WHICH TELESCOPE SHOULD I BUY?

1997 by: Jim Pennington


click on highlighted words for definitions

Introduction

Many newcomers to the hobby of amateur astronomy are often bewildered by the various types of telescopes available on the market. The correct choice of what type of scope to purchase is a personal decision based on one's expectations of the hobby, their budget, knowledge of telescopes, and level of interest. By far, the best way to decide which scope is right for you is to attend an observing session of a local astronomy club. Most club members will be more than happy to let you view through their scopes and answer any questions you may have. If this approach is not available to you, then you must base your decision on information you can glean from books, magazines or information from the Internet or on-line services such as AOL, Compuserve or MSN.

One of the most often questions involves the different types of scopes and what the advantages or disadvantages are of each type:

Refracting Telescopes

The earliest successful telescope design was the refractor first successfully used around 1610 by Galileo. The refractor is the simplest design to manufacture requiring only an objective lens and an eyepiece lens. For this reason, nearly all the "department store telescopes" you see for sale are of the refractor design. Refracting telescopes, however, have a serious drawback due to the nature of light passing through a lens (refracting medium). The different colors of light, having different frequencies, bend at different angles when they pass through the objective (front) lens of the scope. This results in not all the available light coming to a focus at exactly the same point. As a result, what is seen in the eyepiece are "halos" of the spectrum surrounding the brighter stars instead of all the colors merging into a tight focal point

To correct this deficiency in the refractor design, makers of high quality refractor telescopes use what are called achromatic or apochromatic lenses in the optical path. These lens designs utilize multiple lens systems made with special types of optical glass and anti-reflection coatings to correct for chromatic aberration and increase light transmission. Properly designed refractors, therefore, inch for inch of aperture, will deliver the sharpest and highest contrast images to the eyepiece. Astro Physics, one of the leaders in the manufacture of refractor telescopes, sells their 4 inch aperture model for around $2200. Add to this the necessity of having a good quality equatorial mount and the cost soars to $4500 or more!

The jest of what was just mentioned is that the buyer gets exactly what he pays for. A department store refractor telescope generally will not perform optically due to the inexpensive optics used in their construction. An even more serious drawback to department store scopes is the shaky and almost totally useless mounts they come with. A high quality mounting for a telescope actually is more important than the optical quality of the telescope itself. To use any telescope effectively, the mounting should be steady, smooth and precise to operate. The frustrations of trying to use a telescope on a shakey mounting will often cause the novice to either use the scope infrequently, or even worse, to give up trying altogether.

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Reflecting Telescopes

The idea of using a concave mirror to bring light to a focus was known for several years before Sir Isaac Newton built the first reflector telescope. Newton happened to be the first to use this idea to successfully build a reflecting telescope. His design today is known as a Newtonian reflector. Newton's main contribution to this design was the use of a mirror tilted at a 45 degree angle to the light path to send the converging beam of light from the primary mirror out the side of the telescope where the image can be viewed using an eyepiece.

Due to the fact that a large, high optical quality achromatic objective (front) lens is difficult and expensive to make in a refractor design, most amateur astronomers opt for the newtonian reflector. The "newt", as it is commonly called, utilizes a concave parabolic mirror to capture the light entering the telescope tube, reflect it back upward through the tube on a converging path to a focal point. Before coming into focus, however, the light encounters a flat mirror tilted at a 45 degree angle to the light path which reflects the light out through the side of the scope tube to an eyepiece. The diagonal mirror is supported inside the telescope tube by a spider mounting which, by its design, keeps the obstruction of any light entering the scope at a minimum while still providing a stable mounting for the diagonal.

The main advantage of the newtonian reflector over other designs is that inch for inch of aperture, the newtonian is the cheapest and simplest to make for apertures over about 3 inches. Mirror sizes of commercially made newtonian scopes range all the way from 3.5 inches up to 30 inches. Mirrors much larger than this are available for those who wish to make or assemble their own scopes provided they are willing to spend large sums of money. Telescopes have a nasty habit of getting big and expensive very rapidly as the aperture size goes up.

