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Kinds of Telescopes

There are many different kinds of telescope.

In the refracting telescope, the objective lens is usually composed of two lenses, made out of different kinds of glass. Such a lens is called an achromat. A glass prism can be used to produce a rainbow of colors from white light. This is because glass bends different colors of light to different degrees.

When one wants to have a clear and sharp image of something one is looking at, this effect becomes an annoyance, known as chromatic aberration. An achromat is designed to cancel this effect, by using lenses made of two different kinds of glass. One lens is convex, made of crown glass. The other is concave, made of flint glass, which is denser, and which bends light more strongly, if made into a lens of the same shape, than crown glass. However, not only does it bend light more strongly, but the difference in how it bends lights of different colors is also more pronounced, even in proportion to the increased amount of bending.

Thus, two lenses close together, one of flint glass, and one of crown glass, bending light in opposite ways, can be made so that the difference in bending different colors cancels out, but the lens itself still performs a net function of bending the light that goes through it in one direction.

However, because the behavior of light of different colors in glass doesn't follow a simple fixed law, this cancellation can only be exact for two colors. This still gives a great improvement over a plain lens. But sometimes a larger improvement is desired, and then a lens is designed out of three elements of three different kinds of glass. Such a lens is called an apochromat, and these are used as objective lenses on some more expensive telescopes.

The objective lens on a telescope, instead of being like an ordinary magnifying glass lens in profile, equally fat on both sides, usually has a crescent-shaped profile, and such a lens is called a meniscus lens.

This is done to minimize another aberration, called spherical aberration. The bending of light by a lens is due to the mathematical law called Snell's law, and is due to the fact that light travels more slowly in glass than it does in air.

A spherical surface is relatively easy to produce when grinding lenses, but it is only an approximation to the shape of a surface that would focus incoming rays of light to a single point in an image.

Sometimes, particularly when many lenses are being manufactured for a fixed purpose, by being molded from plastic, it is worth the expense to make the mold the exact shape needed to produce the ideal surface for bending light to form an image. Lenses like this are called aspheric lenses. Sometimes such lenses are even made from glass for special purposes, but such lenses are expensive, and thus not commonly used.

The term aspheric, because it means "not spherical", is sometimes applied to other kinds of lenses which are not as difficult to make. They still have curved surfaces which are circles instead of the complicated curves needed to make perfect images. For example, you may have seen cylindrical lenses that can make a line of printing taller, even though they don't make it wider. Such lenses can make optical instruments that do one thing in one direction, and a different thing in another.

One application of this is the anamorphic lenses that are used to squeeze in the wide picture on a movie screen (2.35 times as wide as it is tall) in the frame of motion picture film designed for the original motion picture aspect ratio that is 1.33 times as wide as it is tall, like the screen on your TV set. (Actually, since Edison, motion picture standards were changed slightly to make the regular film aspect ratio 1.37:1; however, movies in the 2.35:1 aspect ratio are recorded in an area on the film that is actually only 1.175 times as wide as tall, because not only is the picture wide and spectacular, but the film also carries several extra sound tracks as well.)

Another application of this is eyeglasses. The lenses in eyeglasses are usually toroidal rather than spherical, so that they can correct not only for a wrong overall focal length in the lens of the eye, but for differences in that focal length in different directions, or astigmatism.

Usually, telescope objective lenses in refracting astronomical telescopes don't use aspheric elements.

A refracting telescope is illustrated below:

A thin lens has less spherical aberration than a thick one. Even after correcting for chromatic aberration has made the two elements of the objective considerably thicker, spherical aberration is still fairly low. Making the objective a meniscus lens minimizes it, because then the overall shape of the lens follows the curve of the surface where the incoming rays of light would be bent into their new desired direction while retaining a uniform spacing between them. (Of course, why that should make a difference is complicated.)

But the next common type of telescope, the Newtonian telescope, does usually make use of an aspheric element. In the Newtonian telescope, the place of the objective lens is taken by a concave mirror, which can magnify and form images in much the same way as a convex lens. An extra mirror, a small flat mirror called the diagonal, is used to keep the head of the person using the telescope out of the way of the incoming light.

