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Handling Eyepieces

Eyepieces

If you have purchased a telescope, the next thing you're going to want is another eyepiece.  (And after that, some filters, and after that...)  I recommend getting the best eyepieces that you can afford and to be patient.  There are good deals out there, but like anything else, you have to know what you want BEFORE you buy!

When I was starting out, I was completely lost when presented with all of the choices in eyepieces.  With that in mind, the purpose of this article is to educate beginners on the merits of many of the types from a Huygens to a Nagler. 

Eyepieces come in several diameters: 0.96 inches, 1.25 inches, and 2 inches.  The 0.96 inch eyepieces are generally found on cheap department store telescopes, I would not recommend buying a telescope only capable of using them, as they can be very difficult to find.

The more common sizes are 1.25" and 2.0". 

Eyepieces not only vary in size, and focal length but also in design. The simple designs give acceptable performance and in some cases even excellent performance.  An extensive list of eyepieces and their specifications, type and cost may be found in the Advanced section of this website, under the "Technical Data" area.  See Eyepiece Specifications.

The cost of an eyepiece is generally determined by its complexity. The market is literally flooded with eyepieces, all of which are geared to achieving the best field of view, magnification without sacrificing image quality.

Galileo, Kepler and Newton didn't have too many choices when they were picking out their eyepieces.  The eyepiece that Galileo used was a single concave lens.  The images it produced were very small and full of aberrations.  Kepler improved the design a bit when he developed a convex lens that produced a wider field of view, but the image was inverted and still had a lot of aberrations.  The progress in eyepiece design was very slow and not always in the right direction.

Huygens

Christiaan Huygens designed the first compound eyepiece in 1654.  A pair of plano-convex elements contain both spherical and chromatic aberrations.  Long ago, Huygens eyepieces were standard equipment with telescopes of f/15 or greater telescopes.  At the longer focal ratios, these eyepieces perform marginally well, although their field of view is very narrow.  They lack an overall sharpness and are considered to have poor image quality.  

Considered obsolete, the Huygens eyepiece are not typically used with today's shorter focal length telescopes because the Huygens eyepiece suffers from short eye relief, high image distortion, chromatic aberration, and a very narrow apparent field of view.  Since these eyepieces are inexpensive to make they can often be found on inexpensive telescopes and microscopes.  Because Huygens eyepieces do not contain cement to hold the lens elements, telescope users sometimes use these eyepieces in the role of "solar projection", i.e. projecting an image of the Sun onto a screen.  Other eyepiece designs are cemented and can be damaged by the intense, concentrated light of the Sun.

Ramsden

It was Christian Ramsden who invented this eyepiece in 1783.  It is similar to a Huygens in that it consists of two plano-convex lenses, except both of the convex surfaces have identical focal lengths and they face each other.  In most cases the lenses are separated by two-thirds to three-quarters of their common focal length, which represents a severe compromise between eye relief and aberrations.  This design was an improvement, albeit a small one. in the history of eyepiece design.

Kellner

About 65 years after the Ramsden eyepiece was developed, Carl Kellner introduced the first achromatic eyepiece in 1849.  He placed a cemented two element achromat lens in place of the eye lens of the Ramsden.  He also used flint glass closest to the observer's eye and crown glass in the other elements.

The design reduced most of the aberrations in Huygens and Ramsden eyepieces and had fairly good color correction and edge sharpness.  They also had good field of views (about 40 to 50 degrees).

The biggest problem that plagues the Kellner design is internal reflections.  Today's anti-reflection coatings make these usable, economical choices for small to medium aperture telescopes at low to medium powers.

Orthoscopic

Ernst Abbe invented the orthoscopic eyepiece in 1880, and it has become a favorite of amateur astronomers ever since.  Its field lens consists of a cemented triplet matched to a single plano-convex eye lens.  Orthos are close to perfect eyepieces that have excellent eye relief, nearly non-existent chromatic or spherical aberration, have a fairly flat, wide field of view (40 to 50 degrees), and have little internal reflection.  Ortho eyepieces remain one of the best for nearly all amateur telescopes.

