Which telescope should you buy? Let's examine the different types of telescopes that you might purchase—to help you select the telescope that's best for you.
Every telescope is composed of the same basic components because the main purpose is the same: to gather the light from celestial objects.
This could be light from objects that produce it such as stars and galaxies. Or light from bodies such as planets, comets, and our Moon that reflect the light from the Sun. Give some thought to which one would be best for you by asking yourself these questions:
- How deeply into astronomical observing do I expect to go?
- How motivated am I?
- Is astronomical observing something that I will do occasionally, or will I want to get out under the stars every chance that I get?
- Do I enjoy learning how mechanical devices work, or am I more of an “appliance operator” who simply wants to look at things?
The “best” telescope is the one that you will actually use. If it’s so complicated that you dread using it, then it’s a poor choice for you.
Before you buy, understand how telescopes work and the differences in types.
A main lens or a mirror is the main optical component of a telescope. The larger it is, the more light it will gather and the easier it will be for you to see faint objects.
The diameter of the main lens or mirror is called the aperture. The size of the aperture determines how much light a telescope will gather. (Even a small telescope gathers more light than the naked eye/s.) All else being equal, the larger a telescope’s aperture, the more you can see with it.
Fig. 1. Achromatic lens used in a Refractor telescope (left). Parabolic mirror used in Reflector telescope (right).
The light gathered is projected through the telescope’s eyepiece. This is a small, barrel-shape device containing one or more lenses. It magnifies the image of the object in view. Typically, the eyepiece can be quickly and easily changed out for one of different magnification.
Fig. 2. The magnification of a telescope can be changed by swapping out the eyepiece. The eyepiece on the right is homemade by the author. The others are commercial products.
Magnification is expressed as a number indicating how large an object appears in the telescope as compared to how large it appears to the unaided eye. If an object appears to be 50 times larger through the telescope, the magnification is “50x,” or “50 times.”
The maximum useful magnification for any telescope is 50x (50 times) the aperture in inches. For example, a telescope with a 3-inch main lens or mirror will have a maximum useful magnification of 3 x 50 = 150x (150 times). Most backyard telescopes are between 50x and 100x.
You can get an eyepiece that will deliver more than 50x per inch. However, the image will almost certainly be dim and fuzzy—or “empty.” (Magnification beyond 50x per inch is known as “empty” magnification.) Don’t be drawn in by magnifications claiming upwards of 200x per inch; these will almost surely disappoint.
The greater the magnification, the narrower the user’s field of view—and the more difficult it can be to accurately point a telescope at an object. This is why you use a finder.
Most telescopes have a finder. Often it is a small telescope with low magnification and a large field of view. When an object is in the center of the finder’s field of view, it is also in the main telescope’s field of view. In recent years, “red dot” finders have become popular. A red dot finder superimposes a small glowing light on the sky. You move the telescope to place the dot on the desired object, which then appears in the main telescope’s field of view.
Fig. 3. This small, low-magnification finder scope is about 6 inches long and is normally attached to a larger telescope to aid in pointing the main telescope at astronomical objects.
A diagonal mirror fits between the telescope and the eyepiece and bends the light path through a 90-degree angle. This makes viewing more comfortable.
Fig. 4. The 1.25” Star Diagonal from Celestron is a diagonal mirror that bends the light path 90° to place the eyepiece in a convenient position. Photo courtesy of Celestron: http://www.celestron.com.
A focuser makes the image as sharp and clear as possible.
Different types of telescopes arrange these basic components slightly differently. Here’s what you need to know.
When most people think “telescope,” they think of a refractor—essentially, a long tube with a lens at the end nearest the sky and an eyepiece at the other end. Refractors were the first telescopes to be invented, beginning in the early 1600s.
Fig. 5. The PowerSeeker 70EQ Telescope from Celestron is a example of a small refractor can show you the moons of Jupiter, the rings of Saturn, the craters of the Moon, and thousands of galaxies and star clusters. This refractor has a 70 mm objective lens, an equatorial mount, and a 5x finder. Photo courtesy of Celestron: http://www.celestron.com.
The earliest astronomical telescopes—such as the ones used by Galileo—were refractors. So were the “spyglasses” used for centuries by mariners.
Light enters a refractor through the “objective” lens. This bends, or refracts, the light slightly, causing it to strike the diagonal mirror, which reflects it to the eyepiece. The eyepiece magnifies the image and presents it to your eye. You can improve the clarity of the image by adjusting the focuser. (An adjustment moves the diagonal mirror toward or away from the objective lens.)
Fig. 6. A refractor is the simplest of telescopes. Lights enters through the objective lens at left, travels down the tube until it strikes the diagonal mirror, which bends the light and sends it to the eyepiece. CLICK TO ENLARGE.
