Astronomical Telescope Eyepieces:
A Discussion for the Beginner
Copyright © 1998, 1999, 2000, 2001, 2002, Jay Reynolds Freeman (Jay.Freeman@Sun.COM)
Beginners sometimes wonder what eyepieces to get for a new telescope, and what eyepieces are all about in general. Eyepiece design and performance is fascinating, and many technical sources explain what's what, but I would like to give some simple, practical comments that might otherwise get lost in minutia.
Basic terminology, arithmetic, and how-does-it-work:
- The magnification of a telescope is the ratio of its focal length to the focal length of the eyepiece in use.
- Thus a telescope with a 1000 mm focal length, used with an eyepiece of 25 mm focal length, has a magnification of 1000 / 25, or 40. It makes things look 40 times wider, or if you prefer, 40 times closer. Put in an eyepiece with 4 mm focal length, and the same telescope now has magnification of 1000 / 4, or 250.
- Focal lengths of commercially available telescope eyepieces range from 2.5 mm to 60 mm or more.
- Magnification is sometimes symbolized by the letter "X", or "x". Thus we might speak of 40x, or 250x, and a 7x50 binocular magnifies seven times. (The "50" is the diameter of its front lenses, in millimeters.)
- I have the eccentric habit of not using the word "power" to refer to the magnification of a telescope/eyepiece combination. "Power" conveys the impression that more magnification is always better, or that magnification is all that matters. Both of those notions are wrong, as we shall shortly see.
- When you look into an eyepiece, the width of the apparent field of view is the angle through which you must turn your eyeball to transfer your gaze from one side of the field of view to the other. It varies with eyepiece design, from as little as 30 degrees to more than 80 degrees. The width of the actual field of view is the angular width of the patch of sky you are looking at, before it is magnified. It is more or less equal to the width of the apparent field of view divided by the magnification.
- Thus if you are using an eyepiece with an apparent field of view of 50 degrees, in combination with a telescope such that its magnification is 100x, the width of the actual field of view will be about 50 degrees / 100, or 0.5 degree -- about the width of the full Moon.
- Eyepieces come in many different designs, and they all have names -- Huygenian, Ramsden, Kellner, Orthoscopic, Erfle, Plossl, Koenig, Nagler, and many others. Don't worry about what the names mean, just remember that they do mean something. Some cost more than others, some work better than others. The ones that cost more aren't always the ones that work better.
- Eyepieces come with different barrel diameters -- the diameter of the cylindrical part of the eyepiece, that fits into the telescope's eyepiece holder. There are three common sizes on the market today, and one less common one. The common barrel diameters are 2.00 inches, 1.25 inches, and 24.5 millimeters (0.965 inch). The less common one is 23 mm (0.917 inch).
- Barrel diameter has nothing to do with magnification. But too small a barrel may restrict the apparent field of view of a long focal-length eyepiece.
- A device called a Barlow lens, or sometimes a telextender, may be used with eyepieces to change their magnification. The best way to think of a Barlow lens is as a device which multiplies the telescope's focal length. Thus if you insert a 3x Barlow lens into the back of a telescope with 1000 mm focal length, the combined focal length of the telescope and Barlow lens becomes 3000 mm, and the magnification of any eyepiece used with the telescope will be tripled when it is used with the Barlow lens, compared to the magnification without.
- Barlow lenses on the commercial market come in at least the three common barrel diameters, and have focal-length multiplication ratios from 1.75 to 5.00. Some have adjustable multiplication ratios.
- Eye relief is the distance between the final glass surface of the eyepiece and the lens of your eye when you are looking through it. It is the space into which your glasses must fit, if you wear them when you observe, and is the clearance which keeps your eyelashes from smearing the outermost lens surface of the eyepiece, and your eyebrow ridges and cheekbones from jiggling the telescope.
- Sufficient eye relief is a good thing. Too little is vexing. Too much can be vexing, too -- sometimes you can have trouble figuring out where to put your eye.
- In general, for eyepieces of the same design, eye relief increases in proportion to focal length. But at constant focal length, it varies enormously from design to design. Several lines of eyepieces have been designed specifically to provide the same, ample, eye relief over a wide range of focal lengths.
Generalities about Eyepiece Selection:
- Hands-on experience is more valuable than the printed word -- join an astronomy club, take your telescope to star parties, and ask to try out other people's eyepieces in it.
- A small number of good eyepieces is better than a large number of bad eyepieces.
