The most important thing about a telescope is its aperture. Because of the high magnifications used with telescopes, diffraction limits their performance. Normally, one would think of diffraction as a phenomenon associated with things like the microscopic rows of pits on the inner reflective surface of a Compact Disc.
Aperture, which means "opening", is normally determined by the narrowest part of the telescope up to and including the main lens or mirror of the telescope. Thus, many telescopes will have a front opening somewhat larger than the main mirror, so that objects in the edge of the area seen by the telescope do not have some of their light blocked from reaching the mirror (an example of vignetting). A Schmidt camera, on the other hand, has a mirror that is larger than the corrector plate as another way to achieve a uniform image area, and in that case the corrector plate determines the effective aperture of the telescope.
A common rule of thumb is that the maximum reasonable magnification with a telescope is 60x per inch of aperture. Thus, an 8-inch (20cm) telescope could be used with magnifications up to 480x. Even this is considered quite high, although one can use larger magnifications to make fine detail easier to see, even if after that point no new details will become visible.
A magnifying glass makes things bigger, but it doesn't make them any brighter. The same is true for a telescope, for example, when you use it to look at the surface of the Moon. But for point objects like stars, making them bigger is effectively the same as making them brighter; their brightness per unit area has not increased, but even after the highest possible magnification is applied to them, their area is still too small to see. If one uses a telescope as a camera lens, however, as in prime focus astrophotography, where film or a CCD is placed at the spot where the telescope forms the image that an eyepiece normally would magnify, the film registers light hitting it from all directions, unlike our eyes, which see only the light coming in from the right direction for any object, and so the required exposure time for film decreases as aperture increases.
One reason why amateur astronomers will emphasize the importance of aperture to newcomers is that sometimes inexpensive small-aperture refractors are advertised as having high magnifications, and this can be impressive to the unwary.
If aperture is what really counts, then does it follow that the cheapest kind of telescope is the best? Although money is certainly a big limit on what most people can do, other things must be considered besides the cost of the telescope itself. Most people live in brightly-lit cities. Since a house in the country costs considerably more than a telescope that is easy to take with you on a short trip out of town, one can't consider the telescope's cost in isolation.
The Newtonian telescope is perhaps the cheapest telescope per inch of aperture. A general-purpose Newtonian, like the one shown in the diagram above, might be an f/6 or f/8 design, depending on size; that is, the focal length might be 6 or 8 times the aperture. (Thus, a camera lens that can be opened to f/1.4 is one which has a very wide aperture compared to its focal length, allowing it to put a very bright image on the film: if the focal length were 49mm, then the aperture would be 35mm.)
An 8" telescope with a focal ratio of f/6, or a 6" telescope with a focal ratio of f/8, has a focal length of 48 inches, or four feet. The actual length of the telescope will be a little larger than its focal length. This makes it somewhat cumbersome to move from place to place.
The particular combinations of aperture and focal length used as an example in the previous paragraph are those of the most popular type of telescope used by serious amateur astronomers up to the 1960s, usually mounted on a German Equatorial mount.
An equatorial mounting, particularly if motorized, enables a telescope to rotate in the direction opposite to the Earth's rotation, and thus naturally follow stars and planets as they appear to move in the night sky. An ordinary Newtonian telescope may make use of the "German equatorial mount", which involves the use of a counterweight.
However, another form of Newtonian telescope combines increased portability with even lower cost. This is the Dobsonian telescope (named after John Dobson of the San Francisco Sidewalk Astronomers) which places a large but thin mirror in a telescope with a simple altazimuth mounting making use of ultra-high molecular weight plastics (such as DuPont Teflon). This kind of telescope has a short focal length in proportion to its aperture, which requires a larger diagonal mirror, meaning a larger central obstruction, and the wider cone of light places extra demands on some eyepiece designs.
