The Three Main Types of Astronomy Gear
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The three most important types of astronomy gear are the telescope, the mount, and the eyepiece.
But before I dive into describing the main types of gear, it’s worth considering something even simpler. Some say that binoculars are a great first “telescope” in that they provide a low-cost, easy-to-use set of optics for astronomical observing. Some love using their binoculars for taking in wide-field views in a simple, handheld instrument.
Personally speaking, sometimes I find myself grabbing the pair of 8 x 56 binoculars that I got during my early days in the hobby. But I’ve never really been drawn to observing with binoculars on a regular basis. My main objection is my inability to hold them steady when they’re pointed up and preventing the shaking that’s obvious when I’m using them to observe the heavens. You can get a binocular tripod, but at that point you’re getting beyond the realm of what a simple setup should be. Also, with a few exceptions, my preference is to observe most objects at a magnification higher than what handheld binoculars can offer.
If you already have an unused pair sitting in a closet, by all means get them out and point them up at the night sky. You’ll be shocked by how much you’re able to see even in the city. I know I was my first time.
After using binoculars, you may find that you’re ready to take the plunge into getting your hands on gear that will give you a deeper look into the heavens.
The Telescope
Three Key Characteristics
Aperture
This is really important: aperture is by far the most important characteristic to consider. A telescope’s main job is not to magnify but to gather light and bring it to a point of focus. The more a telescope gathers light, the brighter and better resolved it can render an object especially at higher magnifications.
A telescope’s ability to gather light is directly related to the aperture of the thing that gathers light (a lens or a mirror). Remember how to calculate the area of a circle? The area of a six-inch circle has an area that is over twice as large as that of a four-inch circle, so a six-inch aperture will gather that much more light than a four-inch aperture.
But the quality of your optics plays an important role, too. A telescope with smaller aperture and excellent optics is more of a pleasure to use than one with bigger aperture and second-rate optics. Some amateur astronomers I know will sacrifice aperture for optical quality if the choice is forced upon them.
Generally speaking, though, remember the most important thing: APERTURE, APERTURE, APERTURE.
Focal Length
The focal length of a telescope is the length (usually measured in millimeters) between the point at which the telescope begins to gather light and the point at which it brings it to a single point of focus. The longer the focal length of a telescope, the more it magnifies and the narrower its field of view.
Focal Ratio
A telescope’s focal ratio is simply its focal length divided by its aperture. For instance, a telescope with a focal length of 1200m and an aperture of 150mm will have a focal ratio of f/8.
Unlike in photography, where a lower or “faster” focal ratio results in a brighter exposure as compared to what a higher or “slower” ratio accomplishes using the same exposure speed, telescopes of the same aperture but different focal ratios will produce an image with the same brightness when used visually. The main effect that focal ratio has is by determining the lowest power that the telescope is capable of operating under. The lower the focal ratio for a given aperture, the higher that lowest magnification power is.
Also, telescopes with a higher focal ratio—a “slower” scope, to borrow photography language—tend to be more forgiving on eyepiece performance since the cone of focused light entering the eyepiece is less steep. When used in “faster” scopes with a lower focal ratio, simpler and less expensive eyepieces will exhibit visual aberrations especially around the edge of the field of view.
Another consideration to take into account is that making well-figured optics for telescope with a higher focal ratio can also be a very difficult and thus a rather expensive undertaking. Telescopes with a higher focal ratio tend to have optics that are easier to make well and are thus less expensive than scopes with a lower focal ratio but equal optical quality.
Telescope Types
Unfortunately, there is no such thing as the perfect telescope for all applications. It’s important to note that all telescope designs represent compromises among various factors: aperture, the length of the telescope’s tube, portability, optical quality, and price. If you get some benefits here, you lose some other benefits there.
