The Eyepiece
You’ll need certain accessories with your telescope. You’ll want at least two good eyepieces. You will need some sort of finder scope or sighting device. A Barlow lens can provide extra magnification for your eyepieces. And, of course, there are optical filters of all kinds, some for looking at the Sun, others for the Moon, some for the planets, and others for more sophisticated purposes.
There are many different designs for telescope eyepieces. All make use of two or more lenses to optimize the apparent field of view, to provide good focus from the center of the view field to the edge, and to make it easy to look through the device. Most eyepieces have focal lengths between 4 and 40 mm.
In general, the longer the focal length of an eyepiece taken as a whole, the lower is the telescope magnification, all other things being equal. Remember how to calculate telescope magnification: Divide the focal length of the objective by the focal length of the eyepiece in the same units. If a telescope has a focal length of 2 m (or 2,000 mm), then a 4-mm eyepiece provides 500× and a 40-mm eyepiece provides 50×. The overall focal length of a telescope eyepiece is not necessarily the same as the focal length of any of its individual lenses.
Eye relief is an important specification of any telescope eyepiece. This is the maximum distance, in millimeters, that the surface of the eye can be away from the surface of the eyepiece lens on the observer side while still letting the observer see the entire apparent field of view. In general, the longer the focal length of an eyepiece, the greater is the eye relief. Larger eye relief numbers translate into easier viewing.
Some people find it difficult and unpleasant to look through short-focal-length eyepieces (6 mm or less) because the lens diameter is more or less proportional to the focal length. A few eyepieces have observer-side lens diameters smaller than the diameter of the pupil of the eye itself. This makes it necessary to bring the eye very close to the eyepiece. If the observer wears glasses, viewing through such eyepieces is compromised. People who don’t wear glasses will flinch away from the eyepiece if its surface comes into direct contact with the eyeball.
An eyepiece’s outside barrel diameter always should match the inside diameter of the telescope’s focusing mount . This is 31.75 mm (1¼ in) in most telescopes, but some instruments have focusing mounts that are 50.80 mm (2 in) across as measured through the inside. Adapters can be found to get small-diameter eyepieces into large-diameter mounts. However, if you want to use a large-diameter eyepiece in a small-diameter mount, you’ll have to improvise.
Eyepiece Designs
Here are four different types of eyepieces you are likely to find on the amateur market. Of these, the Ramsden and the Kellner are the simplest and therefore the cheapest. The orthoscopic and the Plossl are more sophisticated and expensive.
Figure 20-6 A is a cross-sectional diagram of a Ramsden eyepiece. It consists of two planoconvex elements that have the same focal length. The larger lens is toward the telescope objective, and the shorter element is toward the observer. (This is true of virtually all telescope eyepieces.) The convex surfaces of the lenses face inward toward each other, and the flat surfaces face outward. This is an old design, dating all the way back to the 1700s. The Ramsden eyepiece is difficult to optimize because the spacing between the lenses is always a tradeoff between eye relief and the effects of lens aberration.
Figure 20-6 B shows the Kellner design. It is similar to the Ramsden, except that the observer-side lens is a compound element consisting of a convex lens glued to a planoconcave lens. The compound element, when designed properly, eliminates the chromatic aberration inherent in the Ramsden design. The lenses must be coated to minimize reflection of light inside the eyepiece. Kellner eyepieces work best at the longer focal lengths, providing low to medium telescope magnification.
The orthoscopic eyepiece (see Fig. 20-6 C ) is among the most popular designs in use today. Image distortion and chromatic aberration are eliminated by the three-element compound lens on the objective side. This type of lens is noted for its excellent contrast and its ability to maintain focus from the center of the view field to the edge. In addition, the view field appears relatively flat as compared with some eyepiece designs that give the view field a concave (bowl-shaped) appearance. Orthoscopic lenses work well at all focal lengths.
Figure 20-6. Common telescope eyepiece designs: Ramsden ( A ), Kellner ( B ), orthoscopic ( C ), and Plossl ( D ). In each drawing, the observer’s eye is at left and the incoming light arrives from the right.
