Understand the advantages and disadvantages of popular scope types.
If you want to start a war, just ask a group of astronomers what the best type of scope is. Everyone agrees that “junk” scopes should be avoided, of course, but that’s about the extent of the agreement. There are many types of scopes, all of which have advantages and drawbacks relative to the other types. Each type of scope has proponents and detractors, and the debate can become quite heated. In this hack, we’ll attempt to provide unbiased advice about the strengths and weaknesses of each type of scope.
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Just so that you’re aware of it going in, we confess that we’re “Dob bigots.” We tried to get an “SCT fanatic” we know to help us with this hack, but he’s not speaking to us. Neither is a “refractor maniac” we know. And they’re not speaking to each other. We’re only kidding, of course, but feelings do run high when people start debating the best scope type. The Coke–Pepsi, PC–Mac, and Linux–Windows wars are nothing compared to the scope-type wars.
Here are the three most important characteristics of a telescope:
- Aperture
The aperture of a telescope is the diameter of its primary mirror or objective lens, which may be specified in inches or millimeters. Amateur telescopes have apertures ranging from 60mm (2.4”) to 30” or more. Aperture determines the amount of light a scope can gather, the fineness of detail it can resolve, and the maximum and minimum useful magnifications for the scope.
Light gathering is proportional to the square of the aperture. For example, a 10” scope gathers four times as much light as a 5” scope. The amount of light gathered determines how “deep” the scope can go. Larger aperture allows you to see dimmer objects (and more detail in all objects) than a smaller aperture.
Resolution is proportional to the aperture. For example, a 10” scope can resolve detail twice as fine as a 5” scope (assuming equal optical quality and steady seeing conditions).
- Aperture
See above.
- Aperture
See above.
Above all, aperture rules. Here are some other important characteristics:
- Focal length
The focal length of a scope is the actual or virtual distance from the optical center of its primary mirror or primary objective lens at which it brings an object located at infinity to focus. The focal length of amateur telescopes ranges from 400mm (~16”) for short-tube refractors and other small scopes through 4,000mm (~160”) or more for the largest amateur instruments.
For a given focuser size, focal length determines the maximum possible true field of view (TFoV) of a scope, which is to say how wide a swath of sky is visible in that scope. For example, a scope of 400mm focal length with a 2” focuser has a maximum possible TFoV of about 7°, while a scope of 2,800mm focal length with a 2” focuser can never show more than about 1° of sky.
Conversely, a short focal length makes it difficult to achieve high magnification. Magnification is calculated by dividing the focal length of the scope by the focal length of the eyepiece. For example, using a 14mm eyepiece with a scope of focal length 2,800mm yields 200X because 2,800/14=200. To get that same 200X magnification in a scope of only 400mm focal length, you’d need a 2mm eyepiece (400/2=200). But such short focal length eyepieces have problems of their own—typically including tiny eye lenses and very short eye relief—that make them uncomfortable to use.
Accordingly, short focal length telescopes are best for low-power, wide-field applications, such as scanning Milky Way star fields—while long focal length telescopes are best for high-power, narrow-field applications—such as Lunar and planetary observing. Typical general-purpose amateur telescopes have focal lengths ranging from about 1,000mm to 2,500mm, which allow using high magnifications easily while maintaining reasonably wide fields of view.
- Focal ratio
The focal ratio of a scope is the ratio of its focal length to its aperture. For example, a scope of 250mm (~10”) aperture and 1,250mm focal length has a focal ratio of 1,250/250=5, expressed as f/5. A typical 8” (203.2mm) SCT (Schmidt-Casssegrain Telescope) has focal length of 2,032mm, and accordingly a focal ratio of f/10. Mainstream amateur telescopes have focal ratios ranging from f/4 to about f/16, with the vast majority in the lower half of that range. A scope with a focal ratio of f/6 or less is considered to be a “fast” scope. An f/6 to f/9 focal ratio is considered medium, and a larger focal ratio is considered slow.
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Fast, medium, and slow as applied to telescope focal ratios is a holdover from photography, where a lens with lower focal ratio allows a shorter exposure time than a lens with a higher focal ratio. Focal ratio has no relationship to the brightness of the image a scope provides when used visually. For example, an 8” f/5 scope provides the same image brightness as an 8” f/10 scope, if the two scopes are used at the same magnification.
Focal ratio is important because, for standard reflector and refractor telescopes, the focal ratio determines the length of the optical tube assembly (OTA). For example, a 10” f/5 scope has a tube about 50” long, whereas a 10” f/10 scope has a tube 100” long. Long tubes are heavy, hard to transport, and require heavy, expensive mounts. All other things being equal, then, a fast focal ratio is clearly desirable.
Unfortunately, all other things are not equal. Fast focal ratios require deeply curved mirrors and lenses, and these deep curves are much harder and more expensive to produce to the required level of precision than are the shallower curves used by instruments with longer focal ratios. Fast focal ratios are also hard on eyepieces. Nearly any modern wide-field eyepiece provides excellent image quality in an f/8 or slower scope, but only modern, complex (read “expensive”) eyepiece designs can provide a wide-field image that’s sharp edge to edge in a fast scope. In exchange for greater portability, those who buy fast focal ratio scopes resign themselves to buying expensive premium eyepieces or putting up with soft edges in inexpensive wide-field eyepieces.
- Optical quality
As odd as it sounds, optical quality is not a major issue with modern commercial telescopes. There are differences, certainly. Premium telescopes have optics as accurate as it is humanly possible to make and are priced accordingly. But even mass-produced Taiwanese and Chinese scopes have surprisingly good optics. So good, in fact, that only a very experienced observer on a night when the atmosphere is extremely stable will be able to tell the difference in optical quality between a premium scope and a mass-produced model. For most observers in most locations on most nights, atmospheric turbulence will be the limiting factor, not the quality of the optics.
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There are a couple of caveats about mass-produced scopes. There is, of course, a great deal more variation in optical quality among mass-produced scopes than among premium models. When you buy a premium scope, you get superb optics, period. That’s part of what you’re paying for. When you buy a mass-produced scope, the optics may be anything from mediocre to excellent. That variability is part of the reason the scope costs much less than a premium model. Quality control costs money.
Also, when we sing the praises of mass-produced scopes, we’re not endorsing all of them. Some models are excellent, but there are many “junk” scopes available—including some with well-known names like Meade and Celestron—that have simply terrible optics. The trick is knowing the difference, but then that’s why you’re reading this.