When viewing deep sky objects which include galaxies, nebulae, and planetary nebula, nothing substitutes for aperture. The more aperture (light collecting area) the better these objects can be seen. The light from deep sky objects as a rule is quite tenuous and faint and a large surface to collect these few photons of light and concentrate them at a point of focus is mandatory if they are to be seen well.

The main drawback to newtonian scopes is the fact that the secondary mirror (the diagonal) and its mounting hardware (spider) being in the light path, casts a shadow on the primary mirror and the resulting defraction effects tend to reduce contrast somewhat and makes viewing of small planetary detail more difficult under high magnifications. This statement is a bag of worms when brought up in the presence of both refractor and reflector lovers but let it suffice to say that in general, a well made reflector will hold it's own with a refractor of roughly one-half to two-thirds the aperture. A second disadvantage of the newtonian scope is that they tend to be large and bulky. The weight isn't nearly the factor for portability considerations as is the size. A 12 incher will completely take up the back seat space of a medium size automobile. A third disadvantage involves the mounting required for a newt. Being large and bulky and great wind catchers, a heavy and well designed mounting is called for. If the scope is used on a motorized equatorial mount, the cost of such a mount (especially if astrophotography is involved) can be prohibitive.

But not all astronomers are astrophotographers. Most enjoy the scope simply for purposes of observing the heavens. In this case, a motorized equatorial mounting of far less cost is more than adequate for observing purposes. For those who demand the utmost in simplicity, an even cheaper alternative to the equatorial mount is available- the Dobsonian.

In the last few years, the Dobsonian mount has become more and more popular. With the Dobsonian (or "dob" for short), the user only has to worry about setting up a swivel base on the ground and setting the optical tube of the reflector scope on the base. The dobsonian mount is a type of altazmuth mounting. With this mounting, there is no motor drive and the scope must be manually "nudged" along to keep an object within the field of view of an eyepiece. At first glance, this method seems awfully crude and unhandy but be assured many amateur astronomers swear by their dobs. The technique is a learned one and they have no trouble at all using their dobs to view anything in the night sky. In addition, dob users, on the average, have a far more intimate knowledge of the night sky because, to be an expert dob user, one must have a workable knowledge of where things are located in the night sky. The big advantage of the dob is that the money spent on the scope goes toward the optics and not the other expensive hardware other scope mountings require. Large dobs of 16 inch aperture are available commercially for a little over $1200 and entry level dobs of 6 to 8 inch aperture typically cost in the neighborhood of $350 to $450. One can expect to pay out an additional $100 or so for a more usable finderscope than those included by the manufacturer with the scope. Later on, a wider selection of eyepieces may be desired and their costs can run anywhere between $45 to $250 each depending on make, model and type of eyepiece.

If a prospective scope buyer wants a large aperture scope with good optics and has no need of all the "bells and whistles" other type scope mounts have, then the dob is the only way to go.

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Cassegrain Reflectors

The Cassegrain reflector is a member of the "folded light" class of telescopes. The advantage of the cassegrain system is that a fairly long focal length telescope can be built in an optical tube that is much shorter than would normally be required. This feat is accomplished by using a parabolic primary mirror with a hole manufactured into it's center and a convex mirror for the secondary . The cassegrain reflector works like this: Light entering the front of the scope strikes the primary mirror. The primary reflects a converging beam of light back toward the secondary which is supported in the center of the optical tube much as the diagonal is in a pure newt. Unlike a pure newtonian, however, the secondary does not direct the light out the side of the scope but rather reflects it back down the center of the scope, through the hole in the primary and on into an eyepiece at the rear of the scope.

A very popular variation of the cassegrain reflector is the Schmidt Cassegrain commonly referred to in the hobby as "SCTs". The main difference in the Schmidt Cassegrain is that instead of the parabolic primary mirror in the cassegrain, the SCT uses a spherical primary that creates a very sharply converging beam of light directed upward toward the secondary. The secondary mirror then redirects the light back down the optical tube where it passes through a hole in the primary and then on to an eyepiece. Schmidt cassegrains are sometimes referred to as catidioptric scopes since they utiilize both refractive and reflective optical elements in their design. The spherical primary allows for an even shorter optical tube to be used and this is why most Schmidt Cassegrain telescopes are often found mounted on equatorial fork mounts. This allows the scopes to be very portable and less demanding of storage space and still provide the owner with a fairly large aperture instrument.

