Hack 49. Optimize Your Eyepiece Collection
Choose eyepieces that match your budget and observing habits.
Astronomers tend to collect eyepieces like dogs collect fleas. We know astronomers who have owned 50 and even 100 eyepieces over their observing careers. Some constantly buy and sell eyepieces in an eternal search for the perfect set. That gets expensive fast, though, and there is a better way to do it. Read on to discover what you need to know to assemble a perfect eyepiece set for your own scope and your own needs at a price you can afford.
4.7.1. Eyepiece Characteristics
Here are the important characteristics of eyepieces:
Eyepieces are made with barrel sizes designed to fit standard telescope focusers. For some reason, eyepiece barrel sizes are always denominated in inches, even outside the U.S. The most common barrel size is 1.25". Low-power, wide-field models use 2" barrels. The larger diameter of 2" eyepieces allows them to show more of the sky, but 2" eyepieces are more expensive to produce than 1.25" models. Accordingly, with few exceptions, 2" models are available only in longer focal lengths.
A telescope with a 1.25" focuser accepts only 1.25" eyepieces. A telescope with a 2" focuser accepts 2" eyepieces, and, with an adapter, 1.25" models. Figure 4-10 shows a 1.25" Tele Vue Radian eyepiece on the left and a2" Tele Vue Panoptic eyepiece on the right.
Figure 4-10. Tele Vue 1.25" Radian (left) and 2" Panoptic eyepieces
Eyepieces with 0.965" barrels were formerly common, and remain popular in Japan. In the U.S. nowadays, only the very cheapest of telescopes have 0.965" focusers. We recommend avoiding 0.965" eyepieces because their small barrel diameter provides a very constricted view at anything less than medium magnification.
The focal length of an eyepiece is stated in millimeters and determines the magnification that eyepiece provides with any particular telescope. To determine the magnification, divide the focal length of the telescope bythe focal length of the eyepiece. For example, a 25mm eyepiece used in a scope of 1,000mm focal length yields 1,000/25=40X.
Mainstream eyepieces are available in focal lengths from 2.5mm to 56mm. The optimum range of eyepiece focal lengths for a particular scope depends on the focal ratio of that scope and the size of your dark-adapted entrance pupil [Hack #7]. The exit pupil provided by an eyepiece is calculated by dividing the eyepiece focal length by the focal ratio of the scope. For example, a 10mm eyepiece used in an f/5 [Hack #9] Dob provides a 2mm exit pupil, while that same eyepiece used in an f/10 SCT provides a 1mm exit pupil.
The ideal range of eyepiece focal lengths for a particular scope is one that provides exit pupils no smaller than about 0.7mmanything less than that greatly reduces visual acuityand no larger than the size of your fully dark-adapted entrance pupil. For example, if you have an f/5 scope and your maximum entrance pupil is 6mm, the ideal range of eyepiece focal lengths for you is 3.5mm (3.5mm/5=0.7mm) to 30mm (30mm/5=6mm). An exit pupil in the 2mm to 3mm range provides maximum visual acuity, which allows you to see the most detail.
When you select focal lengths for your eyepieces, keep in mind the effect of your Barlow lens [Hack #46]. For example, if you already own a 20mm eyepiece and a 2X Barlow, it makes little sense to buy a 10mm eyepiece because a10mm eyepiece duplicates the effect of using the 20mm eyepiece with the Barlow.
Apparent field of view
The apparent field of view (AFoV) of an eyepiece is the angular size of the circular image it presents. At any given magnification, an eyepiece with a wide AFoV shows more of the sky than an eyepiece with a narrower AFoV. For example, Figure 4-11 shows simulated views of M42, the Great Orion Nebula, in two eyepieces of the same focal length but with different apparent fields. Because the focal length of the two eyepieces is the same, so is the magnification, and the nebula appears to be the same size. But the eyepiece on the left has an 82° AFoV, and so it shows much more of the surrounding sky than the image from the 50° eyepiece, shown on the right.
Eyepieces that provide a wide AFoV are popular for two reasons. First, many people find the larger field of view pleasing esthetically. Second, if you are using an undriven scope, such as a Dobsonian, the larger AFoV allows the object to remain in view longer before you have to "bump" the scope to recenter it. The AFoV of mainstream eyepieces ranges from 40° to more than 80°. All other things being equal, eyepieces with a wide AFoV cost more than those with a narrow AFoV.