- Not all Barlow lenses work well with all telescopes and eyepieces. You have to try the combinations you have in mind, and find out what works.
- It is ultimately desirable, to have eyepieces that provide a nicely-spaced sequence of magnifications along a useful range for your telescope. Yet some magnifications are more useful than others. It makes sense to buy those first. I have an opinion on what they are:
I recommend starting with two eyepieces, one with magnification of about one-fifth the aperture of your telescope, expressed in millimeters, and the other with magnification about equal to the aperture in millimeters. That is, for a six-inch telescope -- 152 mm -- I recommend you start with magnifications of about 30 and about 150. I won't complain if they are 25 and 120, or 40 and 200.
The more powerful eyepiece gives a magnification most of us would call "medium". It will be the one you use in decent seeing, to look at the Moon, planets, and double stars. The other one will give brighter images of faint nebulae and galaxies, and so make them easier to see than if their limited light were spread wide, by high magnification.
The low-magnification eyepiece will also do double duty for finding things. Thus it should have a field of view as wide as possible, and that means that its front lens should be as big in diameter as possible, subject to two limits -- your budget (wide-field eyepieces are expensive), and the diameter of the focus tube of your telescope (wide lenses won't help if telescope parts get in the way). If you have a telescope with a long focal ratio (big "f" number), with a small-diameter focus tube, then you won't be able to get a wide-field view, but you will still want a low magnification for faint fuzzies.
If you have money left after buying these two eyepieces, and if you absolutely cannot wait until you have joined a club and tried things out, then the next two magnifications you will want will probably be one a little less than the half-way point between the first two -- say, 65x to 75x on a six-inch -- and one at not quite twice the magnification of the more powerful of the first two -- say, 250x on a six-inch.
Here are recommendations for the "first two", for common telecopes.
- f/5 Dobson -- Get a 5 mm Orthoscopic for the "high" magnification. For the low one, if you are on a budget, get a 20 mm Erfle from Pocono, or find a used one with another label. If you are not on a budget, and if you have a two-inch focuser, you will probably look at Tele Vue Panoptics, Meade Super-Wides, and the Tele Vue 20 mm Nagler Type 2, but you will have to decide for yourself which one you like, because I don't like any of them -- I'd go with the Erfle. I hasten to add, that all these high-price eyepieces have advocates among experienced users. I'm just weird.
- f/10 Schmidt-Cassegrain, Maksutov, or refractor -- Get a 10 mm Orthoscopic or Plossl. Then, if you have a 1.25-inch focuser, a 40 mm Plossl or Kellner in that diameter will do nicely -- several vendors carry them -- or maybe a 32 mm if you find the narrow field of the 40 mm aesthetically unappealing. With a two-inch focuser, Meade, Orion, Pentax and University Optics have suitable 40 mm eyepieces. Some of those eyepieces are expensive.
- Many of these telescopes come with a decent 25 mm or 26 mm eyepiece as standard equipment. If so, keep it for a while, and just buy the 10 mm.
- 4.5-inch f/8 Newtonian -- 8mm Orthoscopic or Plossl, and 32 mm Plossl.
- Small "beginner" refractor, f/10 to f/12, with 0.965-inch focuser -- The right answer for 0.965-inch focusers is a "hybrid" star diagonal that fits the focuser and accepts 1.25-inch eyepieces. The focuser restricts the field of view, but there are lots more eyepieces to choose from at 1.25-inch diameter than at 0.965. You can then go for a 10 or 12 mm Orthoscopic or Plossl, and a 40 mm Plossl or Kellner. But be careful, not all small refractors will come to focus with a hybrid diagonal in place, and a 40 mm Plossl may be too big and clumsy for such a small telescope. If the hybrid diagonal does not work for you, you will have to hunt down eyepieces with 0.965-inch barrels in appropriate focal lengths. 10 or 12 mm should be no problem, but you will be lucky to find a decent eyepiece of that barrel diameter with a focal length over 30 mm.
An Approach to Technical Minutia:
Switching eyepieces enables a tradeoff among three important, related quantities; namely
- Image brightness of extended objects, like nebulae, and
- Width of field of view.