This type of telescope is illustrated below:

In the Newtonian telescope, the mirror which functions as the objective, called the primary mirror, usually is not left as spherical, but is carefully adjusted during grinding to take on a parabolic shape. (Thus, this adjustment step is called parabolizing the mirror.)

A very expensive and high quality kind of telescope is known as the Maksutov telescope. In this telescope, the primary mirror is left as spherical. A thick glass element at the front of the telescope, with the same curvature on the front and back, acts as a corrector for the spherical aberration of the mirror. It also has a circular spot in the center that is coated on the inside to be a mirror; this mirror reflects the light that would normally be brought to a focus shortly beyond it, and, because of its curvature, delays the focusing of the reflected light until it goes out the back of the telescope through a hole in the center of the primary mirror.

Some effort has been made to draw this diagram to scale, based on the actual design in Dimitri Maksutov's notebooks. However, the distance from the mirror surface out of the back of the telescope to the focal plane in the eyepiece is still exaggerated.

A similar, but less expensive, and thus highly popular kind of telescope is called the Schmidt-Cassegrain telescope. Here, instead of a thick piece of glass with two spherical surfaces, correction is provided by a very thin piece of glass, flat on one side, and with an aspheric surface on the other.

It happened that the aspheric surface needed for this kind of telescope could be very closely approximated by a simple and inexpensive method: take a thin, flat piece of glass, and put it on top of a round hole, behind which you pump out the air. This bends the glass slightly, and while the glass is thus bent, you grind it flat in the normal manner.

Here, the mirror for reflecting the light back through the end of the telescope is bolted to the front element.

The Schmidt-Cassegrain telescope is an example of a catadioptric telescope, one that uses both mirrors and lenses. A refracting telescope, which uses lenses only, is called dioptric, and one which uses mirrors only (not counting the eyepiece) such as the Newtonian is called catoptric.

The fundamental types of reflecting telescopes are illustrated in the diagram below:

In each case, the primary mirror is ideally a paraboloid. In the Cassegrain, the secondary mirror should be a hyperboloid, and in the Gregorian, the secondary mirror should be an ellipsoid. However, there are modified versions of these designs that allow one element to have some spherical aberration, with the other element counterbalancing it.

In the case of the Cassegrain telescope, the possible forms are:

The Schmidt-Cassegrain telescope combines the principle of a Cassegrain reflecting telescope with that of the original Schmidt camera:

This form of telescope used the symmetry of a spherical mirror, and a curved, spherical field of focus at which a curved photographic plate was placed, to allow a telescope with a very wide field to be constructed. The fact that the corrector plate, located at the center of these two concentric spheres, made the symmetry slightly less than perfect did not prevent it from giving good images.

To illustrate the principle, the diagram shows a very large focal field, which could not be fully used in practice, because a photographic plate at that location would block too much of the incoming light.

Note, though, that in the typical Schmidt-Cassegrain telescope, the corrector plate cannot be located at the center of curvature any longer, because the Cassegrain secondary is mounted directly on the corrector plate, which requires that the corrector plate be located closer to the mirror than the focus, which is at half the distance of the center of curvature.

The Maksutov design originally involved a similar symmetry principle:

note that there are two possible positions for the corrector plate, both shown in this diagram, which preserve the symmetry.

In practice, of course, only one of the two meniscus correctors is present. However, one telescope design,

known as the Super-Schmidt telescope, actually includes two symmetrical meniscus correctors, as well as a doublet lens at the center of curvature of the primary mirror. The flint element of the doublet corrects for the chromatic aberration of all three crown components; as well, the doublet's spherical aberration supplies part of the correction needed by the spherical primary mirror, which is, presumably, the reason for not calling it a Super-Maksutov telescope. Splitting the correction in this fashion between the two meniscus correctors and the lens allows a very fast focal ratio to be achieved. This form of telescope was designed by Dr. James Baker in 1947, and was used for a system of specialized cameras used for taking photographs of meteors.

A variant, with only the rear meniscus present, also exists, known as the Meniscus-Schmidt camera.