- Mount
Every telescope requires a mount of some sort. If you’ve ever used a binocular, you know how unsteady the image can be, even at only 7X or 10X magnification. Telescopes typically operate at magnifications in the range of 50X to 300X, which makes a stable mount imperative.
There are two broad classes of telescope mount. An altitude-azimuth (alt-az) mount is simple, inexpensive, light, and intuitive to learn and use; however, it is not designed to track the apparent motions of the stars unless you add supplementary equipment. An equatorial (EQ) mount is complex, more expensive, heavier, and difficult to learn to use properly; but it is designed to track the stars.
Unfortunately, as scopes have gotten better and less expensive, equatorial mounts have gone in the opposite direction. The typical cheap Chinese equatorial mounts supplied with low-end scopes are much too light to support the weight of the scopes they’re bundled with for anything more than rudimentary visual observing. They’re crude, shaky, and shoddily constructed. Most beginners who attempt to use such mounts believe the problems they have are their fault. They’re not. The mounts themselves are the problem. Even an experienced astronomer can’t do much with them. For an inexpensive scope, an alt-az mount is usually a better choice.
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If you are thinking of buying an inexpensive equatorial mount because you want to do astrophotography, disabuse yourself of that notion. Cheap EQ mounts have neither the accuracy nor the precision needed for long-exposure astrophotography. The least expensive EQ mount suitable for serious astrophotography is the Vixen GP-DX, which costs $1,300. That’s for the mount only—no scope included.
Some alt-azimuth and equatorial mounts provide a go-to feature, either standard or as an optional upgrade. Go-to mounts include drive motors and a hand controller with electronics that calculates the current position of a specified object and automatically moves the scope until it is pointed at that object.
To use a go-to mount, you initialize it by choosing two or three bright “guide stars.” (A go-to scope can usually be initialized against any of 25 or more guide stars, so there are always guide stars available from any location and at any time of year.) As you point the scope to each guide star, you press a button to indicate that the selected guide star is centered in the eyepiece. After the scope is initialized, you simply choose an object on the hand controller, and the scope moves automatically to that object.
A go-to mount can be very useful, as long as you don’t use it as a crutch to avoid learning the night sky. On nights when you’re more interested in looking at objects than pursuing the challenge of locating them manually, a go-to scope allows you to spend your time looking instead of finding. A go-to scope is also very useful for urban observing [Hack #10] because it can be very difficult to locate objects manually under bright urban skies.
Unfortunately, inexpensive go-to scopes are typically quite unreliable, both mechanically and in terms of locating objects. They seldom center the desired object in the eyepiece, and quite often fail entirely to put the object anywhere in the field of view. They use cheap motors, plastic gears, and other low-end components that are likely to fail sooner rather than later. Because so much of the cost of an inexpensive go-to scope goes toward the motors and electronics, the optics are usually small and of low quality, which means that even if the go-to succeeds in locating the object, you won’t be able to see much detail, if the object is visible at all. Unless you are willing to spend at least $1,500 to $2,000 on a go-to scope, we suggest you avoid go-to.
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Some telescopes, notably Orion IntelliScope Dobsonians, include digital setting circles (DSCs), which serve a similar purpose—helping you locate objects automatically. Unlike go-to scopes, these “push-to” scopes don’t have motors, so they can’t move to the object on their own. Instead, when you enter the object to be located, the hand controller displays arrows to tell you which direction to move the scope. You push the scope in the direction indicated, and when it’s pointed at the object, the hand controller display “zeroes out” to tell you that you’ve arrived.
- Portability
Don’t underestimate the importance of portability, which is determined by the size, weight, and bulkiness of the various parts of the telescope and mount, as well as the ease or difficulty of setting up and tearing down the scope. A scope that is light, portable, and can be set up quickly and easily will be used much more than one that is heavy, awkward, and requires more time and effort to set up and tear down.
Some scopes are obviously very portable, for example small to mid-size refractors. Other scopes are just as obviously difficult to transport, such as large tube Dobsonians. A 16” tube Dobsonian, for example, may require a van to transport and two or three people to set up and tear down, both because of its bulk and its weight. Although mid-size (8” to 10”) SCTs can be handled by one person, large SCTs suffer from portability problems. An 11” to 14” SCT really needs two people to set up safely, and the 16” and larger models might as well be considered observatory instruments.
Despite their bulk, 10” and smaller tube Dobsonian scopes are very portable, assuming your vehicle is large enough to accommodate a tube that’s a foot or so in diameter and four feet long. Tube Dobs have only two parts—the optical tube and the base—and can easily be set up and torn down by one person in a minute or less. We consider 12” to 12.5” the maximum practical size for a tube Dob, although tube Dobs are made in sizes up to 17.5”. For larger Dobs, tubes are simply too heavy, large, and unwieldy. Fortunately, various companies manufacture trusstube Dobs, which replace the single large solid tube with a structure of thin tubes. Truss Dobs are made in sizes as large as 30”, and even 20” models can be assembled and torn down by one person. Unfortunately, truss Dobs are very expensive compared to a tube Dob of the same size.
- Cool-down time required
In order to provide its best images, the mirrors and/or lenses in a telescope must be allowed to equilibrate to ambient temperature. (For some reason, amateur astronomers call this process “cool down,” even when the telescope starts out cooler than the outside air.) Until the optics reach air temperature, the scope does not provide its best resolution and, in some types of scopes, tube currents cause wavy images, distortion, and other visual anomalies. All other things being equal, a larger scope always takes longer to cool down than a similar smaller model, but different types of scopes have different cool-down characteristics.
In general, small refractors cool quickly. Unless the temperature differential is large, a refractor is ready for use within a few minutes after it is set up. A 6” to 10” Newtonian reflector (such as a Dob) may require 30–60 minutes to cool down, depending on the temperature differential, and less if fans are used to expedite cooling. An 8” to 10” SCT (Schmidt-Cassegrain Telescope) might require between one and two hours to cool completely, and a 5” MCT (Maksutov-Cassegrain Telescope) might require two to three hours to cool fully. Large scopes, including 12” or larger SCTs and large Newtonian reflectors, may never cool fully, even with fans, because their massive mirrors simply cannot lose heat fast enough to keep up with the decline in air temperature over the course of the evening.
