Want to get an earful? Ask any practicing amateur astronomer his or her opinion about eyepieces (oculars). If there is one thing that provides a topic for endless discussion and even argument in the amateur ranks, it is oculars. There is a wealth of information about eyepieces available online and in books—almost too much. Endless reviews and comparisons ("shootouts") are available on telescope review sites such as Astromart.com and Cloudynights.com. Unfortunately, these reams of data tend to confuse the situation more than clarify it for the novice. Fortunately, just as with SCTs, oculars sort themselves into several groups, which makes it easier to pick eyepieces to fit a particular budget and observe style without getting too deep into esoteric performance specs.
Choosing an eyepiece wisely has more to do with what will be done with it than how much it costs. For that reason, this roundup of oculars is divided into categories based on an optical characteristic, the eyepieces' apparent fields of view (AFOVs), rather than price: narrow field, medium-wide field, and ultrawide field. Before doing any serious eyepiece shopping, though, it is mandatory to learn a little eyepiece language. Knowledge of the specifications and terminology used in the ads and by fellow amateurs makes picking oculars simpler.
Eyepiece Terminology and Technology The most basic commercial eyepieces available today consist of two lens elements. No matter how many lenses make up an ocular, the one looked into is the eye lens, and the one at the other end of the eyepiece barrel, the telescope end, is the field lens. Eyepieces are produced in several barrel sizes. The 0.965-inch models are referred to as Japanese standard format. They are not often seen today, although some top Japanese scope makers such as Takahashi still make 0.965s. Much more common are the 1.25-inch diameter American standard eyepieces. No matter where a scope is made, its focuser will usually be able to accept 1.25-inch eyepieces. The 2-inch diameter eyepieces used to be rare and only used by "advanced" amateurs. Today, they are more common, and even some discount store scopes have focusers that will accept 2-inch oculars.
A feature of almost all eyepieces is an eyecup, a rubber shield around the eye lens that prevents stray light from striking it and helps with eye placement. Another common feature is filter threads on the field lens end of the barrel (28.4-mm diameter, 0.6-mm tpi [threads per inch] thread pitch on 1.25-inch eyepieces). To suppress undesirable reflections from stray light entering the telescope end of the barrel, the inside surface there is usually painted a flat black. The lenses of eyepieces are coated to reduce reflections and increase light transmission. Coatings can be as simple as a single layer of magnesium fluoride or as complex as multiple coatings of rare-earth elements. Modern eyepieces can be either multicoated (both air-glass surfaces are coated) or fully multicoated (all lens element surfaces are coated).
Focal Length An eyepiece's focal length is the distance from the eye lens to the point where the rays of light converge on the focal plane of the scope. Eyepiece focal length is usually expressed in millimeters these days, and oculars are commonly found in focal lengths from about 4 to 40-mm. What is most important about eyepiece focal length for you is to be aware that it determines the magnification of an eyepiece.
Magnification Every beginner knows about magnification; it is a telescope's power and determines how big the Moon, a planet, or other object will look. Magnifications below 50x (usually pronounced "50 power") are considered low magnifications. Powers between 50x and about 150x are thought of as medium magnification. Above x150 is the realm of high power.
How is a telescope's magnification changed? This is done by swapping out eyepieces. A long focal length eyepiece like a 40 mm produces low magnification. A short focal length eyepiece like a 4 mm produces high magnification. A simple but vital telescope magnification formula was given in Chapter 2, but it is so important for astronomers, it is repeated here:
(power)Magmfication _ (mm)
A 12-mm focal length eyepiece in an 8-inch SCT—which usually has a focal length of about 2,000 mm—gives a magnification of 167x (2,000/12).
Eye Relief Eye relief, another important eyepiece specification, especially for those who wear glasses, is the distance the observer's eye can be from the ocular's eye lens while observing. Normally, the eye is not jammed up against the eye lens of an ocular; it is held a comfortable distance away, just far enough so that the whole eyepiece field can still be seen. The distance the eye can be from the lens and still take in the full field is the eyepiece's eye relief. The amount of eye relief available in an ocular depends on its design, and it is sometimes the case that expensive eyepieces have less eye relief than cheaper ones. Generally, the shorter an eyepiece's focal length, the smaller its eye relief.