Figure 20-6 D is a cross-sectional diagram of a Plossl eyepiece. This design first gained widespread acceptance among amateur astronomers in the 1980s. It has all the assets of the orthoscopic eyepiece. The eye relief of a well-made Plossl is adequate even at the shortest focal lengths. The observer-side lens has a relatively wide diameter. Plossls with long focal lengths (25 to 40 mm) are physically bulky, projecting some distance out from the telescope’s eyepiece tube, but they offer the ultimate in viewing comfort. Some have rubber eye guards to keep out external light.
Other eyepiece designs you might encounter are the Erfle , the zoom , the RKE , and the Huygens . The sheer variety of eyepieces can confuse the novice amateur astronomer. You can get on the Internet, enter eyepiece designs as keyword phrases (for example, Kellner eyepiece ), and see what various folks have to say about the different designs. A salesperson at a hobby shop sometimes can help, but beware. A salesperson may be more motivated to get you to spend a lot of money than to sell you the best eyepieces for your needs.
Star Diagonal
With refractors and SCTs, the eyepieces are normally in line with the telescope tube. Viewing can be uncomfortable when such a telescope is aimed at objects high in the sky; you have to crouch down and crane your neck. However, there’s a simple and common solution to this problem: the star diagonal . This device bends the light path without introducing distortion, although it flips the image laterally, as a mirror flips your reflection.
A simple star diagonal employs a prism that causes the light to turn a 90-degree corner because of total internal reflection. The principle is the same as in binoculars. A cutaway view of a basic 90-degree star diagonal is shown in Fig. 20-7. More sophisticated star diagonals provide smaller angles, such as 45 degrees. Some star diagonals use two prisms rather than one, so the image is not laterally reversed.
Figure 20-7. A star diagonal uses a prism to bend the light, making it easier to view some celestial objects.
The lateral-reversal feature of basic star diagonals makes it rather inconvenient when you try to find objects in the sky using a star map. You have to imagine everything on the map backwards. With a little practice, however, most people can overcome this mental obstacle.
Sighting Device
If you’ve ever used a telescope in an attempt to locate a planet or star and you didn’t have some sort of aiming or sighting device, you know how frustrating such an exercise can be. Except at the very lowest magnifications, you can end up searching for a long time. The simplest sighting devices are similar to gunsights. You aim the telescope as if it were a high-powered rifle. The more advanced type of sighting device has a small laser diode inside; it shines on a slanted glass to produce a variable-brightness red dot in the center of the view field. This dot is used to align the telescope with the object you want to observe.
Before you use it for celestial observations, the sighting device first must be aligned on a terrestrial target that is at least a couple of kilometers away. Find some object on the horizon that is large enough to see through the sighting device (that is, at 1×) yet small enough to fit into the view field of the telescope. Get the object centered in the view field of the telescope, fix the telescope in position, and then adjust the sighting device until the object lines up in it. Then check the view through the telescope again to be sure the object is still centered there. For good measure, go back and check the sighting device again too.
Finder Scope
A more precise device for telescope aiming is a finder scope , often called simply a finder . This is a small Keplerian refractor. Most finders have objective diameters of 40 to 60 mm and magnify several times. The eyepiece has a pair of fine threads or wires, called cross hairs , placed at its focus. These produce a + or × pattern in the view field. The intersection point of the cross hairs is at the center of the view field. The finder position is adjusted until a star that falls at the cross-hair intersection point also shows up in the center of the view field of the main telescope at high magnification.
A finder can be aligned using the same technique as is used for a simple sighting device. The best finders are mounted in a pair of rings, both of which are attached to the main tube of the telescope near the eyepiece. Each ring has three or four adjustment screws. These should be fairly tight (but not so tight that the finder is damaged or the screw threads are stripped). A few finders have single-ring mountings. These are unstable. It is best to stay away from them.
Barlow Lens
A concave or planoconcave lens can be inserted in any telescope between the eyepiece and the objective, and the effect is to increase the apparent focal length of the objective. This type of lens is called a Barlow lens . It is placed close to the eyepiece. The lens is mounted inside a cylinder designed to fit into the eyepiece barrel of the telescope at one end and around the barrel of the eyepiece at the other end (Fig. 20-8).