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Proper cool down is particularly important when you view Luna or the planets. For observing faint fuzzies, cool down is less critical. You can’t see much fine detail in them anyway, so the resolution degrading effects of an uncooled mirror don’t matter much. We often observe DSOs [Hack #22] while we wait for our mirror to cool down.
All of these characteristics are important, and all of them pertain to any scope of any type you might buy. In the following sections, we examine the details of the various types of scopes available.
There are actually dozens of different optical designs used in scopes, but all of them fall into one of three broad categories:
In the following sections, we examine each of those categories and explain the advantages and disadvantages of each.
When most people hear the word “telescope,” a refractor is what they think of (which is why we used a refractor on the front cover). When Galileo first turned his telescope to the heavens in 1610, it was a refractor, and refractors have remained popular with amateur astronomers ever since. Figure 1-6 shows a typical refractor. Galileo would have felt right at home with it. (Actually, he would have killed for one anywhere near this good.)
In its simplest form, a refractor comprises a tube with an objective lens on one end and a focuser with an eyepiece on the other. The objective lens gathers and focuses the light to a point about midway in the focuser’s travel. The tube of a refractor is therefore roughly as long as the focal length of the objective lens, plus the length of the lens shade on the front end and the focuser mechanism and drawtube on the rear. Our 90mm f/11.1 refractor, for example, has a focal length of 1,000mm. The tube, including lens shade and with the focuser mechanism fully extended, is roughly 1,200mm long.
For practical reasons, most modern traditional-style (“long-tube”) refractors have focal lengths of 1,000mm or less, regardless of their apertures. This is true because tubes much longer than one meter are unwieldy and require tall (and expensive) mounts. Accordingly, most manufacturers design their refractors to have a focal length of no more than 1,000mm. But if the aperture varies and the focal length remains constant, the focal ratio of the scope must also vary. And that is exactly how manufacturers handle the problem. For example:
A 60mm (2.4”) refractor has a focal ratio of f/10 to f/16, and a focal length of 600mm to 1,000mm.
A 70mm (2.8”) refractor has a focal ratio of f/10 to f/14, and a focal length of 700mm to 1,000mm.
An 85mm (3.3”) refractor has a focal ratio of about f/12, and a focal length of about 1,000mm.
A 100mm (4”) refractor has a focal ratio of about f/10, and a focal length of about 1,000mm.
A 127mm (5”) refractor has a focal ratio of about f/8, and a focal length of about 1,000mm.
A 152mm (6”) refractor has a focal ratio of about f/6.7, and a focal length of about 1,000mm.
Notice the pattern? As the aperture increases, the manufacturer reduces the focal ratio to keep the focal length no longer than about 1,000mm. (Actually, a few 6” f/8.3 refractors are made, with focal lengths of about 1,250mm and correspondingly longer tubes, but these scopes are extremely unwieldy because of their length and weight.) It might seem that the manufacturers could continue that game forever. Why not, for example, make an 8” f/5 refractor or even a 10” f/4 model? Either would have the same 1,000mm focal length, and a reasonably short, albeit heavy, tube.
Alas, the laws of optics don’t allow it. The problem is false color (otherwise known as chromatic aberration), which manifests as a colored fringe around bright objects, or even as an overall color cast. False color is not just esthetically displeasing; it actually reduces the amount of visible detail significantly. And, in any refractor design, false color increases as you increase the aperture and as you decrease the focal ratio. Our imaginary 8” f/5 or 10” f/4 refractor would be hit by a double whammy: much too much aperture, and much too short a focal ratio. (Actually, the false color in a standard 6” f/6.7 refractor is hideous. Most people consider the false color even in a standard 5” f/8 refractor unacceptably high. The false color in our imaginary 8” or 10” refractor would be positively kaleidoscopic.)
The subject of false color is always on the minds of refractor owners. Even people who own $5,000 and $10,000 premium refractors debate whether this or that model has a bit less false color than an equally expensive competing model. The best refractors show only slight false color even on bright objects, but some false color is inherent in the optical design of any refractor.
Broadly speaking, there are two classes of refractors:
- Achromatic refractor
An achromatic refractor (or achromat) typically uses a two-element objective lens, with elements of crown and flint glass. An achromat is reasonably well corrected for most optical aberrations, but it shows noticeable chromatic aberration on bright objects such as Luna, the planets, and bright stars. False color in an achromat can be nearly eliminated by using a longer focal ratio (and thereby increasing the length of the tube), but that is impractical for any aperture larger than 90mm to 100mm because the length of the tube becomes excessive.
- Apochromatic refractor
An apochromatic refractor (or apochromat) typically uses a three-element objective lens, or a two-element objective made from expensive rare-earth glasses or calcium fluorite. An apochromat is well corrected for optical aberrations, including chromatic aberration.
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Whether the scope is an achromat or an apochromat, chromatic aberration increases with increasing aperture and decreasing focal ratio (although the level is much less in an apochromat of the same aperture and focal ratio.) For example, a 5” refractor shows more chromatic aberration than a 3.5” refractor of the same focal ratio. Conversely, an f/5 refractor has much more chromatic aberration than an f/10 refractor of the same aperture.
Achromats are inexpensive to moderately priced for their aperture, typically $35 to $100 per inch of aperture. Small achromats—those in the 60mm to 100mm range—are popular beginner scopes, although we believe they are seldom a good choice.
Apochromats are very expensive, typically $250 to $1,000+ per inch of aperture, with the cost per inch climbing rapidly as aperture increases. Apochromats in the 60mm to 127mm (2.4” to 5”) range are popular among advanced amateur astronomers, particularly those who do imaging. Apochromats of 6” to 10” aperture are available, although in larger sizes the prices become— dare we say it?—astronomical. For example, a 6” apochromat might cost $7,500 (tube only), and a 10” model might cost $40,000.
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Some refractors are described by their manufacturers as “semi-apos” or “neo-achros.” These are marketing terms rather than technical categories. Both indicate a scope that isn’t quite well corrected enough to be honestly described as apochromatic, but has better color correction than standard achromats. A refractor that does not claim to be an apochromat but is described as using “ED glass” or a “fluorite element” is usually a semi-apo, although true apos also use ED glass and/or fluorite elements.
