Best Planetary Eyepieces

Jack Kramer

The planets and moon are bright and detailed; if the atmosphere is steady, they can take a lot of magnification. One of them is often in view, so they're good targets for the general observer and for those who have adopted the Solar System as their favorite destination. If you happen to ask dedicated planetary observers which eyepiece they prefer to use, they'll all agree that they need the highest possible contrast...but that may be the only point of agreement.

First of all, any top-notch eyepiece needs good optics - highly corrected lenses with smooth finish and the very best coatings. Add to that blackened lens edges and well-baffled barrels to reduce internal reflections. Some manufacturers (such as Brandon, Zeiss and Clavé) employed run-of-the-mill designs, but made a name for themselves purely because of the high quality of their optics. As an example of the value attached to such eyepieces, in 2002 the asking price for some used Zeiss Abbe orthoscopic eyepieces was $800 each!

The second consideration is the optical design. Here is where many observers have a distinct preference, despite the fact that each design presents a compromise of some sort. So let's take a look at a few of the different eyepiece designs and try to sort out which ones are especially well suited for "shallow sky" observing.

Kellner

Veteran planetary observers often say that the best high power eyepiece is the one having the fewest number of optical elements. This is because every additional lens in the system slightly diminishes the light throughput. So a three-element design like a Kellner will pass more light than a seven element Nagler, assuming they both use comparable high-transmission coatings. However, there are a few problems with the Kellner (and other simple designs):

  • Narrow field of view (~ 40o )
  • Edge-of-field distortion
  • Ghost images
  • Short eye relief

But planetary observers do not need wide fields of view, and since the image is centered in the eyepiece, edge distortion is ignored. In 1975, Edmund Scientific came out with the RKE eyepiece, which is a modified Kellner. (RKE stands for "Rank Kellner Eyepiece", named after its inventor David Rank.) By using a computer-aided design for the lenses, low dispersion glass, and reversing the order of the lenses, the severity of the Kellner problems was lessened (though not eliminated). The drawback of the RKE is that the shortest focal length available is 8mm, so to get really high magnification in some telescopes you'd have to use a Barlow. Adding those extra Barlow lenses to the system will slightly compromise some advantages of the RKE design. Finally, in most side-by-side comparisons, the RKE eyepieces fare poorly against other designs.

Plossl and Orthoscopic

Two designs well suited for planetary work are the Plossl and Orthoscopic. Both have freedom from ghosting, plus fields that are a bit wider - typically 50o and 45o, respectively. In the case of the Orthoscopic, there is the added advantage of a virtually distortion-free field of view, edge-to-edge. Both of these designs use four lenses. It was these designs that were chosen by the premium manufacturers Brandon, Zeiss and Clavé. The Brandon Plossls are still available, but are expensive; several users have commented that while they treasure their Brandons for image quality, they question whether they're worth the cost in comparison to some of today's newer designs. Celestron's "Ultima" and Orion's "Ultrascopic" are basically Plossls to which a fifth element has been added to better control edge-of-field distortion. The University Optics Orthoscopics (manufactured in Japan by Kokusai Kohki) are highly regarded for their image sharpness and generally rated only slightly inferior to premium-priced Orthoscopics.

Monocentric

A design that we don't hear very much about is the monocentric. Many veteran observers consider it the ultimate lunar-planetary eyepiece. These are a variety of designs that consist of three elements cemented together into a single group. They're called monocentric because the radius of curvature of each surface is roughly in the same place. The advantage is that with only two air-to-glass surfaces, the number of internal reflections is kept to a bare minimum. This in turn reduces glare and ghost images on bright objects such as the moon and planets, and it allows low-contrast detail to be well seen. Another advantage of the two air/glass surfaces is very high light transmission, especially if good coatings are used.

Among the cemented 3-element eyepieces are the Hastings triplet, Cooke triplet, Steinheil triplet and Zeiss monocentric. None of these are currently available, except in the used market. The Zeiss monocentric was an improvement of the Hastings design and was made from sometime before WWII until the 1960s. But it had an apparent field of view of only 25 to 30 degrees. Telescope maker Thomas Back has resurrected an updated variation of the monocentric eyepiece using modern optical glasses and coatings. But it does suffer from a 30o AFOV and very short eye relief. The price range is around $200 apiece. Astro-Physics is also now in the three-element short-focus eyepiece game with a more ergonomically comfortable version.

