Choosing Eyepieces - Part 1
The following refers to Alan Dyer's feature on eyepieces that appeared in the June, 1993, issue of Astronomy Magazine. Rick Blakley was asked for comments on the Dyer article with greater depth for the more serious amateur astronomers. It resulted in this article, published in a past issue of SACNEWS, and it comes to us courtesy of Lenny Abbey of the Atlanta Astronomy Club. Rick Blakley is a mathematics and optical specialist, and contributing editor to ATM Journal. The entire article is quite lengthy, so we'll publish it in two parts, with the final part in a future issue of NightTimes.
Paul Dickson has asked me to look at Mr. Dyer's article and offer comment so as to better advise the committed amateur. While part of this review will cover myths and misconceptions, I am not intending to make a criticism of Mr. Dyer's text, and I must say in his defense that he has not claimed to be an expert on the subject. His knowledge comes from his active participation as an amateur with amateurs, and I thank Mr. Dyer for his efforts.
Paragraphs from Mr. Dyer's article will be shown in boxes. My comments will follow each of his paragraphs. Let us begin.
Happiness is a sparkling new eyepiece. Or better yet, a matching set of them. Eyepieces are the most important accessories you'll ever buy for your telescope. Although it is the main lens or mirror of your telescope which forms the image, it is the eyepiece which magnifies the image. A good eyepiece does so without adding to or subtracting from the image. With a set of fine eyepieces you'll be able to resolve all the celestial wonders your telescope is capable of revealing.
Actually, both the objective (the "main" lens or mirror) and the eyepiece magnify. Magnification for any optical focusing element is a function of the distance of the object from the element and the element's focal length. The objective of a telescope, the eyepiece, and the eye, all are optical focusing elements. Of course, only the eye has a detector array, the retina, which presents the brain with the image information that the brain requires to build a model of the space observed. The size of the image produced by the telescope objective is larger then that made by the eye's lens. But the eye can accommodate only relatively narrow object angles (thus, the average person of thirty years +/- can read comfortably at a minimum distance of about ten inches), and one sees only a blur when looking directly at the magnified image provided by the telescope objective. (Some of the early, incredibly long, simple refractors of the time of Christian Huygens produced magnified images that could be clearly viewed without eyepieces!) The most critical effect the eyepiece accomplishes for the observer is to alter the highly angular cone of rays produced by the objective to become a series of nearly parallel rays that the eye can comfortably receive. This cannot be done without modifying the image, but the best eyepieces do the least harm.
To get the most out of your telescope you should have three or four different eyepieces because on an astronomical telescope you switch magnification by switching eyepieces. You need to change magnification because celestial targets come in various sizes and brightnesses. A big object such as a sprawling cluster of stars is best framed with a low-power (25X to 50X) eyepiece. Inspecting details on a planet's tiny disk calls for a high-power (150X to 200X) eyepiece. Hunting galaxies or planetary nebulae might be best with a medium-power (80X to 120X) eyepiece.
"Many observers who have had long experience of planetary work...have been of the opinion that, with amateur instruments generally, from 200X to 400X is the best planetary magnification range, more or less irrespective of aperture"- J. B. Sidgwick, on page 102 of his OBSERVATIONAL ASTRONOMY FOR AMATEURS. Also, on page 100, "...for regular planetary work in this country [Britain] less than 5 inches provides insufficient resolving power." I concur with this statement, which fairly describes my own experience.
A figure of 375X sounds terrific. It's not. Although your import telescope may boast three or four eyepieces, the 6-mm and 4-mm models produce so much magnification that images through them look faint and fuzzy. Under the best conditions (using good optics in a steady atmosphere), the highest power you can usually employ on any telescope is about 50X to 60X per inch of aperture. For a 60-mm (2.4-inch) telescope this works out to a maximum power of 120X to 150X. On typical 60-mm aperture refractors with focal lengths of 700-mm to 900-mm, a 4-mm eyepiece gives an excessive power of 175X to 225X. A 6-mm eyepiece produces 120X to 150X, right at the upper limit. Under most conditions, you'll find these eyepieces of little use.
For planets, 30 to 40 power per inch of aperture is appropriate. These figures result from considering 1/30" as the minimum exit pupil diameter, the diameter at which interference effects just begin to become significant. Closely spaced multiple stars may take as much as 60 power per inch, since interference effects take less of a toll on bright, high contrast objects.
