N.A.A.'s "Choosing a Telescope" Guide

current page:
More Resources>
About Telescopes>
Choosing A 'Scope

WHAT IS MAGNITUDE?

Magnitude is the measure of the brightness of an object. Most commonly for observers, it is how bright the object appears to us in the sky ("visual magnitude"). The brightest stars are around 1st magnitude; they give us about 2.5 times as much light as second magnitude stars, which are 2.5 times brighter than 3rd magnitude ones, and so on (The actual step is 2.512 or the 5th root of 100 between magnitudes). Brighter objects go into the negative numbers (the full Moon shining at about -12.7 mag.); in other words, the higher the number, the fainter the object is.

All the light from an object is totalled together in its regular magnitude measurement; an 8th magnitude star and an 8th magnitude galaxy both put the same amount of light into your telescope, but while that of the star is focused into a (relatively) bright point, that of the galaxy is smeared over an area, which causes it to look much fainter. The Surface Brightness measurement takes the spreading into account.

The magnitude scale is a logarithmic one; a star of mag. 1.0 is delivering 100 times more light than a star of mag. 6.0. This works out well for us, since the response of our eyes is also fairly logarithmic; a star of mag. 2.0 appears "twice as bright" as one of mag. 4.0.

Choosing A Telescope

Picking out a new telescope can be a daunting experience, whether you're a beginning stargazer or a seasoned amateur astronomer. There are many types of optical systems in many sizes, and an equally large array of mounting types; a host of manufacturers and suppliers, too. If there was one “best” telescope, that's the one everybody would get; in reality, there are many variables to consider, and each setup will be better at some, and worse at others. On this page, we'll look at some of those variables; if you decide which are most important to you, you will be on your way to choosing the best setup for your observing pleasure.

Does Size Matter?

If you've read the "How Telescopes Work" and played with the Telescope Calculator pages that accompany this one, you've learned that there are two different sizes we consider in looking at the optics of a telescope. First is the diameter of its objective lens or mirror, which is its light-gathering component. Since light gathering is the #1 function of a telescope, you can definitely say that the larger the diameter of the telescope's objective, the more “powerful” it is for viewing faint objects. But if you are not too interested in faint objects, or are concerned about portability and ease of use, perhaps Huge isn't for you... The second size we're concerned with is the focal length of the telescope; the longer that is, the higher the useful magnification range of the telescope will be shifted (see the section on “optimum magnification range” on the Telescope Calculator page). Since longer focal lengths can deliver higher magnifications, you could say that they're more “powerful”; but they can't achieve the lower magnification, wider field views of shorter focal lengths, and perhaps you like to view objects which need those to be seen well...

We'll take a look at those and other properties, and try to provide some information and examples which might help you analyze them more to focus on what your needs are. Let's look at ever-popular Magnification first:

How Much Magnification?

Never buy a telescope because someone says (or you computed) that it can achieve a super-sounding magnification. In reality, the amount of magnification you can actually apply out under the stars will always be limited by the blurring effect of turbulence in the atmosphere; no matter how good your optics are, if the turbulence (called “seeing” by astronomers) is limiting you to 200x, any increase in magnification over that will just provide a blurrier image. In our location in the central U.S., seeing often tops out at 150-200x (and sometimes below that); a night which provides sharp viewing at 300x or more is a rare pleasure. But thankfully, many of the things we observe are big enough to be seen well at much lower magnifications. Remember that the higher power you use, the less sky you'll see through the eyepiece; if the object you want to observe is large, you'll have to use lower powers to fit it in your field of view (and that it is much easier to locate objects using a low power, wide field!) Also remember that the more you magnify them, the fainter extended (non-stellar) objects will appear. So, selecting a magnification range (the focal length of the telescope) will be dependent on what types of objects you most want to observe.