Here’s a brief comparison of the most common telescope types:
| Type | Brief Description | Advantages | Disadvantages | Best Uses |
|---|---|---|---|---|
| Refractors | A group of lenses spanning the front of the telescope brings light to focus at the back. | Unobstructed optics provide sharp, high-contrast views. Simple, rugged design. Relatively lightweight. | Smaller aperture limits how visible dimmer objects are. Achromatic refractors can show false color especially at lower focal ratios. Apochromatic refractors reduce or altogether eliminate false color but carry a very high price relative to aperture. | Lunar, planetary, and double-star observing. Refractors with lower focal ratios offer wide-field observing opportunities. |
| Reflectors | A concave primary mirror at the back of the telescope brings light to a focus as it reflects it back to the front. A secondary mirror near the front reflects light up to the user. | Largest aperture and best light grasp for the money. Simple design. Well suited to tinkerers. | Bulky and cumbersome, especially 8-inch scopes and larger. Secondary mirror obstruction reduces sharpness and contrast especially in faster scopes. Requires collimation, which can be harder to achieve in scopes with a lower focal ratio. | Observing fainter deep space objects under darker skies. Reflectors with lower focal ratios offer wide-field observing opportunities. |
| Catadioptrics | A combination of a correcting lens in the front of the telescope, a concave primary mirror at the back, and a secondary mirror in the front work together to focus light in a compact tube. | Very compact and easy to handle. Perhaps the most versatile design. | Somewhat higher price relative to aperture compared to reflectors. Schmidt-Cassegrains may require infrequent collimation. Secondary mirror obstruction reduces sharpness and contrast. Narrower field of view. | Situations where a jack of all trades is best. Catadioptrics are all-around instruments that are often satisfyingly good at everything, but not truly excellent at anything. A favorite of urban observers. |
Refractors
Refractors are what would come to mind for most people when they imagine a telescope. And perhaps for a good reason: they’ve been around at least since the seventeenth century. In 1608, Dutch spectacle maker Hans Lippershey attempted to patent a design for a refracting telescope. The next year, Galileo Galilei learned of the design and built his own refractor, which he used to make a number of astronomical discoveries.
Quite simply, modern refractors are usually comprised of two or three lenses that are mounted in the front of the telescope. Light enters the front of the telescope, and those lenses bring light to a focus on the other end of the tube, where the observer looks into the eyepiece.
The following diagram illustrates a basic refractor figured at f/7 (i.e., the focal length is seven times as long as the aperture):
Generally speaking, refractors provide one with a crisp, high-contrast visual observing experience. But inexpensive refractors can also suffer from a problem known as chromatic aberration.
The problem of chromatic aberration or false color arises from the nature of how different colors in the spectrum of light come to focus at different points after it passes through some lenses:
Here is an illustration of an extreme case of what false color looks like in a very crude single-lens homemade refractor that I cobbled together using spare parts I had on hand:
Note the blue, purple, and yellow fringing around the edges of the cross member of this utility pole and the power lines. Also notice how out-of-focus this image appears. This is due partly to the fact that it’s impossible to achieve focus because different colors of light are coming to focus at different points. What a disaster.
By contrast, this is a photograph of the same pole that I took using my good-quality 4-inch Maksutov-Cassegrain telescope, which, to be sure, is not a refractor but which nonetheless demonstrates what one would rather see at the eyepiece:
Some types of refractors handle this problem better than others. Amateur astronomers typically use two basic types of refractors: achromats (or achromatic refractors) and apochromats (or apochromatic refractors). Each type handles chromatic aberration differently.
Achromats
Achromats (pronounced “ACK-row-mats”) bring some colors to focus at the same point but bring other colors to focus at different points. To compensate for this, the higher the focal ratio of the telescope (that is, the longer the focal length relative to the aperture), the less apparent this effect is. Generally speaking, achromatic refractors whose focal ratio is f/10 or higher have less noticeable false color. In the middle of the twentieth century, it was not uncommon for amateurs to use long-tube achromatic refractors whose focal ratio was f/15 or even more.
After having shown you the above photo of what is an extreme case of chromatic aberration, I don’t want to leave you with the impression that achromats are completely incapable telescopes. In my experience, even inexpensive achromats can perform well as long as their focal ratio is f/10 or greater—that is, as long as the telescope tube is long. One of my favorite telescopes is my 90mm f/11 Meade Series 390 achromatic refractor. I’ve spent many hours at the eyepiece with this telescope. I reach for it whenever I do lunar observing in particular. It does show some false color, but it’s negligible.
All in all, long-focus achromatic refractors with focal ratios of f/10 and higher can be ideal telescopes for use in observing the Moon, the planets, and double stars.
Apochromats
Apochromats (pronounced “uh-PAH-chro-mats”) appeared as a viable option in the amateur astronomy market in the last decades of the twentieth century. When they’re well made, they have negligible if any false color. As one might expect, apochromatic refractors can be very expensive especially relative to the aperture size they offer. Those amateur astronomers who have a taste for the very best of the best will spend thousands of dollars in pursuit of the perfect apochromat.