Wide Field Designs

Beyond the favored three and four-element designs, we get into the more complex eyepieces with wider fields of view and six to eight lens elements. Some examples are the Vixen Lanthanum, Takahashi LE, Pentax XL, Meade Ultra Wide Angle, and Tele Vue's Nagler and Radian. Most of these also use exotic glasses to help enhance the images. Despite the fact that some serious planetary observers shun these eyepieces, many others find they do an excellent job. What they have going for them are wide fields of view and long eye relief, which makes them more "comfortable" to use. I've always felt that the more comfort you have while observing, the longer you will observe an object and the more you will see. The eyepiece is part of that equation. I had some of the brightest and sharpest views of the moon and planets with my 5mm Orthoscopic, but found it annoying that with the short eye relief my eyelashes constantly brushed against the eyepiece faceplate and that I had to really work at keeping my eye centered over that tiny eye lens.

One factor that has given a boost to wide field eyepieces is the use of Dobsonian telescopes without motor-driven mountings. If you have to nudge your telescope along to follow the apparent motion of the moon and planets, then a wide FOV lets you look a bit longer before having to disrupt your concentration with the old "armstrong" drive.

What the complex designs have going against them is scattered light and lower light throughput due to the greater number of lenses. Another rap sometimes heard is that their images are not quite as sharp because of the contrast-robbing aspect of that extra glass.

Having never made a side-by-side comparison of the different designs, my impressions are somewhat anecdotal. I have a couple of Radians, which to my eyes provide very sharp images and are comfortable to use. Images as sharp as in my old Orthoscopic? They seem pretty close. From many reports, the Radians provide slightly sharper views with less scattered light than the wider field Naglers.

Zoom Eyepieces

Several new short focal length zoom eyepieces have come on the market. Reports on these say that the fields of view are not as large as comparable single focal length eyepieces, eye relief is shorter, and slight refocusing is sometimes necessary when changing magnification. The Nagler zoom, however, does not require any refocusing and provides better images than other zooms. Many users really like it. Another criticism leveled at zoom eyepieces, including the Nagler version, is that the image sharpness is never quite comparable to a single focal length eyepiece and in the shorter focal length ranges, the images lose a lot of brightness.

Conclusions

Some reviewers comment that due to the lens coatings, certain eyepieces, such as the Pentax, impart a slightly "warmer" color tone to objects. I also have a long focal length Pentax XL, but can't say I've noticed any so-called warmer tone. Then I've read other reviews that say the Tele Vue or Vixen eyepieces have a warmer tone. So some impressions may be rather subjective, perhaps colored by preconceptions or the fact that eyes differ from person to person and we really do see things differently. Certain eyepieces may also give different results depending on the telescope in which they're used. Apart from obvious factors such as field of view, ease of use, and edge distortion, many differences are quite subtle. Since some reviews in the astronomy press and on the Internet contradict one another, this suggests that opinions indeed can be affected by a number of factors.

Even within the same design, some focal lengths will perform better than other focal lengths. This has been confirmed by a number of observers. One of my eyepieces is a 6.7mm Meade Ultra Wide Angle, which is a decent eyepiece except for an excessive amount of scattered light. Others have made the identical comment about this eyepiece, yet the same design in the 8.8mm focal length doesn't have nearly as severe a problem. One reason for the difference isattributable to the fact that within certain designs, for example Tele Vue's Radian, the layout of the lenses varies somewhat with different focal lengths. Also, Meade reportedly uses various different manufacturers to make its eyepieces, and this could introduce further variations.

Looking at low contrast planetary features under high magnification is pushing both the telescope and eyepiece to the ultimate. Dedicated planetary observers have different requirements than many of us. Their concern is how an eyepiece performs on-axis, meaning what the image looks like in the center of the field. They don't need a wide FOV because their scopes are invariably on motor-driven mounts that keep the objects centered. And they usually have telescopes with longer focal ratios, so eyepieces don't have to contend with coma.

So which is the best planetary eyepiece in terms of image quality? Maybe the simpler designs are the best. This would be a monocentric, or an Orthoscopic if you're on a budget. But the Radian, Pentax XL, and Takahashi LE are also highly regarded, plus they provide a comfortable viewing experience. It's largely a matter of personal preference.

Update on Last Month Eye Piece Article

When I wrote the article in the September NightTimes on the best eyepieces for planetary observation, the monocentric eyepieces by TMB were too new on the scene to get any meaningful feedback. Since then, I've read a couple of comments on the Internet from those who have had a chance to put them to the test. The reviews were all favorable, noting that the contrast and definition visible on the moon and planets noticeably surpasses any other eyepieces, even the premium ones.

The monocentrics are also quite small and lightweight. Each reviewer mentioned that while the narrow field of view is not much of a factor for this kind of observing, the very short eye relief may be a problem for many people. The cost is about $200 and the TMB monocentrics are available from Astronomics and a few other suppliers.