Some folks think high magnification is what telescopes are all about, but there is a lot more to it. If an object is sufficiently bright to begin with, then increasing magnification up to certain a point will generally allow you to see more details. But, where is that "certain point"? There is no single answer. If you increase the magnification of your telescope in small increments, there are several reasons why you might want to stop after a while. They include:
- Poor seeing. At sufficient magnification, images may look as if you were viewing through swirling water -- the effect of turbulence in the atmosphere. How much magnification it takes before the swirling gets objectionable will vary from night to night, and perhaps from minute to minute, from place to place, and from telescope to telescope -- large apertures are often bothered by poor seeing more than small apertures. And if you are patient and persistent, you may wish to try a high magnification even in poor seeing, in the hope that things may settle down for a second or so every now and then, so you can get a good look.
- Jiggly mounting. Not only must the atmosphere be steady, but so also must be the mounting of your telescope, if the images are not to look wiggly.
- Less than perfect telescope optics. Other things being equal, poor optics will give a blurrier image than good optics. There's no point magnifying detail that isn't there!
- Even a telescope with good optics may temporarily misbehave when it is first set up, before all the parts have cooled to the temperature of the surrounding air. Large amateur telescopes may take hours to settle to temperature. The problem is worse in winter than in summer.
- The wave nature of light itself. There is a limit to the amount of detail in the image of even a perfect telescope, and there is no point using more magnification than it takes to see all of it. The amount of detail present in the image formed by a perfect telescope, in perfect conditions, is proportional to its clear aperture -- doubling the aperture produces twice as sharp a view. Thus the maximum amount of magnification that is useful, is proportional to aperture: As a rule of thumb, it is extremely rare for an observer to have any use for a magnification much greater than approximately twice the telescope's clear aperture, in millimeters -- that would be 300x on a 150 mm telescope.
- The actual upper limit for useful magnification depends on the visual acuity of the observer, which of course varies from person to person. People who have "sharp eyes" won't need as much magnification, to see all the detail that is present in the images of their telescopes.
- This matter is rarely understood, so I repeat: There is a limit to the amount of detail in the image of even a perfect telescope, and there is no point in using more magnification than it takes to see all of it.
- Not enough light. When you look at any particular object, the amount of its light entering the front of your telescope is fixed. If you are looking at an extended object, like the surface of the Moon, or a planet, or a galaxy or nebula, then as you increase magnification, that light is spread out over an ever-greater area of the retina of your eye, so the image looks dimmer and dimmer. Spread it out too much, and it will become too dim to see at all. What's more, various kinds of fine or low-contrast detail become hard to see before the object itself vanishes. If the object is dim to begin with, then the right magnification to see detail of a given size and contrast will be different, and almost certainly lower, than for a bright object. You will have to try many magnifications to see which one works best.
This discussion is closely related to the material in the paragraph about "not enough light", immediately above. The image brightness of an unresolved point of light -- like a star -- does not change as you change magnification, at least, not until the magnification is high enough so that you can begin to see the diffraction pattern of the star. The brightness of an extended object, however, declines as the square of magnification -- twice as much magnification reduces the image brightness by a factor of four. Reducing brightness will eventually make objects and their details harder to see, but on the other hand, increasing magnification does make those details larger. It is not always obvious which way the tradeoff works -- does increasing magnification gain more by enlarging details than it loses by making them fainter? You have to try it and see.
You might think that the best way to see faint extended objects is therefore to use a very low magnification, which makes their images as bright as possible. That doesn't always work, either. The eye behaves in a more complicated manner: Sometimes the tradeoff between image size and brightness favors a larger, dimmer image: I often have best luck looking at distant, faint galaxies at a magnification of about two-thirds the telescope clear aperture in millimeters (100x on a 150 mm telescope), which is a lot more magnification than many books say to use for deep-sky observation. Sometimes, also, it seems to help to have a magnification great enough that the background sky looks black -- at very low magnifications, it may look gray, and that seems to make faint things harder to see.
The tradeoff between image size and brightness often varies dramatically with only small changes in magnification. A change as little as ten percent can sometimes make a big difference.
At very low magnifications, another thing begins to happen. The diameter of the beam of light coming out of an eyepiece -- which is called the "exit pupil" -- is equal to the telescope clear aperture divided by the magnification. Thus a 150 mm telescope used at 100x has a 1.5 mm exit pupil. It turns out that the exit pupil can also be calculated as the eyepiece focal length divided by the focal ratio of the telescope. Thus any f/10 telescope used with a 15 mm eyepiece will have a 1.5 mm exit pupil. When the exit pupil is bigger than the pupil of the observer's eye, not all the light that comes out of the telescope can make it to the observer's retina -- some will be blocked by the iris of the eye. Once magnification has dropped so far that that has started to happen, then further reductions in magnification do not make the images of extended objects any brighter. Extended object surface brightnesses stay the same, and images of point sources, like stars, get dimmer, as magnification decreases further.