Reflectors
While refractors use lenses to gather light and bring it to a focus, reflectors do the same thing by using mirrors. Sir Isaac Newton built the first reflecting telescope in 1668.
Light enters the front of the telescope, reflects off the concave primary mirror, reflects again off of a secondary mirror positioned near the front of the telescope, and goes into the eyepiece.
The following illustration of a basic reflector figured at f/7 shows how this works:
Reflectors offer the cheapest way to get the most aperture. They also typically have the simplest design of any other type of telescope. I can’t count how many times I’ve completely disassembled a reflector and put it back together again with absolutely no ill effect on the telescope’s performance. They are well suited to folks who like to tinker with things. If you’re mainly interested in using a simple setup to observe faint galaxies and nebulae, then a reflector with a generous aperture is definitely the way to go.
In spite of their advantages, reflectors can suffer from some drawbacks. One optical aberration that can sometimes turn up in reflectors is called coma, a type of aberration that makes stars at the edge of the field of view appear like drawn-out points of light.
Reflectors also require periodic collimation, which involves fine-tuning the alignment of the primary and secondary mirrors so that the path of focused light goes straight and squarely into the eyepiece without deviating to one side or another. It’s not hard to do, but it does need to be done, and it requires the use of a simple collimating tool.
Additionally, the secondary mirror creates a central obstruction that, while not obvious when your using the telescope, does have the effect of reducing the contrast of bright objects like the planets and introducing diffraction spikes.
Cheaper reflectors will often have primary mirrors whose curve is spherical in shape. One way to compensate for this is to figure the mirror with a long and thus more forgiving focal length. If you encounter an inexpensive reflector with a long tube, chances are it has a spherical mirror.
More expensive ones will typically have a parabolically-shaped primary mirror. The latter helps keep stars sharp towards the edge of the field of view. But the more complex a primary mirror’s figuring is, the more expensive it will make the telescope. Generally speaking, a reflector with a parabolic mirror is one important indication of its quality particularly in the case of those with a short focal length.
Catadioptrics
Catadioptric telescopes combine lenses and mirrors to fold a path of focused light back and forth inside a compact tube.
Maksutov-Cassegrain Telescopes
Catadioptric telescopes made their debut on the commercial market with the venerable Questar telescope in 1954. Their design stems from work that Russian optician Dimtri Maksutov did around the time of the Second World War.
Light first passes through a relatively thick meniscus corrector lens at the front of the scope, reflects off of a concave primary mirror at the back of the scope, reflects again off of a silvered secondary mirror spot usually positioned on the inside surface of the corrector lens, and passes out of a central hole in the primary mirror to the eyepiece. The following illustration demonstrates this design figured at f/15:
It’s an ingenious design that fits what would otherwise be an extremely long focal length into a compact telescope. If the focal length of an f/15 Maksutov-Cassegrain telescope were implemented as a refractor of equal aperture and focal ratio, this is how the two would compare:
Note how the width of the telescope tubes and cones of focused light exiting the back of both designs are the same proportion, but the Maksutov-Cassegrain is significantly shorter in length.
Since Maksutov-Cassegrains have a long focal length packed into a short tube, they are excellent performers for observing objects that require high magnification, particularly the planets. They are often the favorite design of urban astronomers who often observe under light polluted skies and who most commonly go for objects that don’t necessarily require dark skies to see well (again, the planets as well as the Moon and, when properly filtered, the Sun).
But because of the thickness of both the primary mirror and corrector lens coupled with the closed-tube nature of the design, longer cooldown times can sometimes be an issue especially when you’re taking your telescope from inside a warm house to the cold outdoors. Convection currents radiating off of the optics may cause a boiling visual effect at the eyepiece until the telescope comes to thermal equilibrium.
The central obstruction, while typically smaller than either reflectors (discussed above) or Schmidt-Cassegrains (discussed below), can still reduce the contrast of an object. Out of all the telescope designs that have an obstructed aperture, however, Maksutov-Cassegrains have the smallest obstruction and thus offer the best contrast.
And since the corrector lens is so thick, producing apertures larger than five inches involves figuring a large and thick lens blank, often an expensive undertaking.