How much eye relief is good? For an eyeglass wearer, anything less than 15-mm means not being able to see the whole field. Because of the presence of glasses, the eye will be positioned beyond the eye relief "limit," and the full field will not be visible; only the central area will be seen. The need for at least 15-mm of eye relief unfortunately eliminates many wonderful eyepieces from these folks' consideration. Luckily, many eyeglass wearers can observe without their glasses. Only in cases of severe astigmatism is glasses wearing mandatory for astronomers.
If there is such a thing as too little eye relief, can there also be too much? Yes, indeed, there can. Get much past 20-mm, and it becomes difficult to position the eye correctly for viewing. If the eye is not in the correct place, an eyepiece can suffer from blackout— portions of the field will suddenly go dark.
How is the eye relief distance of a particular eyepiece determined? To do this, either take the manufacturer's word for it—if it is listed in the eyepiece's specifications—or find out. Technically speaking, eye relief is the distance from the eye lens to the eyepiece's "exit pupil." To measure this distance, point the scope at a bright object (not the Sun)—maybe a bright terrestrial scene. Hold an index card or a piece of white paper behind the eyepiece so that an illuminated circle is visible on the card. Move the card toward the eyepiece or away from it until this bright circle of light is as sharply focused as possible. The point where it is sharpest is the exit pupil, and the distance from card to the eye lens represents the eyepiece's eye relief. Measure this distance with a ruler's millimeter scale (do not touch the lens with the end of the ruler; hold it beside the eyepiece).
Apparent Field of View Apparent field is something amateurs go on and on about when they talk eyepieces. AFOV is a simple concept, but beginners often find it a difficult one to grasp. Apparent field is the diameter of the circular field of an eyepiece expressed in degrees. An eyepiece that has an AFOV of 50° will not show a swath of sky 50° across. A 50° 25-mm eyepiece shows something less than 1° of sky. Is this clear as mud? The situation with apparent field is analogous to the size of a television screen. Comparing an eyepiece with a small AFOV and one with a large AFOV is like comparing a 12-inch screen portable television set showing an image of the Grand Canyon to a 70-inch flat-screen monster showing the same scene. The big TV displays a much more expansive version of the same stretch of landscape. Using a large AFOV eyepiece is like viewing the universe through a spaceship porthole rather than a little peephole. Modern eyepieces usually have AFOVs from around 50° (Plossls) to 80° (ultrawide-field eyepieces). Apparent field also determines the true field shown by the ocular.
True Field of View True field is easy to understand. It is the expanse of sky visible through an eyepiece, expressed in angular degrees. If, for example, an eyepiece just fits the entire disk of the Moon into its field of view, it has a true field of 0.5° or 30 minutes of arc. Why? Because the Moon itself is half a degree, 30 arc minutes, across in the sky. If only half the Moon fits into the eyepiece, then its true field is 0.25°, or 15 minutes of arc. The true field of an ocular will vary, depending on the focal length of the telescope in which it is used. Long focal length telescopes produce higher powers and smaller true fields with a given eyepiece than short focal length telescopes.
There are several ways to calculate an eyepiece's true field of view. The easiest method uses the AFOV figure from the eyepiece manufacturer's specifications:
A 35-mm eyepiece with a 68° apparent field in an 8-inch f/10 SCT yields a true field of 1.19°, over two full Moons wide: 68(AFOV)/57 (magnification).
Eyepiece Aberrations When amateur astronomers sit down to talk eyepieces, there is also a lot said about optical aberrations—optical problems. All eyepieces suffer from one or more optical defects. There are no perfect telescopes, and there also are no perfect eyepieces. Some common aberrations are discussed next.
Astigmatism Just as an observer's eyes can be astigmatic, so can eyepieces. Severe astigmatism manifests as oddly shaped stars. Rather than small points of light, they appear as ovals, crosses, or "seagulls." There is one sure way to diagnose astigmatism: observe a slightly defocused medium-bright star. If its diffraction rings are not round but elongated on one side of focus and the direction of this elongation changes 90° on the other side of focus, there is astigmatism somewhere—in the eyepiece, the telescope, or the astronomer's eyes. See Chapter 7 for some pointers on diagnosing the source of astigmatism. Don't worry too much if it seems an ocular is astigmatic. Many, if not most, eyepieces suffer from some astigmatism, and it will not do much more than make stars at the edge of the field look a little less pretty than they would otherwise.