The diameter of the pupil of the dark-adapted human eye varies from person to person, but tends to decline with age. As a rule of thumb, persons younger than about 40 will likely have dark-adapted pupils of 7 mm diameter; older people will have smaller ones. But I do not generally recommend magnifications that produce exit pupils larger than four or five millimeters anyway.
There is a special case: If you have a fast (small f-number) telescope, and need a very wide field eyepiece to find things, or for spectacular views of very large objects, then perhaps you have cause to get a very low-magnification eyepiece, even if it has an over-large exit pupil. You will be throwing away light, but perhaps the wider field is worth it to you.
Field of View:
For eyepieces of a given design (Kellner, Orthoscopic, Plossl...) the width of the field of view is generally inversely proportional to the magnification -- doubling the magnification cuts the field width in half. But at the same magnification, different eyepiece types have dramatically different fields of view -- expensive modern designs, like Naglers, may have twice as wide a field as such old standbys as Kellners and Orthoscopics. So if you want a wide field of view, perhaps for finding things or for looking at some of your favorite large objects, you must either get a long focal-length, low-magnification eyepiece, or spend some money for a fancy design, or both.
There is a catch, too: There is no magic to getting a wide field of view in an eyepiece -- look down the barrel of a short focal-length, high-magnification eyepiece, and see how tiny the lenses are compared to those in a low-magnification eyepiece. Wide fields of view require wide lenses. But wide lenses won't do any good if the eyepiece barrel, or the focuser tube, or the baffle tube of a Schmidt-Cassegrain, prevents light from getting to their outer edges. For good use of a low-magnification, wide-field eyepiece design, it must be mounted in a large eyepiece barrel and used with a large-diameter focuser and baffle tube. A 40 mm eyepiece will give a nice low magnification in an f/10 refractor or in a Schmidt-Cassegrain, but it must be have a two-inch barrel and be used in a two-inch focuser, if it is to give a wide field of view, as well.
Eyepiece Errors of Color -- Some Basics:
Many people know that designers of the objective lenses of refracting telescopes have engaged in a centuries-long battle against longitudinal chromatic aberration, which is also called longitudinal color. The nature of that aberration is that different colors of light do not come to the same focal point. With too much error of this kind, the image seen through a refractor is not in sharp focus for all the colors of light that the human eye can see; rather, the image will likely be in pretty good focus for yellow and green light, but will have a violet blur composed of the out-of-focus red and blue images in combination.
You might think that eyepieces would have the same problem -- after all, they use lenses, too. You might conclude that the image seen through a poor eyepiece might not be in simultaneous focus for all colors, even if the main telescope were a reflector, or some other design that reduced longitudinal chromatic aberration to a minimum.
Surprisingly, that's not so. It's not that eyepieces lenses don't have longitudinal chromatic aberration -- they certainly do -- rather, it is that the amount is so small that it is generally far smaller than the focusing tolerance for the telescope. Other things being equal, the length of the region along the optical axis where the various colors of visible light come to focus, is proportional to the focal length of the lens in question. For a long focal-length lens, like a 1-meter focal-length refractor objective, that length is large enough to matter. For a short focal-length lens, like a 10 mm eyepiece, it's too tiny to make a difference.
The most common error of color introduced by eyepieces is a different one, called chromatic difference of magnification, or lateral chromatic aberration, or just lateral color. This problem also stems from the fact that lenses do not bring all colors of light to the same focal point, but lateral color involves a different consequence of that fact than does longitudinal color: If an eyepiece, used as a lens, does not bring all colors of light to the same focal point, it follows that the focal length of that eyepiece will vary with color, and consequently, that magnification will vary with color.
The image seen while looking through an otherwise perfect telescope, while using an eyepiece that has noticeable lateral color, will in effect comprise different-colored images of slightly different sizes, superimposed. Most eyepieces with this problem have images whose size increases from the red to the blue end of the spectrum. With such an eyepiece, if the object you are looking at is a white circle, or something like it -- perhaps a planet or the Moon -- then when it is nicely centered in the field, the edge of the circle will look blue, where the large blue image overlaps beyond the edge of the smaller images of other colors. If you are looking at a test pattern of black and white stripes, then the sides of the white stripes that are away from the center of the field will have blue edges, and the sides toward the center of the field will have red edges.
