Your basic eyepiece set.
Your first decision is what tradeoff to make between wide fields of view on the one hand and price, weight, and extra lens elements on the other. Fantastically wide views about 80 [degrees] in apparent diameter - wider than you may be able to see at once without moving your eyeball around - are available if you can spend $600 to $900 for a set of three eyepieces. To use these at low and medium powers, you'll need a 2-inch eyepiece holder, a telescope that illuminates a very broad image plane, and a convenient way to rebalance your scope when you add up to 2 1/4 pounds of glass and metal to its eye end.
The next step down in field size, price, and weight are "wide-field" eyepieces giving views roughly 65 [degrees] across. Finally there are the standard, mainstream eyepieces with fields in the 50 [degrees] range. A generation or more ago a view this wide was a luxury, but nowadays most observers consider 50 [degrees] about the minimum acceptable field.
Some buyers assume that field size and optical quality (sharpness and clarity) go hand in hand. But these are separate issues and in fact may even conflict. Sure, a wide view provides dramatic images and helps to sweep up sights when you're hunting through the sky. But as soon as you find an object, you naturally center it. Some observers also wonder if the extra lens elements used to achieve broad views may degrade the center of the image slightly.
For this review we considered $50 to $90 eyepieces having fields of about 50 [degrees]. We tested 10 sets of low-, medium-, and high-power eyepieces from seven suppliers. All have 1 1/4-inch barrels, all that's required for eyepieces shorter than 32-mm focal length with apparent fields in the 50 [degrees] range.
Our testing was done in two parts. We performed bench tests to measure each eyepiece's focal length, eye relief, apparent field of view, distortion, and light throughput. We also obtained real-world impressions by viewing astronomical objects. Our test telescope was a top-quality 12.5-inch f/6 Newtonian reflector in excellent collimation. The table on pages 40 and 41 presents our measurements and considered opinions.
The eyepieces within each set were at least roughly parfocal, meaning that only a little refocusing was necessary when changing eyepieces. So if parfocality is a concern, you are probably better off buying one set rather than mixing brands. We chose at least three eyepieces per set, but some observers buy just two - low and medium power - and rely on a Barlow lens to provide a second medium power and a high power.
Overall, we found that most choices would be good choices. Nearly all of these eyepieces gave views of the Moon, stars, and planets that were far more alike than different. In fact, as we began some 300 eyepiece swaps at the telescope, it became evident that distinguishing among the views would be tough, at least near the center of the field. Differences were plainer near the edge. After 11 1/2 hours of comparisons, however, most of the sets began to show personality quirks, even on axis.
Measured focal length. We first determined the actual focal length of each eyepiece. We wanted to check the accuracy of the number engraved on each unit and determine the eyepiece's exact magnifying power for later tests. We measured the exit pupil that each eyepiece formed in a telescope of known clear aperture and focal length. The measures, made with a micrometer reticle, yielded eyepiece focal lengths accurate to 0.1 or 0.2 millimeter.
All the eyepieces turned out to have essentially the focal lengths advertised, with the exception of the University Optics Plossls.
Field of view. The view in most of these eyepieces was about equally wide. Orion's Explorer Orthoscopics, however, gave a substantially narrower window on the universe than the Plossl and RKE designs, while the University Optics 12-mm Plossl stood out for its unusually wide field.
Aside from aesthetics, there's a little-known reason for having a moderately large field of view: the ability to see fainter objects. You might think the opposite would be true - that viewing a small circle of sky surrounded by the total blackness of an eyepiece barrel would improve your dark adaptation. But in practice, a small sky area looks brighter against black surroundings, and this effect tends to hide very faint objects.
Apparent field is the diameter of the bright circle you see when you hold an eyepiece up and look through it. The width of the view is normally defined by the edge of a metal ring called the field stop. We measured the apparent angular width by comparing the edges of the field stop against a measuring tape set at a known distance from the eyepiece's exit pupil. The results, accurate to 1 [degree], are in the column headed Apparent field (by field stop).
In addition, we measured the field that each eyepiece gave in the test telescope by timing how long a star at the celestial equator took to cross the view. Contrary to popular belief, the apparent field determined by the first method is not necessarily the same as that given by the star-drift method. Distortion and other geometric effects can make them somewhat different.
There has been controversy over which of these two techniques best indicates an eyepiece's field of view, so we list both results in the table. As can be seen, the differences are practically nil for these relatively narrow-field eyepieces. Differences are apparently greater for eyepieces with the most field distortion, as might be expected. Other discrepancies could result from slight measurement and rounding-off errors. The disparity between such results would become more significant in eyepieces with ultrawide fields.
Sharpness: fine-scale contrast. How crisp and clear is the image at the center of view? Saturn, with the dark, unlit side of its rings facing Earth, offered a critical test. We judged sharpness and contrast by crisp rendition of the planet's edge against the sky, the clarity of the thin black line of the dark rings on Saturn's disk, the visibility of the very dim rings just beyond the disk's edge, faint satellites in Saturn's glare, and contrast in the planet's belts and zones.