Blackout and Kidney Beaning Blackout, already mentioned, is another common eyepiece problem. It usually happens with long eye relief eyepieces when the observer's eye is not in the proper position for viewing. Get too close or move the eye too far off axis, and the field will go dark. Sometimes, only part of the field will black out in small bean-shaped areas. The cause of this kidney beaning is usually the same as that of blackout: improper eye placement. It can also be a symptom of spherical aberration in the eyepiece. Both of these problems become more pronounced when the eye's pupil is small, as when viewing bright terrestrial scenes.
Field Curvature When stars at the center of an eyepiece's field are in focus and stars at the edge are out of focus and vice-versa, an eyepiece is showing field curvature. This is a familiar effect for the SCT owner because the focal plane of a Schmidt Cassegrain, its "field," is not flat but strongly curved. This defect may not be entirely the telescope's fault. Many eyepieces at least contribute to field curvature. More expensive and complex oculars such as the Naglers and Panoptics tend to be better corrected for this aberration than cheap, simple ones.
Apparent field(degrees) Magnification
Lateral Color Have you ever observed a bright planet like Jupiter and noticed one of its limbs (disk edges) was blue and the other red? That is lateral color. Do not be too quick to accuse the eyepiece of this sin, however. Lateral color can also be caused by observing a planet that is too low in the sky. Wait until Jupiter is at least 30° to 40° above the horizon before laying blame for lateral color.
Pincushion and Barrel Distortion Pincushion and barrel distortion are two different but similar aberrations. They are easiest to see in terrestrial objects that include "lines," such as a rooftop's shingles or, even better, a fence's boards. If the lines of the boards appear curved rather than straight, diverging at the center and converging at top and bottom, that is pincushion distortion. If they do the opposite, come together at the center of the field and curve apart at top and bottom, the problem is barrel distortion. These aberrations are common in wide-field eyepieces and are usually not very obvious unless the scope is panned across a rich star field.
Eyepiece Image Orientation A telescope orients its images differently in the eyepiece, depending on its particular configuration of lenses and mirrors. One scope may place north at the top of the field and east on the right. Another may put south at the top and east at the left. It is sometimes important to know which direction is which in the eyepiece when searching for objects. The SCT, when used with a star diagonal, presents an image oriented just like a terrestrial map. North is up, and east is on the right, 90° from north. If this seems hard to remember, just keep in mind something my students have christened "Rod's rule": A telescope with an even number of mirrors or no mirrors (a Newtonian reflector or a refractor) yields an image that is inverted (upside down) but mirror correct. A telescope with an odd number of mirrors (an SCT or Maksutov Cassegrain telescope [MCT] or refractor with a star diagonal) gives an image that is right-side up, but reversed right to left.
Eyepiece Buyer's Guide Enough of the dad-gummed technical mumbo-jumbo. How many and what kind of eyepieces does an SCT owner need? A set of three is a good number to begin: a low-power eyepiece for big objects, maybe a 32 mm (62x in an 8-inch SCT); a medium-power ocular, like the 25 mm that probably came with the scope (80x), for most observing tasks; and a high-power 10 mm (200x) for small deep sky objects and for the Moon and planets. It might be nice to supplement this basic set with a really high-power ocular in the 6-mm (333x) range for use on the planets on nights of good seeing.
What about eyepiece design? And, how much are these things going to cost? Fortunately, we are living in a time when cheap, good eyepieces are the rule, mostly thanks to Chinese imports. Stop and think before filling an eyepiece case with cheap oculars. It is impossible to go wrong by buying good eyepieces. As has often been said, an eyepiece makes up half the scope's optical system. You agonized over choosing a good CAT, so why limit its performance with less-than-excellent eyepieces? If buying top-quality eyepieces means getting along with two instead of three oculars for a while, that is still the way to go. Without further ado, let us choose a few nice eyepieces.
Narrow AFOV Oculars: Plossls and Orthoscopies "Narrow"
AFOV eyepieces are those with apparent fields of 50° or smaller. This group, over the years, has had quite a few members: Kellners, Erfles, Orthoscopics, and Plossls. Thanks to the wide availability of Chinese eyepieces and the decision by most of their makers to concentrate on the Plossl design, however, the Kellner and the Erfle have all but disappeared. The Orthoscopic has not been embraced by the Far Eastern optical manufacturers, but it is still widely available—if not in the huge numbers of the Plossl—due to its enduring popularity with amateurs.
