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Telescopes: What the Buyer Needs to Know
| WARNING!!! |
| Never use a telescope to look directly at the Sun. Doing so will lead to permanent blindness! Very special equipment is required to do solar observing safely. |
Regardless of specific design, there are several optical characteristics that relate to the successful purchase of an instrument which will provide years of future enjoyment in astronomical observation. Likewise, there are misconceptions that some manufacturers exploit in order to sell substandard equipment to the public. We have compiled helpful information in the following areas:
| Power-- A Major Misconception |
Most inexperienced telescope buyers suffer from the misconception that "power" or "magnification" is the most important criterion in judging a telescope. The uninitiated person almost always assumes that the higher the magnification of an instrument, the better. Realizing this, some manufacturers emblazon their telescope advertising and packaging with such claims as "475 power" or "475x", and the like. This sort of claim is most often found with small "department store" refractors.
In reality, most experienced observers gravitate toward lower magnifications, rather than higher ones. The reason is simple: the lower the magnification, the sharper and brighter the image. In addition, lower magnification displays a wider area of the sky, and this in turn, slows the tendency of an astronomical object (star, planet, etc.) to drift out of the field of view-- an apparent motion caused by the Earth's rotation. Also, any instability-- or jiggle-- in the telescope and its mounting is amplified with higher and higher power views. This makes higher powers more difficult to use, as even the slightest touch of a finger to the instrument-- particularly one on a flimsy mount-- can cause the image to bounce around annoyingly. Furthermore, atmospheric turbulence plays havoc with higher power viewing, while this "boiling" effect is much less prominent under lower magnifications.
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| Light-- Gathering and Focusing |
Rather than magnification, a much more important telescope specification is its aperture. Simply put, aperture is the diameter of the main optical element, or objective, of the instrument (the main lens in a refractor telescope, or main mirror in reflector). In even simpler terms, the fatter the telescope, the larger the aperture. A major purpose of a telescope is to gather more light than the eye alone can see, and the bigger the scope's objective, the more light-gathering ability it has. The larger the aperture, the fainter the sky object that can be seen. This is particularly important when considering that the light-gathering of a telescope objective is proportional to the area of the lens or mirror (and to the pupil of the human eye). Therefore, a two-inch objective will gather four-times the light of a one-inch objective, and a three-incher will gather nine-times the light of the one-inch model. This effect takes on added significance when you factor-in that a one-inch lens-- having a diameter three-times that of the pupil in a dark-adapted human eye (about 1/3-inch) gathers nine-times the light of the eye alone. Using this relationship, it soon becomes apparent that a three-inch objective will gather about 90 times the light of the eye alone, and a ten-incher can gather 900 times the light of the naked eye. Of course, telescope size also has a practical impact upon convenience. A big scope is great for viewing, but is of little real use if-- given your particular circumstances-- its large size makes you unable or unwilling to transport and set it up.
Aperture is also a major factor in determining the maximum magnification of a given scope. Although the selected pairing of a telescope and an eyepiece can theoretically gain you extremely high magnification, there is a practical limitation to power. For most telescopes, the rule of thumb is that you can only expect to get up to about 50 power (or 50x) per inch of scope aperture (or about 2x per millimeter of aperture) of usable magnification. And this rule is only for ideal viewing conditions, which is seldom the case. In fact, turbulence in the atmosphere generally lowers this to an average limit of about 25x per inch of aperture (1x per millimeter). Going back to the example of the typical department store scope-- which usually has an aperture of 2.4 inches (60 mm)-- the maximum usable magnification of such an instrument is really only about 120x under ideal viewing conditions, or about 60x under more average viewing conditions. This is a far cry from the 400-500x claims of these scopes' manufacturers!