All this makes me wonder why no one has ever designed a telescope like this:

or this:

or even like this:

a silvered spot on a deeply curved meniscus provides a Cassegrain mirror of a convenient focal length, but most of the correction for the spherical shape of the primary may come, for example, from a thicker meniscus which either is concentric with the primary or is slightly less steeply curved to compensate for the excessive curvature of the thin meniscus. Of course, the only purpose for any of these designs, which attempt to return to the original forms of the Maksutov, Schmidt, or Super-Schmidt designs with their spherical symmetry or near-symmetry, is to reduce coma, and as the fact that these are Cassegrain designs breaks the spherical symmetry, and the spherical secondary mirror introduces other complications, correcting for coma may not be as simple as would lead to designs like these.

Since the corrector plate of a Maksutov telescope, in the more common orientation, is closer to the mirror than the corrector plate in a Schmidt camera, yet Gregory-Maksutov telescopes tend to be longer than Schimdt-Cassegrain telescopes of similar aperture, this means that the mirrors in the Maksutov telescopes are less strongly curved, and therefore have less spherical aberration to correct. This is another reason for their superior optical quality. In addition, less of a compromise is required in regards to the overall symmetry of the basic Maksutov design for a Gregory-Maksutov telescope, although it is likely that some compromise is required, in order that the silvered area serving as the secondary mirror have enough curvature to function in an effective manner as a Cassegrain secondary.

One could have a Schmidt-Newtonian telescope, or a Schmidt-Gregorian telescope as well, of course.

Similarly, in addition to the type of Maksutov telescope shown above, which is a form of Maksutov-Cassegrain telescope, there are also Maksutov-Newtonian telescopes offered for sale.

Also, the following illustration:

shows how the large meniscus of the Maksutov design can be replaced by small meniscus lenses in two possible locations; either just behind the Cassegrain secondary, where light passes through it twice, as in a design by the Australian amateur astronomer Ralph W. Field, and (using two elements for color correction) in some designs offered commercially by the Novosibirsk Instrument-Making Plant of Russia, or at the exit of the telescope tube. In that position, however, other kinds of corrector lenses, such as coma correctors or focal reducers are also used, so I am not certain if a particular brand of inexpensive telescope with an achromatic two-element lens in that position uses it to correct for a spherical primary mirror or not.

The Gregorian reflecting telescope was designed by one James Gregory in 1663. The particular form of the Maksutov-Cassegrain telescope in which the secondary is created by placing an aluminized circular area on the corrector, to simplify construction, was designed by one John Gregory, during the decade of the 1950s. (In fact, Maksutov did design such a version of his telescope in 1943, but it did not become generally known until later. On the other hand, the basic idea of a telescope with a meniscus corrector was discovered by Bouwers in 1941, shortly before it was discovered by Maksutov.) Hence, the most popular form of Maksutov-Cassegrain telescope is also referred to as a Gregory-Maksutov telescope, although it most definitely is not a Maksutov-Gregorian telescope. It is important not to allow oneself to be confused by this difficulty in telescope nomenclature.

Incidentally, when an aluminized spot on the corrector is used as the secondary mirror, it is necessary to deviate from the spherical symmetry of the basic design in order to correct aberrations more effectively, and to avoid a secondary mirror with an unreasonably long focal length.

The Schmidt camera design and the basic Maksutov design both exploit the symmetry of the spherical primary mirror as a means of allowing off-axis rays to be focused correctly. The usual Schmidt-Cassegrain and Maksutov-Cassegrain telescopes encountered, however, no longer retain this symmetry. The Schmidt-Cassegrain telescope gives it up so that the secondary can be located on the corrector plate, rather than somewhat closer to the primary than to the corrector plate, thus avoiding the need for vanes holding the secondary that obstruct light. The Maksutov-Cassegrain often has a secondary that is an aluminized spot on the corrector plate; in a spherically symmetric design, that spot would simply reflect the light back out of the telescope, and so the corrector plate has to have a steeper curve.