Eye relief is the distance an eyepiece projects its exit pupil from the outer surface of the eyepiece eye lens. When you view with an eyepiece that has short eye relief, you have to press your eye right up against the eyepiece. Using an eyepiece with long eye relief allows you to maintain some separation between the eyepiece and your eye. If you must wear eyeglasses while observing, or if you just find longer eye relief more comfortable, look for an eyepiece with 20mm or so of eye relief. If you observe without glasses, 12mm or so of eye relief is adequate.
Figure 4-11. The Great Orion Nebula (M42) with an 82° eyepiece (left) and a 50° eyepiece
Too much eye relief can be as bad as too little. Some long focal length eyepieces have excessive eye relief, sometimes 40mm or more. When eye relief becomes longer than 25mm or so, it becomes very hard to get and hold the exit pupil, which causes blackouts as your eye moves around, seeking the exit pupil. Eye cups and other aids can minimize this problem, but in general we recommend avoiding any eyepiece with eye relief greater than 25mm.
Mechanical and optical quality
The mechanical fit and finish of eyepieces varies dramatically. In general, all Japanese-made eyepieces and some Taiwanese models have excellent mechanical quality. Name-brand eyepieces, regardless of where they are made, also have excellent fit and finish. Low-cost eyepieces, referred to generically as "Chinese eyepieces," are of noticeably lower quality. Labels are painted on rather than engraved, tolerances are looser, lens edges are not blackened, internal baffling is poor or absent, and so on.
Optical quality is even more important than mechanical quality. The major reason for the price difference between cheap generic eyepieces and expensive name-brand models is the level of attention given to details such as lens polish and coatings. More expensive eyepieces generally have much better lens polish and coatings, which translates to sharper images, higher contrast, and less ghosting and flaring.
Edge performance in fast scopes
In fast focal ratio scopes, the light cone converges over a shorter distance than in slower scopes. This fast-converging cone of light means that individual light rays arrive from significantly different angles, which is difficult for an eyepiece to handle without showing visible aberrations. Those aberrations are minimal at the center of the field, but become increasingly apparent as you approach the edge of the field, where the incoming light rays must be bent more sharply by the eyepiece. Edge performance is particularly problematic for eyepieces that provide a wide apparent field of view.
In an f/10 or slower scope, for example, nearly any well-made, wide-field eyepiece provides a sharp, pleasing image across most or all of its field. In an f/5 or faster scope, only top-notch eyepieces of modern designsuch as Pentaxes, Naglers, Radians, and Panopticsare capable of providing a good image across a wide apparent field of view.
If you have a slow focal ratio scope, even older wide-field designs such as Erfles and Königs work reasonably well (although the modern premium eyepieces are still better). If you have a fast focal ratio scope, you basically have three choices: (1) pay the price for premium wide-field eyepieces; (2) limit yourself to Plössls and similar older designs, which have narrower apparent fields but are sharp to the edge even in fast scopes; or (3) buy inexpensive wide-field eyepieces and resign yourself to very poor edge performance.
Physical size and weight
Even within one barrel size, eyepieces differ dramatically in physical size and weight. Physical size is usually not an issue, but it may become important if you need to use an eyepiece on a small telescope with little physical clearance or if you plan to install two identical eyepieces in a binoviewer. For example, the body diameter of Pentax XL and XW eyepieces is so large that even if you adjust the binoviewer so that the eyepieces touch, most people's eyes are too close together to see through both eyepieces simultaneously.
Weight may be an issue with a Dobsonian scope, where changing from a very light eyepiece to a very heavy one may cause balance problems.
Mainstream eyepieces range in weight from a few ounces for a typical 1.25" eyepiece to 35 ounces for the 31mm Tele Vue Nagler. Various companies manufacture eyepiece weight rings that slide onto the barrel and lock with a set screw. These rings allow you to increase the weights of your lighter eyepieces to avoid this balance problem.
Different eyepiece designs come to focus at different points, which is to say that you may have to rack the focuser in or out a significant distance when changing eyepieces. What you want are eyepieces that you can simply swap in and out without having to refocus. Such eyepieces are referred to as being parfocalwith each other.
