helps you identify constellations. Certainly you should be adept at using one before embarking on telescopic astronomy (our companion article gets you started). However, a planisphere alone will no more help you find the Cat's Eye Nebula, say, than a freeway map will help you find the shoe store at the corner of Park Avenue and Elm Street. To mine the heavens' riches, you need a set of astronomical star charts.
Most astronomical atlases display all the stars above some specified brightness level, along with an assortment of nebulae, star clusters, and galaxies. An atlas that reaches 6th magnitude (roughly the brightness limit of the unaided eye) suffices for users of binoculars or small telescopes, while an 8th-magnitude atlas like our famous Sky Atlas 2000.0 (shown here) will better serve the user of a large-aperture, narrow-field telescope.
If you haven't used star charts before, there's no better way to get started than with binoculars (see our primer on binocular astronomy). Stargazing with binoculars offers two bonuses: views are right-side-up, and the field of view is wide enough to take in recognizable formations of stars.
"Smart" Telescopes
You might think that with computers in everything from appliances to automobiles, someone would have put a computer in a telescope by now. And you'd be right! Actually the computer doesn't go in the scope itself, but in the mount, along with electric motors on both axes. A motorized telescope on a "smart" altazimuth mount can track celestial objects as accurately as one on a more bulky and complicated equatorial mount. Even better, once you set up the scope and initialize the computer with the current date, time, and location, it can automatically point to thousands of celestial objects.
Until recently such futuristic capabilities would set you back many thousands of dollars. But a new generation of battery-powered "smart scopes" has come onto the market at very affordable prices. These instruments can do things no commercial telescope has ever done before. A keypress or two gives the times of sunrise and sunset, moonrise and moonset, and the dates of meteor showers, solstices, equinoxes, and eclipses. Or choose a guided tour of the best celestial showpieces, complete with a digital readout describing what we know about each object. These scopes literally give you a beginner's course in astronomy.
Too many new owners of telescopes — even with decent, well aligned finders — can't locate anything but the Moon and a few bright stars or planets with their instruments. Eventually they throw their hands up in frustration and their telescopes into a closet. This problem could become a thing of the past in an era when a budding astronomer can point his or her telescope at virtually any object with the push of a button.
Still, these new scopes aren't for everyone. For one thing, the affordable models have smaller apertures than similarly priced entry-level scopes that have no electronics. Second, in an altazimuth telescope the image in the eyepiece rotates as the instrument tracks objects rising and setting. If you want to pursue deep-sky (long-exposure) photography, you need a sturdy and precisely aligned equatorial mount instead — computer or no computer. Third, these telescopes consume lots of electricity, and some models will exhaust a set of batteries in less than one night's observing. Finally, if your "smart scope" fails to show you a particular object, you may have trouble figuring out whether your eye or your telescope is to blame — that is, unless you know the sky and your charts well enough to confirm that the instrument is pointed to the intended spot on the sky.
Be An Informed Buyer
Now that you're up to speed on some of the most important concepts and terms, take the time to peruse the ads and product reviews in recent issues of Sky & Telescope or SkyWatch magazine. Then go ahead and call or write to anyone who manufactures instruments you might be interested in. Their brochures and catalogs should tell you much of what you want to know; if not, call the manufacturers or their dealers and ask away.
However, nothing substitutes for firsthand experience. By far the best way to acquaint yourself with the wide world of telescopes is to participate in an astronomy club's nighttime observing session, or "star party." There, you can try out and ask about a wide variety of telescopes. (Find an astronomy club or star party using our Resources section.) You may also be able to buy a used telescope from someone in an astronomy club. Used telescopes carry some risks, including undisclosed damage by the previous owner and a lack of warranty coverage. However, they can also be spectacular bargains. You can also find used telescopes on the Internet. (Be sure to take reasonable precautions if buying from a private party online.)
Of course, many buyers will find that a new instrument best suits them. This should be bought from a source specializing in astronomical telescopes. Many camera stores are excellent sources of astronomical products as well.
If you're set on buying a new instrument, be prepared to spend at least $200. If this is beyond your means, your astronomical aspirations will probably be best served by buying a decent pair of binoculars and a sturdy lawn chair. At the same time, realize that many excellent beginners' telescopes are available for well under $1,000. Remember that whatever investment you make may serve you well for several decades.
Kicking the Tires
You generally can't test a telescope's optical performance in a store, and many telescopes are sold by mail order. As a result, you should ask any vendor to spell out return policies (preferably in writing). Make sure you'll be given enough time to try a scope out under the stars, and the opportunity to return it for a full refund if it doesn't meet your needs. (One more caveat: if you bought the telescope by mail order, be sure to determine whether you or the vendor will pay for shipping in the event you wish to return the telescope for a refund. You should be prepared to pick up at least part of the shipping tab in exchange for the privilege of being able to "test-drive" the telescope.)
Once you obtain a telescope, new or used, you can immediately scrutinize its mechanical features and its mount, even in daylight. Any telescope mount, be it a camera/video tripod or a computer-controlled equatorial, should be stable enough to remain standing even if someone bumps into it in the dark. Give the mounted scope a gentle tap while viewing some distant target. Does the view jump around briefly, then stabilize? Good! But if it hops around for several seconds or more, it'll be endlessly frustrating. Finally, you should be able to move your telescope easily and smoothly, whether by pushing the telescope's tube, turning a knob, or switching on an electric motor.
It's harder for a newcomer to visual astronomy to critically assess a telescope's optical performance. But even an inexpensive telescope should pass the following nighttime tests. Point the telescope at a starry region in or near the Milky Way. Use your telescope's lowest power. Stars at the center of your telescope's field of view should focus to points of light without any distracting flares or colored halos. (Flares or halos may appear at the edge of the field of view, but they shouldn't be prominent until at least halfway out.)
Now set your sights on a fairly bright star and switch to high power. Focus the star, then turn the focus knob one way, then the other. The two out-of-focus images should resemble one another and be more or less circular. (Keep this important note in mind: eyeglass wearers with astigmatism should keep their eyeglasses on while performing this test and the preceding one.)
Finally, examine the Moon: it should look crisp, not hazy, and it shouldn't produce numerous ghost images (the consequences of inadequate coatings somewhere in the optics).
Keep in mind that perfection is expensive, and that a lot can be seen with less-than-perfect equipment. (It's the only kind I've ever owned!) Be patient with your new telescope and with yourself. At the same time, don't be afraid to ask for help! As time goes on, the wonders of the heavens will become familiar friends.