Pros: Backyard refractors are seldom more than 4 or 5 inches (100 to 125 mm) in aperture. Generally, refractors are best suited for viewing relatively bright objects such as planets, star clusters, and the Moon. Refractors can provide exquisite views of these sorts of objects. Refractors are easy to use, seldom go out of adjustment, provide sharp images with good contrast, and are generally very sturdy.
Cons: Large objective lenses are expensive to make, so affordable refractors are limited to those with relatively small apertures. In addition, refractors with large apertures are fairly heavy and thus require exceptionally sturdy and expensive tripods and mountings (we discuss these below). Note, also, that quality varies: Two refractors that look similar on the outside can be vastly different in quality. To view galaxies or other faint objects, you will want more aperture than a typical refractor offers.
Expect to pay $100 to $300 for a starter refractor.
The main light-gathering component in a reflector telescope is a mirror. The “primary” mirror is located at the end of the telescope farthest from the sky and has a precise curve, or parabola. This reflects light as a narrow beam to a secondary mirror, which redirects the light into the eyepiece and to your eye.
Fig. 7. This Sky-Watcher 8” Collapsible Dobsonian reflector uses the same basic telescope design as the reflector invented by Sir Isaac Newton. The Dobsonian mount is a type of Altazimuth mount discussed later in this article. Photo courtesy of Sky-Watcher USA: http://www.skywatcherusa.com.
Sir Isaac Newton invented the reflector in 1668, and this type of telescope is often called a Newtonian Reflector or simply, a Newtonian. There are other types of reflectors, but the Newtonian is by far the most common.
Fig. 8. A Newtonian reflector collects light with a parabolic mirror and bounces it off a flat secondary mirror before it enters the eyepiece and reaches your eye. CLICK TO ENLARGE.
Whereas the lens of a refractor is located at the end of the telescope nearest the sky, the primary mirror of a reflector is located at the opposite end of the telescope, farthest from the sky. So where a refractor tends to be top heavy when pointed upward toward the sky, a reflector is bottom heavy.
The primary mirror of a reflector has a very specific shape, that of a parabola. Light striking anywhere on the surface of a parabolic mirror is all reflected toward a single point. In a reflector telescope, the light beam is intercepted by the flat secondary mirror and redirected sideways to the eyepiece and to your eye. The focuser is built into the eyepiece holder.
Parabolic mirrors are easier and less expensive to make than lenses: Only one side of the glass must be shaped; lenses are shaped on both sides. The front surface of a parabolic mirror is coated with a reflective material. It must be smooth and shiny for viewing, but the backside and interior glass of a reflector’s mirror can be dirty or cloudy. By contrast, a lens must be pristine because light passes through it. Due to these advantages in fabrication, an ambitious amateur astronomer can make a parabolic mirror and put it into a Newtonian reflector—and thousands have!
Pros: The aperture of a reflector telescope is typically much greater than that of a refractor (that is, it gathers more light) and enables you to see fainter objects. You will nearly always get more aperture for your dollar with a reflector.
Cons: Reflectors require regular adjustment and maintenance for best performance. Many owners spend a few minutes of every observing session adjusting the mirrors. Reflectors tend to be bulky, and they aren’t as intuitive as a refractor: The eyepiece position requires you to look into the side of the telescope in order to look up at the sky.
Expect to pay $150 to $500 for a starter reflector.
Can’t decide between a refractor and a reflector? Compound (or, to be technically proper, catadioptric) telescopes combine both lenses and mirrors in their design.
Of the several different designs for compound telescopes, the most popular is the Schmidt–Cassegrain Telescope (SCT). This combines elements of the Schmidt camera, developed in 1931 by Bernard Schmidt, and the Cassegrain telescope, attributed to French priest Laurent Cassegrain in 1672. Astronomer James Gilbert Baker combined the two designs into the Schmidt–Cassegrain Telescope in 1940.
Fig. 9. A Schmidt-Cassegrain Telescope is notable for its short, stubby design that packs many features of a large telescope into a compact tube. The Celestron NexStar 6SE features a computerized “Go-To” drive to find objects automatically. Photo courtesy of Celestron: http://www.celestron.com.
The sophisticated optical design of the SCT has a complex light path. Light enters through a corrector plate (a lens that bends light), then strikes the primary mirror at the back of the telescope and bounces forward, where it encounters the secondary mirror mounted on the inside of the corrector plate. Instantly, the light is reflected to the back of the telescope again, where it passes through a hole in the middle of the primary mirror. A diagonal mirror typically mounted behind the primary mirror directs the light through the eyepiece and into your eye!