Most eyepiece series are parfocal within the series. For example, allTele Vue Radians are parfocal with each other, as well as with most Tele Vue Plössls, most Tele Vue Naglers, and some Tele Vue Panoptics. There are often exceptions at very short or very long focal lengths. By happenstance, an eyepiece from one manufacturer may be parfocal with an eyepiece from a different manufacturer. For example, our 30mm Orion Ultrascopic is parfocal with our Pentax eyepieces.
Just as you can adjust the weights of your eyepieces by adding weights, you can adjust them to be parfocal by adding parfocalizing rings, which are available from various manufacturers. To parfocalize your eyepieces, you first determine which of them requires the most in-focus. With that eyepiece focused sharply on a star, you lock your focuser. You then add a parfocalizing ring to each of your other eyepieces and physically slide them in and out in the focuser until they reach focus. At that point, you slide the parfocalizing ring down to the top of the focuser and lock the set screw. Other than the cost of the parfocalizing rings, the only drawback to this workaround is that some of your eyepieces may end up not having much of their barrels held by the focuser and so may be at risk of falling out.
Most eyepieces are threaded for filters on the field lens end. Some are also threaded for filters on the eye lens end. Filter threads are generally standardized for 1.25" and 2" eyepieces, but there are exceptions. Meade eyepieces and filters, for example, use a thread that is not quite standard. Meade filters appear to fit standard eyepieces, and vice versa, but they don't, really. It's quite easy to strip or crossthread a Meade filter in a standard eyepiece or a standard filter in a Meade eyepiece, which for us is reason enough to avoid Meade eyepieces and filters. There are other minor exceptions to thread standardization, but all of those are "system" designs, such as the Questar scopes and Brandon eyepieces.
4.7.2. Real-World Eyepieces
Obviously, we'd like the best of all worldsa wide apparent field of view, plenty of eye relief, an image that's sharp all the way to the edge of the field even in a fast scope, and top-notch mechanical and optical quality. Oh, yeah, and it shouldn't cost much, either. Unfortunately, no such eyepiece exists.
There is a class of eyepieces that comes close, though, on everything except price. These eyepiecesTele Vue Naglers, Panoptics, and Radians, and the Pentax XWs, along with the recently discontinued Pentax XLs, which are still widely available to purchase usedare what astronomers call premium eyepieces. They are world-class in every respect, including price. Many would also include the Celestron Axiom, Meade Super Wide, Meade Ultra Wide, and Vixen Lanthanum Super Wide eyepieces in this category, but we consider them a small step down from the other premium models. Table 4-1 lists the important characteristics of premium eyepieces.
Table 4-1. Important characteristics of premium eyepieces
Focal lengths (mm)
15, 19, 23, 34, 40, 50
Meade Super Wide Angle
16, 20, 24, 28, 34, 40
Meade Ultra Wide Angle
4.7, 6.7, 8.8, 14, 18, 24, 30
5.2, 7, 10.5, 14, 21, 28, 40
3.5, 5, 7, 10, 14, 20, 30, 40
Tele Vue Nagler
2.5, 3.5, 4.8, 5, 7, 9, 11, 12, 13, 16, 17, 20, 22, 26, 31
Tele Vue Panoptic
15, 19, 22, 24, 27, 35, 41
Tele Vue Radian
3, 4, 5, 6, 8, 10, 12, 14, 18
Vixen Lanthanum Super Wide
3.5, 5, 8, 13, 17, 22, 42
a 50mm Axiom has 52° AFoV.
b The original Meade Series 4000 Super Wide Angle and Series 4000 Ultra Wide Angle eyepieces have been replaced by the Series 5000 models. Meade specifies only "long eye relief" for these new models. The 5000-series eyepieces had not yet shipped when we wrote this, so we have been unable to determine eye relief figures for them.
c All Pentax XL eyepieces have a 65° AFoV, except the 28mm, which has only a 55° AFoV.
d Pentax XL eyepieces were discontinued in 2004, but remain widely available used. The 2" 40mm Pentax XL typically sells for $225 to $250 used, if in good condition. All other focal lengths are 1.25" models, and typically sell for $200 to $225 used.
eThe 42mm Vixen Lanthanum Super Wide has a 72° AFoV.