The width of the red and blue edges will increase in proportion to the distance of the edge from the center of the field. Lateral color, from the eyepiece, goes away when an small object is centered. Longitudinal color, from the objective, does not.
Lateral color is often particularly easy to see in binoculars. Try looking at a dark telephone pole silhouetted against the bright sky, or at a white picket fence against the dark interior of a garden. Colored edges will often be easy to see when the object is out at the edge of the field. Stars will be drawn out into short, radial spectral streaks, with the red ends pointing toward the center of the field.
It turns out that eyepieces don't need to be very fancy to correct pretty well for lateral color. The centuries-old Ramsden eyepiece, composed of two identical plano-convex lenses placed convex sides toward one another at a separation equal to their focal length, has essentially no lateral color. (Strictly, it is corrected for lateral color through first order in the longitudinal color of each of the lenses it is made from.) Good Ramsdens eyepieces are essentially color-free, even though they do not use achromatic lenses. That sounds like something that ought to be impossible, but it's not: Jesse Ramsden was a smart man.
Many people confuse lateral color with another color problem which doesn't have to do with the telescope at all. Dispersion of the different colors of light by the atmosphere causes stars and other objects near the horizon to be drawn out into very short spectra -- the atmosphere works like a big, weak prism. The spectra will all point in the same direction, no matter where they are in the field, but at a quick glance you might not notice that they were all the same. With high magnification, these effects are detectable surprisingly far above the horizon. So when you are testing an eyepiece for lateral color, be sure to look only for color variations in the horizontal direction, so that you will not be confused by atmospheric dispersion.
Good Eyepieces and Bad -- Generalities:
Whether an eyepiece is good or bad depends on what you want it to do. What observers want from eyepieces often includes the following:
- Good images when used with telescopes with fast f numbers,
- Wide apparent field of view, and
- Ability to reveal low-contrast, fine detail, if there is any in the image in the first place.
Some warnings: First, you probably are not going to find any one eyepiece that delivers all these things. Second, some of these qualities are expensive.
It is harder to make eyepieces that work well at fast f-numbers (that is, at small f-numbers) than at slow ones. To see why, let's take an eyepiece with 10 mm focal length. Suppose you use it with an f/10 telescope -- perhaps a refractor or a Schmidt-Cassegrain. Consider the bundle of rays of light from a single star, as it passes through the telescope and then the eyepiece. The bundle comes to focus at a single point, or very nearly so, in the telescope's focal plane, then expands outward into the eyepiece.
A 10 mm eyepiece used with an f/10 telescope will have a 1 mm exit pupil diameter: The bundle of rays leaving the eyepiece will be 1 mm in diameter. Thus a chunk of eyepiece only about 1 mm wide will have been used to get the rays from that star set up to go into your eye. The eyepiece designer will only have had to worry about aberrations being small across a 1 mm wide hunk of glass. Of course, the bundles of rays from other stars in the field of view will go through different 1 mm hunks -- that's why the eyepiece has lenses that are more than 1 mm wide -- and for a good eyepiece, the aberrations in each individual 1 mm hunk must be small. But a slow variation in some optical property across the full width of the lenses will be okay, as long as 1 mm worth of it doesn't add up to too much aberration.
Now put the same eyepiece in an f/5 telescope -- perhaps a fast Newtonian. The bundle of rays from one star, leaving the eyepiece, is now 2 mm in diameter, and the designer has a correspondingly harder task -- the aberrations must be kept small over a 2 mm wide hunks of glass, instead of smaller, 1 mm hunks.
Thus it is no surprise that eyepieces that work well at fast f-numbers are fancier and more costly than ones that don't. There are inexpensive centuries-old eyepiece designs, like the Ramsden and Huygenian, that have only two pieces of simple glass, and work perfectly well at f/15 or more, as in classic refractors. But don't try them in fast Newtonians.
Wide Fields of View:
As I said before, there is no magic to getting a wide apparent field of view -- it takes wide pieces of glass. The labors of the eyepiece designer thus increase, for the task is to provide good images over a bigger area of the focal plane than with a narrow-field eyepiece. To do so may take lots of pieces of fancy glass, processed to tight tolerances.
Fast F-Numbers and Wide Fields:
If you want an eyepiece that works with a wide field at a fast f-number, then you are compounding the two previous problems. Only in the last decade or two have there begun to be wide-field eyepieces that work decently with f-numbers down around 5 -- as you find in many Dobsons and fast refractors. They cost an arm and a leg. The first was the Tele Vue Nagler; Tele Vue, Meade, and perhaps some of the Japanese manufacturers have produced others.
