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.
 |
| Coma... |
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... |
Coma
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.
Astigmatism
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... |
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.