Although astronomers argue about which of the premium eyepieces are "best," the truth is that all of them provide superb images. Whatever their preferences, most would agree that the Pentax and Tele Vue lines are the best of the best, and those are the eyepiece lines we recommend unreservedly.
When choosing among them, we suggest you not worry too much about image quality. There are very minor differences, yes, but the similarities are much greater. Where these premium eyepieces differ is more a matter of emphasis. Naglers, for example, offer an extremely wide 82° apparent field, but have only 12mm of eye relief in most of the current models. Pentax XWs, on the other hand, offer a narrower (but still very wide) 70° apparent field, but have a generous 20mm of eye relief across the line. Radians have the same 20mm of eye relief as the Pentaxes, but only a 60° apparent field. On the other hand, Radians cost less than Pentax XWs. And so on.
There are variations within the lines, as well, which is why many astronomers have a mix of eyepieces. For example, some prefer Pentax eyepieces for high magnification, but use Panoptics or Naglers for medium and low magnification. It's not unusual for one astronomer to have a mix of Pentax, Nagler, Radian, and Panoptic eyepieces. It's more common, though, to have only one or two lines. For example, many astronomers use all Naglers, all Pentaxes, or Radians for medium and high magnifications and Panoptics for medium and low magnifications.
A step below the premium eyepiece lines are what we call "semi-premium" eyepieces. Like premium eyepieces, these semi-premium eyepieces have world-class fit and finish, and their optics have excellent polish and coatings. But they lack one or more of the important (and expensive) characteristics of premium eyepieces, such as very wide apparent fields, long eye relief, or suitability for use with fast focal ratio scopes. Table 4-2 lists the important characteristics of semi-premium eyepieces.
Table 4-2. Important characteristics of semi-premium eyepieces
Focal lengths (mm)
7.5, 10, 14, 14L, 18, 18L
Antares Elite Plössl
5, 7.5, 10, 15, 20, 25
5, 7.5, 10, 12.5, 18, 30, 35, 42
3.8, 5, 7.5, 10, 15, 20, 25, 30, 35
Tele Vue Plössl
8, 11, 15, 20, 25, 32, 40
University Optics König
12, 16, 24, 25, 32, 40
University Optics Orthoscopic
4, 5, 6, 7, 9, 12.5, 18, 25
2.5, 4, 5, 6, 7, 9, 10, 12, 15, 18, 20, 25, 30, 40, 50
a The 35mm Ultima has a 49° AFoV. The oddball 42mm Ultima has an AFoV of only 36° and eye relief of 32mm.
bThe 35mm Ultrascopic has a 49° AFoV.
cThe 40mm Tele Vue Plössl has only a 43° AFoV and, like all 1.25" 40mm Plössls, should be avoided. Tele Vue also offers a $228 2" 55mm Plössl, with a 50° AFoV and 20mm of eye relief. That eyepiece is suitable only for f/8 or slower scopes with 2" focusers, such as SCTs.
dThe 2.5, 4, 5, 6, 7, and 50mm Vixen Lanthanums have a 45° AFoV.The 42mm Vixen Lanthanum has a 42° AFoV. The 30mm Vixen Lanthanum has a60° AFoV.
The Antares Speers-WALER (wide-angle, long eye relief) eyepieces are a mixed bag. Speers-WALER eyepieces have a reputation for spotty manufacturing quality, and the fit and finish level is a bit lower than the other eyepieces in our semi-premium group. The 7.5mm and 10mm are reasonably good optically even in fast scopes, although not quite up to the optical standards of premium eyepieces. The 14mm is somewhat inferior optically to the 7.5mm and 10mm, particularly in fast scopes, and the 18mm more so. The 14mm and18mm aren't bad eyepieces by any means, but they don't have the sharpness, contrast, and edge performance we expect from a semipremium eyepiece. On the other hand, no other eyepieces in this price range come close to having the apparent field of view and eye relief of a Speers-WALER in a design that's usable down to f/5 or faster.