Schmidt-Cassegrain Telescopes
Schmidt-Cassegrain telescopes use the same basic principles as Maksutov-Cassegrains but with a slightly different design. Like with Mak-Casses, light enters the telescope through a corrector lens, although an SCT’s corrector lens is significantly thinner. Light then reflects off a concave primary mirror back toward the front of the scope, reflects again off a secondary mirror mounted into a hole in the lens, and finally makes its way to the back of the scope and to the eyepiece. The following illustration demonstrates this design figured at f/10, which is the most common focal ratio for SCTs:
It’s another clever design that squeezes a long focal length in a short tube. If this f/10 SCT were a refractor of equal aperture and focal ratio, this is how they would compare in size:
Like the Maksutov-Cassegrain above, note how the width of the telescope tubes and the cones of light exiting the back of both designs are the same proportion, but the SCT is significantly shorter in length.
Many of the same advantages and disadvantages of the Maksutov-Cassegrain design apply also to Schmidt-Cassegrains. SCTs pack a long focal length into a short tube. They are a favorite for urban observers. Cooldown times can be a problem just like with Maks. But the thin corrector lens of SCTs makes their weight more manageable especially in larger apertures. The eight-inch SCT is a staple of the amateur astronomy world and is not an uncommon sight on the observing field, but I have never encountered heavy Maksutov-Cassegrains larger than five inches at star parties. Any SCT that is eleven inches or larger is starting to get big and cumbersome. Schmidt-Cassegrains have a larger secondary mirror obstruction than the ones found in Maks, and so the contrast they offer is somewhat diminished than that of that other catadioptric design. And unlike the secondary mirror of Maksutov-Cassegrains, which are most commonly affixed directly to the back of the corrector lens as a mirrored spot, the secondary mirror of SCTs are mounted in a housing that can require periodic collimation especially if the telescope is transported frequently and/or is jostled about.
All in all, though, I really like the Schmidt-Cassegrain design especially in its eight-inch form. It does most everything I ask of it with a reasonable level of ability.
For an interesting commentary on Celestron’s Schmidt-Cassegrain telescopes, visit Ed Ting’s Celestron Schmidt-Cassegrain overview.
The Mount
When I was first getting started in the hobby, I endlessly wrung my hands over what type of telescope to get. Once I finally got over my analysis paralysis and made a decision, the mount was an afterthought. I went for a relatively inexpensive, lightweight mount. It was a big mistake.
Don’t underestimate the importance of the mount. It will make or break an observing experience. A good scope sitting on a cheap, flimsy mount makes for a very frustrating experience. If you’re considering the purchase of a telescope that doesn’t include the mount, make sure the mount can adequately handle the scope. If the telescope already includes the mount, pay attention to whether the mount can in fact do its job. Especially in the case of the cheapest packages, many manufacturers include mounts that are not really up to the task of steadily holding the telescopes they come with.
Just like there are different types of telescopes, so, too, are there different types of mounts.
Alt-az Mounts
Alt-az (i.e., altitude-azimuth) mounts represent simply. The movement is up-down and side-to-side. Simplicity itself. They tend to sit on tripods, are most commonly found with smaller short-tube refractors and reflectors, and are very good choices for casual scanning or grab-and-go setups.
Dobsonian Mounts
Equally simple are Dobsonian mounts, named after their chief proponent John Dobson. They are most common with moderate to large reflectors that have two-armed swivel bases that sit on the ground. One can also think of Dobsonians as a large version of an alt-az mount.
Equatorial Mounts
Equatorial mounts represent a more complex mount design. Because they have one of their axes of movement pointed at the north pole and thus enable a telescope’s movement to mimic the rotation of the Earth, they are well-suited for tracking objects especially at higher magnifications (e.g., the planets). But their counterweights make them a little more cumbersome to work with, and they can be tricky to figure out at first. But with a little thought and effort, they can be very intuitive mounts to work with.
The Eyepiece
An eyepiece is a set of small lenses one looks through when using a telescope. Since an eyepiece is the piece of gear that you are physically closest to when you use a telescope, they are indeed one of the most critical parts in the optical chain of a telescope.
Focal Length
In combination with the focal length of the telescope, the focal length of the eyepiece determines the magnification. The lower the eyepiece’s focal length, the higher the magnification.
Here is the critical formula:
Magnification = focal length of the telescope รท focal length of the eyepiece.
For example, using a 25mm eyepiece with a telescope whose focal length is 1200mm yields a magnification of 48x.
