Wide Field Eyepieces
Update: Since this article was first written, many of the eyepieces referred-to have been superceded by newer versions. However, these eyepieces often are on sale in the used marketplace.
After using wide field eyepieces, you can appreciate how much their long eye relief and large lenses add to the pleasure of observing. Low-power versions allow observers to take in a large swath of the sky when searching for faint objects or looking at a large object that doesn't fit within the confines of a "normal" eyepiece. Short focal length versions are popular too because they allow un-driven telescopes to keep a planet in the field of view longer while examining it at high magnification.
Wide field eyepieces ("oculars") also work well in eyepiece projection photography with today's video and digital cameras. They help minimize vignetting - a fall-off in brightness at the edge of the image. Plus they allow you to capture larger objects that lesser eyepieces would cut off. Just make sure to take advantage of the wide field by getting the camera lens at the optimum position with respect to the eye lens of the ocular.
Bear in mind that in order to have a wide true field of view in a focal length of 32mm and above, the eyepiece should ideally come in a 2" barrel, not the 11/4" format. That's because the narrower eyepiece barrel will itself restrict the field of view. The laws of physics prevail!
But to get an apparent FOV (field of view) of 60o or more you pay the price of either poorer image quality or higher cost. Most wide-field eyepieces suffer from some level of field curvature and/or distortion, which are two different types of aberration. (We often lump these together as an edge-of-field "softness".) Field curvature means that you have to refocus the eyepiece in order to bring either the center or the edge of the field into focus - if it's sharp at the center of the field, the edges are fuzzy, and vice versa. The word "distortion" means that while the whole image may be sharp, the magnification varies over the field. This results in two types of distortion: pincushion and barrel. Using the example of two parallel lines:
| | - no distortion,
) ( - pincushion distortion,
( ) - barrel distortion.
Distortion is usually more noticeable in terrestrial viewing than in astronomical use, because we don't encounter parallel lines in the sky. But it does become noticeable astronomically while panning across the sky or while watching a planet drift across the field of view. In the latter case, a circular planet might appear slightly oval when close to the edge of the field.
Up until about fifty years ago, astronomical eyepieces had pretty narrow fields of view by today's standards. And the simpler designs, such as the Kellner, performed reasonably well because back then most telescopes had relatively long focal ratios in the neighborhood of f/8 to f/10. The Erfle design is one of the oldest wide-field oculars, widely used in bombsights during the Second World War. Erfles often showed up during the 1950's as "war surplus" items. It generally had an apparent FOV of around 60o, which was considered huge in its day. Later they were produced specifically for the astronomy community, many with the addition of a fifth or sixth lens element that served as a field flattener. This was necessary because the original four-element design could not compensate for aberrations introduced in the wide field, especially when used in telescopes of more recent vintage with shorter focal ratios. (The Konig design is a variation of the Erfle.) In fast scopes their performance still leaves much to be desired. I used a 32mm Erfle for many years, and even though it had a field flattener lens, the combination of distortion and field curvature was enough to give you vertigo when panning across the sky!
Some wide field eyepieces are now available at quite reasonable prices, but to various degrees they suffer from image softness. Still, many observers have been pleased with these eyepieces, depending on their tolerance for the aberrations, which show up much more prominently in a fast scope (around f/6 or below).
Expanding the FOV requires some slight of hand by the eyepiece designer. In order to correct field curvature, they're sometimes forced to introduce a bit of distortion, as in the case of Tele Vue's Panoptic. The trick is to keep this distortion as small as possible. The Panoptic does this quite well, and is highly regarded as an excellent wide-field (65o) ocular. The comparable Pentax XW provides a 70o field. Some observers comment that the Pentax eyepieces are a little softer at the edges, but have better contrast. Reflecting personal tastes, some users prefer the Tele Vue while others prefer the Pentax.
Totally eliminating distortion while providing an ultra-wide FOV demands many lens elements and a complex design. This is the case with the Nagler, which has a field of about 82o. The largest version of this eyepiece (31mm focal length) is about the size of a can of Coke! Plus its weight may cause balance problems on some scopes. But the view through one of these is nothing short of stunning, with virtually zero aberration. Initial reviews of the newest version of the Meade Ultra Wide Angle eyepiece indicate that Tele Vue still rules.
