Classification of Telescopes

Disclaimer

The comments I include here are to help beginners understand the main characteristics of the telescope and mount types they will encounter. This is not intended to be an in-depth study. There are factors such as optical distortion, field flatness (scopes) and periodic error correction (mounts) that you will start to understand and care about much later, but they are not important factors as a beginner.

Your Telescope is Really a System

Although your beginner telescope will most likely come “everything included” in one package, you should understand that there are really several separate components involved. These could be purchased separately, and often are when you get into more advanced equipment. For example, many amateur astronomers will buy a telescope and mount as a set, and then later upgrade the mount while keeping the same telescope.

Types of Optical Tube Assembly

All the variety of telescope optics can be divided into three simple categories, depending on whether they are based on mirrors, lenses, or a combination of the two. Your telescope will be one of the following major types.

Reflector

A reflector has no lenses (other than the eyepiece). It uses a curved mirror, at the back, to focus the incoming light, and a small flat mirror tilted 45 degrees, at the front, to deflect the focused light out the side into the eyepiece. The eyepiece is located high up on the tube, near the front, sticking out at a right angle to the tube.

Advantages

• Reflectors have good optics for a good price, since it is easier to make good mirrors than good lenses (only one surface has to be precision-ground, and the glass doesn’t have to be clear since it will be coated). (Beware, it is still possible to make bad mirrors, as many junk scopes have managed to do.)
• You can also get larger apertures (diameter) with reflectors, since it is much easier to make large mirrors than good-quality large lenses. In all, you tend to get better optics or larger optics, for a given price, than with a refractor (a lens-based telescope).
• Since the main mirror is located at the bottom of a deep tube, usually closed on the sides in beginner-level gear, it resists dew much longer.

Disadvantages

• Low-quality mirrors in low-quality reflectors produce badly distorted images that cannot be adjusted. (But don’t panic – this applies to toys, “department store telescopes”, and the like. Even fairly inexpensive reflectors from reputable brands have good quality mirrors.)
• Reflectors require “collimation” at regular intervals. This is an alignment technique that ensures the mirrors are properly centred and aligned. It’s not difficult with the appropriate tools, but can be a little intimidating for the beginner.
• The eyepiece is at the front, which means it can be high off the ground if you are looking at something high overhead. It may require a stool or stepladder for shorter people or children to reach the eyepiece.
• They are long, both physically and optically. Being physically long can make large reflectors awkward to store and move. Being optically long means you get high magnification, which is not always an advantage; reflectors can have trouble producing low magnification wide-field views for observing large objects like certain star clusters.
• Finally, a reflector is an astronomy-only instrument, not suitable for terrestrial use. The mirrors cause images to be inverted or reversed. While this is not a problem when looking at objects in space, it would make terrestrial objects such as birds or people look upside down or reversed left-to-right.

Refractor

Refractors use lenses, not mirrors, to focus the incoming light. Usually there is a large lens near the front of the tube and another midway or near the back, followed by a diagonal (to bend the light 90 degrees) and the eyepiece. The eyepiece is at the back, or bottom, end.

Refractors come in a wide range of quality levels, with corresponding different prices. For example, a refractor with a 100 mm main lens might cost between $300 and $6000, with the use of exotic glass and higher-quality mechanical parts accounting for the price of the expensive units.

Advantages

• Some observers find that good refractors can have cleaner images, showing better contrast and less distortion than a similar sized reflector. High-quality refractors give stunning clear images and pinpoint stars, and have cult-like followings. (But they are very expensive.)
• Refractors have no obstruction inside the tube (like the secondary mirror in the other two types), which results in their higher contrast, and also eliminates the cross-shaped patterns that appear on bright stars in the other types. (These effects are minor, even pretty, and should not stop you from considering the other designs).
• Small refractors are highly portable, by far the easiest unit to take in a small vehicle, backpacking, or on an aircraft.
• Since the lens is permanently mounted, refractors rarely need maintenance or collimation.
• Short-focal length refractors can produce better low-power, wide-field views, yet can still produce moderately high magnification for planetary viewing.
• The eyepiece, being at the back, is lower to the ground and may be more easily accessible.
• With an appropriate diagonal, a refractor can be used for terrestrial viewing, or even hooked directly to a camera as a high-quality telephoto lens.