It is interesting, that many people who think they have seen the off-axis coma and astigmatism which we are all taught characterize a fast Newtonian, are wrong. What they have actually seen is the off-axis aberrations of some hapless eyepiece that can't hope to work well at -- say -- f/5. The coma and astigmatism of the eyepiece are greater than that of the mirror.
I said earlier that I don't much like the modern whizzy wide-field designs, that do work with fast f-numbers. That's because I am willing to fuss to keep an object centered, even in a big Dobson -- I need wide fields of view only for finding things, and that doesn't require pinpoint star images at the edges of the field. From that point of view, the likes of Tele Vue Naglers and Panoptics, or of Meade Super Wide Angles and Ultra Wide Angles, are all too big, too heavy, and too expensive. I don't like lugging them around, I don't like rebalancing the telescope every time I change eyepieces, and most of all, I don't like paying for them. But your preferences may vary: Many observers disagree with me. And if I had a Dobson too big to steer easily, or if I wanted to use one regularly at high magnification, I might well want the extremely wide well-corrected fields of some of these eyepieces.
Fine, Low-Contrast Detail:
Suppose you have many types of eyepieces, with many focal lengths of each type, all well enough corrected to work at the f number of your telescope. Which one will show the most low-contrast detail on Mars or Jupiter, or will have the best chance to find the close, faint companion of a double star? It should be clear from what I have said before, that selecting the right magnification has a lot to do with the answer, but suppose you still have a choice: For instance, suppose you think a 6 mm eyepiece is best for what you have in mind, but you have several different 6 mm eyepieces in your collection. Which will work best?
The likely problem here is scattered light -- light from the bright parts of the object itself, which is not getting to the point in the image where it is supposed to be, but instead ends up a little way away, on top of the image of some fainter part of the object. The image as a whole looks a little foggy, or seems to have extra glare. We've all seen this kind of effect in non-astronomical circumstances, perhaps looking through real fog, or through a dusty or dew-covered window or spectacle lens.
Scattering in the eyepiece generally comes from irregularities or defects at the surface of a lens, where light passes from air into glass, or vice-versa. Dust or dew may be present, but for purposes of comparison testing, let's assume the eyepieces are clean and dew-free. Other irregularities may come from incomplete or flawed polishing of the lens surface -- that is, from pits or scratches -- or from defects in the lens coatings. Places where one lens is cemented to another are less likely to cause problems -- these surfaces will not have coatings, and the lens cement itself does a pretty good job of filling surface irregularities in the glass and masking their effect. Thus what counts is the number of air/glass interfaces in the eyepiece.
Lest we not see the forest for the trees, let me stress one obvious fact: The fewer the air/glass interfaces, the fewer the places where scattering can occur. At any given level of quality of polish and quality of coating, simple eyepieces, with few air/glass interfaces, will have less scattering than fancy eyepieces, with lots of air/glass interfaces. Of course, lots of simple eyepieces are made to sell at low prices, and perhaps do not have as good polish and coatings as fancier types, but do not let the workings of the marketplace obscure what's going on: Other things being equal, more air/glass interfaces means more glare.
Many complicated eyepieces have six, eight, or even ten air/glass interfaces, but such simple designs as Orthoscopics, Kellners, Plossls, Ramsdens, and Huygenians have four. There are rarer designs with only two air/glass interfaces -- the optics are one solid or cemented piece of glass. They include the Coddington, Tolles, Hastings Triplet, and several different designs called Monocentric. Any of these simple designs will have a leg up over whizzy types with lots of air/glass interfaces, for fine, low-contrast detail. If the simple eyepiece has well-polished lenses, and if the coatings -- if any -- are first-rate, then the simple eyepiece will probably show more low-contrast fine detail.
This effect is significant. I have done a moderate amount of comparison testing of eyepieces, looking for scattered light, and eyepieces with few air/glass interfaces almost always have noticeably less scattering than eyepieces with many air/glass interfaces. I have some old Ramsden eyepieces -- four air/glass interfaces -- which don't work well at fast f-numbers. Yet at long focal ratios, they are embarrassingly good planetary eyepieces, and often put to shame more modern, fancier designs. The embarrassment is heightened by the fact that my Ramsdens cost only ten or twenty dollars each, when I bought them in about 1980.