The classic or long-tube refractor has been around for hundreds of years. Recently, the short-tube refractor, shown in Figure 1-7, has become quite popular. These scopes are typically 70mm to 90mm in aperture with focal ratios of f/5 to f/6, although some models are as large as 150mm. The short focal lengths provide wide fields of view and a short, easily mounted optical tube, both of which are desirable features. Short-tube refractors are popular as grab-'n-go scopes [Hack #10] and for scanning Milky Way star fields, open star clusters, and other large astronomical objects. Short-tube refractors are generally a poor choice for high-magnification viewing, such as Lunar and planetary observing.
Refractors have the following advantages:
- Simplicity and durability
Assuming that you take reasonable care, there’s not much that can go wrong with a refractor. The optics are collimated at the factory and seldom if ever need to be recollimated [Hack #38]. There’s no twiddling necessary with a refractor. You simply install it on its mount and start viewing.
- Portability
Small refractors, particularly short-tube models, are extremely portable. They are relatively short and light, and so they do not require a heavy or complex mount. Easy portability is one reason why refractors are very popular as grab-'n-go scopes.
- Fast cool down
Small and mid-size models require little or no cool-down time to provide their best images. You can simply set them up and start observing.
- Pristine image quality
Refractors have no secondary mirror or other central obstruction in the light path to produce diffraction spikes and reduce contrast. A well-made refractor provides bright pinpoint stars on a velvet black back-ground. Most astronomers agree that refractors provide the most esthetically pleasing images of any telescope type.
- Usable for terrestrial viewing
Astronomical telescopes, including refractors, provide an image that is flipped left-to-right and/or inverted, and so is useless for terrestrial observing. However, you can convert an astronomical refractor to a terrestrial scope simply by installing a correct-image diagonal or eyepiece, allowing the scope to serve two purposes.
- Ideal for astrophotography
Refractors, particularly apochromatic models, are well suited for astrophotography and are the scope of choice for many professional astrophotographers. Their absence of diffraction effects and high contrast makes refractors an ideal match for photography.
Warning
Most astronomers feel an urge to photograph the heavens, but astrophotography is an expensive hobby that requires total dedication. Don’t expect to take good astrophotographs with inexpensive equipment. Those “amateur” astrophotographs you see in the astronomy magazines often represent weeks or months of effort using equipment that costs $5,000 to $50,000. Failing at astrophotography is one of the main reasons people leave the hobby. If you are determined to shoot high quality astrophotographs, plan to spend a lot of money on equipment and months or years learning how to do it.
Refractors have following disadvantages:
- Small aperture
Practical refractors have apertures of 60mm to 150mm. These relatively small apertures limit both the light-gathering ability and the resolution of refractors. If your observing is limited to Lunar, planetary, and double stars, a refractor may be a good choice. But if you have any interest in observing DSOs, you need a larger scope.
- False color
As we said earlier, all refractors exhibit false color to some extent. You can limit the problem by choosing a scope of relatively small aperture and long focal ratio or by spending the money necessary to get an apochromat.
- Inconvenient eyepiece location
Because the eyepiece of a refractor is on one end of a long tube, the eyepiece position changes dramatically with the elevation of the scope. If you are observing an object near zenith, you may find yourself lying on the ground to get your eye low enough to see into the eyepiece. Conversely, if you are observing an object near the horizon, you may find yourself standing erect, or even on a stepladder or stool [Hack #60], depending on the height of the mount.
- high price
Refractors, particularly apo models, are the most expensive type of scope in terms of dollars spent per inch of aperture.
In the 1970s, John Dobson started the Sidewalk Astronomers (http://www.sidewalkastronomers.com) in San Francisco. Dobson set out to bring telescopes to the people. His goal was to build large telescopes at low cost. He achieved that goal by scrounging, begging, and recycling materials to build his scopes. He ground his primary mirrors, for example, from salvaged ship portholes, and recycled old binoculars for finder scopes.
The real problem was the mount. Traditional mounts for scopes the size of those Dobson was making would have cost thousands of dollars, and it was impossible to produce home-made versions of these mounts with adequate precision. In a moment of inspiration, Dobson came up with a simple but brilliant idea. Instead of using a traditional tripod or pier mount, Dobson designed a simple alt-azimuth box mount that sat flat on the ground and rode on Teflon bearings. Such mounts could be produced cheaply from inexpensive, easily worked materials such as plywood and kitchen counter laminate, and were stable enough to support even the largest scopes.
Commercial telescope makers grabbed the ball and ran with it, and nowadays Dobsonian reflector telescopes are ubiquitous. If you attend a large star party [Hack #2], you’ll probably see more Dobs than all other types of scopes combined. Figure 1-8 shows our 10” f/5 Orion XT10, a typical commercial Dobsonian telescope. Similar models are produced or sold by numerous companies, including Orion, Celestron, and others.
Dobsonian-style scopes were responsible for the proliferation of large-aperture scopes among amateurs. When Robert started observing in the mid-60s, the standard amateur instruments were commercial 60mm refractors and 6” home-made reflectors on home-made equatorial mounts. People would drive for hours for a chance to look through an 8” scope, and if you had a 10” scope you probably had one of the largest amateur instruments in the state. Nowadays, 10” and even 12” Dobs are considered mid-size instruments, suitable even for beginners, and many serious amateurs own 15”, 20”, and even 30” Dobs. And it’s all thanks to John Dobson.
As scope sizes began to increase, a problem became apparent. In Dobs up to 10” or 12”, the scope tube is awkward but manageable. Most people can, with little or no assistance, handle a 4-or 5-foot tube that’s 12” or 14” in diameter and weighs 30 to 50 pounds. In larger apertures, though, a solid tube becomes impractical. The largest tube Dobs have tubes 8 feet long that weigh more than 300 pounds. You need a crane to move them, or at least a couple of strong friends.
With the tube putting a practical upper limit on aperture, some alternative was needed if Dobs were to continue growing larger. That alternative is called a truss Dob. In a truss Dob, the solid tube is eliminated, replaced by a structure of light aluminum tubes that connect the focuser cage to the mirror box. Figure 1-9 shows a typical truss Dob; this one is a 17.5” f/5 model built by contributor Steve Childers (at left), with Paul Jones center and Robert on the right.
To give you an idea of the scale of large truss Dobs, all of us are about 6’4” tall, and this is “only” a 17.5” model. So-called “monster Dobs” are 30” to 40” in aperture. To see through the eyepiece when these huge scopes are pointed near zenith, you must stand atop an 18’ or 20’ stepladder. (Standing on a stepladder two stories off the ground in pitch blackness—now there’s our idea of a good time.)
