Sunday, September 06, 2015


Are my Eyepieces Good?

Measuring eye relief...
Well, are they? Sure they are if you enjoy using them, but are they as good as they could be? Are you getting your money’s worth? What got me started thinking about testing eyepiece quality was that I recently completed a magazine review of a new series of oculars and had spent several weeks putting them through their paces. Frankly, despite what the eyepiece fanboys on the Internet forums would have you believe, it’s easy to test eyepieces, and I thought you might like to learn how to do so if you don’t already know.

Eyepieces or “oculars” are fairly simple things in the grand scheme. They are essentially sophisticated magnifying glasses. Your telescope produces an image without an eyepiece, but it is really too small to show much detail to the eye. You need a magnifying glass to bring out details, and that is the role of the eyepiece.

Let’s get ocular vocabulary definitions out of the way before we begin. Apparent field of view (AFOV) is the size of the eyepiece field circle measured in angular degrees. It is not how much area of the sky the eyepiece takes in. That is true field of view (TFOV), which is also expressed in degrees. Apparent field is analogous to the size of your TV screen, while true field is the size of the scene shown on the TV. A 12-inch TV can display a shot of the whole globe of the Earth, but the view is much more immersive on the 60-inch. Apparent field also determines how much true field you get; more AFOV equals more TFOV in eyepieces of the same focal length.

The lens you look into is the Eye lens, the out-facing lens of the eyepiece. The field lens is the lens element closest to the telescope. In modern wide-field eyepiece designs there may be as many as six (or more) internal lenses between eye lens and field lens.

Eye relief is an important ocular characteristic. That is the measure, always in millimeters these days, of how far you can hold your eye from the eye lens of an eyepiece and still see the whole field of view circle. If you must wear glasses while observing (because you suffer from astigmatism), you need as much eye relief as you can get.

Focal length of an eyepiece is the number that determines its magnification. Magnification is equal to the focal length of the telescope divided by the focal length of the eyepiece. So, a shorter focal length ocular delivers more power. A 10mm focal length eyepiece in a 2000mm focal telescope yields a power of 200x.

One last term you will hear occasionally is “Field stop,” which is the metal ring inside (some) eyepieces that limits the size of the field. It is analogous to the iris diaphragm in a camera lens, but unlike a camera iris it cannot be changed.

Now, let's have some ground rules as to what to expect. First, simpler eyepieces, eyepieces with fewer lens elements, generally produce brighter images. That is still true despite modern lens coatings, but not as true as it used to be. Simpler eyepieces perform less well in fast focal ratio telescopes, though, with “fast” beginning at about f/5.

Wide-field eyepieces tend to be harder to make well. It’s difficult to make an eyepiece that has a wide field of view and few optical aberrations, especially at longer focal lengths. Wide field eyepieces also tend to have shorter eye relief. So, the best reasonably priced eyepieces are often of medium focal length (15mm – 20mm) and possessed of moderate apparent fields (less than 70-degrees or so). If you want “long” and “wide,” expect to pay for it.

How about color rendition? Some eyepieces tend to produce warmer tones, and some cooler. Saturn might look more yellowish in one design, and have a slightly blue cast in another. This color cast is usually subtle and not disturbing.

OK, let’s start testing. The first things you’ll probably want to know are the eyepiece’s eye relief and its apparent field. The manufacturer probably lists these things on their website, but if you’re like me, you want to know for sure.

Eye relief is easy to determine. Hold a white card up to the eye lens of the eyepiece (out of the telescope, natch) under scrutiny, and point it at a distant light source. Move the card back and forth until the image projected on the card is sharp. Now, measure the distance between card and eye lens as precisely as you can with a ruler.

Apparent field is also easy to determine, if a bit more work. The simplest way to figure it is to work backwards from true field. How do you calculate the true field of your scope/eyepiece? You can calculate it from the eyepiece field stop diameter. Divide the field stop size (in millimeters) by the telescope's focal length and multiply the result by 57.3. Unfortunately, you probably won’t know the field stop diameter; most manufacturers don’t publish that figure and it can be hard to measure. Luckily, there is another way to arrive at true field, by timing the drift of a star near the celestial equator across the center of the eyepiece’s field.