Also, while a parabolic mirror provides full correction for the second most serious imperfection to avoid in a telescope, spherical aberration, the next most serious one, coma becomes more of a problem with a wider aperture in proportion to the focal length (which is also referred to, from photography, as a faster focal ratio). A reflecting telescope, by its very nature, is immune to an even more serious aberration, chromatic aberration,
Refracting telescopes do not have a diagonal mirror in the middle of the light path, so they avoid the diffraction caused by the central obstruction. A refracting telescope with an objective made from a single piece of glass has enough chromatic abberration as to be useful only at low magnifications: while, centuries ago, this was the only kind of telescope there was, today such a telescope would tend to be regarded as little more than a toy. Even when a two-element achromatic lens is used, there is enough chromatic abberration remaining that this is the main limiting factor of the telescope. A typical amateur refractor might have an objective lens two inches in diameter, a focal ratio of f/15, and hence a length of 30 inches, or two and one-half feet.
Larger refracting telescopes with a two-element achromatic lens have been, and still are, legitimately regarded as being as much serious scientific instruments as any other kind of telescope.
A 2" telescope with a long 2 1/2' tube still looks like most layperson's idea of a telescope. At one time, larger refractors with achromatic lenses and focal ratios in the neighborhood of f/15 were reasonably common and popular, despite being much more expensive than a reflecting telescope. Due to their bulk, and improvements in other kinds of telescope, larger achromatic refractors are much less popular than was previously the case. However, 3" achromats are still commonly available, and objectives of up to 6" can still be ordered by mail from some firms.
As noted, having a longer focal length for the objective lens reduces abberrations, including chromatic abberration. At f/15, a 3" achromatic telescope performs very well, providing performance comparable with that of an apochromat. A 6" telescope at f/15 has some noticeable chromatic abberration, but is still quite satisfactory. The required focal ratio for an achromatic refractor increases in proportion to the aperture, so the focal length for a given level of performance increases with the square of the aperture.
However, just as it is possible to add supplementary lenses to large Newtonians to correct coma, or to Schmidt-Cassegrain telescopes to correct curvature of field, lenses to correct the remaining secondary chromatic abberration in an achromatic refractor are available.
Since even a telescope with four inches of aperture, when used with eyepieces that yield no more magnification than is genuinely available from such a telescope, gives only a small image of Jupiter or Mars in the eyepiece, many amateur astronomers will feel that a pair of 7 by 50 binoculars would be a better instrument for looking at the Moon and the constellations than a telescope with no greater aperture. As well, some smaller telescopes are advertised as being usable with magnifications in excess of what diffraction will really allow to be effective, or packaged with photographs of Saturn on the box, and so on. So ordinary refractors with an aperture of two inches (50 mm) or so tend to be disparaged as "department store telescopes", likely to lead to disappointment and loss of interest on the part of the budding astronomer. Some of that criticism is due to other potential quality issues, for example the stability of the mount.
The Moon provides exciting detail in even a pair of binoculars or a very small telescope. The planets, being very bright, can be viewed from a city backyard despite some degree of light pollution, and they're a very obvious target for the beginning observer. A telescope with 3" (75mm) to 6" (150mm) of aperture, when used at a high, but reasonable, magnification, will show the easier planets to view (Mars, Jupiter, and Saturn) as disks with some detail visible. Mars will still be quite a small disk in a 6" telescope, and even Jupiter will be quite a small disk in a 3" telescope. But they will be sharp and clear at those sizes.
The apochromat is a refracting telescope which uses a three-element objective lens, and usually one of the elements is made from a special type of glass instead of simply normal crown and flint glass. This reduces chromatic and other aberrations enough to allow a shorter telescope to be made, but such telescopes are quite expensive. Apochromatic telescopes are typically made with focal ratios such as f/6 or f/8.
Using more exotic forms of glass in a two-element lens also allows chromatic aberration to be reduced significantly, and some telescopes with lenses of this type are now being advertised and sold as apochromats. Although this is not in line with what has been understood as the original technical meaning of the term, the telescopes so advertised are still, at this time, generally comparable in quality to many three-element apochromat designs. However, those same glasses are also used in three-element designs to provide even better quality.
