Home / Eyepiece Qualities and Aberrations
by Don Pensack 01/09/06 | Email Author
Voice your opinion about this topic in the forums
EYEPIECE PARAMETERS AND ABERRATIONS
EYEPIECE QUALITIES AND ABERRATIONS
By Don Pensack
eyepiece market, there is a lot of confusion about eyepiece characteristics and
What follows will be a basic guide, and I owe much of my text to
innumerable threads from Cloudy Nights and other web sites: "It's easy to stand
tall when you stand on the shoulders of giants".
My thanks go out to all of
you who have made these points clearer.
In my humble way, I hope to pass along
what I have learned.
I will not
review eyepiece types, here, nor go into every possible design parameter for an
I will merely define some terms in QUALITIES, and describe the usual
problems we see at the telescope in ABERRATIONS.
This can be measured by the use of a grid
or line test, but, if those tests are performed, the results are not
published, so we do not have tests to compare.
Our own eyes tell us which
eyepiece is sharper and which is less sharp.
There is more to sharpness
than merely the center of the field, however, and it is the resolution of
the outer perimeter of the fields of view that really set apart the superb
eyepieces from the good.
If you'd like to make your own test, use small
details on the planets and Moon to compare one eyepiece with another.
Subtle though they may be, differences will soon display themselves.
- Apparent Field of View.
This is the size of the field
our eye sees when looking into the eyepiece.
The manufacturers can quote
larger fields of view if allowing distortion at the edge of the field, but
that is for later discussion.
Suffice it to say, 30-45 degrees is
considered a narrow field, 50-60 degrees considered a normal field, 65-70
degrees considered widefield, and 78-85 degrees considered an ultra-wide
field of view.
Many people have trouble seeing the entire field at once
if the field is ultra-wide.
- True Field of View.
This is determined by the
Essentially, the field stop of the eyepiece encompasses a
linear field of view from the telescope.
The true field of view depends
on the linear scale of that field stop, as determined by the magnification
of the eyepiece in that telescope.
Telescope focal length divided by
eyepiece focal length equals magnification, and (approximately), the
eyepiece apparent field divided by the magnification equals the true
I say approximately because distortion at the edge of the field in
an eyepiece changes this relationship somewhat.
- Focal length.
Eyepieces have intrinsic focal lengths
and work as simple magnifiers of the focal image of the telescope.
Shorter focal lengths produce higher powers.
- Eye relief.
This is the distance above the eye lens
(the lens closest to the eye in the eyepiece) that the eye can be held and
still allow the observer to see the entire field.
There is no true
standard for measurement of this parameter, but it is usually described as
the distance the cornea is from the center of the eye lens.
glasses wearers will need 18-20mm or more of this parameter, whereas
otherwise eye reliefs can be as short as 8mm without being short enough to
cause the eyelashes to continually brush the eyepiece.
Eyepieces can be
found with shorter eye reliefs than that, but they are not comfortable to
use for long periods.
This is the most misunderstood of all
parameters of performance.
It will be very difficult to measure or come
up with a system of measurement for this characteristic, because it
describes many different characteristics all lumped together into one
It encompasses light scatter (the reflected/scattered light in the
eyepiece FOV), quality of coatings, light transmission, light absorption,
surface accuracy of the lenses, and all the aberrations eyepieces are
Ultimately, even the resolution (sharpness) of the eyepiece
comes into play when attempting to see the faintest stars.
I don't like
this term as it is applied to eyepieces, because so many external factors
come into play (the mirrors, seeing, transparency, fatigue, eye quality,
etc.) that have nothing to do with the eyepiece.
At best, a direct
comparison of two different eyepieces with the exactly same focal lengths,
used in the same scope, on the same night, on the same target, at nearly
the same time, and seen by more than one observer, can show you some valid
But change any of the factors I mention, and the outcome is
Apparent field even enters into the picture, as eyepieces of
small apparent fields seem to have their fields of view surrounded by
blackness. This elimination of peripheral light may allow the maximal
opening of the eye's pupil, reduce field brightness, and give a perceived
edge to the narrower field of view.
Dark adaptation is critical for
evaluating contrast, as the better dark adaptation will see fainter
And finally, eye problems such as cataracts will have profound
effects on the contrast evaluation.
Contrast is an elusive parameter, yet
we all use the word, so I put it in here as a descriptor of eyepieces.
- Chromatic Aberration.