Who forms the audience for this class of eyepieces? Folks who do not care about large AFOVs. An amateur who mainly looks at the planets, for example, does not need the "spacewalk" field offered by ultrawide AFOV oculars. Simpler eyepieces can also often produce brighter, sharper planetary images than can complicated wide-field designs. Price used to be an attraction of the narrow apparent field eyepieces, but that is less true now. Medium AFOV and even ultrawide eyepieces are now available for not much more money than the narrow types.
Plossls There is no doubt that the Plossl (Plate 36) is the "standard" amateur eyepiece of today because of the way it has been aggressively marketed. Is it the best eyepiece? Probably not because its design is far from perfect. Its strengths are its low price and its "reasonable" performance characteristics. The Plossl, which is sometimes referred to as the symmetrical, is a classic eyepiece design that dates to the nineteenth century; it was invented by an Austrian optician, G. S. Plossl, in 1860. Its optical design, seen in Figure 2, incorporates four convex lens elements placed back to back in two groups. Designers have played around with the Plossl formula a
Plate 36. (Narrow/Medium Field Eyepieces) A collection of narrow to medium field eyepieces, (l- r) Pro-optic 40mm Plossl, Orion Expanse 20mm, Orion Expanse 9mm, Celestron 9mm Orthoscopic. Credit: Author.
lot over the last decade or two, and it is not uncommon to see Plossls with different numbers and types of lens elements being advertised as "modified Plossls."
What is the Plossl like in a telescope? It is made in a wide range of focal lengths and is a genuinely good performer in most of these focal lengths. Plossls are commonly available from 55 mm down to at least 6 mm. If properly manufactured, this eyepiece's color correction is good, as is its edge-of-field performance across the entire range of focal lengths (at least in SCTs, whose high focal ratios help eyepieces perform better). How about AFOV? Coming from someone who was raised on the 30° apparent field Ramsden eyepieces of the 1960s (do not ask), it almost seems a sin to call Plossls narrow-field eyepieces. The main drawback to Plossls is that in shorter focal lengths eye relief tends to be small. A 12 mm will likely have about 10 mm. As for a 6 mm, do not expect more than 3 to 4 mm of eye relief.
Then, there is the question of which Plossl to choose. If you are on a budget, go for the low-price leader. You will not be disappointed. Even $30 imported Plossls do a respectable job in SCTs. If possible, however, spend just a little more. Eyepiece industry leader TeleVue's Plossls, for example, are noticeably sharper than the chea-pos. They are also much better built mechanically, incorporating good eyecups and rubber barrel grip rings. Despite these pluses, the TeleVues are only about $50 more than the lowest-priced imported eyepieces.
Orthoscopies Back in the 1960s when Uncle Rod was a young'un, the ne plus ultra of eyepieces was the Orthoscopic, sometimes referred to as Abbe Orthoscopic in honor of its designer, optical guru Ernst Abbé. These were the top-dog oculars we all wanted but could not afford. How things have changed. Although the Ortho is still a good performer, it is hardly considered tops these days—but it is no longer exorbitantly priced, either. The Orthoscopic (Plate 36) is, like the Plossl, a four-element design. Also like the Plossl, it was developed in the nineteenth century. The design of the Orthoscopic, unlike that of the Plossl, is pretty standard and consists of a single-element eye lens, a convex lens with the flat side facing out. At the other end of the barrel is a three-element field lens consisting of two convex lenses with their curved sides facing each other and separated by a double-concave negative element (Figure 2).
Orthoscopic apparent fields are nothing to write home about, usually being in the 40° to 45° range, but this weakness is easily offset by the design's strengths. The Orthoscopic is sharp all across its field and especially at the field edge, where many less-expensive oculars have trouble. Plossl edge sharpness is good, but Orthos are better, especially at shorter focal lengths. Orthoscopic color correction is excellent, and eye relief is fair—sometimes a little better than that of Plossls of the same focal length. When it comes to focal length choices, Orthos do not cover the whole range, like Plossls do, instead tending to concentrate on the shorter end. Usually, "long" for an Orthoscopic eyepiece is 25-mm. On the other hand, Orthos can be had in very short focal lengths: 4-, 3-, and even 2-mm.