Figure 20-8. A Barlow lens increases the magnification obtainable with a given eyepiece.
Because the Barlow lens increases the effective focal length of the objective, it provides increased magnification when a given eyepiece is used. Most Barlow lenses are rated at 2×. This means that they double the magnification for each eyepiece used. Some Barlow lenses are rated at 3×; these triple the magnification.
A Barlow lens can be useful in two situations. First, it eliminates the need for using eyepieces with extremely short focal lengths when high magnification is desired. An 8-mm eyepiece can be used in place of a 4-mm eyepiece, for example, when a 2× Barlow is inserted in the light path. Most people find an 8-mm eyepiece more viewer-friendly than a 4-mm eyepiece. Another asset of the Barlow lens is that it can double the number of obtainable levels of magnification, provided that you have chosen your eyepieces wisely. Suppose, for example, that you have a telescope whose objective has a focal length 1,000 mm and you have eyepieces whose focal lengths are 20 and 28 mm. This provides magnifications of 50× and 36×, respectively. If you obtain a 2× Barlow lens, you can obtain magnifications of 100× and 72× with the same two eyepieces. This gives you four well-spaced degrees of magnification.
A Barlow lens should not be used in an attempt to get extreme magnification. For example, if you have a telescope whose objective has a focal length of 2,000 mm and you use a 4-mm eyepiece with a 3× Barlow, you can theoretically obtain 1,500×. However, Earth’s atmosphere generally makes it futile to try for anything more than 500×, even with the largest telescopes. The slightest vibration will cause terrible wobbling of the image. In addition, the brightness of an observed image in any particular telescope decreases as the magnification increases. Remember the formula for the highest useful power you can get out of a telescope: approximately 20× per centimeter of objective diameter, or 50× per inch, with a maximum of 500× at sea level and most land-based locations.
Focal Reducer/corrector
Most hobby SCTs have f -ratios of around 10. This is all right for viewing planets, lunar surface features, and some star clusters. However, when looking at nebulae or galaxies, often you will want to reduce the magnification as much as possible. By so doing, you can concentrate the light so that dim, diffuse features show up more clearly against the background of the sky. Reducing the magnification with a given eyepiece also increases the absolute field of view.
A focal reducer/corrector is a convex lens that shortens the effective focal length of the SCT objective by a certain amount, usually 37 percent. This means that the effective focal length and the f -ratio are both cut to 63 percent of their values without the device installed. You might think of it as the opposite of a Barlow lens. With a 37 percent focal reducer/corrector, an f /10 telescope becomes an f /6.3 instrument. A focal reducer/corrector is larger in diameter than a Barlow lens and is equipped with a threaded mount that can be screwed into the opening in the objective mirror that passes light into the eyepiece holder.
Suppose that your SCT has an objective with a focal length of 2,000 mm. If you have a 40-mm eyepiece, a focal reducer/corrector shortens the effective focal length to 1,260 mm. This reduces the magnification from 50× to a little more than 30×. It also increases the absolute field of view by a factor of about 1.6. The corrector feature helps to ensure proper focus throughout the apparent field of view.
Solar Filter
You can use a telescope to look at the Sun, but there are some precautions you must take to avoid damage to your telescope, your eyesight, or both. Before you point a telescope toward the Sun, get a solar filter that fits over the entire skyward opening of the telescope. The filter must be as large in diameter as the objective and is called a full-aperture solar filter . With such a filter, direct sunlight does not fall on any of the telescope optics. Only certain types of filters are acceptable; these block ultraviolet (UV) rays that otherwise could damage your eyes even if the image is not uncomfortably bright. The brand-name telescope manufacturers such as Celestron supply excellent solar filters. They’re not cheap, but neither are your telescope or your eyesight.
If you have a finder that uses lenses, such as a Keplerian refractor with cross hairs, cover it before aiming the telescope at the Sun. Otherwise, you risk damage to the finder’s eyepiece and cross hairs.
