1. Telescopes 101
In this guide you’ll learn how to choose a telescope, including how to choose a telescope for stargazing and some tips on how to choose a telescope for a child. A telescope makes a wonderful gift for someone who has an interest in the night sky. Even a majority of the smallest inexpensive telescopes are capable of revealing the rings of Saturn, the cloud belts of Jupiter, hundreds of star clusters, and even distant galaxies. With a medium or large telescope, amateur astronomers rarely see everything there is to see with a telescope, even after years of observing. Astronomy is truly an interest that can last a lifetime, especially if you know how to pick a telescope that will continue to be used as the hobbyist picks up more skills.
So how do you choose a telescope for a new astronomer? With so many telescope models designed for the beginner, it is sometimes hard to figure out how to pick a telescope, especially if you are not an astronomer yourself. Here are some frequently asked questions about telescopes for the person buying gifts for an astronomer. We have also included some telescope recommendations for beginning astronomers to make shopping easier. Learn how telescopes work with OpticsPlanet and become an avid astronomer for life. We’ll tell you all about the best telescope to buy when choosing between reflecting telescopes, refracting telescopes, Schmidt-Newtonian telescopes, Schmidt Cassegrain telescopes, Dobsonian telescopes, and the Maksutov-Cassegrain telescopes.
Sizes and Measurements
Since any telescope, even a small one, can produce any given magnification with the right telescope eyepiece, magnification is not a useful way to rate a telescope. Therefore, the size of the lens or mirror (the heart of the telescope) is used as a general way to measure telescope potential. This is usually part of the telescope’s model description.
Size of the Lens or Mirror
The larger the telescope lens or telescope mirror, the fainter the objects a telescope will reveal (the more objects that will be visible), and the greater the telescope magnification it can use and still produce a good image. For example, at 120x, a ten-inch telescope will reveal hundreds of objects not visible in a three-inch telescope. In addition, at 120x, the 10-inch telescope will produce images that are brighter and sharper than the 3-inch telescope.
Large vs. Small Telescopes
For amateur astronomers, a small telescope generally refers to a telescope of 4 inches or less. A medium telescope generally means a telescope of five to eight inches and a large telescope usually means anything over ten inches in size. However, this is a very general way to look at a telescope. Optical quality and special features may allow a small telescope to outperform a larger telescope for some types of observing.
Is Larger Always Better?
When it comes to telescope sizes, not always. Large telescopes do translate into more objects seen and better images, but they are also more expensive and also less portable. Many astronomers choose smaller telescopes because they need to transport them to locations where the sky is darker and more stars are visible. Even if observing will be done at home in the backyard, portability is still a factor. When a telescope becomes a chore to move and set up because of its size, it’s doomed to collect dust in the closet. If you are buying a telescope for a new astronomer, especially a child, it also makes sense to start with a telescope that is suitable to their size, age, and interest.
What to Look For
You need a starfinder or planisphere. It will help you locate the most common things to look at in the sky. A great tool for the beginner is a finderscope, a piece of equipment that will help you find various objects in the cosmos. It also provides interesting facts and information about astronomical phenomena. It’s a great telescope accessory to get started in stargazing.
You probably don’t need as much as you think. Few astronomical objects require more than 350x magnification and most observing is done at less than 200x, even with a large telescope. It is essential to understand that as magnification goes up, telescope image quality goes down and that it goes down much quicker in a small telescope.
500x Magnification with Small Telescopes
Yes, you can get 500x or 600x magnification, even in a small telescope, if you use the right telescope eyepieces. However, at such extreme magnification, the images produced will be useless. Worse yet, especially for the beginner astronomer, as magnification goes up, the ease of astronomical telescope use quickly goes down. At 500x, a small telescope is impossible to use, despite what it might say on the box about magnification.
Minimum Magnification With a Good Image
That depends partly on Mother Nature. On nights when the sky is clear and stable, you can use higher magnifications than on nights when the sky is hazy or unsettled. Assuming good conditions, a basic rule of thumb is 30-50x of magnification for every inch of a telescope as a maximum. Thus, the top useable magnification for a 2.4 inch (60mm) will be in the neighborhood of 120x on nights when the sky allows it. This is still more than enough to see the rings of Saturn, cloud belts on Jupiter, and many star clusters and nebulae. Magnifications above this in a 2.4-inch telescope, even if you have the telescope eyepieces to obtain it, will produce images of low quality. Above 350-400x, even large telescopes have trouble seeing through the atmosphere above us.