Which specific Orthoscopic should you choose? You will not have to fight the allure of cheap imported Orthoscopics as there really are not any. On the other end of the price range, premium manufacturers such as Zeiss offer or have offered Orthoscopics that are crazy expensive and highly sought after by discriminating planetary observers. When talking "Orthoscopic," though, one maker's name gets mentioned more than any other, University Optics (Appendix 1). This U.S. firm's Abbé Orthoscopics are of outstanding optical quality and are very reasonably priced at $59.95 for all focal lengths from 25- to 4-mm.
Medium AFOV Designs Medium AFOV eyepieces are those oculars that provide apparent fields of about 65° to 70°. Although this is considerably smaller than the spaceship porthole fields of the ultrawides, it is still a big increase over Plossls. For amateurs on a budget, medium AFOVs are more optically forgiving than inexpensive 80° or higher ultrawides. It is pretty easy for a bargain 68° AFOV eyepiece to present good-looking stars at the edge of the field; it is much more difficult for a cheap 82° eyepiece to do the same thing.
Despite their considerably more expansive fields, these eyepieces are not always better than the narrow AFOV eyepieces. Most medium-wide designs use from 6 to 8 lens elements (Figure 2). Despite modern lens coatings, their images are usually slightly dimmer than those of narrow AFOV eyepieces. Most medium users think this sacrifice is acceptable to gain that extra apparent field, however.
Who likes the mediums? Many SCT owners do. Since our telescopes are driven, the big apparent fields of the ultrawide eyepieces are not as necessary for us as they are for Dobsonian owners, who must nudge their scopes along. Eyeglass owners may prefer the medium oculars as well since many offer excellent eye relief, better than what is usually found in the ultrawides. All observers may prefer the medium AFOV experience because it is easier to take in the whole field without moving the eye around than it is with the huge fields of the ultras.
It has only been in the last 5 years that 65° to 70° eyepieces have become available at popular prices. As with other bargain equipment on the amateur scene, the source of these eyepieces is Taiwan or mainland China, and the factories there are now turning out container ships full of 65° to 70° oculars, most of which are good performers.
Synta Ultrawides One of the first series of reasonably priced medium AFOVs to hit the United States was the Synta Ultrawides from Taiwan (Plate 36). These are all 1.25-inch barrel format eyepieces, and despite the name, all yield 66° apparent fields. These fields are satisfyingly sharp out to at least 90% of this field in f/10 SCTs. The prices of these eyepieces are very attractive, that is for sure, with all focal lengths available for about $50 apiece, depending on the seller. Like most Synta products, they are never sold as "Synta" but as various house brands, such as Skywatcher, Orion (as the Expanse series), or Pro Optic.
As nice as the Syntas are, they are not fault free. The shorter focal lengths are prone to light scatter and internal reflections—although this will not be much of a problem when observing the deep sky. Another annoyance is that the focal length range available is limited to four choices: 20-, 15-, 9-, and 6-mm. Don't try the longer focal length pair in fast scopes; edge-of-field stars fall apart badly in the 20-mm especially. One thing these eyepieces do have going for them is decent eye relief: 17-mm for the longer focal lengths and 13-mm for the shorter ones.
Vixen Lanthanum Superwides A step up from the Synta eyepieces is the Vixen Lanthanum Superwide series. The eyepieces in this group have been on the U.S. market for over a decade and, at a price of about $200 each, have been attractive to amateurs looking to save money over more expensive brands such as TeleVue. These are good eyepieces, although they lack the "snap," the excellent contrast, of more costly mediums such as the TeleVue Panoptics. One of the hallmarks of the Lanthanums (which refers to the rare-earth element used in their coatings) has always been good eye relief—20 mm across the board for the whole series, from 3.4 to 42 mm. Depending on focal length, the AFOVs of these eyepieces range from 65° to 72°. All feature good mechanical build quality.
Orion Statuses With the budget medium-field eyepiece explosion, the Vixens have lost some ground in the astronomy marketplace. They do seem to be making a comeback of sorts lately—in a way. Orion has begun selling a line of eyepieces called the Stratus Wide Fields. The barrels of these eyepieces look very much like those of the Lanthanums, they offer similar eye relief, and their AFOVs are about the same. One thing the Stratus series lacks compared to the Lanthanums is longer focal lengths, with the longest Stratus a 24-mm. Are these eyepieces produced under license from Vixen, or are they "clones"? We do not know, but they are good values at $130 each.