Comparing Object Sizes
	View:

The above graphic compares the apparent sizes of some celestial objects as they appear from Earth (hover your mouse over the Solar System box to see some of the planets, and over the Deep Sky box to compare some objects beyond our Solar System). As we see, the other planets look pretty small in our sky, while some of our other target objects are relatively much larger (the inner planets are shown in two sizes, largest as when we're closest to them, smallest when we're farthest; the outer planets just at their largest, but they don't vary as much). Indeed, some deep sky objects are much bigger yet; the Orion Nebula (M42) is about 1° across, the Beehive Cluster (M44) about 1.5°, the Pleiades cluster (M45) 2°, and the Andromeda Galaxy (M31) a whopping 3° across!

What this means is that we'll frequently want to use relatively high magnifications (150-300x) to study things in our solar system, and relatively low powers (20-150x) to view objects beyond. While the magnification a telescope delivers can be changed by puting in different eyepieces, each 'scope has its own usable range of magnifications, determined by its focal length and by the range of eyepiece focal lengths which is commercially available (see the Telescope Calculator page for more discussion of this); make sure the 'scope you're considering can reach the magnification ranges you're most interested in.

How Much Light Gathering Power Is Needed?

Some observers would quip that you can never have a big enough telescope. While the views of small, faint objects will probably be more spectacular through a huge telescope than a small one, that doesn't always make the larger one superior. If the object is large, the view in a smaller 'scope, which takes in the whole object and “frames” it nicely, might be prettier than the one through the large 'scope, which can only look at a small portion of the object at a time. And the view through a smaller telescope, which you popped out into the backyard for a quick observing session, wins by default over that through a large 'scope which you didn't bother to lug out and set up!

Just like the wide range of sizes represented by the different astronomical objects we observe, there is a large range of object brightnesses, too. Larger diameter 'scopes are needed to view fainter objects, but not all objects are faint. Let's take a look at a comparison of the brighness of some types of objects. While point-like stars are simple (see the Theoretical Magnitude Limits section of the Telescope Calculator page, which gives you the magnitude of the faintest star you should be able to see with any given telescope), we also want to observe non-pointlike objects like planets, nebulas and galaxies, which get fainter the more you magnify them (magnifying spreads out their light). For those extended objects, we look at their surface brightness, which is the amount of light that shines from a given unit of area of their “surface”, as it appears to us.

The Moon, at an average brightness of 3.3mag./sq", is a bright object, as are all the naked-eye planets. They can be observed well with a 3- or 4-inch telescope, even at fairly high powers (if the 'scope has a long enough focal length to reach high power). But there is quite a jump down in brightness when we get out of our Solar System; for example, the Ring Nebula (M57), which is a small but bright planetary nebula in the constellation Lyra, has an average surface brightness of only 17.7mag./sq". M82, a comparatively bright galaxy in Ursa Major, has a surface brightness of just 21.5; many galaxies are in that range or are fainter yet. The 3" telescope, under a reasonably dark sky, can pick up objects as faint as galaxies (as long as the objects are large enough in angular size), but this is the area where bigger instruments really have an edge.

The diameter of the telescope objective also relates directly to the instrument's “resolving power”; that is how small details can be discerned through it. In theory, the larger the telescope's aperture (the size of its objective lens or mirror), the finer details it can resolve. This is assuming perfect conditions, though; a small telescope of high quality may beat a larger one that's just so-so. Also, as noted before, the atmospheric conditions (seeing) will limit resolving capability; oddly enough, sometimes in bad seeing, a small 'scope will resolve more than a larger diameter one, because the large one “catches” more turbulence!

Beyond The Tube

Wait a minute, we're not finished with our discussion of choosing a telescope optical system already, are we? I don't know what to buy yet! Well, it is beyond the scope (pardon the pun) of this page to look in depth at the pluses and minuses of each particular optical design (see the "How Telescopes Work" page for more on that). Using the two above tools (analyzing objective diameter and focal length) which apply to all telescopic systems is a big start. But there are some other important factors to consider in choosing a telescope.

Mountings

Astronomical telescopes really cannot be hand-held; they must be mounted on some sort of machinery which holds them still and stable, yet allows them to be aimed wherever the user wishes. The one exception would be binoculars, but even those benefit greatly from being stabily mounted when used for stargazing. It may not be obvious to the beginner, but the experienced observer learns that the quality of a telescope's mounting is just as important as the quality of its optics; what good is a fine telescope if you can't get objects into view and keep them there long enough to see them?