There are other tradeoffs for a wider FOV. All those lens elements result in some light scatter. This is seen as a background that isn't quite as dark as in the simpler designs. You'll notice this especially if you switch between one of the wide-field eyepieces and a good Orthoscopic or Plossl. It's also logical to expect that the multiple elements diminish the light throughput; however, the newest fully multi-coated lenses greatly minimize light absorption. In fact, I read that the average human eye is incapable of detecting any light loss between a simple ocular such as a three-element Kellner and a new eight-element design. Then there's the matter of internal reflections, which may show up as a faint ghost image of bright objects. But the better wide-field eyepieces eliminate this.
Image sharpness is another item that's negatively impacted by the number of lenses. Thus you might find that the images are very slightly sharper in a good quality Orthoscopic (four-element) eyepiece. Of course, the Ortho isn't nearly as comfortable to use. From the reviews I've read, it seems that only the most astute observers are able to detect any difference, however.
One problem that has plagued wide field oculars is the "kidney bean" effect. Depending on where you place your eye, you'll see a black spot floating in the field. These oculars tend to have long eye relief, meaning you have to position your eye farther back from them. When you try to move your eye closer to the eyepiece in order to see the edge of the field, parts of the field between the center and the edge are cut off, because part of the converging light beam misses the eye's pupil. In the mid-80's I tried a first generation Nagler on my Newtonian and absolutely hated it. The wide field was nice but I couldn't avoid that infernal dark blob. The problem has been solved in many of the newer oculars by design refinements and by incorporating an adjustable eyecup that helps position your eye at the optimal point.
Another factor is the exit pupil of an eyepiece. Wide field eyepieces with long focal lengths give expansive low magnification views, which typically yield a larger exit pupil. This is the virtual image of the telescope's objective that is formed by the eyepiece - in simplest terms, the diameter of the light cone that strikes your eye at the point of best focus. There are a couple of ways to determine the size of the exit pupil: - divide the diameter of the objective (mm) by the magnification yielded with a certain eyepiece - divide the eyepiece focal length by the telescope's focal ratio. As we age, the pupils in our eyes become less elastic and don't dilate as much in the dark. If your pupils expand to only 5mm and the eyepiece presents an exit pupil of 7mm, not only is light output being wasted, but you may see distortion that manifests itself as somewhat bloated star images. In addition, you have to strain to keep the exit pupil and your eye aligned; otherwise you may see the kidney bean effect, with odd shadows moving around the periphery of your vision.
Despite what I had always believed, I read that the kidney bean effect is not the same as what is often referred to as a "black spot" or "blackout". In a reflector, a low powered eyepiece with a large exit pupil produces a large image of the secondary mirror obstruction. When the pupil of the eye is small during daylight and the size of the secondary obstruction image approaches the size of the pupil, it will appear as a darkened region in the center of the field. At night the darkened region is not as noticeable, provided the pupil of your eye is able to dilate large enough.
Finally, there's the dollar cost of the complex eyepiece designs. It's not unusual to have as much money invested in eyepieces as you have in the telescope itself. A wide field eyepiece that adequately controls all the various aberrations is going to cost over $200. This cost escalates as the focal length increases. The 31mm Nagler Type 5 is a prime example; it lists for an astronomical $620! Although lower priced eyepieces don't correct the aberrations quite as well, they're still good quality eyepieces. Many observers are happy with the University Optics MK-70, Celestron Axiom, Orion Optilux, Vixen Lanthanum Superwide, etc. The price difference between a well-corrected eyepiece and one that is exceptionally well corrected is pretty steep, and for many people, the extra cost simply isn't worthwhile. Be aware, however, that you can't go much below about $100 and expect to get a wide field eyepiece that will have a tolerable level of aberration in all telescopes, unless you're willing to restrict your observing to the very center of the field. But then why use an eyepiece with a wide FOV?
So first determine how much edge-of-field softness you find acceptable. If possible, try different wide field eyepieces in your own scope. If you have a short focal ratio telescope then the differences will be more apparent. However, less expensive wide-field designs work just fine on an SCT or others with a longer focal length. You will love a wide field view, but there's no sense spending more money than you have to.