After much eyepiece-swapping, differences began to appear, mainly at high power. The Tele Vue Plossls (a recent redesign) and Celestron Ultimas gave views just a trace clearer and sharper than the rest. The Explorer Orthoscopics came in last in this regard.
The rankings were essentially the same for lunar detail (craterlets in Plato, contrast in a rough area near the terminator) and for the double star Rigel. We paid special attention to the quality of the dark space that separates Rigel and its 7th-magnitude companion 10 arcseconds away. This spacing provided a fine reference for judging the image compactness of the dazzling, zero-magnitude primary in the 12.5-inch reflector. We were severe in our judgments. The difference between "excellent" and "good" in the ratings might not be detectable except in careful side-by-side tests.
Scattered light: large-scale contrast. For this test we half-filled the field with the gibbous Moon and examined the other half for scattered light. We also looked at contrast between sunlight and shadow along the Moon's terminator. Again the Tele Vues and Celestron Ultimas did very well; so did the Tuthill Plossls. The Sirius Plossls lagged perceptibly, and the 7-mm University Optics Plossl exhibited a bothersome, greenish ghost image as well as greenish scattered light.
Light transmission was measured on Sky & Telescope's optical bench using an International Light IL1400A Radiometer/Photometer. We sent a very narrow, parallel beam of white light to the center of the field lens and measured, at the eye point, the quantity of light exiting. We measured the beam again with the eyepiece removed, and took the ratio of the two measurements as the throughput percentage. All of the readings were done several times; they proved to be repeatable to better than 2 percent.
Light throughput measured this way is a good indicator of eyepiece quality for several reasons. Before coated lenses, a 4 percent reflection by each bare glass surface caused a total light loss of up to 30 percent or more (0.3 magnitude of starlight or more). Worse, this reflected light bounced around between lenses to create ghost images or a background glow that washed out contrast.
High-quality optical coatings (either single-layer or multilayer) have reduced reflections greatly. In a good eyepiece every air-to-glass surface is coated. Find out by reading the supplier's literature carefully or ask whether every surface is single- or multicoated. The latter is several times better if properly applied.
To check for coatings, you can examine an eyepiece at reading distance with the far end capped and a bright white light bulb behind your shoulder. You'll see many reflections of the light bulb with various sizes. A colored reflection comes from a coated surface; a white reflection indicates an uncoated surface. The only white reflections we saw were from both sides of the eye lens of the Edmund RKE 28-mm. Indeed, its light throughput tested lower than the rest.
Every maker seems to follow a different coating philosophy; even in eyepieces of the same basic design, there was little consistency in the colors of the reflections from coated surfaces. We saw purple, blue, green, amber, and reddish reflections.
When comparing the mostly small differences in throughput, remember that beyond a certain level, the limiting factor is your uncoated, debris-filled eye. Modern coatings and well-thought-out optical designs have almost banished the age-old problem of ghosts - except for the ghosts due to reflections from the eye itself. (They quiver when your eye quivers, not when the eyepiece or telescope is moved.) Much of the scattered light around a bright star or planet also arises inside your eyeball.
Edge quality, sweet spot. The image deteriorated away from the middle of the view in all the eyepieces reviewed. Astigmatism, field curvature, coma, false color, and other aberrations - arising both in the eyepiece and the rest of the telescope - become increasingly bother-some off axis. Short-focus (low f/number) telescopes place especially severe demands on an eyepiece's corrections for aberrations.
All of the off-axis aberrations were lumped together in the "edge quality" and "sweet spot" tests. We moved a bright star back and forth from center to edge and watched its image deform and defocus. In some eyepieces, especially at high power, stars stayed nearly starlike even close to the edge. In others they turned into big spears or seagulls. The "sweet spot" is the approximate area, expressed as apparent-field diameter, where during normal observing most people probably wouldn't notice degradation. This test was done at f/6; your mileage may vary.
Distortion. All the eyepieces magnified more powerfully near the edge than near the center, producing so-called pincushion distortion; test squares showed concave sides. We ranked the eyepieces by the squares' appearance and also by how flat the sky looked as we swept the telescope across star fields. The only effect of distortion visual astronomers usually notice is that the sky takes on an odd, curved-looking appearance when you are sweeping. The Explorer Orthoscopics showed the least; the Edmund RKEs and Tele Vue Plossls showed the most by a slight margin.
We measured eye relief as the distance from the eyepiece's top metal surface to the exit pupil or eye point, the place where you put the pupil of your eye to see the full field of view. Poor eye relief, a problem in most eyepieces of very short focal length, is one reason why many observers prefer to get their highest power with a Barlow lens. If you wear glasses to correct for strong astigmatism, you can't take them off when observing, so you need enough room to fit them between the rear metal surface and your eye.
The field stop should be in good focus (for someone with normal distance-vision) in order to provide a clean, sharp-edged view.