The 14mm and 18mm also require a great deal of in-focus, so much so that they will not come to focus in most Dobs (although they work fine in SCTs). For this reason, Antares introduced "L" models of the 14mm and 18mm SW eyepieces, which are designed for low-profile focusers and differ optically from the standard models. Unfortunately, the "L" models are optically inferior to the standard 14mm and 18mm models. If you're on a budget and want a 7.5mm or 10mm wide-field eyepiece, and if 12mm of eye relief is sufficient for you, the Antares Speers-WALER eyepieces are a good choice. We'd consider the 14mm and possibly the 18mm acceptable choices if we owned an SCT or a Dob with sufficient in-focus to handle them. We would avoid the "L" models entirely.
We consider the Antares Elite Plössl, Celestron Ultima, Orion Ultrascopic, and Tele Vue Plössl eyepieces essentially interchangeable. All have excellent fit and finish, top-quality optics and coatings, a similar range of focal lengths available, and similar apparent fields of view. The Antares Elite Plössl, Celestron Ultima, and Orion Ultrascopic eyepieces are in fact identical except for their labels and minor differences in focal length. They are Japanese-made, 5-element Masayuma designs, which add a fifth correcting element to the standard 4-element Plössl design. The Tele Vue Plössl is a standard Plössl design, superbly executed. All of these eyepieces offer sharp, flat fields and excellent edge performance in scopes as fast as f/4. They are less costly than premium eyepieces because they have only 50° apparent fields and very short eye relief in shorter focal lengths.
The University Optics Königs have the excellent fit and finish, polish, and coatings typical of most Japanese optics. The inexpensive modelsthe 12, 16, and 24mm 1.25" models and the 32mm 2" modeltypically have eye relief of about 2/3 to 3/4 their focal lengths, and 60° to 68° apparent fields of view. Their downfall is their short eye relief in shorter focal lengths and their hideous edge performance in fast scopes. If you have an f/8 or slower scope, such as an SCT, these eyepieces provide excellent value for money, but don't even think about using them in a fast Dob. The more expensive 2" models25mm, 32mm, and 40mmare much better corrected for fast focal ratios, although their edges are still soft in fast scopes.
If you wonder why we class a $296 eyepiece as only "semi-premium," it's because that eyepiece, the 32mm MK-80, competes head-on with the $620 31mm Tele Vue Nagler. Like the 31mm Nagler, the 32mm MK-80 provides an 82° apparent field and is of excellent mechanical and optical quality, but a $296 eyepiece simply can't match Al Nagler's "TermiNagler" in providing crisp images edge to edge in fast scopes. There ain't no such thing as a free lunch.
University Optics (UO) has been selling their Abbe Orthoscopic eyepieces for 45 years now, and they're as good as ever. Based on a 19th-century design, but using modern production methods and coatings, the UO Orthos provide absolutely top-notch optical performance and are the favorite of many observers for planetary work. The downside to Orthos is their relatively narrow fields of view, typically 40° to 45°, and their limited eye relief. The popularity of Orthos began to wane with the advent of Dobsonian reflectors and other undriven scopes, in which using a high-power eyepiece with a narrow apparent field makes it difficult to keep the object in the field of view. Still, for driven scopesor Dobs with very smooth motionsan Ortho is an excellent and economical choice for high-power observing. In the 4mm to 12.5mm range, the UO Orthos compare favorably to the Tele Vue Plössls, Celestron Ultimas, and Orion Ultrascopics. The Orthos cost a bit less, provide slightly greater eye relief, and (some believe) slightly sharper and higher-contrast images.
UO actually sells three lines of Orthos. Their original line, the University Abbe Orthoscopics, cost $60 each and are available in 4, 5, 6, 7, 9, 12.5, 18, and 25mm focal lengths. The University H.D. Abbe Orthoscopics sell for $80 and have several minor physical enhancements, along with slightly better coatings. They're available in 5, 6, 7, 9, 12.5, and 18mm focal lengths. Finally, the 9, 12.5, and 18mm Orthoscopic Planetary Series (O.P.S) eyepieces sell for $120 and have specialized coatings optimized for Lunar and planetary observation.