Compounding the problem is that many entry-level telescopes come with eyepieces of poor quality. Even the lower-power 20-mm and 12-mm eyepieces often provide poor images. The worst offenders are the ones marked AH, HM, or SR. These letters refer to the optical design of the eyepiece (AH = Achromatic Huygenian, HM = Huygenian Mittenzwey, SR = Symmetric Ramsden). All are simple two or three-element designs that add fringes of false color around images of bright objects and have narrow, tunnel-like fields of view. Far better are the Kellner, Modified Achromat, and Orthoscopic eyepieces that manufacturers such as Celestron, Meade, Orion, and Parks are now supplying with their entry-level telescopes.
All of the eyepieces named perform well when used with telescopes having the appropriate focal ratios. The Huygens and the Huygens-Mittenzwey are the only old types that will not show "fringes of color" (read "lateral color") when properly executed. In fact, their correction in this regard is better than that of the Nagler! The Huygens works well on systems as fast as f/12. Ramsdens work well to about f/9. These designs can be used effectively on fast telescopes with good Barlows. They possess apparent fields of about 35 degrees to 45 degrees. Many professional observatories still make use of their old, classic Huygens eyepieces, and I am hoping to construct a good Mittenzwey eyepiece some day (this design can produce a well-corrected apparent field of up to 50 degrees). Unfortunately, most cheap telescope manufacturers simply assemble eyepieces out of any convenient lens set on hand and name them with no regard as to design. This is the primary reason these eyepiece types have bad names.
The next step up is to an orthoscopic. Although "orthoscopic" can mean any highly-corrected eyepiece, the term usually refers to a specific design invented in 1880 by Ernst Abbe, an optician with Zeiss in Jena, Germany. Orthos contain four elements and correct optical aberrations better than 3-element designs do. Orthos are fine eyepieces for viewing the planets.
"Orthoscopic" refers to the fact that the distortion aberration in the field is well corrected. The lines of brick walls in the field are rendered straight and square, rather than scalloped like the sides of a pin cushion or bowed like the walls of a barrel. The term means nothing in regards to general, overall correction. The Abbe "Orthoscopic" is an excellent eyepiece when well executed. But, like the Huygens, Mittenzweys, and Ramsdens, some poorly manufactured examples exist. Sophistication in "design" is no guarantee that the eyepiece fabrication is well executed.
Although not an officially sanctioned term, Modified Plossl is our name for a new family of eyepieces which adds a fifth element between the Plossl's two pairs of lenses. Examples include Celestron's Ultima line, Meade's Super Plossls, Orion's Ultrascopics, and both Parks' and Roger Tuthill's premium Plossl series. All provide superb color correction, sharp on-axis images, and excellent suppression of ghost images, the term for annoying internal reflections of bright stars and planets.
The new Vixen Lanthanum LV design has a 5-element Modified Plossl at its heart, but adds a 1-, 2-, or 3-element lens called a Barlow ahead of the main group. The advantage is that all LV models share a valuable characteristic - long eye relief. Eye relief is the distance your eye needs to be from the top of the eyepiece in order to see the entire field of view. Short focal-length eyepieces usually have short eye reliefs; you have to place your eye uncomfortably close to the eyepieces to look through them. But each LV eyepiece boasts a generous eye relief of 20 mm.
One may consider the "modified Plossls" as pseudo-Erfles that have accepted slightly less apparent field coverage for better overall performance. The middle "fifth" lens acts as a collecting lens for the eye lens which allows the field lens to transmit rays of greater angular extent that otherwise would have been lost. This is precisely Erfle's contribution in the design of the eyepiece that carries his name. The "Barlow" that is added to the front of the Vixen eyepiece mentioned, and to the Nagler and Pretoria as well, is actually called a "Smyth" lens. Barlows are not designed to participate in the correction of the aberrations of the eyepieces they are intended for use with. Smyth lenses, by definition and practice, are.
But a number such as 80 degrees doesn't mean you see 80 degrees of sky when you look through that eyepiece on your telescope. How much sky you do see is called the "actual field of view." To determine the approximate actual field of an eyepiece, divide its apparent field by the magnification that eyepiece provides on your telescope. For example, an eyepiece with a 50 degrees apparent field which produces 50X will show you 50 / 50 = 1 degrees of sky. An 80 degrees apparent-field eyepiece that also produces 50X on your scope will show you 80 / 50 = 1.6 degrees. The magnification hasn't changed, but with the 80 degrees eyepiece you see more of the sky.
An eyepiece with an apparent field of 80 degrees will show you more of the sky, but you have to look around to see it. The healthy eye can accept a field of about 48 degrees in extent, and this figure was, until the advent of the Nagler and the unguided Dobsonian, considered about the ideal for the observer. Every extra degree of apparent field above about 40 degrees that one tries to design into an eyepiece requires extra measures, generally, to gain the excellent performance expected. Thus, lenses, stops, coatings, and other extras are added to make the gain.Published in the February 1996 issue of the NightTimes