Along with aperture, the most critical specification for a telescope's optical performance relates to its resolution, or how accurately the lenses or mirrors are manufactured. This is usually referred-to as the lens' or mirror's wave-number. For a telescope to work, its lens(es) or mirror(s) must be able to bend light in a precisely-controlled fashion. A good lens or mirror should be able to bend the light intersecting its edges as well as at its center-- and all points in between-- in such a way that all that light is accurately focused to almost precisely the same point from the lens or mirror. However, no lens or mirror is perfect, and the measure of this imperfection is expressed in fractions of a wavelength of light. The smaller the fraction, the better the lens or mirror. For instance, a mirror rated at 1/16-wave accuracy is considered much better than one measured at 1/2-wave. A good telescope objective lens or mirror should always be rated at 1/8-wave or better. In fact, since wave-error is cumulative within a telescope, any smaller secondary optical surfaces-- such as the flat diagonal mirror in a Newtonian reflector-- should be rated at 1/16-wave or better to maintain good optical performance.
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| Long or Short, Fat or Skinny-- Focal Length and Focal Ratio |
There are other telescope characteristics to watch for, as well. These are focal length and focal ratio. Unlike wave-number, however, these specifications have less to do with absolute optical quality than with the types of observing for which the instrument is best suited. Focal length refers to the distance from the objective lens or mirror where the light converges to a focus within the scope. see diagram Together with a suitable eyepiece, a longer focal-length telescope objective can deliver a higher-power view than a scope with a shorter focal length and a similar eyepiece. (Note that the final image magnification is also dependent upon the eyepiece's focal-length.)
Focal ratio, or f-ratio refers to the relationship between a telescope's focal length and its aperture. A telescope with a focal length many times longer than its aperture-- a comparatively long, skinny scope-- will have a high f-ratio. On the other hand, an instrument with a comparatively short, fat tube will have a low f-ratio. see diagram (Rough assessment of a scope's f-ratio by external visual appearance becomes more complicated, though, in the case of instruments such as a Schmitt-Cassegrain. In this case, the folded light-path within the telescope means that the instrument can have a higher f-ratio but a short, fat tube.) F-ratio is calculated by dividing the scope's focal length by its aperture. For example, a telescope with a 1000mm focal length and a 125mm aperture has an focal ratio of f/8. On the other hand, an instrument with a 560mm focal length and a 125mm aperture has a focal ratio of about f/4.5. Telescopes which are around f/10 or higher are considered optimal for higher-power lunar and planetary viewing and for splitting double stars, while instruments that are f/6 or less-- particularly those with larger apertures-- are optimized for lower-power, wide-field (rich-field) views of nebulae, star clusters, and other deep-sky objects. Falling right in between, f/8 scopes are considered a decent compromise between the other two types, and make good instruments for all-around viewing.
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| Telescopes-- The Major Types |
Telescopes available for amateur astronomers these days can be divided into three major categories: the refractor, the reflector, and the catadioptric. Each design has its advantages and disadvantages, including issues relating to observing application, maintenance, convenience, and cost.
The refractor uses a large lens for its objective, placed at the front of the telescope tube. This lens focuses the light down into a small image near the rear-end of the scope, where the eyepiece enlarges that image. Refractors-- especially those with higher f-ratios-- are optimized for higher-power views of the Moon and planets, and are good for splitting double stars, because they have no internal obstructions like secondary mirrors which can diffract light. Good-quality refractors are probably the best telescopes that money can buy. On the down side, a high-quality refractor tends to be quite expensive, due to the larger number of optical surfaces present in its achromatic lens which must be precisely manufactured. In fact, good refractors carry with them the highest cost-per-unit-of-measure of aperture of all telescope types. Although there are inexpensive, small-diameter refractors which can be found as "department store" telescopes, these are often of substandard quality, and should be avoided, if possible.