This kind of telescope is quite expensive, although not totally unaffordable if one is content with a limited aperture.
The Schmidt-Cassegrain telescope, although more expensive than a Newtonian, is still relatively inexpensive, and is very convenient. The tube is sealed, so both dust and accidents are less of a worry. Even more important, because the tube is sealed, problems inherent in taking the telescope from a warm storage area out into the cold night are reduced, at least initially. (This is actually a somewhat complicated tradeoff: the worst effects of a temperature difference are eliminated with the sealed telescope, but not all of them, while the open telescope will eventually match the temperature of the surroundings, so for a patient observer, the open telescope comes out ahead.) Although the Cassegrain secondary mirror increases the effective focal ratio of the telescope as a whole to what is normally f/10 (a 4" Schmidt-Cassegrain might be f/12 instead), the telescope is short, lightweight, and portable. Usually, an equatorial fork mount had been used with such a telescope; this holds it very steady, and requires no counterweight, as the telescope is centered within the fork. Of late, this has changed, however; now, the tendency is to either use a computer-controlled altazimuth fork mount, because computerized control can take the place of equatorial orientation, or a German Equatorial mount.
A drawback of this telescope type is that the central obstruction is relatively large. Also, Schmidt-Cassegrain telescopes had for many years a reputation for less-than-ideal optical quality. Part of this may have stemmed from the difficulty in making the corrector plate with the correct complicated aspheric curve. This seems to have improved of late; first one major maker of such telescopes, and now its principal competitor, have found it worthwhile to switch to a design with an aspheric secondary mirror so as to eliminate coma in addition to spherical abberration. This, however, requires an additional increase of central obstruction, although apparently not quite enough to render the instrument suitable only for photographic, and not visual, use, as is generaly thought to be the case for real Ritchey-Chrètien telescopes.
Maksutov-Cassegrain telescopes are very similar to Schmidt-Cassegrain telescopes in their convenience, but the design has tended to offer better optical quality. This has led to these instruments being considered premium telescopes, and this has meant that their higher prices have translated into a much higher quality of manufacture, offering advantages in addition to the inherent ones of the design.
Because they only require spherical surfaces, however, recently less expensive Maksutov-Cassegrain telescopes have become available from countries that do not yet export Schmidt-Cassegrain telescopes. As well, at least in smaller apertures, most mainstream companies selling Schmidt-Cassegrain telescopes now also offer Maksutov-Cassegrain telescopes, so this design is no longer the exotic premium design it once was.
Both Schmidt-Cassegrain and Maksutov-Cassegrain telescopes are examples of catadioptric optical systems, systems that use lenses (dioptric) and mirrors (catoptric) as integral parts. The types of telescopes mentioned above are the most common and popular types; we have already met some other types, and we will be doing so later on as well.
A Richest-Field Telescope (RFT) is a phrase used for a particular kind of telescope. One doesn't hear this phrase used too often to describe telescopes offered for sale these days, because it describes a condition that involves the magnification of a telescope, and thus is only really applicable to a telescope which doesn't allow you to change its eyepiece.
As has been noted, 60x per inch of aperture is considered to be the usable limit of magnification on a telescope. There is also a limit to how low the magnification on a telescope can usefully be made.
The objective lens on a refracting telescope, or the aperture of a telescope in general, is acting on behalf of the pupil of your eye when you are looking through the telescope. The light that enters the telescope must also enter your eye as well, if that light is to serve to allow you to see the heavens.
The ratio between the focal length of the eyepiece, and that of the telescope, determines not only the magnification of the telescope, but also the factor by which the stream of light entering the telescope is compressed before it enters your eye.
A pair of 7x50 binoculars has an aperture (in each half) of 50 millimeters, and, because it has a magnification of 7 times, compresses the light entering that aperture down to one-seventh of its width, which is just over 7mm. This is a value typically used for the size of the dark-adapted pupil of the eye. The actual value actually changes with age, but 7mm is an average for a 30-year-old adult, who is considered typical for many applications of optical equipment.