The benefits of the SCT come a a price however. The spherical primary, by nature of its figure, has a good deal of spherical aberration and coma. Spherical aberration manifests itself as showing stars at the edge of the field of view in the eyepiece as not being the sharp pinpoints of light that a well focused stars should be but rather having "tails" of smeared light. The stars would look more like tiny "comets" would be a better description. (Actually stars are not pinpoints of light in a well designed optical system but rather are described as Airy disks which is a subject we won't get into here). The Schmidt Cassegrain, however, has a "fix" for this problem. Instead of having an open ended optical tube like the newtonian or pure cassegrain, SC's utilize a corrector plate at the front of the scope. The corrector plate is a lens that intentionally distorts the light passing through it to an amount equal but opposite to the distortion caused by the spherical primary mirror. The end result is that the two optical surfaces working together more or less nullify the distortion caused by the spherical primary. Like the pure newtonian, however, cassegrains, Schmidt cassegrains, and Maksutov cassegrains still have the shadow problem of the secondary mirror which somewhat degrades contrast but this problem is offset to a degree by the larger apertures of the reflector designs over that of the refractor.

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Focal Length and Focal Ratio
Another topic of major confusion when buying a telescope is what focal length scope do I need and what does focal ratio have to do with anything? As mentioned previously, the focal length is simply the distance (usually measured in millimeters) from the objective to the point where all the light rays come to a sharp focus. The point of focus is called the focal plane.

If the new scope buyer thinks that planetary, solar, or lunar observing would be his main interest, then he should consider buying a scope with a high focal ratio. Before you panic, let me explain what focal ratio is. It simply is the ratio of the scopes aperture to that of its focal length. For example, an 8 inch aperture scope with a focal ratio of 1 to 10 (expressed as F/10) would have a focal length of 2000mm (8 inches approximates 200mm). The focal length is determined by multiplying the aperture in millimeters by the focal ratio (200mm X 10) to arrive at 2000mm. Now what you may ask does this mean? OK, let's assume you have a 9mm eyepiece. By simply dividing the focal length of the eyepiece (9mm) into the focal length of the scope (2000mm), you arrive at a figure of 222 which just happens to be the magnification that the scope will operate at with the 9mm eyepiece. Now if you happen to own a 2X Barlow lens, you can insert it between the scope and eyepiece and the scope magnification will jump up to 444X. Now a magnification of 444X can be considered an upper limit for magnification in an 8 inch aperture scope. A general rule of thumb is that good quality optics will support 50x to 60x of magnification for each inch of aperture the scope has. For example, a 12 inch scope can be operated at a maximum of 560x under conditions of good "seeing" as an upper limit. Generally telescopes are operated at far below their maximum capability of magnification because seeing conditions simply won't permit it.

Now you may begin to understand why serious amateur astronomers frown on "department store" telescopes. How many times have you seen a scope advertisement on the side of the box saying something like 550 power? Take a look at the size of the objective lens and you will rarely see one larger than about 3 inches. Three inches times 50x per inch equals 150x or 150 power. A telescope with excellent optics can only operate, as a rule, at a maximum of 60x per inch of aperture so how can a 3 inch aperture "department store" scope with less than excellent optics operate at 550x? When a scope is operated at magnifications far beyond its maximum sharp image capabilities, the only thing you will see in the eyepiece is a very mottled and indistinguishable image of the object you are looking at. Save your money and stay away from department store scopes- they are nothing but junk!