Dobsonian reflectors have the following advantages:
- Price
Dobsonian reflectors are the least expensive type of telescope in terms of dollars spent per inch of aperture. Mid-size tube Dobs—those in the 6” to 12” range—typically sell for $50 to $75 per inch of aperture, a small fraction of the cost of other types of mid-size to large telescopes. With a tube Dob, most of what you pay goes for the optics rather than the mount, motors, and electronics for which you pay a high price with other types of scopes.
Truss Dobs, although they cost considerably more than tube Dobs of similar size, are also a bargain. Although $3,000 or more for a 15” truss Dob sounds expensive, when you compare that to the cost of a traditionally mounted 15” scope, the cost advantage of the Dobsonian mount becomes apparent.
- Stability
A well-built Dob is inherently immensely stable. The optical tube sits atop a rigid plywood rocker box that rests on a groundboard, which sits flat on the ground. The center of gravity is very low, and the weight of the scope is distributed to three widely spaced feet on the groundboard. With other inexpensive mounts, just touching the focuser causes the image to bounce around for several seconds. With a Dob, vibrations damp out almost instantly.
- Intuitiveness
Nearly everyone intuitively understands the up-down-left-right motions of a Dob. When you want to point a Dob at a different object, you just grab the tube and move it to the object. Even a complete newbie can learn to use a Dob in about one minute flat. (Of course, that doesn’t mean a newbie can learn to locate objects that quickly, but intuitive operation is a real advantage nonetheless.)
- Portability
Despite its large tube, a tube Dob is easily portable, at least if the tube fits your vehicle. A tube Dob has only two parts—the tube and the base—so setup is a simple matter of placing the base on the ground and putting the tube on the base. Tear-down is equally quick and easy. It takes us literally one minute to unpack and set up our 10” Dob at the beginning of a session, and another minute at the end of the session to tear it down and repack it.
Truss Dobs are even more portable than tube Dobs, although they do take longer to set up and tear down. We know owners of 15” to 20” truss Dobs who carry their scopes in sub-compact cars. Depending on size, a truss Dob takes 5 or 10 minutes to set up and a similar time to tear down. Truss Dobs 20” and smaller can usually be set up and torn down by one person. Larger models may require a helper.
- Fast cool down
Newtonian reflectors, including Dobs, cool down faster than any other type of scope except a refractor. Our 10” Dob, for example, normally stabilizes within 30 minutes or so. When the differential between the scope temperature and the ambient air temperature is extreme, a small or mid-size Dob may require an hour to cool, versus two to four hours for a catadioptric scope of similar size. Because the primary mirror on a reflector is exposed, it’s easy to add fans to larger models to shorten cool-down time.
- High image quality
Although the secondary mirror and spider vanes on a Newtonian reflector inevitably contribute diffraction effects, the image quality of a good Newtonian is second only to a refractor. Well-designed Newtonians with long focal ratios and correspondingly small secondary mirrors provide refractor-like images, with few diffraction effects and very high contrast. Even faster Newtonians, such as f/5 and f/6 Dobsonian models, exhibit contrastier images and finer detail than catadioptric scopes of similar aperture.
Dobsonian reflectors have the following disadvantages:
- Frequent collimation
Collimation is the process of aligning the mirrors and/or lenses in a telescope so that they share a common optical axis. All scopes must be properly collimated to provide their best image quality, but reflectors (including Dobsonians) require more frequent collimation than do other types of scopes, and the collimation process is somewhat more complex for reflectors.
Beginners sometimes shy away from reflectors because they fear they will be unable to collimate properly. In fact, a full collimation [Hack #38] of the secondary mirror and primary mirror takes only a few minutes, and it is usually required only when you first assemble the scope. You collimate the primary mirror [Hack #39] every time you set up the scope, but that takes only a minute to do. Finally, you tweak the scope into perfect alignment [Hack #40].
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As evidence of how trivially easy it is to collimate a reflector, we forgot to include collimation as a disadvantage in the first draft of this chapter, and we had to come back and add it during an editing pass.
- Lack of tracking
By design, a Dob is an unmotorized alt-azimuth mount, which means the scope doesn’t track the apparent motion of the stars automatically. You have to move the scope manually to keep objects from drifting out of view. If your Dob has smooth motions [Hack #42] [Hack #43], it’s no problem to track manually, even at high power. However, the absence of motorized tracking does make it difficult, for example, to sketch at the eyepiece.
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There are ways to add tracking to a Dob. You can install a commercial motorized tracking system such as the Dob-Driver (http://homepages.accnorwalk.com/tddi/tech2000) or the ServoCAT (http://www.stellarcat.biz), or you can use an equatorial platform [Hack #63].
- Unsuitability for astrophotography
Even if equipped with motorized tracking, Dobsonians are generally poorly suited for astrophotography other than perhaps Lunar and planetary photography using eyepiece projection. The focusers on most Dobsonians have insufficient in-travel to accommodate even CCD cameras at prime focus, and film cameras are usually out of the question. Just as important, serious astrophotography requires hanging a lot of equipment on the scope—camera, autoguider or guidescope, and so on—and that often causes insurmountable balance problems with a Dob. If you want to do serious astrophotography, a Dob is about the worst possible choice of scope.
Although all Dobs are Newtonian reflectors, not all Newtonian reflectors are Dobs. Rather than mounting a Newtonian reflector optical tube on a Dobsonian base, it’s possible to install it on a traditional equatorial mount supported by a tripod or pier. In fact, until the advent of the Dobsonian, an equatorially mounted reflector was the most common amateur instrument.
An EQ Newt shares the optical advantages of a Dobsonian—large, high-quality aperture, fast cool down, and so on—and also has the advantage of motorized tracking (as an extra-cost option, if not always as a standard feature). Despite these advantages, EQ Newts aren’t very popular nowadays because the EQ mount itself adds significantly to their price relative to that of a Dob of similar size. For example, Orion sells their 6” XT6 Dobsonian for $249. Their similar EQ version, the SkyView Pro 6LT Equatorial Reflector, sells for $548. Similarly, their 10” XT10 Dobsonian sells for $499, while their Atlas 10 EQ Reflector—which is essentially the same optical tube mounted on an EQ mount—sells for $1,299.