A baffle tube inside the telescope keeps stray reflections from interfering with the view. Focusing an SCT is easily done with a control knob. Despite their rather complicated design, Schmidt-Cassegrain telescopes stay in adjustment fairly well. They require occasional alignment, but not as much as a Newtonian reflector.
Fig. 10. The light path inside a Schmidt-Cassegrain Telescope (SCT) is far more complex than other popular designs. CLICK TO ENLARGE.
Most SCTs being sold today are “Go-To” telescopes: They have a computerized motor drive system that simplifies the search for objects in the sky: You tell the telescope which object you wish to view, and it moves to the object.
Pros: Many backyard astronomers consider the SCT to be a good compromise: It offers almost as sharp a view as a refractor but has a large enough aperture to see faint objects such as can be seen with a reflector. The SCT is lightweight (one with an 8-inch, or about 200mm, aperature is about 18 inches long and weighs 15 to 20 pounds; a similar refractor would be about 8 feet long and weigh 75 pounds or more), compact, and easily handled by one person.
Cons: An SCT is expensive, and the view it provides is not quite as sharp as that of a refractor.
Expect to pay $700 and up for a compound telescope.
Tripods and Mounts
A telescope is only as good as its mounting. When you are magnifying objects 50, 100, 200 times, any wobble in the mount or tripod results in a shaky view in the telescope. Every telescope needs a solid mount. There are three popular types.
“Altazimuth” is shorthand for Altitude and Azimuth. These words relate to the ways you can move the telescope on it when searching for an object in the sky: Altitude is up and down. Azimuth is left and right. Many beginner refractor telescopes are supplied on this mount.
Fig. 11. The Altazimuth mount is a simple mount often supplied with beginner refractors.
The mount sits atop a wooden or aluminum tripod, the legs of which can be adjusted for each user. This mount’s movements are simple and intuitive, although some dexterity is required to move the telescope in two axes at the same time, and it usually has controls for moving and locking the telescope in place.
Pros: The Altazimuth mount is easy moved, easily positioned for viewing, and common to many refractor telescopes.
Cons: The Altazimuth mount’s motions do not match the motions of objects in the sky. (See Equatorial mount, below.)
The Dobsonian mount is, essentially, an Altazimuth for large, heavy telescopes, such as reflectors. It was popularized by the late John Dobson, cofounder of the San Francisco Sidewalk Astronomers.
In late 1960s, Dobson believed that large telescopes were expensive, so he sought an inexpensive yet simple mount for them—one that could be built by anyone who was handy with tools. He took the concept even further and designed entire telescopes that could be home-built.
Fig. 12. The Dobsonian mount is a solid and stable platform for large reflectors. It is a type of Altazimuth mount.
The Dobsonian mount, usually constructed from wood and laminate countertop material, sits on the ground and does not need a tripod. Its low center of gravity makes it stable. The telescope is positioned on the mount, and it functions like an Altazimuth mount: It moves up, down, left, and right.
Thousands of amateur astronomers have built Dobsonian mounts for their large reflectors. Many commercial vendors of large reflectors now supply Dobsonian mounts with their telescopes.
Due to Earth’s rotation on its axis, objects in the sky appear to be in constant, albeit slow motion. However, when magnified 50 or 100 times, even slow-traveling objects quickly drift out of view; you must constantly move the telescope! Equatorial mounts are designed to match the motions of objects in the sky.
Fig. 13. The Equatorial mount has its Polar Axis aligned with the Earth’s axis of rotation by pointing the axis toward the Celestial Pole, located near Polaris, the North Star.
The Equatorial mount has one of its axes of motion, the Polar Axis, always aligned with, or parallel to, Earth’s axis. You would point it at the Celestial Pole (in the Northern Hemisphere, this is located near Polaris, the North Star). To view an object, you would move, or point, the telescope in both the Polar Axis and the Declination Axis. (The Polar Axis always remains pointed at Polaris.) This is both the essence of an Equatorial mount and the part that is most difficult to understand.
Once the object has been found, you would lock the Declination Axis and follow the object by moving the Polar Axis only. By eliminating the need to constantly move the telescope in two axes at once, the Equatorial mount makes it much easier to keep an object in view.
The movements of an Equatorial mount are not simple or intuitive! The best way to understand it is to use one. There are many types of Equatorial mounts.
So, which telescope should you buy?
Again: The best telescope is one that you will actually use. A telescope that is too heavy or too complicated or does not show you what you want to see eventually ends up gathering dust in a corner of your garage.
A telescope of any type and size can keep you busy for a lifetime. But whether you use a telescope, binoculars, or simply your own two eyes, there is plenty to see on every clear night. You just have to go outside and look up.
Jeff DeTray, Astronomy Boy