Eyepieces are made with lenses,
and as such do not focus all colors at exactly the same point or with the
It is a very poor eyepiece that displays "blur circles"
of different sizes for different colors on axis, but it is not at all
uncommon for even excellent eyepieces to do so at the edge of the field.
chromatic aberration is chosen as the "lesser of two evils" when
correcting a nasty edge-of-field aberration that may be worse.
aberration can express itself as a color fringe to a bright object like a
star or the edge of the Moon, or it can express itself as a slight
prismatic effect seen when viewing sideways through the eyepiece (usually
on the edge-of-field objects).
Also, if an eyepiece design is created to
correct for color up to a lateral field size of, say, 50 degrees, but is
created with a 60 degree field, that final 10 degrees of field may display
color beyond the parameters of the designer, sometimes unpredictably.
Daytime use will often show the extent of the problem easier than any
nighttime viewing (except, perhaps, on the Moon).
Some early eyepiece
designs, such as Ramsden or Huyghens, do not correct well for this
aberration, and should be avoided.
- Field Curvature.
Most telescopes' final focal plane
is positively curved—that is, the focal plane is convex toward you and
slightly curves away as the field of view widens.
curvature is very slight in the narrow fields we observe through the
Nonetheless, it is there.
Eyepieces can have either negative
(concave toward you) or positive field curvature depending on their
designs, and this brings into play the idea of a telescope-eyepiece
interaction that points out that eyepieces will perform differently in
If the negative curvature of the eyepiece matches the
positive curvature of the telescope exactly, this can result in a
perfectly flat field of view through the eyepiece.
Neither the eyepiece
nor the telescope is perfect—merely the interaction between the two.
is why evaluating field curvature in an eyepiece is merely a statement of
how the curvature appears in your scope.
There may be other telescopes in
which the eyepiece does not present the same field curvature (though the
differences will be slight).
Field curvature is seen as a
defocusing of the star images at/near the edge of field.
infocusing (racking the focuser toward the objective) focuses the stars at
the edge, then the field curvature is positive.
If outfocusing is
necessary, then the curvature is negative.
Young people, whose eyes can
accommodate more focal differences than older people, will be bothered
less by this aberration.
Ideally, if an eyepiece has no field curvature,
all that will display is the very slight curvature from the scope itself.
Note that this is NOT the same aberration that causes a feeling
of "looking through a fishbowl" when panning the scope.
- Angular magnification distortion.
This is where the
magnification factor at the edge of the field is not the same as it is in
I see this all the time in well corrected binoculars, and it
is usually applied to correct for rectilinear distortion (I'll discuss
In an astronomical usage, this is not a severe aberration,
but it does play havoc with trying to figure out what the apparent field
or true field of an eyepiece is.
The speed with which an object will
drift across the FOV in an undriven scope is not linear, but changes with
distance from center.
This can result in a star-drift timing (to
determine the true field of view) that results in a true field that cannot
be derived from the apparent field quoted for the eyepiece.
As an extreme
case, picture sitting in space and watching a city on the Earth as it
first appears on the limb, then traverses the disc and finally exits the
It will move fastest when moving across the center, yet slowly when
entering and exiting the FOV.
A timing of the passage would lead you to
calculate a wider field for the image of the Earth than actual.
comparison, this would mean, in an eyepiece, that the true field of view
is wider than the apparent field would calculate.
And the reverse can be
true as well.
Fortunately for us, this distortion is usually small in
Most star drift timings of true field result in only small
discrepancies from the apparent field predictions of true field.
- Rectilinear distortion.
This is the distortion at the
edge of the field that causes straight lines to bow in toward the center
(called pincushion distortion) or bow out away from the center (called
It is usually unnoticeable in star fields, and is
often tolerated to correct for astigmatism, an aberration that is much
more easily seen.
It does mean that the geometric arrangement of the
stars in a field of view will be different at the edge of the field than
in the center, but this is easily tolerated.
This is a horrible
aberration for an eyepiece in binoculars, when used on land objects in the
daytime, but is no big deal in an astronomical setting.
who pan their scopes back and forth often get nauseated by the "passing
through a fishbowl" effect this can cause in the field of view.
usage involves a lot of scanning of the skies, this may be an aberration
you won't want to tolerate.
For most of us, though, it is so hard to see
that it is quite preferable to astigmatism, the aberration it is designed
to hold in check.
This is caused by the vertical
(sagittal) curvature of the eyepiece field being different that the
horizontal (tangential) curvature of the eyepiece field.
That this is
tolerated at all is due to the fact that not all forms of distortion can
be corrected at once.