Never use a “sun filter” that screws into telescope eyepieces! Such a device is at the prime focus of the telescope objective, so it will heat up. Such filters have been known to melt or crack. If one of these “filters” fails while you’re looking at the Sun, you will remember the experience for the rest of your life. You’ll be lucky if your retina is not injured permanently.
Have you heard that you can aim a telescope at the Sun without a solar filter, with the eyepiece installed, and let the brilliant light shine onto a white piece of paper or a screen to see details of the Sun’s surface? In theory, this scheme works, and you can in fact get a decent image without risking damage to your eyes. Several people can view the image at the same time. But this is a bad idea. It subjects the eyepiece to direct focused sunlight, which can permanently damage the eyepiece. Besides this, as the Sun moves in the sky or as you move the telescope around while locating the Sun, the focused spot will strike and heat up interior components of the telescope.
Think of the focused, unfiltered rays of the Sun as the business end of a blowtorch. Would you turn a hot flame on anything you value? Of course not. So follow the universal rule: Always filter sunlight before it gets into a telescope (Fig. 20-9). Treat your telescope as kindly as you treat your own eyes.
Figure 20-9. A solar filter should keep direct sunlight out of the interior of a telescope.
Moon (lunar) Filter
If you have a telescope whose objective lens or mirror is larger than about 10 cm (4 in), the Moon will appear extremely bright at low magnification when it is near the full phase. In fact, at the lowest obtainable magnifications with SCTs using focal reducer/correctors, the full Moon can appear so brilliant that it hurts your eyes to look at it. A Moon filter , attached to the eyepiece, renders the Moon’s image tolerable under these conditions.
A Moon filter, also called a lunar filter , is tinted grayish, gray-green, or brownish, like the lenses in a high-quality pair of sunglasses. It is mounted in a threaded ring that screws into the objective side of the eyepiece. This placement is all right; the Moon’s light is not intense enough to cause damage to a telescope’s interior components or to an eyepiece. You’ll know when you need a lunar filter and when you don’t. At high magnification levels or when the Moon is a thin crescent, you won’t want one. At low magnification, after sundown, and when the Moon is gibbous or full, you will.
Other Filters
It is not easy for most people to find places where the nighttime sky is not polluted by airglow. Airglow doesn’t interfere very much with viewing of the Moon or the planets, although dust and particulate pollution, along with convection currents rising from the day-heated land that roil the evening air, can blur even these images and reduce their contrast. If you want to see nebulae, globular clusters, and galaxies, you will have trouble with airglow unless you take some measures to reduce it.
A light-pollution-reduction (LPR) filter reduces the effects of airglow at night. Most big outdoor lamps are sodium-vapor devices that emit most of their radiation at well-defined wavelengths in the yellow part of the visible spectrum. Mercury-vapor lamps are less common, but they too emit most of their light at certain discrete wavelengths. A line-type LPR filter is designed to transmit light at all visible wavelengths except specific ones. In this way, the airglow from sodium-vapor and mercury-vapor lamps can be attenuated, whereas light at other wavelengths passes through the filter unaffected. Other LPR filters include narrowband and broadband types. The particular filter that will work best in a given situation must be found by trial and error. The folks in your local astronomy club can give you advice based on their own experiences. All LPR filters, like Moon filters, are designed to be screwed into the objective side of an eyepiece.
Planetary filters are simple color filters that are screwed into eyepieces in the same manner as are Moon filters and LPR filters. They are available in almost any tint you can imagine. You can use an orange filter to look at Mars, a yellow-green filter to look at Jupiter, or a red filter to look at Venus. You can even use these filters ( in addition to a full-aperture solar filter—never all by itself! ) to look at the Sun. Experimentation is the key. Try all the filters you can find. Look at anything you want with them. See if you can borrow some from friends, so that you don’t spend a lot of money unnecessarily. You’re bound to see some interesting things.
Practice problems of this concept can be found at: Your Home Observatory Practice Problems
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From Astronomy Demystified: A Self-Teaching Guide. Copyright © 2003 by The McGraw-Hill Companies, Inc. All Rights Reserved.
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