TeleVue Panoptics Do you want the best medium-AFOV eyepieces money can buy? Try the TeleVue Panoptic series. These oculars (Plate 37) are available in a wide range of focal lengths, including 19-, 22-, 27-, 35-, and 41-mm. Eye relief ranges from 13 to 19 mm across the series, except for the big 41, which has a whopping 27-mm. All the "Pans" feature the same 68° AFOV. The 19-mm model has a 1.25-inch barrel, the 22-mm has a hybrid 1.25-inch/2-inch barrel (for use in either size focuser), and the 27, 35, and 41 are 2-inch format only eyepieces.
One other characteristic of the Panoptics is optical excellence. In f/10 SCTs, these things are dead sharp from field edge to field edge, and they perform nearly as well in faster telescopes. They are also extremely comfortable to use, lacking much of the blackout and field distortion found in less-expensive mediums. The only problem with the Pans may be their prices, which begin at about $200 for the shorter focal lengths, climb to $300 for the medium focal lengths, and top out at over $500 for the 41-mm.
Plate 37. (Medium/Wide Field Eyepieces) The upper crust of the eyepiece world, medium and ultra-wide premium oculars, (l - r) William Optics 28mm Uwan, William Optics 7mm Uwan, TeleVue 22mm Panoptic, TeleVue 12mm Nagler Type II. Credit: Author.
Pentax SMC XWs Pentax is a name that is been familiar to photographers for decades, usually as a manufacturer of high-quality 35-mm single-lens reflex cameras. This Japanese company also produces a few astronomy products, including its highly regarded medium-AFOV oculars, the Pentax SMC XWs. These eyepieces, which come in focal lengths from 3.5- to 40-mm, are of incredibly good quality both optically and mechanically. Are they as good as the Panoptics? Their fans will tell you they are better. The optics mounted in Pentax's heavy-duty weatherproof barrels provide generous 70° apparent fields slightly larger than the Pans, and do it without sacrificing eye relief. All focal lengths provide 20-mm. Except for the 30- and 40-mm models, all the XWs are 1.25-inch eyepieces.
There are two caveats concerning the Pentax oculars: Some amateurs have sometimes found it difficult to get needed service from Pentax, and these eyepieces are expensive. In that area, they definitely outdo TeleVue. The 19-mm Panoptic, for example, is $250. The comparable XW, the 20-mm, is $300.
Meade Super Wide Angles Are the Panoptics and Pentaxes, as good as they are, budget busters? If so, consider the Meade alternative. Meade's Series 5000 Super Wide Angles are a couple of cuts above the bargain-bin Syntas and Orions at prices a bit lower than those of the Panoptics. The Meades, available in the somewhat eccentric focal lengths of 13.8-, 18-, 24.8-, 32-, and 40-mm, have good eye relief that ranges from 12-mm on the short end to 31 mm on the long end. They are also sturdily constructed and attractively packaged. Optically, they perform quite similarly to the Panoptics in SCTs. They do give ground to the more expensive medium-AFOV oculars in faster scopes, where their field-edge sharpness deteriorates somewhat. Prices for the Super Wides begin at $180 for the short focal lengths and increase to $400 for the 40 mm.
Burgess Paragons Is the cost still too much? A relatively new medium-AFOV ocular that is garnering rave reviews is the Burgess Optical Paragon. Although this eyepiece is currently only available in focal lengths of 40- and 30-mm, the company apparently will soon be expanding the line to other focal lengths. The Paragon, designed by late apochromatic refractor guru Tom Back, performs as well as much more expensive oculars for a modest price ($250). The Paragon uses six lens elements in four groups in a 2-inch barrel to produce a 69° apparent field and images that, while not quite as sharp to the field edge as a Panoptic's or a Pentax's, are nevertheless satisfyingly good.