Don't forget to count the air/glass interfaces in Barlow lenses, too. Every Barlow has at least two of them, and some have more.
Let's talk about polish. Good polish costs money: Not only must the lens spend extra time on the polishing machine, but also the manufacturer must more carefully control the quality of the polishing materials and the cleanliness of the polishing area. I suspect some manufacturers skimp on polishing. Some of the best polish seems to be on old military-surplus optics. I think the military has always had a no-nonsense attitude about contrast, hence about polishing -- when you are peering into the dark looking for someone coming to kill you, it is useful to be able to see clearly. My old Ramsdens were probably assembled from military-surplus lenses; that's a likely reason why they work well.
The effect of coatings requires more thought. The purpose of coatings is to increase transmission of light. An uncoated air/glass interface transmits only about 96 percent of the light that strikes it, and reflects essentially all the rest. The best modern coatings increase transmission to 99 percent or better. The older, magnesium fluoride coatings transmit about 98 percent. Transmissions multiply, so so the transmission through the ten air/glass interfaces of a Barlow and a fancy modern eyepiece combined will be something like 0.96 to the tenth power, or 66 percent, if they are uncoated. With magnesium fluoride coatings, it will be 82 percent, and with good modern coatings, 90 percent or more . That's quite a difference: Coatings are important for looking at faint objects.
Reducing reflected light is important for another reason: That light may return to haunt you. Some eyepiece types are noted for ghosts -- out-of-focus images of what you are looking at, caused by multiple, back-and-forth reflections from lens surfaces. Kellners and some of the Monocentric designs have notable ghosts. Even with no ghosts, light lost by reflection may scatter off the inside of the eyepiece barrel or focuser tube, and add glare to the image. Good coatings hit the heart of these problems, by reducing reflections. Yet not all eyepiece designs have ghosts, and it is possible to control scattering off the inside of the eyepiece barrel or focuser tube by means of black paint, threading, baffles, and so forth.
Coatings may have one undesired effect on the detection of fine low-contrast detail. I have heard it said that some coatings may increase scattering at the air/glass interface. I don't have any details. It is clear, however, that every time you put something on a lens surface, you have the opportunity to trap dust, or to create irregularities by imperfections in the coating process. Multi-layered coatings have many such opportunities. Yet I prefer coated eyepieces to uncoated ones, across the board, notwithstanding.
In any case, the effect of coatings upon scattering at the air/glass interface seems minimal. Some of my Ramsdens are uncoated, yet they exhibit neither more nor less glare than do other eyepieces with the same number of air/glass interfaces (four) and with coated lenses. The uncoated Ramsdens undoubtedly have less overall transmission than most coated eyepieces, but for bright stars and planets there is usually plenty of light, so the lower throughput is not a big deal.
I also have worked with some Monocentric eyepieces manufactured by the Russian firm, Intes, which have only two air/glass interfaces. Most of them have terrible coatings -- the lens surfaces reflect more light than does uncoated glass. Yet the best of these eyepieces produce less glare than the Ramsdens. I hope Intes gets its coatings together. The Monocentric designs make excellent planetary eyepieces. Well-made ones would be welcome.
I think the bottom line on detection of fine, low-contrast detail is to use an eyepiece with only a small number of well-polished air/glass interfaces, and not worry too much about coatings. But this subject is complicated, and there is probably more that should be said, that I do not know about.
What's in My Eyepiece Box:
We could discuss the pros and cons of various eyepiece types for a long time without coming to any conclusions, so I am just going to tell you what I have in the box of eyepieces that I bring to star parties, and leave it at that. I work with telescopes of various focal ratios from f/5 to f/12, so I need more eyepieces than for just one. Not all of my telescopes have 2.00-inch focusers, so there are duplications in longer focal lengths. I of course have a bushing to adapt 1.25-inch eyepieces to 2.00-inch focusers. What I carry at the moment is three sequences of eyepieces, plus some odds and ends. The first sequence is a run of high-definition eyepieces for use when image quality is paramount:
- 32.0 mm, 1.25-inch barrel, Brandon "Orthoscopic"
- 24.0 mm, 1.25-inch barrel, Brandon "Orthoscopic"
- 16.0 mm, 1.25-inch barrel, Brandon "Orthoscopic"
- 12.0 mm, 1.25-inch barrel, Brandon "Orthoscopic"
- 8.0 mm, 1.25-inch barrel, Brandon "Orthoscopic"
- 6.0 mm, 0.965-inch barrel, Pentax SMC ED Orthoscopic
- 5.0 mm, 0.965-inch barrel, Pentax SMC ED Orthoscopic
- 3.8 mm, 0.965-inch barrel, Pentax SMC XP Orthoscopic
- 2.8 mm, 0.965-inch barrel, Takahashi Orthoscopic
Brandons use two cemented doublets face to face, rather like Plossls -- they are not the kind of "Orthoscopic" most people think of, though I suspect the design is technically entitled to that name. They are excellent high-definition eyepieces, and not cheap, with apparent fields of view near 45 degrees. There are better eyepieces, but they are less cheap -- a lot less cheap -- and some brands are difficult to locate. I used to have some other old orthoscopics in the lineup, but I have replaced them.