If you want low rectilinear distortion, some
astigmatism may appear (and the converse, as I previously mentioned).
This, in daylight use, causes a defocusing/blurring of the edge of the
field of view.
At night, it causes the stars at the edge of the field to
appear as short radial lines on one side of focus, and short circumferential
lines on the other.
In focus, the star images may appear slightly blurry
or appear like seagulls or bats.
This is the most prevalent problem with
inexpensive eyepieces, and is worse when the focal ratio of the telescope
is short (say, f/4-f/5).
Astigmatism can also be caused by tilted elements in the eyepiece
housing, or wedge (faces of lenses not in same axial lineup).
also be caused by an interaction of the eyepiece design with the
astigmatism of the telescope's objective or the improper tilt of a mirror,
which is why we are looking for an aberration that is equal in all
directions from center in the FOV, where the eyepiece is concerned.
- Spherical aberration.
With multiple elements, this
would seem to be tightly controlled in eyepieces, yet it can be an issue.
This would manifest itself as different parts of the axial ray (or all
rays, for that matter) coming to focus at different places.
The result is
a blurred image (one that doesn't focus well or seems to have a long range
of best focus) that cannot be sharply focused.
I will state that the
amount of this present in eyepieces is so small compared to the objectives
that, to all intents and purposes, it is not there.
What tiny amounts are
present would largely go unnoticed.
- Spherical aberration of the Exit Pupil.
This is found
in some eyepieces and is described as having different parts of the
eyepieces exit pupil come to best focus at different distances from the
How you would see it is that at different distances away from
the eyepiece, you would see the outer edges of the field, or the center,
or one edge or the other, but not at the same time.
The field of view
would appear to have kidney bean-shaped dark areas drifting around the
field, depending on where you were holding your eye.
There would be only
one position for the eye that would result in most of the field of view
being visible and in focus at the same time, and you might have to rock
your head from side to side to see the edges of the field, one after the
In essence, the exit pupil of the eyepiece, instead of being a
small, circular plane, is a curved surface, usually curving away from the
eye in all directions from center.
The original Nagler Type 1 eyepieces,
especially the 13mm, displayed this aberration, and the correction for it
was the genesis of the Nagler Type 2.
Most of the other eyepieces
exhibiting this characteristic are long focal length eyepieces, or
eyepieces with long eye reliefs, though it should be noted that blackout
problems with an eyepiece do not necessarily indicate spherical aberration
of the exit pupil.
- Transmission anomalies by wavelength.
This is exemplified
by an eyepiece's not transmitting all wavelengths of light with equal
At best, it means a light rolling off of transmission at the
extremes of the visual spectrum.
At worst, it means a noticeable tint in
the field of view, especially on the Moon.
This is an aberration in all
The difference is only in severity or noticeability.
This is a loss of edge brightness
(transmission anomaly by distance from center) due to improper lens
diameters (one element unable to field the entire set of rays from the
preceding one), barrel diameter (too small an internal diameter to pass
all edge-of-field rays), or simply normal design (a 40mm eyepiece in a
1-1/4" barrel will vignette rays at the edge so that, regardless of lens
diameter, the field of view will be truncated by the barrel's entrance
The causes of vignetting, where it is described as being due
to the size of the secondary mirror, is really an improper relationship of
the field of the eyepiece and the telescope's focal plane.
It is not the
eyepiece that is vignetting, in that case, but the use of too large an
eyepiece for the telescope's illuminated field.
Vignetting in an eyepiece
is harder to see, and can often be seen only by holding the eyepiece up to
a bright sky and looking at the edges of the field.
If it noticeably
darkens at the edge, there is vignetting involved.
If it doesn't
noticeably vignette, it could still be there in lab tests, but is unlikely
to cause problems in viewing.
Yes, eyepieces can have coma.
It is the same
as coma in a short focal length lens or mirror, but is significantly
smaller in quantity.
It expresses itself, usually, as a radial unsharpness
in the star images, as they move from center to edge, that gets gradually
worse toward the edge.
Because astigmatism is likely to be more severe
and more noticeable, I am not sure how you would tell the eyepiece has
coma other than by ray-tracing the design.
It might be possible to notice
it in a completely flat-field telescope that lacks coma (f/30 refractor?),
but in the real world, coma is 99.99% an issue with the primary objective.
Very simple designs can exhibit coma, but you are staying away from
these, aren't you?
- Light loss.
This can be caused by back reflection
from lens surfaces, absorption by the lens elements (lack of transparency
or tint), scatter from the lens surfaces causing destructive interference
in the wavefront, and internal vignetting.