Field of View
When you look through an eyepiece, you’ll immediately notice how wide the field of view is. Some eyepieces have rather narrow and confining fields of view, which isn’t necessarily a bad thing if the object you’re observing doesn’t need a wide field of view to begin with (for example, planets). In fact, many high-end eyepieces have narrow fields of view but high-quality optics that feature high-grade materials. But generally speaking, a generously wide field of view can offer an expansive feeling at the eyepiece and, when well designed and executed, can often be the hallmark of a good, quality eyepiece. Many expensive eyepieces have several lens elements in several groups that, put together, offer a wide field of view with stars appearing sharp from edge to edge. The more glass is in an eyepiece, the heavier, bulkier, and more expensive it can get.
Eye Relief
Eye relief is an expression of how close your eye needs to be to the eye lens (i.e., the top-most lens of an eyepiece) in order for you to be able to take in the entire field of view without having to move your eye around. Longer eye relief means that you can still take in the entire field of view with your eye being further from the eyepiece. Those who must wear glasses while observing (most frequently those who suffer from more than a minor amount of astigmatism) will look for eyepieces with generous eye relief.
I don’t wear my eyeglasses when I’m at the eyepiece. But given the particular shape of my eyebrow and my long eyelashes (I dislike the sensation of my lashes brushing up against the eye lens), I tend to look for eye relief no shorter than 18mm or so. Everyone is different, though. Some don’t mind shorter eye relief especially if the eyepiece’s field of view is wide. Many consider 20mm to be the standard for what constitutes long and comfortable eye relief especially for those who keep their eyeglasses on when observing.
Types of Eyepieces
Two types of eyepieces are typically included with beginner telescopes: Plossl and Kellner eyepieces. Plossl eyepieces feature two pairs of lenses and typically offer a moderately wide field of view. Kellner eyepieces are a somewhat simpler design that feature three lens and have roughly the same field of view size as Plossls. Both types at shorter focal lengths have tighter eye relief.
Beware of Huygens and Ramsden eyepieces—they tend to have narrow fields of view, tight eye relief, poor optical performance, and an overall level of quality that is truly awful. If a scope includes these eyepiece types and/or has eyepieces with 0.965-inch barrels as opposed to the more modern 1.25-inch standard, that’s one sign that the scope may suffer from inferior quality.
Edmund Optics has an excellent discussion of these and other eyepiece types.
Higher-end eyepieces often have 2-inch barrels that allow for wider fields of view and better performance than 1.25-inch eyepieces. Of course, your telescope will need to be able to accept these wider eyepieces.
Minimum and Maximum Magnifications
Where magnification is concerned, don’t go too low for your scope. Generally speaking, I’ve found that taking the focal ratio of a telescope and multiplying it by six will give me an idea of the highest eyepiece focal length and thus the lowest magnification that is useful for a telescope. If I had an f/5 telescope, for instance, I would not get an eyepiece whose focal length was higher than 30mm. For reasons having to do with the size of your eye’s dark-adapted pupil and the optical interaction between the telescope and eyepiece, lower magnifications may not be possible for a telescope with a given focal ratio.
Don’t push your magnification too high for your scope, either. Whatever your aperture will give you in terms of light gathering and resolution will really be apparent as you explore how high you can push magnification up. Making an object look bigger at the eyepiece means that whatever light the telescope gathers will be spread out more as that object appears larger. The object will thus appear dimmer, mushier, and less resolved.
Imagine aiming a flashlight against a wall. When the flashlight is close to the wall, the spot of light appears small and bright. As you pull the flashlight away from the wall, the spot of light grows bigger but also gets dimmer because the same amount of light becomes spread out across a larger area. Something like that happens at any telescope as you increase magnification.
Some say that 50x per inch of aperture is good rule of thumb to use. I’m somewhat more conservative. I find that a scope won’t yield useful magnifications beyond 30 or 35x per inch of aperture. Past that point, the telescope simply runs out of light. That is, a telescope’s aperture won’t gather enough light to resolve an object satisfactorily beyond a certain point.
I also find that anything above 200x is usually not possible no matter what the aperture is. Only on rare occasions am I able to push an 8-inch telescope up to 225x with anything approaching a well-resolved and pleasing image at the eyepiece. Turbulence in the atmosphere is usually the factor that imposes that 200x ceiling.