Even without changes in overall design, the fact of having a Cassegrain secondary means the design is no longer spherically symmetric.

Among the several modified versions of the purely catoptric Cassegrain design we examined, the Pressmann-Camichel design gave the poorest off-axis performance; eliminating coma required going in the opposite direction, to the Ritchey-Chrètien design. Given that off-axis performance is already compromised in the common catadioptric Cassegrains, the idea of a Maksutov-Pressmann-Camichel design where a meniscus corrector of reduced thickness is combined with an ellipsoidal secondary to provide part of the correction for spherical aberration may not be entirely preposterous. Actually, though, existing Schmidt-Cassegrain and Maksutov-Cassegrain telescopes are already making use of that principle in a sense; their secondaries are spherical, and as we saw above, the spherical secondary of the Dall-Kirkham design compensates for an undercorrected primary: a fully-corrected primary would require the hyperboloidal secondary of the classical Cassegrain.

Thus, while the symmetry principles illustrated above for the original Schmidt and Maksutov designs are of historical interest, it would be misleading to conclude that Schmidt-Cassegrain or even Maksutov-Cassegrain telescopes are highly corrected against coma. Some Maksutov-Cassegrain designs are better than others in this respect; having the secondary mirror an aluminized spot on the corrector is one thing that involves a compromise; but the compromise is still a minor one, and such telescopes do deserve their good reputation. Instead, for that one needs to consider a telescope based on the Ritchey-Chrètien design.

The Maksutov telescope, whether Maksutov-Newtonian or Maksutov-Cassegrain, is one example of a telescope that can be well-corrected and yet use only spherical surfaces. Another possible design is the Lurie-Houghton telescope:

At first, this design looks like it offers the worst of both worlds; a doublet lens as the objective, and a mirror as well, so one is paying for the optics of both a Newtonian and a refractor. However, the main mirror is now spherical, not parabolic, and the corrector is made from two lenses both of plain crown glass, so one does not need to use flint glass. Also, even after this is recognized, it might be asked what advantage such a telescope has over a Maksutov-Newtonian, which requires just one corrector element, also with two spherical surfaces.

But for an amateur telescope maker, creating two corrector elements that are relatively thin lenses, rather than the steeply-curved meniscus of a Maksutov design, avoids the need to either work from a very thick piece of glass, and remove a lot of material, or somehow find a blank that has already been molded into an approximation of the desired shape.

In addition, the additional lens does allow an additional correction, for curvature of field. Remember, a Maksutov-Newtonian is essentially equivalent to the original Maksutov camera, which like the Schmidt camera avoids coma by being spherically symmetrical everywhere, including the surface on which the image is produced. As a result, a number of amateur astronomers have made themselves telescopes of this type, with focal ratios of f/5 or f/4. Of course, simple Newtonians with very fast focal ratios have been used as well, despite the coma, which can be corrected with commercial attachments at the eyepiece end of the telescope.

One of the best pages on the Web about the Lurie-Houghton telescope is this page by Rick Scott.

What Kind of Telescope is Best?

Aperture

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.

Differences Between Telescope Types

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.)

An f/6 8" telescope, or an f/8 6" telescope, has a focal length - and (approximately) a length length - of 48 inches, or four feet. This makes it somewhat cumbersome to move from place to place.

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.

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.

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. However, chromatic aberration caused by the objective lens is the main limiting factor for these telescopes. Thus, even with an achromatic objective lens, refractors often have focal ratios of f/12, f/15, or longer. This allows the lens to be less strongly curved, which reduces all forms of optical aberrations.

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.

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.

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 around f/10 or f/12, the telescope is short, lightweight, and portable. Usually, an equatorial fork mount is used with such a telescope; this holds it very steady, and requires no counterweight, as the telescope is centered within the fork. Their drawback is that the central obstruction is relatively large.

Maksutov-Cassegrain telescopes are very similar to Schmidt-Cassegrain telescopes in their convenience, but the design offers 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.

What is a Richest-Field Telescope?

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 50mm 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.

How Important is Central Obstruction?

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.

Copyright (c) 2001, 2002 John J. G. Savard


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