The well-respected Japanese optical company Vixen, now a subsidiary of Tele Vue, manufactures two lines of eyepieces. The Lanthanum Super Wides are true premium eyepieces. The standard Lanthanum series eyepieces retain the 20mm eye relief of the premium line, but have apparent fields of view of 45° to 60°, rather than the 65° to 72° of the premium series. The Vixen Lanthanum eyepieces were formerly sold by Orion and Celestron under their own names, but are now available only from Vixen. The Lanthanum eyepieces are first-rate optically and mechanically, with the fit and finish we've come to expect from Japanese eyepieces. We think of them as Plössls that just happen to have 20mm of eye relief regardless of their focal lengths. The Lanthanums are sharp, contrasty, and have excellent edge performance, but they also have a reputation for being a bit dimmer than premium eyepieces, Plössls, and orthoscopics. That may be true, but if so, we've never been able to detect it.
These are by no means all of the premium and semi-premium eyepieces available. Zeiss, for example, makes world-class eyepieces, as does Takahashi. TMB makes very specialized (and expensive) eyepieces designed for planetary observing. But you won't find any of those in most astronomers' eyepiece cases, and for good reason. The models we've listed offer the best combination of features, performance, and price available.
If you wonder why we haven't talked about economy eyepieces, it's because we don't recommend buying them. Economy eyepieces generally fall into one of the following three categories:
With very few exceptionssuch as the expensive TMB Monocentric eyepiecesa well-corrected eyepiece requires at least four lens elements. Older three-element designs, such as the Kellner (and its modern derivative, the Edmund RKE), the Modified Achromat (MA), and the Super Modified Achromat (SMA) are cheaper to produce than modern designs, but suffer from aberrations, short eye relief, narrow apparent fields of view, and other problems.
Older wide-field designs, including the König and Erfle, use from four to eight elements and are suitable for use in slow focal ratio scopes, but they generally provide very poor edge performance in faster scopes. (Owners of fast Dobs pronounce Erfle "awful.")
Ironically, the excellent Antares Elite Plössl, Celestron Ultima, and Orion Ultrascopic eyepieceswhich are marketed as "five-element Plössls" or "Super Plössls"aren't Plössls at all. They are actually Erfles that use Heinrich Erfle's original 2-1-2 lens element design. But, because they restrict their apparent fields of view to about 50° rather than the 60° to 70° of typical Erfle eyepieces, they are usable without significant aberrations in scopes down to f/5 or so.
Most inexpensive scopes are bundled with one or two Chinese Plössls, usually 25mm or 26mm and 9mm or 10mm. Those eyepieces are usable until you can get something better, but their polish and coatings are noticeably inferior to those of eyepieces that don't cost all that much more. For example, Chinese Plössls typically sell for $40 to $70, depending on focal length, while the much superior Antares Elite, Celestron Ultima, Orion Ultrascopic, and Tele Vue Plössls sell for $75 to $110. If you're on an extremely tight budget, Chinese Plössls offer a lot of bang for the buck. But if paying an extra $20 to $40 for an eyepiece is no big deal to you, you'll be much happier in the long run with the better-quality eyepieces.
Chinese optical factories have begun making knock-offs of some premium and semi-premium Japanese eyepiece lines. For example, when Orion introduced their Chinese Epic ED-2 line of eyepieces, a lot of people got excited.These $70 eyepieces promised 20mm of eye relief and 55° apparent fields of view. There was a lot of debate about whether the Epic ED-2 eyepieces were knock-offs of the $130 50° Vixen Lanthanums or the (then) $228 60° Tele Vue Radians. Everyone wants a bargain.
Then, as Epic ED-2 eyepieces made it into astronomers' hands, reality set in. The Epic ED-2 eyepieces weren't "Radian killers" nor even "Lanthanum killers." They were $70 Chinese eyepieces with mediocre fit and finish and optical performance well below that of more expensive eyepieces. They suffer from ghosting and flaring, and edge performance in fast scopes is poor. Unless you place your eye just so, they also suffer from blackouts, as the image simply disappears. Although some astronomers are happy with them for the price, no one mistakes them for a Radian or Lanthanum.
The moral is, when it comes to eyepieces, TANSTAAFL, There Ain't No Such Thing As A Free Lunch. If cheap eyepieces performed like expensive eyepieces, no one would buy the expensive eyepieces. Duh.
4.7.3. Choosing an Eyepiece Set
When it comes to choosing eyepieces, most beginning amateur astronomers follow a predictable path. They start with the eyepieces supplied with their scopes and soon decide they need more eyepieces. A full set of focal lengths! Wider fields! They buy a lot of cheap eyepieces, hoping against hope to find what they want without paying high prices for premium eyepieces. Then one night they make the mistake of looking through someone else's premium eyepiece. The scales fall from their eyes, and they realize just how much better premium eyepieces really are. So they buy their first premium eyepiece, and they're hooked. But they find themselves with one premium eyepiece and a case full of cheap eyepieces that are pretty much worthless.