Choose a bright star as close to the equator as possible, get your stopwatch (your smart-phone should have one) ready, and shut off the scope’s drive if it has one. The only problem is positioning the star so that it drifts precisely across the center of the field. There’s a trick to get around that. The human eye/brain combo is very adept at centering a bright point, like a star, in a circle, like an eyepiece field. Center the star, let it drift, and time how long it takes for it to leave the eyepiece. Multiply that value by two and you are done with the field work. To reduce error, repeat your drift timing two more times and average the three resulting values.

TeleVue coma corrector...
Back inside, your first task is to convert your minutes/seconds figures to minutes and decimal minutes. 1-minute 30-seconds won’t work in the formula we’ll be using to determine TFOV. That time has to be expressed as 1.5-minutes. Covert to decimal minutes by dividing the seconds figure by 60. When you have done the conversion, determine true field of view by dividing your result by 4. For example, if the time was 7.5 minutes, the TFOV of the eyepiece in question is 7.5/4 = 1.8-degrees.

While it’s nice to know TFOV, what we are really after is the size of the spaceship porthole, the apparent field, the size of the field of view circle displayed before you in angular degrees. To find that, determine the magnification of the eyepiece under test by dividing the focal length of the telescope by the eyepiece’s focal length. Multiply that by the TFOV and you have AFOV. If the eyepiece has a measured TFOV of 1.7-degrees, and its magnification in the telescope is 59x, its AFOV is 100-degrees (59 x 1.7 = 100).

It’s sometimes useful to know the size of an eyepiece/telescope combo’s exit pupil. That is the diameter of the light cone emerging from the eyepiece. Why would you want to know that? If your pupils, like those of most middle-aged folks, don’t dilate more than 5 or 6mm, an eyepiece that delivers a light beam 7mm in diameter is wasting light (which can be the case with small short focal length scopes and long focal length oculars). To get the exit pupil diameter, divide telescope aperture in millimeters by magnification. For example, a 100mm aperture scope with a 10mm eyepiece gives a power of 100x. That means the exit pupil is 1mm in diameter (100/100 = 1).

What else can you/should you check? How heavy is the thing? If you’ve got a postal scale or similar you can find out. How are the lens coatings? When you hold the eyepiece up to an oblique light do the coatings (usually green these days) look even? You should see minimal internal reflections. How about the barrel on the scope end of the ocular? Is it painted a flat, dark shade of black on its interior to suppress reflected light? Filter threads should be there, but be aware that since nobody has ever standardized eyepiece filter thread pitch, not all filters will screw smoothly onto all eyepieces.

Finally, how about the physical properties of the ocular? Does it look professionally made and feel solid? Does the eyepiece's shape get in the way of putting your eye to the eye lens? Unfortunately, a few ultra-modern barrel designs make that hard. Some newer Celestron eyepieces, for example, seem designed to give you a hard time when it comes to eye placement.

After you have determined where your eyepiece stands vis-à-vis its specs and its physical characteristics, it’s time to check it for aberrations.

"The Stig,"  Astigmatism...

Nothing makes Joe and Jane Newbie more depressed and disappointed in their eyepieces than coma, the aberration that makes stars out toward the field edge look like fraking comets. That’s why it’s called “coma,” natch. The irony is that eyepieces don’t typically introduce coma. It’s an effect caused by fast telescope optical systems, mainly in Newtonians. Coma is almost unnoticeable at f/8, visible at f/6, prominent at f/5, and disturbing below that.

So what do you do about coma? With one exception, an eyepiece, no matter its cost or pedigree, won’t help. There was one eyepiece design, the Pretoria, that corrected for coma, but it wasn’t a very good eyepiece design and has long since been made obsolete by coma correctors like the TeleVue Paracorr. To reduce coma in a Newtonian, you use a coma corrector which, to put it simply, is like a very special sort of Barlow. Or you just concentrate on the field center and ignore the "comets" toward the edge. If you can’t do that and don’t want to buy a coma corrector, use higher magnification. Higher power eyepieces and narrower field eyepieces won’t show the hairy edge of the light cone where coma is worst and your field will look better.


Not all of the nastiness at the field edge is caused by coma. Some may be from astigmatism, an aberration found in many eyepieces. You’ll know you have it when you plug in the coma corrector and the stars at the edge of the field of your Acme 100-degree Wonder still look like hell. Now, astigmatism can be in your eye, in the telescope, or in the eyepiece, so how do you know if it’s the eyepiece or not? A tip-off is that in oculars it’s sorta like coma; stars at the center are good, but those at the edge are distorted things that look like lines, seagulls, or even crosses.