Determining Magnification with the Eyepiece
Telescope eyepieces are marked with their focal length in millimeters, not magnification. To calculate the magnification you will get with any telescope eyepiece, divide the focal length of the telescope eyepiece into the focal length of the telescope. A telescope eyepiece with a focal length of 25 mm when used in a telescope with a 1,000 mm focal length, therefore, produces a magnification of 40x.
How to Choose a Telescope By Type
Newtonian Reflector Telescope
A reflector telescope uses a large mirror at the bottom of the telescope to focus light rays on a smaller mirror near the opening which then reflects the light into an eyepiece located at the top of the telescope. The Newtonian reflector is the oldest reflector design in use and still the most popular reflector telescope.
Reflector telescopes cost less to produce than other types and offer the most telescope for the money. Reflector telescopes produce good images and are favored by astronomers who need the largest possible telescope to find distant, faint objects beyond our solar system (deep-sky objects). If you are thinking of giving a telescope that will allow room to grow in astronomy, a medium (four and a half inches or larger) reflector telescope is the least expensive way to go.
Reflector telescopes are more fragile than other types of telescopes and need periodic readjustment of the telescope mirrors (a process called collimation) and, because they are open on the top (Newtonian reflector), the mirrors are exposed to dust and dirt and therefore need periodic cleaning. The reflector telescope is by no means fragile, but it’s the least childproof telescope design. The telescope eyepiece, located at the top of the telescope, may also be more difficult for children to reach.
A Schmidt-Newtonian reflector telescope is a catadioptric telescope (a telescope that uses both lens and mirrors). The SN telescope uses a special plate at the top of the tube to correct the image before it reaches the primary mirror at the bottom of the tube. This makes the SN reflector telescope shorter than a standard Newtonian reflector with no loss of performance.
The Schmidt-Newtonian telescope is a shorter overall reflector than a Newtonian reflector which makes it more portable and easier to use. Because of the corrector plate (lens) at the top of the tube, the optics in a Schmidt-Newtonian telescope are also better protected against dirt and dust than a Newtonian reflector.
Unlike the Newtonian reflector, the corrector plate at the top of the SN telescope can collect dew, and like the Newtonian reflector, the SN reflector still needs periodic collimation of the mirrors. These, however, are minor concerns in an otherwise excellent telescope design.
Refractor telescopes use lenses (not mirrors) at the top of the telescope to focus light rays into an eyepiece at the bottom of the telescope. It’s the same design used in binoculars and most spotting scopes (daytime telescopes).
Refractor telescopes are virtually maintenance-free since the telescope tube is closed (not open as in a Newtonian reflector) and there is no telescope mirror to adjust. The refractor telescope is the most rugged telescope design and in the most expensive versions, a refractor will produce the brightest and sharpest image per inch of a telescope. The refractor telescope is a good choice for children, not only because it’s rugged, but also because the telescope eyepiece is located at the bottom of the telescope where it’s easier for small people to reach. Refractor telescopes are also easier to use as a daytime telescope (with an image erector) than reflector telescopes.
The refractor telescope is the most expensive telescope per inch of telescope size; beyond three inches, the refractor goes up very quickly in price. If you buy a beginner a small refractor, be advised that the beginner may quickly outgrow it if they stay with astronomy for long.
A Schmidt Cassegrain is a catadioptric telescope (a telescope with lenses and mirrors) that uses a combination of mirrors to squeeze an optically very long telescope into a short tube. A Schmidt-Cassegrain telescope has features of both a refractor (eyepiece at the bottom) and reflector (uses lenses) but is considerably shorter and more compact than either one. The Schmidt Cassegrain telescope is very similar to the Maksutov-Cassegrain below, but it uses a thinner, less complex corrector plate at the top of the telescope making it more affordable and lighter in weight when used in large telescopes.
A Schmidt Cassegrain telescope offers some of the advantages of a refractor telescope – durability and less maintenance – but at a lower price. The main advantage of a Schmidt Cassegrain telescope is portability; even a 5-inch Schmidt Cassegrain is small enough to carry on the front seat of your car. A Schmidt Cassegrain is regarded as the best all-purpose telescope for an astronomer who will be mixing visual observing with astrophotography.
A Schmidt Cassegrain telescope has a mirror in the front that partially obstructs the field of view and causes some light to be lost. For this reason, you may notice a slight darkening at the center of the field of view, though the effect on performance is negligible. Schmidt Cassegrain telescopes do not produce images quite as bright as the best reflector telescopes or an image quite as sharp as the best refractor telescopes, but its versatility is better than either.