Such binoculars, therefore, are called "night glasses", because their large aperture (compared to lighter 7x35 binoculars) admits light to the entire area of the pupil of the eye when it is fully dark-adapted.
For a given magnification, therefore, increasing the aperture until it corresponds to 3.5x per inch, or 1x per 7mm, will make everything seen in a telescope brighter, because more light is entering the eye.
For a given aperture, increasing the magnification beyond 3.5x per inch will take the scene presented at 3.5x per inch and enlarge it, without adding any light to it. Extended objects, like the Moon, (or, at much higher magnifications, planets) will become dimmer as that happens. Stars, however, are point objects. So they will remain at the same level of brightness, the magnification will only spread them apart.
This is why a telescope operating at 3.5x per inch of aperture is called a "Richest-Field Telescope"; at (or below) this level of magnification, the sky will appear through the telescope to be filled with bright stars close together, on average, to the same extent as it does when viewed with the naked eye.
Telescopes are often described in terms of their focal ratio. A telescope with an aperture of 6 inches and a focal length of 48 inches is an f/8 telescope, one whose aperture is 1/8th of its focal length. (This is the same rule as used for camera lenses; an f/1.4 camera lens with a focal length of 49mm therefore has an aperture of 35mm.)
A telescope with a focal length of 1000mm and an aperture of 180mm would normally be termed an f/5.6 telescope. If you use a 40mm eyepiece with it, the magnification will be 25x (1000/40 = 25), and 25 times 7mm is 175mm, close to the telescope's aperture. Doubling the focal length would double the magnification and double the aperture required for this condition to be met, so on any f/5.6 telescope, using an eyepiece with a 40mm focal length turns it into an RFT.
Here is a very short table of focal ratios, and the correponding eyepiece focal lengths that meet this condition:
28mm f/4 36mm f/5 40mm f/5.6
The table is short because although one could add more entries, in general eyepieces are not available in focal lengths much above 40mm (at least not those with a 1 1/4" barrel; those for larger telescopes with a 2" barrel can have proportionately larger focal lengths).
One question which often causes animated discussions among amateur astronomers is whether or not there is such a thing as a "planetary telescope". Are refractors and long-focus (f/12, f/16) Newtonians really any better than short-focus (f/5.6) Newtonians and Schmidt-Cassegrain telescopes, as some people say, or is that a myth?
What does central obstruction do?
It isn't a myth that central obstruction changes the character of the little dot into which a telescope focuses the light from a point object. It does do that. What happens is that the brightest part of that dot, in the center, remains the same size, but the ring of scattered light around it becomes a little brighter.
This effect is quite slight, unless the central obstruction is very large, as this table (based on information from Amateur Astronomer's Handbook, 4th edition, Sidgwick and Muirden) of how much light is found in the rings surrounding the Airy disc for various sizes of central obstruction shows:
No central obstruction: 16.2% 10% of aperture: 18.2% 15% of aperture: 20.5% 20% of aperture: 23.6% 25% of aperture: 26.8% 30% of aperture: 31.8%
A central obstruction of 10 or even 15 percent of the aperture is not a significant problem, but one of 30 percent of the aperture would be noticeable, and even then, not too serious a problem.
What this means is that while the overall contrast in the image remains the same, contrast of the finest details in the image is reduced. Since very fine detail is hard to see in any event, it might even mean you will no longer see the finest details.
Because the brightest part of the dot in the center remains the same, though, the resolution of the image isn't changed; all the detail is still present, it's just a bit harder to see.
Back in 1965 or so, most amateur astronomers really had only one choice of telescope, a Newtonian on a German equatorial mount, for apertures of 3 1/2 inches and above, while for smaller apertures, there were refractors with achromatic objectives.
Under those circumstances, the difference in cost and portability between an f/5.6 Newtonian and an f/12 Newtonian was not major. As well, many amateurs ground their own optics, and a shallower curve is simpler to handle. In addition to a smaller central obstruction, a longer-focus Newtonian has less coma, an important optical aberration. Thus, it made sense to recommend the longer focus telescope as being a better choice for planetary observing. Central obstruction may have been a fine point, but it was worth attending to it, because other things either were equal, or else also tended to favor the longer-focus design.