A telescope looks at a tiny spot in the sky. Consider the graphic above, which shows object sizes; look at the .5° circle which represents the size of the Moon, then go outdoors and look at the Moon in the sky. Your telescope is going to be looking at an area of sky only about that size; you have to center that on the object you want to see, and hold that position (remembering that any shaking or shifting of the 'scope will be magnified by as much as the size of the image is.) You need a good mount!

Mount Classes

While there are many designs of telescope mounts, one important distinction is (clock) driven mounts vs. un-driven mounts. The sky rotates over us continually; everything in it rides along as it turns, rising in the east and setting in the west. While this motion looks slow to us, our telescopes (which are fixed to the Earth) are looking at tiny spots in the sky, and the sky shifts past those spots fairly rapidly. If you point your telescope with a half-degree viewing field at a star overhead, that star will drift from your view in a minute or so if the telescope isn't pushed along to follow it.

A clock driven mount will turn the telescope to follow the sky; when you put that star in, it will stay centered (for the rest of the night, if things are working well). A driven mount can be a real pleasure, especially when using higher magnifications (with their correspondingly smaller viewing fields). But they have their drawbacks: Generally, they are more expensive; they are more mechanically complicated, and therefore might take longer to learn to use, or be more succeptible to mechanical problems; they often take longer to set up. So, if you are on a budget, or want a 'scope to pop outside and observe with quickly, maybe undriven is for you, while if you want to spend longer times studying things (especially at higher powers), you might really want a driven mount.

Just a few years back, mounts were generally divided into two classes: equatorial and non-equatorial (the latter group most often being represented by altitude-azimuth or “alt-az” mounts, which allow the 'scope to swing up-down on one axis and and in horizontal circles on the other). Equatorial mounts have an axis tilted to match the angle of Earth's axis; by turning that axis, the telescope is clock driven. That was the only easy way to drive a telescope to track the sky for many years. Now, computers can control motors on both axes of an alt-az mount, changing their speeds and directions as needed to follow the sky in whatever direction the telescope is pointed. Additionally, “equatorial platforms” are now available which entire telescope & mount setups can be placed on, turning the whole package to follow the sky. These changes have blurred the line between the equatorial and non-equatorial mount, and made classifying them by clock driven and not clock driven a more useful one. (As an additional change, some folks now use the term "tracking" in place of "clock driven" when describing mounts, further removing us from the days when actual clock mechanisms were used to turn equatorial mounts.)

Finding Things

A second main mounting class distinction has to do with how you aim the telescope. Some clock-driven, computer-controlled mounts can also take directions from the computer and turn the telescope to point at whatever object is selected by the user. This Go-To type setup may sound ideal -- just sit back, and your telescope will show you things -- but it should be approached with a little caution. First off, it is a mechanically complicated system; do expect to have to spend some time learning how to make it work properly, and remember the engineer's axiom that the more complex something is, the more things there are to go wrong. Secondly, don't expect to be able to use it successfully if you have no knowledge of what's up in the sky (at least some constellation identification ability, etc.); you need to verify that the machinery is sending you to the right places. This brings up the other side of the coin: Many stargazers find that learning their way around the night sky is a joy unto itself; the heavens become your “playground”, not just something you're shown a tour of. Using star charts (maps of the sky) and locating hidden objects on your own can be a great adventure in itself.

Both driven and un-driven mounts can feature what are called “setting circles”; these can tell you the celestial coordinates of the spot the 'scope is pointed to (the sky having a coordinate grid system on it analogous to Earth's latitude-longitude system; in the sky, north-south is measured as degrees of Declination, east-west in hours of Right Ascension). On some mounts, there will be actual inscribed circles or disks with pointers to indicate Dec and R.A.; more commonly nowadays, you see “digital setting circles”, which show the Dec and R.A. information on an electronic display. Setting circles can be a great aid in locating objects which are too faint to see with the unaided eye; there are other techniques which work well, too, based on comparing star charts to the sky overhead.