Ultrawides A first look through an ultrawide AFOV (Plate 37) eyepiece will be surprising, maybe shocking. It may even border on a religious experience. The huge field will not just look "good"; it will be overpoweringly immersive. You will feel as if you are falling into that distant star cluster. Even at higher powers, the 82° (usually) apparent fields of these eyepieces ensure that an observer never feels constricted. That comes at some cost, of course. The TeleVue Naglers, the premiere ultrawides since the first one was introduced in 1980, are pricey. Also, whether the eyepiece is a Nagler or one of the lower-priced alternatives now available, there is a cost in light. Like the medium-AFOV eyepieces, the ultrawide designs incorporate many elements—as many as seven separate lenses—that tend to dim images a bit. Another cost is eye relief. Eyeglass-wearing observers are in for more frustration than amazement with the ultras.
TeleVue Naglers The TeleVue Naglers are the eyepieces that started the spaceship porthole viewing craze, and Al Nagler's revolutionary oculars are still going strong 30 years later. Currently available in focal lengths from 31-mm all the way down to 2.5-mm, all Naglers feature AFOVs of 82°. Eye reliefs vary from 12- to 19-mm. Eye relief also varies according to the "design" type of the Nagler in question. Currently, TeleVue's Naglers are offered in three slightly different designs, types 4, 5, and 6. These design differences are a result of TeleVue's continual and laudable efforts to update and improve their eyepieces.
Innovation costs money, of course, and that means a selling price for these oculars that is a barrier for some amateurs. The least-expensive (shorter focal length) models retail for about $300 in the United States. The most expensive Nagler, the "holy hand grenade," the huge 31 mm, is a daunting $640. Another strike against them is weight. A 12-mm Nagler comes in at about a pound, and the 31 is over 2 pounds. That much weight on the rear cell can make it difficult to balance some scopes.
Amateur astronomers often complain about the cost of Naglers, but they rarely complain about these oculars' optical or mechanical quality. Optically, they are amazing in CATs, offering pinpoint stars all across their huge fields. Aberrations such as pincushion distortion or astigmatism are minor or nonexistent. In fact, objects may often look sharper in the Naglers than they do in narrow-field eyepieces. Yes, targets may be slightly dimmer in the "Nags" due to all that glass, but because of the superb lens coatings TeleVue uses, even that is held to a minimum. Mechanically, the Naglers are also top-notch. Dropping my beautiful 12-mm onto a concrete observing floor resulted in no shattering of glass and only in one tiny mark on the barrel. Replacing the eyepiece in the focuser, it was obvious that nothing was out of alignment; images were as good as ever.
Meade Ultra Wides Meade has been competing with TeleVue in the ultrawide market for many years, and the Meade 82° field oculars have always been acknowledged as good eyepieces—maybe not quite as good optically or mechanically as the Naglers, but good. Certainly, they have always been good values as their prices have consistently undercut those of the TeleVues. The major complaint about Meade's Ultras has been that, unlike Al Nagler, Meade did not continue to update their designs. Then, a few years ago, the company introduced the Series 5000 Ultra Wides that at least looked very different from the old ultras.
The most striking thing about the new Meades is their barrel design (see Plate 38), which is certainly futuristic looking. The eyecup at the end of the barrel is built into the eyepiece and can be extended or retracted to adjust its height as desired. Unfortunately, Meade uses a lot of grease on the eyecup's mechanism, and it tends to migrate onto a user's hands and telescope (the Super Wides use the same scheme and the same grease).
Plate 38. (Meade Ultrawides) Meade's modernistic Ultrawide Series 5000 collection. Credit: Image courtesy of Meade Instruments Corporation.
Optically, the Ultra Wides, which are available in focal lengths from 4.7- to 30-mm, appear to have been at least incrementally upgraded and are quite competitive with Naglers when it comes to raw sharpness and lack of distortion. Where they fall behind a bit is in the areas of baffling and coating. Focal length for focal length, there are more internal reflections with the Ultra Wides than with the Naglers. All eyepieces are subject to some stray reflections, but the problem seems a little worse in the Meades. Coatings on the ultrawides appear to be as good as those on the TeleVues but less carefully applied, with small flaws sometimes apparent.
One area in which the Meades are slightly better than the TeleVues is eye relief. Some Naglers have as little as 12-mm, but the Meade with the least eye relief is the 4.7-mm with 13-mm. Most of the ultrawides have 15 mm or more. The big draw here, of course, is price. The Series 5000 eyepieces are cheaper, with the "king," the 30 mm, selling for $450. Regarding the verdict on the Meade Ultra Wide Angles, they are cheaper and almost—but not quite—as good as Naglers.