Today, the situation has changed, making it necessary to put the importance of central obstruction in perspective, because there are wider choices available to the prospective amateur astronomer.
There are Dobsonian telescopes, which have made very large apertures reachable by amateurs, but which, in these large apertures, need to have a short focal length to be practical.
There is the Schmidt-Cassegrain telescope, which offers convenience and portability to an extent that ordinary Newtonians and refractors cannot hope to match.
Should the potential benefits of these kinds of telescopes be dismissed because they have relatively large central obstructions? And if not, why not?
Since it is no longer true that other things are equal, but one instead can, through saving money or gaining portability, have a larger aperture of telescope in return for accepting the central obstruction, it doesn't make sense to put a fine point in telescope quality ahead of the characteristic that determines its ability. It does not make sense to prefer a 4" refractor over an 8" Schmidt-Cassegrain on the basis that the lack of central obstruction will improve image quality.
As well, two things mitigate the effects of central obstruction.
The Earth's atmosphere limits the amount of magnification that can be put to effective use with a telescope. A telescope with an aperture of more than 10 inches already is in collision with that limit most of the time. Hence, in a telescope of 24 inches aperture, the detail the contrast of which is reduced by central obstruction is largely detail which will not actually be available when looking through the Earth's atmosphere at a celestial object. At 17 inches, a size that many enthusiastic amateurs have, there is still some effect of central obstruction at times of perfect seeing, but it is now only a partial effect.
Because the central bright spot isn't expanded, and the information in the image is still present, it is possible when using a CCD to take astrophotographs with a telescope, to subject the image to digital processing which will remove the effects of the telescope's central obstruction.
That the central obstruction is an issue in telescope quality is not a myth; this is a real effect, and has a limited degree of importance. But it is a serious error to assign too much importance to it. Since it's easy to change "its importance is limited" to "it isn't real", and it's also easy to change "it is real" to "it is very important", when the truth lies in this particular region, it's rather easy for arguments to start.
Also, given that central obstruction is only a concern when a telescope is used near its limit of magnification, its importance depends on how a telescope will be used.
One does not need a telescope to look at the night sky and see stars. A pair of binoculars will, of course, reveal more stars than are visible to the unaided eye. A telescope, in addition to revealing even more stars, will allow one to see other sights in the night sky.
One thing a telescope will reveal are "double stars"; it is actually possible to see, as two separate stars in the sky, two stars belonging to a binary star system, that is, two stars that are in orbit around their common center of gravity.
Telescopes will also reveal distant galaxies. The closest galaxy to us, other than our own, the Andromeda galaxy, M31, is actually fairly large in angular extent in the night sky. But it is also faint. This applies to several more distant galaxies, and it also applies to other interesting night-sky objects such as nebulae. Since they are relatively large in size, but faint, observing this type of object, which is called "deep-sky astronomy", places a premium on aperture, and while some magnification is still needed, it will normally be low relative to the aperture, so that the telescope is operating as a richest-field telescope, and thus not making the object any fainter, except to the extent imposed by fundamental limitations of the telescope, such as the reflectivity of its mirrors and the transmissivity of its lenses. In this case, the only problem a central obstruction causes is that it blocks some of the light.
The planets (and the Moon) are also interesting objects that provide detail that can only be seen in a telescope. They are small, but bright. This makes them accessible for observation from one's backyard in the city, and they are therefore a natural subject of interest to beginning amateur astronomers. For them, the maximum possible magnification a telescope can yield, and even some "empty" magnification besides, is both usable and useful. Thus, telescopes with a small central obstruction tend to be known as "planetary" telescopes; keeping the central obstruction small allows visual observation of the planets to reveal their details in telescopes with the minimum required aperture.
Copyright (c) 2001, 2002, 2006 John J. G. Savard