Disadvantages

• Low-priced refractors, called “achromats”, have an optical characteristic that slightly distorts the colour of bright objects. For example, it is common to see a blue-violet fringe at the edge of the moon or bright planets. High-quality refractors, called apochromats, use exotic glass alloys that eliminate this colour distortion, but they are expensive.
• It is harder to make refractors than reflectors (lenses are harder to make than mirrors, and there are twice as many surfaces to be precision ground). So for a given price, you generally get a smaller aperture in a refractor than in a reflector.
• Large refractors can be very long, making them difficult to store, transport, and balance on the mount.
• The eyepiece, being at the back, is lower to the ground. Generally considered an advantage, this can be a disadvantage when observing an object high overhead with a large refractor – the eyepiece may be very low, requiring you to sit on the ground.
• Because there is a lens near the front, refractors are more prone to dew than reflectors, although the built-in dew shield on most models helps to minimize this.

Compound or Catadioptric

Compound telescopes (also called Catadioptrics, or CATs for short) use both: lenses and mirrors. The most common types use a lens at the front, a curved mirror at the back to focus the light back toward the front, and a small secondary mirror mounted on the back of the front lens to reflect the focused light back down, through a hole in the main mirror, to a diagonal and eyepiece mounted on the bottom end.

The most common types that behave like this are called Schmidt-Cassegrain Telescopes (SCTs) or Maksutov-Cassegrains (Maks). The mass-produced SCTs from the large manufacturers Celestron and Meade are the most common and most accessible mid-priced optics for many amateurs today.

Advantages

• CATs are much more compact than reflectors or refractors for similar optical specifications. This makes them easier to store, transport, assemble, and balance on the mount. Their compactness makes large apertures (diameters) attainable in a somewhat portable body.
• They tend to have long focal lengths for their size, which makes high magnifications available for good planetary viewing.
• They are often integrated with sophisticated mounts (see below) for ease of use.

Disadvantages

• CATs have more complex optics than the other types, requiring more maintenance such as more frequent collimation.
• The large front lens, mounted directly at the leading edge of the tube, is very susceptible to dew. An add-on dew shield, heater, or hair dryer is an essential accessory.
• The secondary mirror acts as an obstruction, like the secondary in a reflector, slightly reducing the contrast as compared to a refractor.
• CATs tend to have very long focal lengths, making low-power views difficult to attain. Low-power use often requires a “focal reducer” — an accessory rear lens that reduces the magnification of the unit.

What’s Best?

I hope you developed the impression there is no clear winner in the above list. Each type of optical unit has advantages and disadvantages. None of them are a poor choice for a beginner, nor are any of them obviously superior. But you can frustrate yourself by trying to use a design for something it does poorly – e.g. bird-watching with a reflector, or low-power, wide-field viewing with an SCT.

Types of Mount

Properly speaking, the “mount” is just the “head” — the metal unit on top of the tripod, to which your scope attaches, and which moves it in two dimensions. In general language, however, we usually say “mount” to include everything under the telescope, both the head and the tripod.

There are only two major varieties of mount, although each has some subtypes. The major two varieties are

Each of these mount types has advantages and disadvantages, and each has its fans and detractors. You should understand both and then make a careful decision on which type you wish to start with. Each type of mount is available with a variety of options, and in subtypes. Here are more detailed discussions of these mounts:

Equatorial Mount

The Equatorial mount is recognizable from the characteristic tilt at which the telescope tube is held, and by the counterweight on a long shaft at right-angles to the telescope tube. The mount moves the telescope in two directions that may not be immediately intuitive. One, called “declination”, is an “up/down” direction roughly equivalent to altitude. The other, however, is unique. Called “Right Ascension”, this dimension is a rotational movement. The mount is designed to have the axis of this rotation pointed at the celestial North Pole (usually at Polaris, the Pole Star). The second dimension of movement is then rotation around the Pole, just as the stars move.

Advantages

There are several advantages to this type of mount:

• It tracks stars across the sky with only a single simple adjustment. This means that you can keep an object centred by adjusting a single control, or automate this with a single motor.
• Because the telescope is offset somewhat from the mount, the mount can point anywhere in the sky, even straight up, without interference.
• Since the mount actually rotates around the pole, it keeps objects not only centred but also oriented in a consistent way. This is required for long-exposure astrophotography, so these mounts are preferred by photographers.
• Since the mount’s motion corresponds directly to the way the sky moves, it can help gain a better understanding of the geometry of the sky, thus being more educational.
• Although they are often motorized, equatorial mounts don’t require a motor, so you can still use the mount if the power fails (assuming it has manual slow-motion controls such as the mount shown above).

Disadvantages

There are, of course, also some disadvantages:

• The directions of motion are not intuitive since they don’t correspond to the directions we usually think about here on earth. You will need time to get used to this system for astronomical use, and you will find it very unsuitable if you also try to use the telescope for terrestrial viewing (such as birds).
• Polar alignment of these mounts is required. This is another skill to learn, and is easiest in locations where the North Star is visible. If North is obscured from your observing location, alignment is more of a challenge.
• They can also require a longer time to set up, since they must be polar aligned and balanced.
  Large equatorial mounts can be quite heavy because of the counterweights.