William Optics Uwans The Meade Ultra Wides seemed to be about as good as it got in the 82° arena when it came to a balance between price and quality. Then came the William Optics Uwans. Although these eyepieces are made in Taiwan, Uwan is not a city in China. It is an acronym for ultrawide angle. The four eyepieces that have appeared in this series thus far—28-, 16-, 7-, and 4-mm—have turned out to be remarkable oculars, seeming to equal the TeleVue Naglers in most ways while undercutting even Meade's prices.
How good are the Uwans? I did not rely on their views in the optically forgiving SCTs to find out. I arranged a "shootout" between the 28-mm Uwan and the comparable 26-mm Nagler under the dark skies of Florida's Chiefland Star Party. The eyepieces were tested in big Dobsonians with focal ratios down to and including an eyepiece-punishing f/3.26. In the opinions of the experienced observers who participated in this comparison, the Uwan was "as good or a little better" than the 26-mm Nagler in the areas of sharpness and field-edge quality. This was on a variety of objects, including the monstrous globular star cluster Omega Centauri, with its countless tiny stars. In fact, the only time the informal panel of testers felt the Nagler pulled ahead was in the f/3.26 scope, and everybody agreed its advantage, even there, was relatively slight. The only area where the Uwans do seem to lag behind the Naglers is in viewing comfort. Eye placement is slightly more critical with the Uwans, with these eyepieces displaying more "blackout" than the Nags.
Mechanically, the Uwans are perhaps slightly better in some ways than the Tel-eVues and ultrawides. Their barrels are striking modern designs (see Plate 37), all black and high-tech looking. Unlike the TeleVues, most of which rely on plain old rubber eyecups, the Uwans integrate a hard mechanical eye cup, which is rotated to extend or collapse. This design, unlike that of the Meade Series 5000s, does not leave astronomers with greasy hands.
Pricewise, the Uwans beat everything in their class. The 30-mm goes for $398, the 16-mm is $238, and the 7- and 4-mm are $198 each. So, what is not to like? The main thing is the limited range of focal lengths. There are currently seven Meade Ultra Wides. The TeleVue Nagler lineup consists of an amazing 14 eyepieces. Despite this paucity of focal lengths, all things considered, the Uwans deserve "best buy" status.
Spaceship Picture Window: The TeleVue Ethos Recently, TeleVue has introduced a remarkable pair of eyepieces, the 13-mm and 7-mm Ethos oculars. Both have apparent fields of 100 degrees. That's right, 100 degrees. In addition to this huge AFOV, the Ethos eyepieces feature 15-mm of eye relief and display the best sharpness and contrast I have ever seen in ultra wide field eyepieces. By the time this book is published, TeleVue will have added two more oculars to the Ethos stable: A 6-mm and a 17-mm. The only bad thing about any of them? Their prices, which range from just under $600 to over $700.
Cheaper than Cheap? With Chinese medium-AFOV class oculars now common, you would think there would also be some imported ultrawides for less than the $100 prices. There are. Unfortunately, although the Chinese medium-wide AFOV eyepieces perform respectably, their ultra analogs are not quite there yet. A few, like the "Bird's-eye" oculars (11-, 15-, 16-, 30-mm), imported by U.S. astronomy retailer Anacortes Telescope and Wild Bird (Appendix 1), do an acceptable job in slow focal ratio CATs but are in no way comparable to Naglers or Uwans. They can at least give the new or cash-strapped astronomer a taste of eyepiece spacewalking at prices less than $100.
Hope for Ultrawide-Loving Eyeglass Wearers TeleVue has been well aware of the problem for eyeglass wearers posed by the Naglers' relatively short eye reliefs, which make it impossible for astigmatism sufferers to see the oculars' entire gigantic field. The solution TeleVue has come up with is an attachment that allows astigmat-ics to leave their glasses off. The Dioptrx can be attached to TeleVue eyepieces longer than 12-mm, including the Naglers and the new Ethos. This corrective lens element is available in different values to match glasses' prescriptions for astigmatism.
The Dioptrx must screw onto the eye lens end of an eyepiece, which is the reason it is only available for and usable on TeleVue oculars with larger eye lenses.
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