Faced with that trade-off—motorized equatorial tracking versus much more aperture for the same money—most people choose the latter. Rightly, we think. For example, given the choice of a 10” Dob for $500 or a 6” EQ reflector for $550, we think most people will be happier with the Dob. The larger aperture of the Dob allows you to see more than one full magnitude deeper—which is an immense difference—and the Dob is much easier to set up and use than the EQ Newt.
For the price differential between the 10” Dob and 10” EQ models, we’d sooner buy the Dob and spend part of that extra $800 on an equatorial platform [Hack #63]. In fact, we’d probably buy the $949 12” IntelliScope Dob instead and spend the remaining $350 buying the parts to build a top-notch equatorial platform.
For larger EQ-mounted scopes, the other consideration is weight, which directly affects portability. The Orion 10” Dob, for example, weighs 55 pounds complete, divided about equally between tube and base. The Atlas 10 EQ Reflector optical tube weighs 27 pounds, but the mount weighs 90 pounds. Most healthy adults can handle the 10” Dob without assistance, but setting up the Atlas mount may require a helper.
If you hadn’t guessed, we’re not big fans of equatorially mounted reflectors.We’re apparently not alone, as few amateurs buy these scopes nowadays.
A catadioptric telescope (“cat” for short) employs both mirrors and lenses. There are many types of catadioptric telescope, including the Schmidt-Cassegrain Telescope (SCT), the Maksutov-Cassegrain Telescope (MCT or Mak-Cass), the Maksutov-Newtonian (MN or Mak-Newt), and the Schmidt-Newtonian (SN). There are other types of catadioptric telescope, but none are commonly used by amateurs.
The Schmidt/Maksutov and Newtonian/Cassegrain nomenclature confuses a lot of people, but it’s really not that difficult to understand. Most catadioptric scopes use a Newtonian or Cassegrainian primary mirror at the back of the tube and a Schmidt or Maksutov full-aperture corrector plate at the front of the tube. The corrector plate, as its name implies, is not simply a flat piece of glass. It is an actual lens, with curves calculated to reduce the aberrations produced by the primary mirror.
Cassegrain variants use a Cassegrain primary mirror, which has a central hole. Incoming star light reflects from the primary mirror to a convex secondary mirror mounted on the rear of the corrector plate, which reflects the light straight back (180°) toward the rear of the tube, out of the central hole in the primary mirror and into the focuser and eyepiece. Because the secondary mirror is convex, it causes the fast-converging light cone from the primary mirror to diverge, which has the effect of increasing the apparent focal length of the scope.
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Because Cassegrain variants use a convex secondary and “fold” the light path, they are physically much shorter for their focal lengths than other types of scopes. For example, a typical 8” f/10 SCT has a focal length of 2,032mm. The primary mirror is actually f/2, which means its native focal length is about 400mm. But the convex secondary mirror functions, in effect, as a 5X Barlow, causing the rapidly converging f/2 light cone to converge more slowly as an f/10 light cone. The severe aberrations, particularly coma, that are inherent in any fast mirror—let alone one operating at f/2—are corrected by the corrector plate.
Newtonian variants use a solid Newtonian primary mirror. Incoming star light reflects from the primary mirror to a flat secondary mirror mounted diagonally on the rear of the corrector plate, which reflects the light at a 90° angle out the side of the tube and into the focuser and eyepiece.
Schmidt variants use a relatively thin corrector plate with shallowly curved aspheric surfaces.
Maksutov variants use a much thicker corrector plate with deeply curved spherical surfaces. This thick glass corrector plate retains heat, and Maksutov telescopes are notorious for taking long periods to cool down.
So, for example, a Maksutov-Newtonian Telescope uses a Newtonian mirror—which means you view from the front of the scope—and a Maksutov corrector plate, which means the scope takes a long time to cool down. Conversely, a Schmidt-Cassegrain uses a Cassegrain mirror—which means you view from the rear of the scope—and a Schmidt corrector plate, which means the scope cools down much faster than a Maksutov variant, although still slower than a refractor or a reflector.
- Schmidt-Cassegrain
SCTs are by far the most popular type of catadioptric scope used by amateurs. In fact, they come close to Dobs in overall popularity. SCTs are the proverbial jack of all trades and master of none. They do nothing that some other type of scope doesn’t do better. On the other hand, they do just about everything reasonably well. SCTs pack a lot of aperture into a physically compact package. They are available in apertures ranging from 4” to 20”. Models up to 12” are reasonably portable, but larger SCTs are for all practical purposes limited to fixed observatory mountings. Figure 1-10 shows a typical SCT; this one is an 8” Celestron model mounted on a Vixen Super Polaris equatorial mount.
The two leading SCT producers, Celestron and Meade, both produce SCT tubes in various apertures and offer them on different mounts of widely varying quality and cost. For example, Celestron offers the 11” C11 optical tube on four different mounts:
The $1,700 C11-S uses a crude, Chinese-made CG-5 equatorial mount. Although Celestron has enhanced the CG-5 tripod with heavy tubular stainless steel legs, the 11” OTA is grossly under mounted on a CG-5. Most experienced astronomers consider the CG-5 to be an adequate visual mount for an 8” SCT, marginal for the 9.25” SCT, and utterly insufficient for an 11” SCT, even for visual observing. Incredibly, Celestron claims the C11-S is suitable for astrophotography. We’ll be kind here, and say we think they’re exaggerating.
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The $2,000 C11-SGT is a C11-S with go-to functionality added. The extra $300 buys you a hand controller. You punch in an object on the hand controller, and the scope locates the object for you automatically. The scope and mount are otherwise identical to the C11-S. The basic C11 model is also available with enhanced XLT coatings for another $250 or so. Other Celestron models have similar options available
The $2,600 CPC 1100 mounts the C11 OTA on an inexpensive alt-azimuth fork mount. Go-to functionality is standard with this model. We consider the CPC 1100 to be Celestron’s real entry-level 11” SCT. Anyone who buys the C11-S will almost certainly be disappointed by the inadequate mount. The CPC mount, on the other hand, is sufficient for visual observing, although not for astrophotography.
The $3,500 NexStar 11 GPS mounts the C11 OTA on a better quality alt-azimuth fork mount. We consider the NexStar 11 to be Celestron’s mainstream 11” SCT, an excellent visual instrument out of the box. You can convert the NexStar 11 alt-azimuth mount to an equatorial mount by installing an optional equatorial wedge. The NexStar 11 isn’t the best choice for long-exposure prime focus astrophotography, even with the wedge installed, but it is usable for casual astrophotography.