Erfle

The original wide field eyepiece was developed for the military in 1917.  Because it has field of views between 60 and 75 degrees, the amateur astronomers quickly adopted it.  Erfles have either five or six elements, with either two achromats with a double convex lens in between or with three achromats.

Erfle are designed to give a very wide field of view, usually from 60-70. The Erfle is usually less expensive than the plossl's. However they can suffer from considerable image distortion near the edge of the field of view when used at high magnification. They are most useful for observing deep sky objects.

Erfles can give panoramic views of the night sky, but at the expense of image sharpness with astigmatism at the edge of the field.  This design doesn't work well for lunar or planetary observations that use higher powers.  They work exceptionally well at low power, wide angles.

Plossl

An optician named G. S. Plossl, living in Vienna, Austria, developed this excellent eyepiece in 1860.  After a hundred years of relative obscurity, the design finally caught on and has resulted in one of the most highly regarded eyepiece designs around.  Excellent on all criteria, it features twin close-set pairs of doublets for the eye lens and the field lens.

Plossl's incorporate at least 4 optical elements. The plossl design is a very well designed eyepiece, I would highly recommend them if you are using a Newtonian telescope with a very short focal length. Plossl's are very costly to make and hence a lot more expensive than the other designs mentioned so far.  Inexpensive plossl's usually exhibit some degree of internal reflection, which is not present in the more expensive models.  Plossls generally have a wide field of view and generous eye relief. If there is such a thing as a good all around eye piece for observing then these are it: planet viewing, lunar observing or for deep sky objects.

In 1980, Al Nagler, owner of Tele Vue Optics, Inc., introduced a line of Plossls that are considered almost legendary today. This set up a cascade of companies that developed and marketed their own line of outstanding Plossls.  Some are better than others and it's fair to say that you get what you pay for.  Things to look for when purchasing Plossls include, fully multi-coated optics, blackened lens edges, and anti-reflection threads for filters.  

Zoom

I include this type, because the beginning amateur astronomer is often tempted to buy one of these eyepieces thinking that it will replace several individual eyepieces.  But the truth is its too good to be true unless you pay for the premium quality parfocal designs.  These are EXPENSIVE.  Resist all urges to purchase cheaper versions.

Advanced Types

Other types include Brandon, Lanthanum LV, Wide-Field, Nagler, Panoptic, Super Plossl, Super Wide Angle, Ultra Wide Angle, Standard Ultima, Wide Field Ultima, Extra Low Ultima, MegaVista, and Ultrascopic.  All of these high-end eyepieces deserve careful research by potential buyers, and are recommended depending on the type of application.

Various improved designs incorporating 6 to 8 lens elements boast apparent fields up to 85-so wide you have to move your eye around to take in the whole panorama. Light transmission is slightly diminished, but otherwise the image quality in these eyepieces is very high. So is their price.

Barlow Lens

A Barlow lens isn't actually an eyepiece, it simply multiplies the amount of magnification of the eyepiece. Barlows come in 1.5X, 2.0X, 2.5X and 3.0X.  Barlows are mostly used for planetary and binary star observation.  Barlows must be of high quality though or the resulting image will be badly distorted. 

Summary

If you're looking to purchase an eyepiece now, I can tell you that I have had remarkable success purchasing high-grade Tele Vue, Nagler and Meade Series 4000 Plossls from the online auction service eBay.   Try searching on "eyepiece", "televue", "tele vue", "Meade", "Celestron", and misspellings like "telvue" or  "mead".  With patience and timing, you can put together a decent cross section of magnifications with quality eyepieces at a fraction of the new cost...  Something to consider if you haven't won the Lotto.  Check out the Magnification page to get an idea of which eyepieces might give you the best coverage.

 

Introduction to the Night Sky - Part I

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11/2011