We suggest you skip the first part and go straight to the second. Make your first eyepiece purchase a premium model. Yes, if you're on a budget, the thought of buying premium eyepieces is pretty intimidating. After all, a full set of premium eyepieces may cost $3,000 or more. But here's the secret: you don't need a full set.
Most experienced amateur astronomers use at most three or four eyepieces regularly, and if you have a good Barlow or Powermate, you can get by with fewer. For example, Figure 4-12 shows the set we use for 99% of our observing in our 10" f/5 Dob:
- 30mm Orion Ultrascopic ($120; 42X magnification; 1.25° true field)
- 14mm Pentax XL ($225; 90X magnification; 0.73° true field)
- 10mm Pentax XW ($310; 125X magnification; 0.56° true field)
- 14mm Pentax XL with 2X Barlow (180X magnification; 0.36° true field)
- 10mm Pentax XW with 2X Barlow (250X magnification; 0.28° true field)
Figure 4-12. Our standard eyepiece set: 30mm Orion Ultrascopic, 14mm Pentax XL, 10mm Pentax XW
Here's what our observing buddy Paul Jones uses regularly in his 8" f/10 SCT:
- 27mm Tele Vue Panoptic ($330; 75X magnification; 0.90° true field)
- 14mm Pentax XL ($225; 145X magnification; 0.45° true field)
- 7mm Tele Vue Nagler ($180; 290X magnification; 0.28° true field)
Here's what our observing buddy Steve Childers uses regularly in his 17.5" f/5 Dob:
- 40mm Pentax XL ($245; 56X magnification; 1.17° true field
- 27mm Tele Vue Panoptic ($330; 82X magnification; 0.83° true field)
- 14mm Pentax XL ($225; 159X magnification; 0.41° true field)
- 10.5mm Pentax XL ($225; 212X magnification; 0.31° true field)
- 8mm Tele Vue Radian ($240; 278X magnification; 0.22° true field)
With the exception of the 40mm Pentax XL and the 27mm Tele Vue Panoptic, all of these eyepieces use 1.25" barrels, and so they can also be used in our various other scopes that have 1.25" focusers.
So, assuming you're starting with only the bundled eyepieces supplied with your scope, how do you begin building an optimum eyepiece collection? We suggest you use the following guidelines.
If budget is not an obstacle, simply buy the eyepieces you need. If budget is an issue, your first acquisition should be a top-notch 2X Barlow [Hack #46]. A Barlow effectively doubles your eyepiece collection at minimal cost.
If your budget permits, buy only premium eyepieces. If you are on a tight budget, buy semi-premium models. You're better off with only two or three premium or semi-premium eyepieces than with an entire case full of cheap eyepieces.
Everyone needs a low-power eyepiece [Hack #48], but, if you're going to make compromises, this is the place to do it. When you're getting started, the low-power eyepiece bundled with your scope will suffice until you've filled out your collection with higher priority eyepieces. If you have no low-power eyepiece, fill this slot with a $50 Chinese 32mm Plössl for the time being.
Your first premium eyepiece should be a moderate-power eyepiece that can be Barlowed to provide a high-power eyepiece. This will be your work-horse eyepiece. For use with a 2X Barlow, choose an eyepiece that provides a magnification roughly half the aperture of your scope in millimeters, but no more than 135X to 150X. For a 6" (150mm) scope, we recommend an eyepiece that provides 60X to 90X; for an 8" (200mm) or larger scope, 90X to 125X. If you plan to make do with just one premium eyepiece and a 2X Barlow for a while, choose an eyepiece that provides a magnification in the upper part of the range, e.g., 90X for a 6" scope or 125X for an 8" or larger scope.
For scopes under 8", aperture determines maximum useful magnification, which is roughly 1.5 times the scope aperture in millimeters. For scopes 8" and larger, atmospheric instability determines maximum useful magnification, which is typically 250X to 300X on steady nights in most locations. Even if you have a large scope, there is little point to having an eyepiece that provides more than 250X to 300X natively. On those rare nights when seeing is good enough to use magnifications higher than 300X, you can simply Barlow one of your standard eyepieces to achieve higher magnification.