The reflector uses a large parabolic mirror at the rear-end of the telescope to focus light back up toward the front of the tube. In the case of the popular Newtonian reflector (named for Sir Isaac Newton, who invented it), that light is intercepted by a smaller, flat, secondary mirror (positioned diagonally), and reflected out through an opening in the side of the tube to the eyepiece. Newtonian reflectors are the best telescopes for those wanting high-quality on a budget. It's not uncommon to find quality Newtonian scopes with six- to ten-inch diameter apertures for just a few hundred dollars. No other telescope can touch the quality/cost ratio of a Newtonian reflector. As far as disadvantages, Newtonian reflectors do require occasional collimation, or optical alignment of their optical elements, in order to maintain peak performance. Reflectors can also suffer from coma, a characteristic which makes stars near the outer edges of an eyepiece field look like slightly elongated blobs rather than pinpoints of light. However, coma is generally not objectionable in f-ratios of f/8 or higher, and in lower f-ratio instruments a higher-quality eyepiece can help minimize coma as well.
Finally, the catadioptric telescope has a lens-- called the corrector plate-- up front, and large mirror at the rear. Additionally, this design has a smaller, convex secondary mirror just behind the corrector plate which folds the light-path and reflects the light back down to the rear of the scope-- through a small hole in the large mirror-- and into the eyepiece behind. Catadioptric telescopes-- Schmitt-Cassegrains and Maksutov-Cassegrains -- offer convenience, in that their folded internal light paths make for very compact instruments. This leads to scopes which are very good for high-power viewing, with high f-ratios, but without the long, cumbersome tubes found in comparable refractors and Newtonians. Though the larger number of optical surfaces in a catadioptric scope tends to lower image-resolution, tight manufacturing tolerances can minimize this problem. Schmitt-Cassegrains are among the most popular of all telescope designs for serious amateurs, and are most-often purchased in eight- to ten-inch aperture models from manufacturers like Celestron and Meade. However, the convenience of this design comes at a price: Schmitt-Cassegrains are generally around three-times the cost of a good Newtonian of comparable diameter, and Maksutov-Cassegrains are even more expensive.
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| Finders |
Finders are important for aiding in sighting-in objects through the telescope. Lack of a good finder can make observing through even the best telescope a frustrating experience. Finders can range from small simple tubes to larger finderscopes. A finderscope is actually a small telescope mounted and aligned with the main scope. A finderscope provides a low-power, wide-field view of the sky, making it much easier to aim at objects than with the relatively higher magnification (and narrower view) of the main scope. Finderscopes have crosshairs within the eyepiece to facilitate precision aiming.
The finderscope is often yet another pitfall associated with department store telescopes. Many times, these finderscopes have very small, low-quality objectives. In fact, some manufacturers of these cheap scopes even try to reduce optical aberrations in their finderscopes by placing a "stop-ring" behind the objective lens. While this practice does cut down on aberrations, it also has the side-effect of making images very dim. Either way, such finderscopes are virtually unusable for most astronomical applications. A good finderscope should always have an "achromatic" objective lens with a diameter of at least 30mm. (For example, a 6x30 finderscope is a six power unit with a 30mm-diameter objective.)
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| Eyepieces and Accessories |
Few beginners give much thought to telescope accessories. But often, the proper accessories can make the difference between enjoying a good telescope and becoming frustrated by it to the point that the instrument merely collects dust. One perceived advantage of the department store telescope is that it usually comes with what appears to be a complete set of accessories. Unfortunately, the inclusion of three or four eyepieces, an image-erector, a finderscope, and other accessories along with a telescope for less than $300, certainly means that quality is sacrificed. In fact, lots of accessories at low cost is a sure sign of trouble. You should generally avoid these instruments.
The first accessories needed to make a telescope work are eyepieces. Usually, quality instruments come without, or with only a single eyepiece (but rather than feeling that you've gotten cheated when buying such a scope, you can probably assume that the scope [and eyepiece] are of much better quality). Eventually, most telescope users will want to add to their eyepiece collections, since eyepieces of different focal lengths will provide different magnifications. Again, remember that you should always start out viewing an object under lower power. This will give you the sharpest, brightest image, and the accompanyingly wider-field view will also make it easier to find objects. As with telescopes, eyepieces come in different designs. They vary widely in price, and some are better than others for certain applications. And there are some eyepieces that are so poor, you'll want to avoid them for virtually all uses.