Rubber eyecups shield your eye from stray light. Some observers like low eyecups; others prefer tall ones or the kind that flare out to one side, especially if glaring lights are nearby. But tall or flared ones may trap the eye's humidity and dew the lens. Astigmatic eyeglass wearers may have to fold down or remove the eyecups, which was easy to do on the units tested.
Mechanical qualities - our final, most general category. It includes having a friction grip; freedom from burrs or dents on the field stop or elsewhere; whether the eyepiece came in a sturdy; reusable box and had two easy-to-use dust caps; and overall quality of manufacture. We gave credit for edge-blackened lens elements if the edges were visible, and for an especially nonreflective barrel interior.
The Tele Vue eyepieces have a recessed band where the focuser's locking screw presses against the barrel, to prevent the eyepiece from falling out. Some consider this a nice safety feature; others think it a nuisance to have to twist the screw an extra amount every time they change eyepieces, especially when wearing gloves.
The Meade Series 4000 Super Plossls came with new quick-screw plastic containers instead of lens caps. The containers are a nifty convenience, especially if you want to keep eyepieces in a pocket or tote bag.
Street price is what a mail-order dealer was charging for single eyepieces in December, not including shipping and handling.
We were a little more impressed by the Celestron Ultimas and Tele Vue Plossls than by the others. In hours of comparisons, these were consistently a trace sharper and more contrasty than the rest.
The Orion Explorer Orthoscopics, which look identical to common orthoscopics sold 25 years ago, were definitely less sharp on axis than the other sets, at least in the short focal lengths. This is in no way an indictment of orthoscopics generally. The design is excellent if well implemented; some of its variants are famous for flat fields and zero distortion, though the field is never very wide.
The Celestron Plossls and Meade Series 3000 Plossls are attractive for those on a limited budget. Meade's more expensive Series 4000s gained off-axis performance but at the expense of a tiny loss of sharpness at the center. We were impressed with the unusually wide field of the University Optics 12-mm Plossl.
Tuthill Plossls are proof that good eyepieces don't all carry a major brand name. In any case, several brands of eyepiece are assembled in unidentified Asian factories and a supplier's name put on them to order.
The 26-mm Sirius Plossl arrived with a loose eye lens that slid more than a millimeter up and down the barrel. An eyepiece with a flaw like this should be sent back. The lunar ghost and green glow in the University Optics 7-mm Plossl have already been described. The edge aberrations in the University Optics 20-mm Plossls were bad enough to be a real annoyance on the Moon and star fields. As for all the rest, there were no outstanding negative qualities found.
Compared to the uncoated, narrow-field Ramsden and Huygens designs that astronomers looked through for most of the history of the telescope, even a basic eyepiece set is a modern marvel.
RELATED ARTICLE: Cleaning Eyepieces
All eyepieces get dirty, and careless cleaning can permanently damage them. We asked manufacturers how they recommend eyepieces be cleaned safely.
The first step is to remove dust, which will scratch the glass if rubbed against it. A quick technique in the field is to lay a finger across the eye end (without touching the glass!) and suck air under it past the eye lens while flipping your finger away. The sudden pulse of air removes most dust. This trick is harmless and takes about a second.
Gently dust off any remaining particles with a genuine camel-hair or sable brush. Then breathe on the eye lens to fog it, and use a Q-tip or small swab of surgical cotton to very gently swirl the moisture around. Resist the temptation to apply pressure stronger than the weight of a Q-tip.
If eyelash grease or fingerprints remain, as they probably will, dampen a Q-tip with isopropyl or methyl alcohol, available in drug- or hardware stores. (Low-water-content [10 percent or less] isopropyl is best, but standard pure rubbing alcohol works fine, too.) Solutions specially made for cleaning fine optics are available but may not work as well on eyepieces as these inexpensive and readily available alcohols. Be sure such solutions contain no silicone (anti-fogging agent), since it is difficult to remove and actually promotes moisture beading.
Gently swirl a tiny amount of the liquid from the Q-tip around the eye lens. Use the other end of the Q-tip to dry it, applying no pressure. This technique should not harm eyepieces even with many years of frequent cleanings.
Never drop liquid directly onto the glass. It is liable to seep around the lens's edge and stain interior surfaces.
Eyelash and fingerprint oil will stain coatings permanently if left on long enough. So will severe, repeated condensation of moisture carelessly sealed in after observing sessions. (Blow-dry your eyepieces if necessary before capping them.) Manufacturers insist that such stains are only cosmetic. If so, they should have no noticeable effect on performance.
The field lens will probably stay clean by itself. Leave it alone except perhaps for an occasional air suck or camel's-hair dusting. If problems develop inside the eyepiece, don't try to take it apart; you are almost certain to tilt and jam a lens! Send it back to the manufacturer for disassembly and cleaning, which some makers will do at little or no cost.
A. M. and M. B. P.
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|Title Annotation:||includes related article on cleaning eyepieces|
|Author:||MacRobert, Alan; Pepin, M. Barlow|
|Publication:||Sky & Telescope|
|Date:||Apr 1, 1996|
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