The $4,200 CGE 1100 mounts the C11 OTA on a top-notch equatorial mount. The CGE is Celestron’s “serious” 11” SCT model. It is well suited for visual observing and serious astrophotography out of the box.
Note that the only real difference in these four models is the mount. For $1,700, you get a Celestron 11” SCT optical tube on a mount that is so crude and under-sized that using the scope is an exercise in frustration. Getting the same optical tube on what we consider to be a usable mount costs an extra $900, and getting it on the mount you really want costs an extra $2,500. The same is generally true of SCTs in other apertures and from other manufacturers. If you decide to buy an SCT, don’t make the mistake of undermounting it. A good mount is essential, but it isn’t cheap.
- Maksutov-Cassegrain
Until relatively recently, MCTs were a rare sight at star parties. Few astronomers owned one because few models were available, and those that were were very expensive. For many years, the only commercially available MCT was the 3.5” Questar, which ranges in price from $4,000 to $6,800, depending on options. (Questar also makes a 7” MCT, but if you have to ask the price, you can’t afford one.) Several years ago, Meade jump-started the market for MCTs when it introduced a series of MCTs ranging in aperture from 4” to 7” and in price from a few hundred dollars to $3,000 or so. In the last couple years, a flood of Chinese-made MCTs has arrived. These inexpensive instruments range in aperture from 3.5” to 5”, are sold by Orion and other retailers, and are surprisingly good optically for their price. MCTs are now relatively common at star parties. MCTs closely resemble SCTs.
We consider an MCT to be a specialized instrument that is generally unsuitable as a first or only telescope. MCTs have very long focal lengths and high focal ratios, typically f/15, compared to the f/6.3 or f/10 ratios of SCTs and the f/4 to f/6 ratios common in reflectors. That means that MCTs have very narrow fields of view relative to other types of scopes, and they are best suited to high-power, narrow-field observing, such as Lunar and planetary work.
Like SCTs, MCTs are physically compact for their aperture, so many astronomers choose an MCT as a grab-'n-go or quick-look scope[Hack #10]. We consider MCTs ill-suited for that purpose, though. Their thick Maksutov corrector plate means that MCTs have long cool-down times. Even small models may take a couple hours to equilibrate, and 7” models typically require several hours of cool down before they can provide their best image quality.
- Maksutov-Newtonian
Mak-Newts resemble standard Newtonian reflectors, but have a full aperture Maksutov corrector plate at the front of the optical tube. Mak-Newts are one of the best-kept secrets in amateur astronomy. The image quality of a top-notch Mak-Newt is superb, indistinguishable from that of an apochromatic refractor. Because Mak-Newts use a very small secondary mirror, they lack the contrast-robbing large central obstruction of traditional Newtonian reflectors and SCTs. The Mak-Newt secondary mirror is affixed directly to the rear of the corrector plate, so there are no vanes to produce diffraction spikes. Like an apo, a Mak-Newt provides pinpoint stars and an extremely high contrast image.
But a Mak-Newt sells for a small fraction of the price of an equivalent apo. For example, a 5” f/6 apochromatic refractor optical tube costs $4,500 to $6,000, while a 5” f/6 Mak-Newt costs about $900. The Mak-Newt takes longer to cool down than does the apo; otherwise, the two are functionally similar. Frankly, we’ve never understood why anyone would pay the high price for a medium or large apo refractor instead of buying a Mak-Newt of similar aperture, except perhaps that the small secondary mirror of a Mak-Newt can cause some vignetting (progressive darkening of the image as you near the edge) when used for film imaging or used with a very low-power, wide-field eyepiece.
None of the mainstream telescope makers produce a Mak-Newt, so they can be hard to find. Most of the high quality Mak-Newts available are made by two different Russian companies with very similar names, Intes and Intes Micro, which unfortunately have limited U.S. distribution channels. The most reliable U.S. source we know of is ITE (http://www.iteastronomy.com), which resells Intes models. Astromart(http://www.astromart.com) is also an excellent source.
Their relatively small apertures of 5” to 8” make Mak-Newts poorly suited for visual DSO observing, but for Lunar/planetary observing and astrophotography, Mak-Newts are unsurpassed. A Mak-Newt requires a mount, of course. The most popular choice of equatorial mount for 5” and 6” Mak-Newts is the $1,300 Vixen GP-DX (http://www.vixenamerica.com). The 7” and 8” models require a heavier mount, such as the $2,000 Losmandy G11. Figure 1-11 shows a typical Mak-Newt; this one is a 6” f/8 Intes MN-68 mounted on a Vixen GP-DX equatorial mount. Figure 1-12 is a close-up of the Mak-Newt front corrector plate.
- Schmidt-Newtonian
A Schmidt-Newtonian is similar conceptually to a Mak-Newt, except that the Schmidt-Newt substitutes a Schmidt corrector plate for the Maksutov corrector plate. Optically, there’s no reason that an SN scope can’t provide excellent images, but most manufacturers have chosen not to produce SN scopes. The one exception is Meade, which a few years ago began producing a series of SN scopes in 6” to 10” apertures. Unfortunately, the Meade SN-series scopes were designed to meet a price point that would appeal to beginning astronomers. Although we have never used a Meade SN-series scope, people whose opinions we respect tell us that the SN-series scopes are acceptable optically but mounted very poorly.
Warning
Yet another category of catadioptric telescope is commonly available, unfortunately. These hybrid scopes, sold under many brand names, look like standard equatorial Newtonian scopes, but with a shorter tube. Rather than a full-aperture corrector plate, these junk scopes have what is usually described as a “corrector lens” or “built-in Barlow” mounted inside the focuser mechanism. The primary mirror is typically of very short focal ratio—f/3.0 or so—and the built-in corrector lens extends that to an effective f/5 or longer, often f/8 or f/9. The optics of these scopes are terrible, and they are usually mounted on a very poor equatorial mount. Don’t waste your money on one of these scopes.
If you’ve made it this far, you probably have a pretty good idea of which type of telescope would best suit your own needs and preferences. Before you actually buy a telescope, though, we suggest you consider the following guidelines:
Avoid "department store” scopes. Actually, there aren’t many department stores left these days, but we use the term generically to mean junk telescopes sold by department stores, big-box retailers, cable TV shopping channels, “nature” stores at the mall, and so on. Whatever you do, don’t buy a telescope on eBay or a similar online auction site. You willget burned. About 99% of the telescopes sold on auction sites are junk.