Keep in mind that magnification is dependent upon both the telescope's focal length and the eyepiece's focal length. Simply divide the telescope focal length by the eyepiece focal length to calculate the magnification. For example, a 1000mm focal-length scope paired with a 20mm eyepiece will provide a magnification of 50x. On the other hand, a 10mm eyepiece on the same scope will yield a magnification of 100x.
There are three other things to consider when buying eyepieces: optical quality, field size, and eye relief. Of course, optical quality is pretty self-explanatory, though there are some specific things to keep in mind, such as image brightness, sharpness, freedom from aberrations (such as chromatic aberration, in which images suffer from rainbow hues around the edges), and flatness of field, in which stars at the edges of the eyepiece field are focused at the same point of focus-mechanism movement as stars at the center of the field. Eyepiece field size refers to how large a visual "window" the eyepiece provides, but is independant of magnification. Instead, field size is a lot like seeing the world with good peripheral vision as opposed to the view that you'd get looking through toilet-paper tubes. Cheaper eyepiece designs provide narrow field sizes, and give the observer a restrictive "tunnel vision" effect. Eye relief is the distance you must place your eye from the eyepiece to see the entire field. Better eyepieces of a given focal length will provide a longer eye relief, which is particularly useful for eyeglass wearers. But shorter focal-length (higher-power) eyepieces nearly always provide shorter eye relief than longer focal-length models within the same class of optical design. In addition, always look for eyepieces which are "fully coated" or multi-coated". These refer to special coatings applied to the glass elements of the eyepiece to enhance light transmission (which will make objects look brighter) and minimize objectionable "ghost" images when observing.
Also, pay attention to an eyepiece's barrel size. Most good telescopes use eyepieces with 1-1/4-inch barrels, while cheaper scopes tend to have smaller .965-inch barrels. The two sizes are not directly interchangeable.
The cheapest and simplest eyepieces are Huygenians and Ramsdens. These have only two lens elements each, and are rated poor at best in optical quality, field size, and eye relief. Both designs date back to the 1700s, but amazingly, are the eyepiece types that are nearly always supplied with small refractor telescopes. (Most experienced observers see these eyepieces as good only as small paperweights, and therefore, should be avoided for use in all but the most non-critical observing!) The shorter focal-length units are simply unusable, as their images are too dim and fuzzy. Eye relief is extremely short with Huygenians and Ramsdens, and field sizes of both are extremely narrow.
The Kellner eyepiece is a three-element design good for f/10 or higher telescopes, but doesn't perform as well on rich-field scopes (f/6 or lower). While Kellners' field sizes tend to be a bit narrow, they are a big improvement over Huygenians and Ramsdens. Also, short-focal-length Kellners have uncomfortably short eye relief, making them less desirable for high-power applications. Kellners are fairly low-cost, compared with many other eyepiece designs.
The three-element RKE is an Edmund Scientific proprietary design, and is an improvement over the Kellner eyepiece. Eye relief is good, and the field size is moderately so. This design represents a good balance between performance and economy.
Orthoscopics, Erfles, and Plossls all have four or more elements each, and are considered excellent eyepieces in most respects. Of the three, the four-element Orthoscopic has a narrower field, but gives some of the sharpest images available. The five- or six-element Erfle provides very wide, picture-window views of the universe, but suffers from lack of edge-sharpness in the shorter focal lengths. Erfles are generally harder to find these days, as they tend to be rather expensive and are surpassed in performance by some newer wide-field designs. Four-element Plossls are among the most highly-regarded eyepieces around today, and while more expensive than Kellners and Orthoscopics, are quite reasonable when compared to the wide-field "connoisseur" eyepieces. Optical aberrations in the Plossl are minimal, and eye-relief and field size are very good. The Nagler is the "Rolls Royce" of telescope eyepieces. It is a super performer, providing some of the best views of the night sky that can be experienced. Of course, this performance comes at a steep price-- one that few observers can justify.