Altitude-Azimuth Mount

Usually called “alt-az” mounts, these are simple mounts that hold the telescope and move in two directions: up-down (“altitude”) and left-right (“azimuth”).

Advantages

Advantages of this type of mount include:

• Because the left-right, up-down motions correspond to the way we think of motions here on earth, many people find these mounts much easier to understand and use.
• They are much simpler, mechanically, meaning they are easier to transport and take less time to set up. This makes them especially well-suited for “grab-and-go” astronomy: observing when you have only a brief window of time available, and want to spend that time observing and not setting up.

Disadvantages

However, they also have some shortcomings as astronomical mounts:

• Up-down and left-right is not how objects in the sky appear to move. So although this intuitive system makes it easier to find objects, it makes it harder to track objects once you have found them. As an object drifts out of your field of view, you will need to move the scope in two dimensions to keep it centred.
• Some simpler Alt-az mounts have difficulty pointing straight up, or very high in the sky, because the mount gets in the way of the optical tube.
• Although some of these mounts can be motorized to keep objects centred automatically, this requires two motors and computer control since the two directions of motion needed to correspond to how the sky moves are complex. So motorized tracking versions of these mounts are more complex and more expensive.
• Even when motorized, these mounts are not suitable for long-exposure astrophotography because the telescope in such a mount does not rotate as it tracks objects across the sky. The result is that, in long exposures, objects appear to rotate, changing stars into little arcs. This, too, can be overcome by adding accessories to the mount, but this, again, increases complexity.

Two specialized variations of alt-az mounts are extremely popular:

For an alt-az mount to track objects as they move across the sky, it’s necessary to have a motor drive both the up/down and left/right motions in an appropriate proportion, which requires a built-in computer. And since the computer is there anyway, it is very little effort to make the mount a fully automated “go-to” design.

Such mounts are known as fork mounts because the optical tube is suspended inside the two arms of a U-shaped structure resembling a tuning fork.

(Taking the analogy a little too far, there are also “one-sided fork” mounts available for smaller tubes.)

Advantages

• These mounts retain most of the advantages of the alt-az mount, and overcome some of their disadvantages.
• The up/down, left/right dimensions of movement are intuitive, and easy for a beginner to learn.
• They can be more compact than equatorial mounts of similar capacity.
• They can automatically track objects across the sky.
• With the go-to function that is almost always included, they can also find objects in the sky.

Disadvantages

• Since these mounts require the computer to operate, they are dependent on the electronics and power. There is usually no manual mode and no ability to operate without power.
• Large fork mounts can be quite heavy.
• Depending on the design, they may still have difficulty looking straight up.
• Basic fork mounts are not suitable for long-exposure astrophotography because they don’t rotate while they track. This can be overcome with optional accessories, either a field de-rotator (a motor which rotates the camera to compensate for field rotation) or an equatorial wedge (a tilting platform that changes the fork mount into an equatorial mount, requiring polar alignment).

Dobsonian Mounts

The term Dobsonian refers to a reflector telescope mounted in a special kind of alt-az mount. “Dobs” are considered excellent beginner scopes because the mount is simple, sturdy, and relatively inexpensive, allowing the investment to go into better optics.

A Dobsonian mount consists of a cradle that holds the reflector tube, near the bottom of the tube, in a simple curved holder that allows smooth up/down motion. The entire cradle is then mounted on a turntable that allows left-right motion.

Advantages

The advantages of the Reflector optical tube and of the Alt-Az mount apply. In addition:

Dobsonians are simple and stable.

The simple mount allows most of the expense of a given unit to go into the optics. So, for a given price, you generally get the largest-diameter scope in the Dobsonian style.

Very large amateur telescopes are almost always Dobsonians, with an open framework of trusses replacing the solid tube. The truss-Dobsonian can be taken apart for transport, and might be fifteen feet tall and contain a two-foot diameter mirror, when assembled.

Disadvantages

The disadvantages of the Reflector optical tube and of the Alt-Az mount apply. Like alt-az fork mounts, dobsonians can be motorized and computer controlled, but basic entry-level units usually are not.

Comparison

The following table comments on which combinations of the above optical and mount designs you will encounter.

Conclusion

There are several design types for both telescope optical tubes and for mounts. Each has advantages and disadvantages, and none is clearly superior. You need to understand their characteristics and decide what will best meet your needs.

The classic advice for a typical beginner is to start with a mid-sized (150 to 200 mm6 to 8 inch) reflector on a Dobsonian mount.