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We exclude Astromart (http://www.astromart.com) from our general condemnation of online auction sites. Astromart is run by astronomers for astronomers, and many knowledgeable amateur astronomers regularly buy and sell gear there. Although we wouldn’t advise newbies to buy their first scopes on Astromart, it is the best place we know to buy and sell used telescopes and accessories.
Avoid any telescope that is promoted based on magnification. In a practice that we think borders on fraud, the boxes of junk scopes are often emblazoned with “475X!” or some other meaningless number. In fact, any telescope can provide essentially any magnification if you use an eyepiece of the proper focal length, but the high-power images provided by junk scopes are so dim and blurred that they’re unusable. Not that that matters much, because their mounts are so loose and shaky that you’ll probably not be able to keep an object in the eyepiece at high-power anyway.
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We were about to suggest avoiding telescopes with bright, colorful images of astronomical objects on the box, but even some good telescopes use such packaging nowadays. We consider the practice of using full-color images from the Hubble Space Telescope to be deceptive, to say the least, but even reputable vendors have begun doing so. Just don’t expect to see anything even remotely similar through the eyepiece.
Buy from a retailer that specializes in astronomy gear. If you don’t have a local astronomy store, check the ads in magazines such as Astronomy and Sky & Telescope.
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Although we don’t officially endorse any vendors, we think it’s fair to say that we, our observing buddies, and our readers have generally been happy with the prices, products, and service provided by:
Anacortes Telescope and Wild Bird (http://www.buytelescopes.com)
Astronomics (http://www.astronomics.com)
Hands-On Optics (http://www.handsonoptics.com)
High Point Scientific (http://highpointscientific.com)
Oceanside Photo and Telescope (http://www.optcorp.com)
O’Neil Photo and Optical (http://www.oneilphoto.on.ca)
Orion Telescope and Binocular Center (http://www.telescope.com)
Adorama, B & H, and the other New York City camera stores often have excellent prices on astronomy gear, but you must know exactly what you’re looking for. Don’t expect skilled advice from them. Their shipping charges are often very high, so make sure to get a total price before you order. We prefer to support specialty astronomy retailers. They may charge a few bucks more, but they are run by astronomers for astronomers, and their expert advice is often worth the small extra cost, particularly if you’re not entirely sure what you’re doing.
Try out different types of telescopes before you buy. Join your local astronomy club. Attend star parties and public observations where you’ll be able to see different types of scopes up close and personal. Arrive early and leave late so that you can watch scopes being set up and torn down.
Be realistic about your own physical abilities. A smaller scope that you’re willing to get out and set up shows you more than a large scope that never leaves the closet.
Recognize that no one telescope is ideal for all purposes. Buying two moderately priced telescopes may give you more flexibility than spending the same total amount on one more expensive scope. For example, many people buy an inexpensive 8”, 10”, or 12” Dob or an 8” or 10” SCT as a primary scope, and add a short-tube refractor as a grab-'n-go or quick-look scope.
If in doubt, buy the next size larger. If you’re debating between an 8” and 10” scope, for example, it’s usually better to buy the larger scope (assuming you can transport it and so on). Some people buy the next size down, knowing that they’ll want to spend money on eyepieces and other accessories. But eyepieces come and go, while the scope will probably remain with you for a long time. Buy the larger scope now, and add accessories later as you can afford to.
Conversely, don’t blow your entire budget on the telescope. You will need some accessories right away, and you’ll want many more accessories before long.
Don’t buy a scope based on its perceived capabilities for astrophotography. Nearly every amateur astronomer has aspirations to image the heavens, but unless you’re willing to spend at least a few thousand dollars on your first scope, you will be disappointed in the results. Your goal for your first scope should be an instrument best suited to your own preferences for visual observing.
Do not buy an inexpensive go-to scope. Too much of the cost goes to the mount, motor, and electronics, and too little to the scope itself. Unless you’re willing to spend $1,800 or more on an 8” or larger go-to SCT, we think you’ll be be disappointed. There are reliable 5” SCT go-to scopes that sell in the $1,000 range, but we think these scopes have too little aperture to serve as a general-purpose scope. Their small aperture effectively restricts them to viewing Solar system objects and the brightest DSOs, which most people will quickly find too limiting. If you have any interest in observing DSOs, start with a scope of at least 8” to 10” of aperture.
Don’t fall prey to analysis paralysis.
Still can’t make up your mind? Okay, we’ll do it for you, but upon your head be it. (We’ll be happy to pick a spouse for you, too, as long as you’re willing to live with the results.) Here are the scopes we recommend:
If you’re on a tight budget—or just unsure whether you’ll maintain your interest in the hobby—buy the Orion StarBlast. At $170 complete, the financial risk is minimal. Orion markets this as a children’s scope, but there are probably more StarBlasts used by serious adult astronomers than by children. At 4.5”, the StarBlast provides enough aperture to show pleasing images of Luna, the planets, and the brightest DSOs. You’ll probably want a larger scope soon, but you’ll never outgrow the StarBlast. We know astronomers who have spent many thousands of dollars on large premium scopes who still keep a StarBlast as a quick look scope.
If you’re willing to spend $300 to $1,000 initially, buy a mainstream 6”, 8”, 10”, or 12” Dobsonian scope from Orion, Skywatcher (Canada), Celestron, or one of the many other vendors who resell Synta (Chinese) and Guan Sheng (Taiwanese) Dobsonians. Guan Sheng models, sold by Celestron and others, have better mirrors and mechanicals.
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Synta models are not as good optically or mechanically as Guan Sheng models, but the Synta-manufactured Orion IntelliScope Dobs have one unique feature that makes them worth considering. The Computerized Object Locator hand controller, a $99 option, converts the manual dob into a “push-to” automated scope. The scope still doesn’t have motorized tracking or go-to capabilities, but you can locate objects automatically just by watching the arrows on the hand controller and manually moving the scope as the arrows indicate.
If you want a compact, portable scope that tracks and provides go-to functions, choose the Celestron CPC 800 8” SCT, which costs $1,800 with standard coatings or $2,000 with enhanced coatings.
It’s obviously very difficult for anyone to make specific recommendations without knowing your circumstances or personal preferences, so we hope you’ll take this advice merely as a starting point, and follow the suggestions we made earlier.
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