Avoid the temptation to buy zoom eyepieces. Although it's always interesting to enlarge or reduce the size of an object with the turn of a knurled ring, that convenience is more than offset by degraded sharpness and brightness, making the zoom eyepiece anything but a bargain. Always stay with fixed focal-length eyepieces.
Good eyepieces cost money. Be prepared to spend at least $50 each for quality ones. The good thing is that you can accumulate eyepieces one by one, instead of having to buy an entire set at one time. Don't get cheap after buying a good scope by purchasing substandard eyepieces. There are few things that can make a good telescope perform more poorly than a cheap eyepiece.
A quality Barlow lens is one of the most valuable telescope accessories you can buy. A Barlow is an image amplifier that fits between the telescope and an eyepiece-- effectively turning low-power eyepieces into higher-power ones. In fact, buying a quality Barlow is like doubling your selection of eyepieces, for roughly the cost of a single eyepiece. They usually come in 2x and 3x models. Another advantage of the Barlow benefits eyeglass wearers. Instead of using a higher-power eyepiece which has short eye relief-- a problem for viewing with eyeglasses-- the combination of a low-power eyepiece and a Barlow provides the best of both worlds, in that the observer can have the comfort of long eye relief for wearing glasses and high-power views at the same time. The third advantage of a Barlow benefits rich-field scopes. While these instruments normally aren't optimized for higher-power viewing, the use of a Barlow/eyepiece combination effectively raises the focal-ratio, thereby improving the rich-field scope's high-power performance.
Note that telescopes normally invert their images. While image-erector prisms-- devices that turn a telescope image right-side up again-- are desirable for terrestrial viewing, they are not for astronomical observation. Image erectors tend to dim the scope's image, and often make it less sharp, too. However, since there's no real "up" or "down" in space, astronomers don't care whether the image is inverted. They do, however look for maximum image brightness and sharpness, as should you. Therefore, avoid image erector prisms for astronomical use. Some telescope designs, however, require the use of a star diagonal to place the eyepiece in a comfortable position. While these devices do upright the image, they also produce one that's inverted left-to-right. Good star diagonals don't appreciably degrade their images, though the cheaper ones can.
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| Mounts |
Like eyepieces, telescope mounts are often overlooked as an important issue when purchasing. Even if a telescope possesses good optical properties, its performance will be discouraging if its mechanical parts are flimsy. Once again, cheap scopes usually fall far short in this category. While most telescope mounts will work okay under very low magnification, any shakiness or instability becomes apparent as soon as magnification increases. Always look for a really beefy mount when scope hunting. Spindly-legged tripods and small, light-weight mount-swivels should be avoided. If you can, try checking the rigidity of the mount by putting-in a higher-power eyepiece. If the slightest touch causes the image to jiggle infuriatingly, look for another model.
Telescope mounts come in two basic designs-- altazimuth and equatorial. The altazimuth mount uses vertical and horizontal pivot points. With this mount, the telescope can be turned 360 degrees with respect to the horizon and 90 degrees up and down. The altazimuth mount is good for terrestrial viewing, but can also be used for astronomical observations of shorter duration and at lower powers. Equatorial mounts have a polar axis and a declination axis. When properly aligned with the celestial pole, rotation around the polar axis can directly follow the apparent motion of sky objects caused by Earth's rotation.
Altazimuth mounts are best for beginners and those whose astronomical observing is best described as "casual". Scopes with altazimuth mounts are easy to set up and use, and require no special expertise to use properly. Dobsonian mounts form a subcategory of altazimuths, and are used on medium- or large-diameter instruments. These mounts are inexpensively constructed of materials such as plywood and heavy-duty plastics, and are very sturdy and stable.
Equatorial mounts are best for more-experienced observers, as they require a bit more attention to detail set up. However, these mounts can make tracking sky objects easier-- especially good for extended high-power viewing. Such a mount can even be fitted with a "clock drive" motor, which can automatically track sky objects. The equatorial mount is a must for astrophotography.
It's important to become a knowledgeable consumer before buying a telescope. Following these tips should help you to avoid frustration in your first scope purchase.
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