I talk about each of these
issues separately, but I lump them together as light loss.
the reach of your telescope is dependent on the maximum transmission of
The eyepiece is merely a link in the chain.
- Wavefront aberrations.
This is similar to the
problems caused by an inaccurate mirror surface, except that an eyepiece
has many such surfaces.
The reduction in the quality of the final image
can come from poor polish on the lens surfaces (+/- wavelength %), poor
figure (the lack of correspondence of the surface curves to design
parameters—like the Hubble, originally), or an increase in surfaces.
is where fewer elements is often better.
Unless the surfaces are all
perfect (and that is HIGHLY unlikely), the more surfaces there are the
more likely the final image's quality is likely to be reduced by these
Unfortunately, one manufacturer's 8-element eyepiece may
have a final wavefront that is more accurate than another manufacturer's
3-element eyepiece, so we are truly generalizing when we say fewer
elements are better.
In specific, this may not be true.
- Loss of contrast due to light scatter.
This is not,
technically, an aberration, yet it is a problem in eyepieces that causes a
diminution of the final image quality.
It is caused by light scatter due
to poorly polished optical surfaces, lens reflections due to edge-of-lens
reflection, lens reflections due to poorly applied or absence of coatings,
low angle-of-incidence scattering from the lens coatings, and shiny
internal surfaces in the barrels and baffles.
It is exemplified by a
"graying out" of the background sky in a given eyepiece.
apertures and/or lower powers show lighter background skies, the only way
to really tell about the presence of this one is to compare another
eyepiece of exactly the same field of view and focal length, or to put a
bright object just outside the field of view and see if you can detect any
internal evidence of the direction in which the bright object lies.
if a bright object leaves the field, there should be no visible evidence
of its direction left in the field.
Likewise, there should be no halo
around any object, no matter how bright, that changes the darkness of the
background sky around the object.
In practice, many such
problems are caused by the eye, or dirt or dew on the optics, so steps
should be taken to minimize those issues before any form of evaluation.
Ideally, this is one that should be measured on a test lab's bench, but we
don't have access to such data as of yet.
- Thermal issues.
If an inadequate clearance is left
between the housing and the internal lens elements, as the barrel shrinks
it may squeeze and/or bend the internal elements.
This is more likely to
be a problem in larger eyepieces, where the temperature differential
between the housing and the internal elements is likely to be the highest.
Big eyepieces have to come to thermal equilibrium,
just like telescope objectives, in order to give their best images.
Fortunately, the eyepiece will be at equilibrium before the mirror.
is a valid reason NOT to carry the eyepieces in a coat pocket or to store
them in a closed case until used.
- Design flaws in the eyepiece.
I've lumped these
together, even though it is several issues.
They are indicative of
qualitative issues with the eyepiece design, and can indicate why you may
not want to buy any of said eyepieces.
Field stop not in focus (improperly placed field stop results
in vague field edge.
index of glass used, resulting in aberrations not in the original design—this
can be true of later versions of an earlier design.
Critical f/ratio too high—wherein the eyepiece
manufacturer designed the eyepiece to adequately field the narrow light
cone of an f/15 refractor, but not an f/4.5 reflector.
think all eyepieces should be designed to handle the wider f/4 light cone
Why should we have to become aware of which eyepiece does or does
not work in our f/4.5 reflectors?
This is a problem with many companies,
Improper internal design, leading to
vignetting or internal reflections.
These are issues easily addressed,
yet so many eyepieces do not.
It may mean a poor optical design, a poor
manufacturer, or too tight a budget to produce a good eyepiece.
the reason, this is a good reason to eschew the purchase and use of these
eyepieces until the problems are solved.
very liberal in my use of the word aberration to include any deviation from a
perfect image at the exit pupil of the eyepiece.
It must be noted that these
aberrations, though real, detract less, usually, from the final image quality
than do the grosser aberrations of the primary mirrors and lenses.
address, in designing the optimum optical system for one's budget, the quality
of the mirrors and eyepieces together, for, ultimately, it is the combination
of these elements that produces the final image.
we, in the absence of test-lab data, to come to any conclusion about the
quality of any one of the over 1100 eyepieces currently on the market?
Ask users their opinions about their eyepieces.
Read on-line reviews.
Read magazine reviews.
Read books on the
subject of telescope optics.
Test for yourself by experimentation.
day we'll get quantified lab tests.
Until that day, just be aware that there
is no perfect eyepiece—merely ones that are good enough for you.