Maksutov-Cassegrain telescopes, like the Schmidt-Cassegrain telescopes, are catadioptric telescopes (a telescope with lenses and mirrors). The difference between the Schmidt Cassegrain and the Maksutov-Cassegrain is in the shape and size of the corrector plate (lens) at the top of the telescope. The Maksutov-Cassegrain uses a heavier, more complex corrector plate.
The Maksutov-Cassegrain telescope corrector plate allows for very fine correction of its optics. The Mak is a telescope capable of superb image quality, second only to the finest refractor. The Maksutov-Cassegrain is also a great design for smaller telescopes and is very popular for spotting scopes.
The corrector plate on a Mak is heavier and more costly to produce. Maksutov-Cassegrain larger than 7 inches are, therefore, rare.
A Dobsonian telescope is more of a type of mount than a type of telescope. A Dobsonian telescope is a large Newtonian reflector (six inches or larger) mounted on a simple box-like alt-az mount (see mounts below).
A Dobsonian telescope is the essence of simplicity and a perfect choice for the astronomer who shuns hi-tech gadgetry and electronics. The “Dob” is the telescope for the deep-sky purist who wants to see the faintest objects at millions of light-years beyond our solar system. The secret of the Dobsonian telescope is in its size – it’s the largest telescope for the money you can buy. A huge ten-inch or even twelve-inch Dobsonian telescope will cost less than a fancy computerized telescope half its size and will easily take you to places in the universe no smaller telescope can go. Even an advanced amateur will not outgrow a ten-inch Dobsonian in his or her lifetime.
Dobsonian telescopes above ten inches in size are a problem to transport – a disassembled ten-inch Dobsonian telescope will still squeeze into a hatchback with all seats folded down and no passengers, but a twelve-inch Dobsonian will not. Because a Dobsonian mount is all manual, it requires constant readjusting (though easily done with practice) to keep an object in the telescope eyepiece. For the same reason, it’s limited for use as an astrophotography telescope.
A telescope is only as useful as its mount – the higher the magnification, the truer this becomes. A telescope mount not only steadies the telescope, but it also allows the telescope to follow stars and planets as they move across the night sky. When seen in the eyepiece of a telescope, stars and planets move slowly across the field of view in the eyepiece and disappear (because the telescope magnifies the earth’s rotation, not because the stars move). To keep the object centered in the eyepiece, the telescope must be moved constantly. This is done in several ways, depending on the telescope mount.
An Alt-AZ telescope mount requires you to move the telescope to the object you wish to find. You then move the telescope in two directions (up-down and right-left) to keep the object centered in the eyepiece as it moves. This is because stars move across the sky in a gentle arc or curve, but an Alt-AZ telescope mount only allows the telescope to be moved in a straight line.
Alt-AZ telescope mounts, because of their simplicity, are the least expensive telescope mounts. They are uncomplicated and rugged and the least likely to be damaged by over-eager children.
Because a star or planet follows a gentle curve across the night sky instead of a straight line, a telescope on an Alt-AZ mount must be adjusted both in an up-down as well as a right-left direction to keep a star or planet in the eyepiece. This is not much of a problem at low magnifications, but above 75x or so, an Alt-AZ mount can be frustrating for a beginner to use.
German Equatorial Mount
An equatorial telescope mount requires you to move the telescope to the object you wish to find, just as with an Alt-AZ mount. To keep an object centered in the eyepiece, however, requires only a simple nudge of the telescope because the equatorial mount moves the telescope in the same gentle curve that stars follow across the sky. You can spot an equatorial telescope mount easily because it looks like a couple of pipes laid crosswise against each other. One of these “pipes” often has a set of counterweights to balance the telescope as it moves.
An equatorial telescope mount moves a telescope across the sky in the same curved path that stars and planets move. To follow a star or planet, then, requires only a simple nudge of the telescope rather than two adjustments as needed in an Alt-AZ mount. This makes following objects in the sky much easier and makes higher magnifications more practical. Many beginner telescopes are now available with an equatorial telescope mount.
To use the equatorial mount effectively requires that it first be aligned with true north (near Polaris or the North Star). Children may need assistance for this. In larger telescopes, equatorial mounts become heavy and unwieldy and may require some disassembly when moving the telescope.