OK, the soap box speach is over and let's get back on track. If the new scope buyer thinks that "deep sky" objects such as galaxies, nebulae, globular star clusters, and planetary nebulae are the objects of interest, then he should consider getting a large aperture (6 inches or better) scope with a focal ratio a bit lower than what we previously were describing. Here, a focal ratio of f/7 or lower may be the better choice. A shorter focal ratio will deliver wider fields of view and lower magnification. For example, let's stay with the 8 inch aperture scope and 9mm eyepiece. An 8 inch (or 200mm) scope with a focal ratio of f/6.3 will have a focal length of 1260mm. With the 9mm eyepiece, we divide 9mm into 1260mm to come up with a magnification of 140x. See how this works?

Now before someone misunderstands, the focal ratio of a telescope has nothing to do with the how high the magnification can reliably go. Maximum magnification depends on aperture alone and not focal ratio. Also, short focal ratio telescopes DO NOT deliver brighter images as some manufacturers claim. The images may seem brighter due to the fact that with any given eyepiece, they are smaller and thus the light from them is more concentrated. For example, let's assume an 8 inch f/10 scope with a 20mm eyepiece. With a little math, we determine the magnification to be 100x. Now let's also assume an 8 inch f/6.3 scope also with a 20mm eyepiece: with the same math, we determine the magnification of this scope to be 63x. It is obvious that the f/10 scope will provide the larger image due to the higher magnification and the f/6.3 scope will deliver the smaller image due to less magnification. Here's the cathcher: if a 12mm to 13mm eyepiece was used with the latter f/6.3 scope then the magnification would equal that of the f/10 scope with the 20mm eyepiece and the images would be virtually the same in appearance. The only difference, however, would be that the 20mm eyepiece, due to its larger focal ratio, would (usually) have a much better "eye relief" than would the 12-13mm eyepiece. Eye relief is the distance one has to hold his eye from the eyepiece to get a full field of view. Short focal length eyepieces usually require the observer to hold his eye quite close to the lens of the eyepiece. It is obvious that eyeglass wearers should consider very carefully the focal length of a telescope before deciding which to buy.

When viewing "extended" objects like nebulae, galaxies and most of the planetary nebulae, even lower magnifications are desireable. Many beginners think most objects in the night sky are small and require high magnifications. While this is true in some cases, most objects are not really all that small- they are, however, very dim. To see dim objects, large aperture scopes are the best choice since they can collect more of the few available photons of light and concentrate it into a converged image for the eyepiece. The eyepiece is what does most of the the magnifying. Most galaxies and nebulae are viewed at magnifications ranging from 30X up to about 150X. A good rule of thumb to have in mind when buying a new scope is to buy the largest aperture your budget will allow. For this very reason, the dobsonian telescope has in the last few years become very popular.

This article briefly describes the various types of optical systems available on amateur class telescopes. Most first time scope buyers are amazed at the seemingly high cost of investing in a quality instrument. The reasons good telescopes are so expensive is that astronomical telescopes operate in a very demanding environment. They are expected to deliver sharp images to the eyepiece in conditions of very high light contrast, such as when viewing the moon against a black sky or equally sharp images in conditions of very low light as when viewing a distant galaxy. Add to this the demands of high magnifications and one can begin to appreciate the prices. The quality of the optics for a system with this capability is very difficult and expensive to make requiring many hours of grinding, polishing and figuring. The mechanical hardware also is not inexpensive to produce. For example, the requirements of astrophotography demand tracking errors of no more than just a few arc seconds at most. Building scope mountings with these tolerances is akin to the mechanics in a fine Swiss watch.

Thus, the first time scope buyer should realize that purchasing a truly usable telescope is going to cost considerably more than those available at the local department stores. A telescope, when properly cared for, will last the user a lifetime AND provide enjoyment from its use for just as long.

-end-

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Definition List


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Achromatic lens. A lens design that corrects refractive errors of different light colors

University of Wisconsin Space Place

Airy disk. The image of a well focused star. Due to the nature of light, not all the light rays will focus into a single spot. Well focused star images are composed of several refracted rings of light around the central bright area.

Altazmuth mount. A telescope mounted with axes in the horizontal and vertical planes.

