Eyepieces for planetary observing: Jupiter and Saturn are easy to observe now--but what eyepiece type will give the best views?
Just as some telescope designs provide discernibly superior visual views of solar system targets, so do certain eyepiece types. The planets all subtend very small apparent angular sizes, so devoted planetary observers don't place a premium on wide apparent fields of view (or even 2-inch-format eyepieces) that require a multitude of lens elements, air-glass surfaces, or complex edge correction.
The Virtues of Simplicity
Instead, contrast and definition are the qualities of paramount importance to the fastidious planetary observer. Discerning all of the planetary details that a telescope's optics and the state of the atmosphere allow demands eyepieces that have high light transmission and freedom from ghost images, internal reflections, and scattered light.
Ghost images are caused by double reflections from air-glass surfaces that come to focus at or near the eye's focal plane. The more of these interfaces within an eyepiece, the greater the chance that ghost images will arise. Modern anti-reflection coatings dramatically reduce such spurious reflections, but eliminating scattered light requires optical elements with well-polished surfaces that are free of sleeks and scratches, blackened lens edges, a finely threaded and effectively blackened interior of the eyepiece barrel, and a sharp, well-defined field stop.
Conventional wisdom has long held that the best planetary eyepieces are those with the smallest number of lens elements and air-glass surfaces that can still provide a well-defined image in the center of the field of view. Three optical configurations that satisfy this "minimum glass" paradigm have emerged.
Monocentric: Introduced by Hugo Adolf Steinheil in 1883, the monocentric design consists of a cemented triplet lens with spherical surfaces that share a common center and radius of curvature. With only two air-glass surfaces, monocentrics provide images of unsurpassed brightness and contrast. But they have a very narrow apparent field of view, only 25[degrees] to 30[degrees], that many observers have compared to looking through a drinking straw. In 1911 Charles Hastings patented a refinement of the design that Carl Zeiss produced until the 1950s. The firm TMB briefly revived this design a decade ago, but today monocentrics are only available on the secondhand market.
Orthoscopic: Designed by the brilliant German mathematician and physicist Ernst Abbe, the orthoscopic (from the Greek roots for "straight seeing") was the first eyepiece to offer virtually complete correction of optical aberrations and distortion. Introduced in 1880, it consists of a cemented triplet field lens paired with a single-element biconvex or plano-convex eye lens. Widely available even today, orthoscopic eyepieces offer excellent sharpness, color correction, and contrast combined with a 40[degrees] to 45[degrees] apparent field of view.
Plossl: This brainchild of Viennese optician Georg Simon Plossl originally consisted of a pair of identical cemented doublets. It was also known as the symmetrical eyepiece. Twentieth-century refinements by Rudolf Konig and Chester Brandon of the original 1860 design place the interior surfaces almost in contact to minimize ghost reflections and employ an eye lens of shorter focal length than the field lens. Providing a 45[degrees] to 50[degrees] apparent field of view, the best Plossls rival the performance of orthoscopies. But the quality of today's commercial offerings varies widely.
Importance of Eye Relief
During a typical planetary observing session, you'll maintain a prolonged vigil at the eyepiece, waiting patiently for those fleeting moments when the atmosphere steadies momentarily to provide what Percival Lowell called "revelation peeps." Comfort is of paramount importance if you want to maintain visual acuity for long periods.
To fully exploit the resolving power of any telescope when viewing a low-contrast target, you'll need to use a magnification of at least 25* per inch of aperture (or lx per millimeter)--and, when the seeing permits, you can double that value profitably (S&T: March 2015, p. 54). A classic, long-tube achromatic refractor with a focal ratio off/15 achieves this range of magnification with eyepiece focal lengths of 15 to 7.5 mm. The popular f/10 Schmidt-Cassegrain requires focal lengths of 10 to 5 mm, while the increasingly common "fast" f/5 Newtonian reflector needs 5 to 2.5 mm.
The problem is that such short focal lengths tend to offer poor eye relief, the distance from the outer surface of the eyepiece's eye lens within which you can view the full viewing angle. Although monocentric, orthoscopic, and Plossl eyepieces all provide decent eye relief, about 70% to 80% of their focal length, the short focal lengths required for planetary observing involve squinting through tiny eye lenses located a small fraction of an inch from your eye. Not only does this present an insurmountable obstacle to eyeglass wearers, but the surface of the eye lens is also prone to smearing with eyelash oils.
The time-honored solution to this ergonomie difficulty is to combine an eyepiece of moderately long focal length (and thus comfortable eye relief and an eye lens of reasonable diameter) with a Barlow lens or other image amplifier that doubles or triples the magnification. Most observers find that any loss of image quality resulting from the Barlow's two additional air-glass surfaces is almost imperceptible--and more than offset by the combination's ease of use and comfort.
In fact, many of the best eyepiece designs offered today integrate a Barlow lens to provide a remarkably generous eye relief of 20 mm even in eyepiece focal lengths as short as 3 mm, combined with well-corrected apparent fields of 50[degrees] to 60[degrees]. At one point, TMB offered six-element eyepieces of this form that the designer, the late Thomas M. Back, claimed were the equal of the firm's "gold standard" monocentrics in sharpness, contrast, and lack of scattered light while overcoming their poor eye relief and narrow field of view. If you use a Dobsonian reflector or some other undriven telescope, the wider field is a welcome bonus because the target doesn't drift out of the field as quickly.
Frankly, much of the conventional wisdom about what constitutes virtue in the design of a planetary eyepiece has ceased to be true. Maximizing light transmission while minimizing internal reflections and contrast-robbing scattered light remain essential goals, of course. But achieving them no longer limits optical designers to a small number of lens elements and uncomfortably tight eye relief.
Modern high-index glasses and efficient, broadband, multilayer antireflection coatings allow many eyepieces with as many as 10 air-glass surfaces to rival the performance of traditional eyepiece designs in critical side-by-side comparisons on the most challenging planetary details.
The "minimum glass" paradigm for planetary eyepieces still has its vocal adherents, just as there are ardent audiophiles who disparage digital components in favor of old analog technology. But there's been a marked shift toward complex, high-quality eyepieces --many of which, happily, don't command substantially higher prices than those of older, simpler designs.
Contributing Editor TOM DOBBINS has forsaken his old orthoscopic eyepieces in favor of modern, short-focus offerings by Pentax and Baader.
Caption: A steady hand and steady sky--along with an 18-inch reflector and a binocular viewer with high-quality 4.5-mm eyepieces--all came together for this sketched rendering of Jupiter on March 27, 2017.
Caption: Eyepiece designs with fewer lenses tend to deliver the most light to your eye, but those with complex optical combinations often provide more expansive views and better eye relief.
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|Title Annotation:||OBSERVING: Exploring the Solar System|
|Author:||Dobbins, Thomas A.|
|Publication:||Sky & Telescope|
|Date:||Jul 1, 2017|
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