If a telescope is ever going to ever be manually pointed at objects, it needs some sort of “finder” instrument on it; again, telescopes view too small a spot of sky to be effectively aimed on their own. Often the finder is a smaller telescope, which looks at a wide field of sky at very low magnification, and has a crosshair or other marker at the center of its field; when the marker is aligned on the location of the desired object, the object will be centered in the field of the main telescope (if the user has properly aligned the finder when he or she started observing!) Generally, a finder 'scope needs a fairly large objective lens to be really useful; some observers would say that 50mm diameter is the minimum. Also popular are “reflex” finders (also called "unit power" or "red dot" finders) which do not magnify, but when viewed through, superimpose a lighted target (a red dot or circles) on the sky, centered on the spot which the main telescope is viewing; they can be very effective finders, but like the telescopic ones, they must be kept aligned with the main 'scope to function usefully.

Clock-driven or not, Go-To vs. setting circles vs. “star hopping” and other methods of locating objects manually: These are personal preference choices; each stargazer needs to discover what gives them the most pleasure, and gets them out observing and enjoying the wonders of the skies. But again, the mounting of your telescope is not a part to be overlooked when you are choosing an instrument!

Making Decisions

We have grazed the surface in this look at factors to consider when choosing a telescope, but the basic qualities of the optical and mounting systems are things you're “locked into” with a given telescope, which affect usability for various types of observing, and which will have a notable effect on the amount of pleasure you'll get from using it, so they are worth focusing on. Other points to consider include:

--Your Budget: On the bottom end, plan on spending at least $150US on a starter telescope; on the top, well, there is no limit! Remember that there are definite differences in the cost-per-inch of aperture for different telescope types, and in the base cost of different types of mountings. Used equipment is often available at good prices (see our Astro-Ads page for examples).

--Portability: Where do you want to be able to use the telescope (i.e., in your yard, vs. transporting it to other spots, taking it on vacation)? How heavy are the parts? How long will it take to haul out, set up, take down and put away? Where will you store it (hopefully somewhere clean, without high humidity!) Telescopes which are too big, too heavy or too much trouble to set up do not get used much!

--Easy of Use: Are the potential users interested in applying mechanical and computer skills, or would they rather have simplicity?

--Versatility: Do you want to be able to do other observing besides stargazing (birdwatching, etc.)? Photography through a telescope is a complicated subject; most telescopes can be used for simple “afocal” snapshots, while other types astrophotography are extremely complex; if astrophotography is one of your goals, you need to do considerably more research before you choose a telescope and mounting!

Where to Buy

Unfortunately, most towns don't have a good telescope supermarket where you can go see every model, take them for test drives, etc. In place of that, many areas do have amateur astronomy clubs; they are a tremendous resource for the person considering a telescope purchase. A club observing session will feature all sorts of models of 'scopes and mounts (which you can see in action!); members have lots of real-world experience with many items, perhaps even the very ones you are considering. If you've considered the points on this set of pages, and talked with other observers, you will be much more prepared to make a purchase, even if it is sight-unseen from a catalog or Internet store. On that point, a friendly recommendation is to generally not buy a telescope from a discount or department store; there are poorly made 'scopes out on the market, and a retailer who knows nothing about astronomy (and lots about cut-rate selling) is as likely to sell junk as they are quality items (perhaps leaning more toward the former!)

So, the best advice is to do some research and consideration before buying a telescope. (You apparently already figured that out-- you're reading this!) If you're just getting interested in stargazing, many amateurs advise that you buy two other things first: A simple star chart and/or planisphere (an all-sky chart which you rotate to show the sky for a particular time of night), and a pair of binoculars (10x50 binos being great for stargazing). You can see lots more with the binoculars than with your eyes alone, you can get a good introduction to the sky, you'll see how much you really like stargazing, and you can use the binoculars for other things (in the slim chance that the astronomy bug doesn't last!) Oh yeah, joining that astronomy club is probably a great investment, too; it will likely give you opportunities to view through many thousands of dollars worth of equipment that you didn't have to buy!