For your second premium eyepiece, buy one on the other end of the ranges just given. For example, if you have an 8" or larger scope and bought a 125X eyepiece as your first premium eyepiece, the second one should yield a native magnification on the other end of that range, 90X in this case. With the 2X Barlow, that gives you a very useful range of magnifications90X, 125X, 180X, and 250X.
But not everyone can afford even one premium eyepiece, and there are even some people (most with driven scopes) who actually prefer the 50° apparent fields of Plössls and similar eyepieces. If you're in that situation, here's what we recommend:
Buy a top-notch 2X Barlow, such as the Orion Ultrascopic, the Tele Vue, or the Celestron Ultima.
Use your current low-power eyepiece as your finder eyepiece. If you don't have a low-power eyepiece, buy one [Hack #48].
If you must wear eyeglasses while observing, or if long eye relief is otherwise important to you, buy Vixen Lanthanum eyepieces. If you're not concerned about eye relief, buy the less expensive Tele Vue Plössls, Antares Elites, Celestron Ultimas, or Orion Ultrascopics.
If you wear eyeglasses only to correct for near-or farsightedness, you needn't wear them while observing. Simply use the telescope focuser to accommodate your own vision. If your glasses correct for astigmatism, you'll need them when using an eyepiece that provides a large exit pupil, such as a 32mm finder eyepiece. However, many people find that their astigmatism is limited to the outer portion of their pupil. When you observe at moderate to high powers, which is to say using a 3mm or smaller exit pupil, you are using only the center of your pupil and may not need your glasses.
We don't wear glasses to observe, but we've found, as have many others, that we much prefer the 20mm eye relief provided by the Lanthanums and many premium eyepieces. It's simply more comfortable to observe with an eyepiece that provides plenty of eye relief, and if you're comfortable, you tend to spend more time looking at the objects and teasing out finer detail.
There's a good chance that the focal length you might otherwise choose for your moderate-power eyepiece is already provided by your low-power eyepiece when you use it with your Barlow. If so, choose a focal length somewhat shorter than the Barlowed combination. For example, if your low-power eyepiece is 25mm, that gives you 12.5mm Barlowed. Choose an eyepiece in the 10mm range, which also gives you the equivalent of a 5mm eyepiece when Barlowed, providing both a moderate magnification for general viewing and a higher magnification for planetary and Lunar observing.
Although many inexpensive scopes are bundled with a 9mm or 10mm Plössl, we suggest you not consider it when planning your eyepiece purchases. This eyepiece is generally of lower quality than the bundled 25mm or 26mm eyepiece, andit should be a priority to replace it as soon as possible.
Fill in your range by adding one or two more eyepieces with focal lengths spaced appropriately to give a useful range of magnifications, with and without the Barlow, up to the maximum useful magnification of your scope. For example, for a scope with 1,200mm focal lengtha typical 6", 8", or 10" Dobwe might add a 15mm 50° eyepiece, to give us the following options:
- 25mm (48X; 1° true field)
- 15mm (80X; 0.63° true field)
- 25mm with 2X Barlow (96X; 0.52° true field)
- 10mm (120X; 0.42° true field)
- 15mm with 2X Barlow (160X; 0.31° true field)
- 10mm with 2X Barlow (240X; 0.21° true field)
If we wanted to add another eyepiece to this set, we'd probably choose a 12mm model. That makes the 25mm with Barlow superfluous, but when Barlowed, it also adds 200X to your repertoire. 160X is a bit too little for planetary observing, and 240X may be a bit too much on some nights. Having a 200X option puts you right in the middle on nights when the atmosphere is just a bit too unstable to support 240X.
Finally, some people want to spend as little as possible on an eyepiece collection, either because they simply can't afford good eyepieces or because they're new to the hobby and aren't sure they're going to maintain their initial interest. If you're in that group, we recommend simply adding a good 2X Barlow and using the eyepieces supplied with your scope until you can afford something better or decide that you're hooked on the hobby. With the Barlow, you have a decent selection of magnifications and fields of view that will tide you over until you're ready to buy some eyepieces.