I keep the Pentax and Takahashi eyepieces installed in 0.965-inch to 1.25-inch adapters, so it is easy to use them in my 1.25-inch focusers.
The second sequence has much wider fields of view, for deep-sky observation:
- 55.0 mm, 2.00-inch barrel, University Optics Plossl
- 40.0 mm, 2.00-inch barrel, Vernonscope Erfle
- 32.0 mm, 2.00-inch barrel, University Optics Erfle
- 20.0 mm, 1.25-inch barrel, Brandon Wide-Field
Erfles work fine at focal ratios of eight or more, but begin to show substantial aberrations of their own, at the edges of their fields, at faster f numbers. They have apparent fields of view close to 70 degrees.
Plossls do not have as wide an apparent field of view as Erfles, but the field of the 55 mm is limited by the inner diameter of the two-inch barrel, so who cares?
The Brandon Wide-Field appears to be a variant of the Erfle design, but I wouldn't want to say so for certain.
I used to have some shorter focal-length Erfle eyepieces in this sequence, 15.5 mm and 12.4 mm Meade "Research-Grade" Erfles that I liked a lot, but I found myself using the 16 and 12 mm Brandons in their place. The Brandons seem to give a bit less glare than the Meades, yet the Meades had a much wider field of view, which made it easier to find things with them. However, when I bought copy of Millennium Star Atlas, which has lots more stars than my previous observing atlases, I found that the extra stars made it easy to find things even with the narrower fields of view of the Brandons. Who would have thought that changing atlases would have caused me to change eyepieces?
I have a zoom eyepiece, which is very useful.
- 8.0-24.0 mm, 1.25-inch barrel, Vixen Lanthanum zoom eyepiece
The main advantage of zooming is that the range of magnifications at which an object shows best is sometimes very narrow, and zoom capability can help find it. Furthermore, the Vixen is one of the newer zoom units on the market, which have a wider actual field of view at low magnification than at high: Most older zoom units show the exact same patch of sky at all focal lengths, just stretched or shrunken. Thus when I use the Vixen, I can "zoom out" for a wide field to help find something, then "zoom in" for a better look. It is also fun to say "Quick, Chewie, the jump to lightspeed!" when the stars expand outward as I dial up the magnification. Furthermore, the zoom has enough eye relief to make it a good star-party eyepiece, when there are long lines of visitors and I don't want to have to keep refocusing the telescope when people to take off their glasses to get close to the eyepiece.
The Vixen zoom eyepiece delivers images about as good as a "generic" inexpensive Plossl.
I carry one of the regular Vixen Lanthanum LV eyepieces, too. It has a long eye relief, and serves as a star-party eyepiece when I need more magnification than the zoom provides.
- 5.0 mm, 1.25-inch barrel, Vixen Lanthanum LV
I also have a couple of Monocentrics, that I have been experimenting with:
- 12.0 mm, 1.25-inch barrel, Intes Micro Steinheil Monocentric
- 9.0 mm, 1.25-inch barrel, Intes Micro Steinheil Monocentric
- 6.0 mm, 1.25-inch barrel, Intes Micro Steinheil Monocentric
The Monocentrics are special-purpose planetary eyepieces, with the good and bad points that I discussed earlier.
With my 1.25-inch eyepieces I sometimes use a good Barlow lens:
- 2.0x, 1.25-inch barrel, Celestron Ultima Barlow
If I were going to work with a large, fast, Dobson-mounted Newtonian, I might well want Tele Vue Nagler or Panoptic, or a Speers-Waller, in a focal length of approximately 20 mm, as well as a Nagler or Speers-Waller at about 7 mm focal length. I might want a Tele Vue Paracorr Coma Corrector as well.