Manual GOTO Mount
A manual GOTO telescope mount is an Alt-AZ mount with a computer attached to one arm. A manual GOTO telescope mount requires you to move the telescope manually to find an object, just as in a standard Alt-AZ mount, but the computer gives you directions to find your object and tells you when you have found it. Once found, you still have to move the telescope to keep the object in the eyepiece (with the computer giving directions).
The manual GOTO telescope mount is an inexpensive way to help a beginner find an object in the night sky. it’s a good choice for those who wish to keep things simple but still need some help navigating among the stars.
A manual GOTO telescope mount still requires you to move the telescope yourself and is subject to all the other limitations of an Alt-AZ mount – best used at lower magnifications and for brighter objects.
A motorized telescope mount is a mount that requires you to move the telescope to the object you wish to see but then keeps it in the eyepiece for you by moving the telescope with a motor. In other words, once an object is found, no adjustment is needed to keep it in the eyepiece. A motorized telescope mount can be any type of mount – Alt-AZ, fork, or equatorial – that has an attached motor. The motor moves the telescope at the same rate of speed as objects that move across the night sky.
For visual observing, a motorized telescope mount makes it much easier to use high magnification – the observer spends more time viewing and less time moving the telescope. This is also the best telescope mount to use when sharing the telescope with others – no need to adjust the telescope after each person has had their turn. For serious astrophotography, a quality motorized mount is a must, with a motorized equatorial preferred.
To be effective, a motorized telescope mount must be carefully aligned first. The procedure will vary with the type of motorized mount, and periodic corrections will be needed to keep things in alignment. The best motorized equatorial mounts can also be very expensive and cost as much or more than the telescope.
A GOTO computerized telescope mount is a motorized mount that finds the object for you using a computer (usually with a handheld controller) and then keeps the object in the eyepiece for you.
Astronomy changed in a big way when computers were added to a telescope. To find objects in the night sky without a GOTO mount requires use and expertise with a star map, knowledge of the constellations, and experience (one reason so many beginners abandon astronomy as a hobby). Now anyone with a GOTO telescope mount can do instantly what used to require many nights under the stars to learn. Yes, some beginners will never bother to learn the night sky because of this technology, but for those who develop a real passion for astronomy and want to learn their way around the stars, GOTO technology is a great teacher.
This is not a technology for those who have an aversion to bells and whistles or computers. There is also a matter of expectations to consider. On small telescopes, the computer database in the mount may easily exceed the capability of the telescope. In other words, just because the computer is pointing the telescope at an object is no guarantee that the image in the eyepiece will be impressive or even visible, especially on nights of poor sky conditions. No computer can turn a small telescope into a large telescope.
Fork and GPS Mounts
A fork mount is a type of Alt-AZ telescope mount, usually motorized (read “What is a motorized mount?” below). The fork mount is widely used on Schmidt Cassegrain and Maksutov-Cassegrain telescopes. These are highly portable telescope mounts that can actually be used with or without a tripod if placed on a flat, level surface.
A GPS (Global Positioning System) is a navigational system that uses radio signals from satellites to locate your position anywhere on earth. On a telescope mount, it’s used in conjunction with motorized GOTO telescope mounts to inform the computer of the date, time, latitude, and longitude. For most GOTO telescope mounts without GPS, this information must be inputted manually by the observer.
A spotting scope is a small telescope designed for daytime observing of birds, wildlife, boats, airplanes, and any application where high magnification is needed. Optically speaking, most spotting telescopes are refractors, but several excellent spotting scopes are Maksutovs. Many astronomical objects – the rings of Saturn, Jupiter, star clusters, nebulae – are within the magnification range of a 20-60x zoom spotting scope.
The biggest advantage of using a spotting scope for astronomy is that it can also be used for daytime observing. If you are uncertain that the person on your gift list will sustain an interest in astronomy – particularly children – a spotting scope is a much more practical gift than a standard astronomy telescope. It’s much easier to use a spotting scope for astronomy than it’s to use a standard astronomy telescope by day.
A spotting scope for astronomy is limited by its size – most are 80mm or less – and also by its magnification – most spotting scopes go no higher than 60x. However, a few spotting telescope models use interchangeable telescope eyepieces which makes them more practical to use as astronomy telescopes. (see next question).
Summing It Up
Now that we’ve covered how to choose a telescope, you should be prepared to pick one out depending on your astronomical interests. Whether you’re shopping for yourself or looking for the perfect gift for an aspiring stargazer, you can trust OpticsPlanet to meet all of your needs. Shop our Astronomy Store today!