University of Wisconsin Space Place

Apochromatic lens. A lens design that functions as an achromat but uses special forms of extra low dispersion glass in its construction for improved color correction over that of an achromatic lens.

Aperture. The diameter of the light catching objective, whether it be a lens or mirror of a telescope.

Cassegrain telescope. A telescope using a convex as well as a concave mirror. This increases the effective focal length and gives a large image scale.

Catadioptric telescope. A telescope which uses both reflection and refraction in the formation of its primary image.

Celestial sphere. An imaginary sphere surrounding Earth and carrying all the celestial objects. It rotates in 23 hours and 56 minutes and is inscribed with the celestial equivalents of the terrestrial poles, equator, latitude, and longitude.

Coma. A form of optical aberration in which the light rays reflected off the outer parts of a primary mirror come to an asymetrical focus.

Constellation. One of the 88 defined regions of the celestial sphere.

Corrector plate. A lens located at the front of the optical tube of a Schmidt cassegrain telescope. It corrects spherical aberrations of light reflected off the primary spherical mirror.

Declination (Dec.). The angular distance of a celestial body north (+) or south (-) of the celestial equator.

Dobsonian mount. A simple altazmuth type mounting used with reflector telescopes. The azimuth part of the mount sits flatly on the ground and the altitude part (the optical tube) sits on top of the azimuth part and held in place by the weight of the optical tube. The optical tube in manually "panned".

Equatorial fork mount. A telescope mounting designed primarily for the very short optical tubes of Schmidt cassegrains. A forked structure supports the optical tube on both sides and provides bearings at the support points for north-south altitude adjustments. The fork, in turn, can rotate on an axis that is aligned with the rotational axis of the earth

Equatorial mount. A telescope stand with one axis parallel to that of the earth, making it easy to follow the diurnal motion of a celestial body.

University of Wisconsin Space Place

Eyepiece. A lens or system of lens which magnify the concentrated image focused by the primary objective of a telescope.

Focal point. That point at which ideally all the light rays from an objective converge onto a spot of infinitely small size. In practice, this is unobtainable due to the nature of light.

Focal length. The distance from the objective or mirror to the point where the light rays come to a precise focus. The focal length is usually expressed in millimeters but sometimes in inches.

University of Wisconsin Space Place

Light year.An arbitrary measure of distance, taken as the distance traveled by light in one terrestrial year. It is equal to 5,880,000,000,000 miles.

Maksutov telescope. A type of cassegrain telescope utilizing a corrector plate. The secondary mirror, unlike a Schmidt cassegrain, is an aluminized spot on the back of the corrector which serves to reflect the light rays backward through the hole in the primary.

Newtonian telescope. A reflecting telescope using a concave paraboloidal mirror, with a small plane mirror to reflect the converging rays out of the tube.

Objective. The focusing and light-gathering agent of the astronomical telescope whether mirror or lens.

Primary mirror. The light collecting optical element in a reflector or cassegrain telescope. The primary focuses all the collected light to a focal point.

Refractor telescope. A telescope with an objective lens at the front of the optical tube which converges the light to a focus.

Reflector telescope. A telescope with an objective mirror at the back of the optical tube which reflects and converges the light rays to a focus.

Right ascension (R.A.). The celestial equivalent of longitude, measured eastward from the spring or vernal equinox.

Schmidt cassegrain telescope. A catadioptric scope that uses a corrector plate to correct spherical aberrations of reflected light from it's spherical primary mirror, a convex secondary mirror at the rear of the corrector plate and a focusing device that moves the primary mirror fore and aft to achieve focus at the eyepiece.

Seeing. The steadiness of the telescope image, which is affected by atmospheric currents. Some observers rate it from 1 to 10, 1 being hopelessly bad and 10 unattainably good.

Spider. A device for holding a secondary mirror in the center of the optical tube of a newtonian telescope. The device is designed to make the smallest shadow possible on the primary mirror for that particular objective mirrors focal length.

Terminator. The division between the illuminated and dark hemispheres of the moon or a planet.

Zenith. The point on the celestial sphere directly above the observer.

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