What do you do? What do you do? There’s not much you can do, really, except buy a better eyepiece. TeleVue makes a corrector lens, the Dioptrix, for astigmatism, but that is to correct astigmatism in your eyes, not the eyepiece. If the stars at the center are round, even when thrown slightly out of focus, the problem is likely the eyepiece. If the elongated axis of the star changes its position angle, its orientation as you move your head around the eyepiece, when you change the angle of your head in relation to the eyepiece, the problem is (or is in part) in your eyes. Keep in mind that there can be a combination of the three causes, astigmatism in eye, eyepiece, and telescope. Luckily, most people find astigmatism in eyepieces less disturbing than telescope coma.

Curved field...
Kidney Beaning

You are happily enjoying your view of NGC Umptysquat when suddenly the field blacks out. It’s like what your old Admiral portable TV from the 1970s did when it went on the fritz. Maybe the field doesn’t completely black out. Perhaps what you see are dark patches in the eyepiece, oval, bean-shaped patches.

The cause of kidney-beaning is improper eye placement. Get your eye off the optical axis and you will see these effects with many eyepieces, even rather expensive ones. Cheer up. After some practice with a new eyepiece, you’ll probably find it easy to keep your eye centered. The TeleVue 35mm Panoptic is a wonderful ocular, but when I first got mine I had kidney-beaning out the ying-yang. Wasn’t long before I got used to centering my eye on the new eyepiece, though and all was well. Be aware kidney-beaning is common with longer focal length oculars. In fact it is almost inevitable with them.

Curved Field

If the stars in the middle of the field are in sharp focus, but those at the edge are slightly fuzzy (but not distorted) you are suffering from field curvature. Many if not most scopes, and particularly faster refractors and non-corrected (non Edge design) SCTs, have curved focal planes. But eyepieces can contribute to the problem. How can you tell if the problem is in your eyepiece? The easiest way is to check it in a longer focal length telescope. If the problem is mostly the scope (which it usually is) field flatteners are available for both refractors and SCTs, the scopes that generally need them most.

Lateral Color

What’s troubling you, Bunky? Not only do you have comet shaped stars at the field edge, they have blue and/or red fringes? Most modern eyepiece are well color corrected for objects near the center of the field, but some—especially longer focal length and wider field oculars—are not so good at the field edge.

What can you do? Mainly, position your eye properly. Having your eye off the eyepiece’s optical differential refraction. If you are using an achromatic refractor, check the eyepiece in a known color free telescope like a Newtonian. What can you do about lateral color? Stop obsessing about the field edge or open your wallet is about it.
Lateral color...
Having your eye off-axis, not centered, can exacerbate color fringing on stars and bright objects like the Moon’s limb. Be careful you are not being fooled into thinking your eyepiece has a problem here, though. Test it only on objects over 30-degrees above the horizon to make sure you are not actually seeing an atmospheric effect called "differential refraction."

Pincushion and Barrel Distortion

In Pincushion distortion, parallel lines are bowed in as they cross the center of the eyepiece. Barrel distortion is the opposite; they are bowed outward. You can check for these problems by viewing distant boards on a fence or on the side of a neighbor's house during the day.

What can you do about these things? Get another eyepiece. But keep in mind that you probably won't even notice barrel or pincushion distortion during normal observing. It tends to be obvious only when you are looking through the eyepiece while slewing the scope fairly rapidly for a good distance.

Sharpness and Contrast

Now we come to the subjective stuff. How good is this hunk o’ glass really? How good does stuff look in my eyepiece? After a few years in the game, you’ll come to know what a good image is. If you are a novice, you can learn by looking through your friends’ expensive eyepieces. Yes, that Ethos will probably ruin your Orion MegaDoodle ocular for you, but you need to know what perfection is. Even if you decide you don’t need (or want to pay for) it.

I know what a perfect image is, but if you are a long-time reader here you know I absolutely love my humble Happy Hand Grenade, a 100-degree Zhumell eyepiece I bought when it was one of the few 100s that didn’t cost close to a grand. Not perfect, but I enjoy using it, I can overlook its imperfections (it probably suffers from every one of the above problem to one degree or another), and it makes me happy in spite of them. That’s the real task when it comes to choosing eyepieces, deciding what can make you happy. Do that and you are golden, Ethoses or no.

Thanks for the good reading.
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