THE HUMAN EYEUnderstanding the most important "accessory" in astronomy
Written and Copyrighted by P. Clay Sherrod
During the past years I have written much on how your astronomical telescope is
governed by certain "functions" and have gone into great detail about limiting magnitude
of stellar objects which you can expect to see, sky transparency, the steadiness of the
air, and observing "tricks" for discerning even the faintest of detail on planets and deep
From these I hope you have gleaned some appreciation for the "work" that your telescope
can provide you.....but, on the other hand, we have up until now MISSED discussion the
MOST IMPORTANT component of your telescope system:
EYESIGHT AND TELESCOPE PERFORMANCE
Your telescope can provide the finest of astronomical images, many times approaching and
surpassing its theoretical limits on light grasp, resolution and magnification. Yet,
consider this. A lot of what you see through your telescope: the moon, Venus, Mars,
Jupiter, Saturn, the Andromeda galaxy and Orion nebula, are ALL visible to the eye WITHOUT
Keep in mind that this is not just a "visual observing" testiment to the human eye;
without good eyesight, your image processing, research and evaluations of all that you do would be far more difficult
The telescope simply makes celestial objects easier to see. Indeed, the most sophisticated telescope,
when coupled with visual observing, is nothing more than a fancy pair of eyeglasses. An
extension of the eye itself.
Look at the two charts in the overview of Seeing and Transparency published in this web
site: there are two charts of star clusters, the Hyades (for naked eye and binocular
acuity testing) and the Pleiades (for telescopic acuity). ACUITY is merely your eye's
(coupled with the telescope in the case of the latter) ability to see the faintest star
Imagine this: if all of our eyes were the same, we WOULD NOT HAVE TO TEST to see what our
acuity really is! Y ours would be the same as mine (Heaven forbid), and mine the same as
everyone else's. But they are all considerably different for many reasons including:
1) Natural eye sensitivity - you are born with this, some people having much more acute
eyesight than others; age and color of the eyes actually plays into this;
2) Age - the older you become, the less acute your vision can be;
3) Eye Fatigue - just like when reading a book or using a computer, your eyes can become
fatigued when using a telescope for high intensity or prolonged viewing;
4) Position of the eye - in relation to what it is being viewed (averted vs. direct viewing);
5) Dark adaptation - the eye becomes considerably more sensitive to subtle light
variations when totally dilated and equalized to its dark surroundings; and,
6) Eye defect and disease - AND DON'T WRITE THIS ONE OFF! We all have at least some degree
of defect in our eyes as you will come to agree!
THE NATURE OF OUR "FINEST EYEPIECE"
Your eye is an eyepiece that you do not need to carry in the accessory case. You never
need to clean it, but it gets dirty. It does not have an accessory case, but it is very
well protected. It is subject to the extreme cold and heat, but never fogs over. It is not
multi-coated "nine times" but provides the ultimate in light transmission and a minimum of
The only drawback to this marvelous telescope accessory, this human eye? You cannot trade
it in for a bigger one.
Before moving on, let's briefly look inside this accessory and see how it works. Do not do
this at home, or your eye's warranty will likely be voided.
In the accompanying figure you will note that the eye gathers light (just like the
telescope - see the red lines), focuses the image through a LENS and transfers it to an
awaiting screen, the RETINA. The retina is much like a piece of photographic film which
receives the light of the image (upside down) and passes the data through the OPTICAL
NERVE into the brain where it is turned upright (at least in our minds) and fine tuned,
just like a computer imaging program does for CCD recording of deep sky objects.
In some of my reports covering viewing fine planetary detail or very faint stars near the
threshold of your telescope's capability, I have mentioned many times a technique known as
"averted vision," whereby an observer can not look DIRECTLY at an object (which focuses
the light on the eye's blind spot) but rather slightly to one side of the object, yet
still be perceptible of the image itself. Such "averted vision" allows one to use the
eye's more sensitive receptors.
The blind spot in your right eye, if that is the eye you observe with, is located to the
right hand side of direct center, so concentrating your vision slightly to the left will
increase your ability to see finer detail and color. It is the opposite for the left eye.
The very center of your eye is known as the YELLOW SPOT which contains only slightly
sensitive CONES. Surrounding them are the more sensitive rods in combinations with more
The light falls upon RODS (about 120 million of them in each eye) and CONES (only 7
million); Rods are more than 10,000 times more sensitive to light than are the cones,
which are color sensitive in daylight and only in very good light.
So, literally speaking, it is best to look "out of the corner of your eye" when observing
planets, faint deep sky objects and very faint stars; attempt to keep the light focused
away from the "blind spot" immediately centered in your eye!
The idea of eating carrots for Vitamin A is not taken lightly; vitamin A allows you to
maintain the sensitivity of both rods and cones as your age increases. There are many easy
maintenance steps that you can do on a regular basis to MAXIMIZE your eyes' ability to
provide the very best image possible, even if you are not born with exceptionally good
Such steps are discussed in the section "EYE MAINTENANCE" following.
1) NATURAL EYE SENSITIVITY
Some people are born with more sensitive eyes than others. Indeed, although the jury is
still out on this one, there appears to be evidence to suggest that people with "lighter
color" eyes (i.e., blue and green) are more sensitive to subtle color and detail than
those with darker eyes. Eye color, as we all know, is NOT an effect of the actual focusing
nor image forming (the retina area), but rather due to the pigmentation of the eye's IRIS.
Hence, it may be difficult to accept that a lighter colored IRIS (which serves primarily
to "stop down" the eye for bright light and "open it up" for darkened conditions) could
have any direct bearing on retinal activity.
Nonetheless, some people are far more visually acute than others; indeed, if using a
six-inch telescope, some people can see stars of magnitude 13.5, while others of average
vision may be doing good to see a 12.5 magnitude star. The same discrepancy will occur in
planetary viewing of fine detail as well.
Many times such lack of visual perception is due only to eye defect or disease which is
discussed in Section 5.
2) AGE OF THE OBSERVER
It is unfortunately true that our eyes become less sensitive as we age, primarily from
loss of receptors (physically deteriorating) and from loss of sensitivity of those that
still work! Many times sensitivity can be DECREASED by any person doing repetitive
conditions such as:
a) working or playing in bright sunny conditions without eye protection;
b) untreated internal eye disease such as Glaucoma;
c) exposure to prolonged eye focus, such as computer work or assembly line work;
d) excessive reading and/or drawing;
e) very close activity such as sewing, model-building, electronics, jewelry making and
One fact of the loss of eye sensitivity due to excessive rod and cone loss is plain and
indisputable: once they are gone....they are gone. Eyesight is something that cannot be
"revitalized" with a pill or a drop if there is slow degradation of rod or cone reception.
One final thought on aging eyes: the eye tends to age more rapidly beginning at the very
center, in the aforementioned "yellow spot," and moves gradually outward. Consequently,
amateur astronomers with less-than-perfect eyesight might attempt to sharpen their
"averted vision" skills even more and greatly improve their observing prowess.
3) AND 4) EYE FATIGUE - EYE POSITION
Probably the most plaguing of all eye issues for amateur astronomers is the EASIEST TO REMEDY: "eye fatigue." Not that I have to tell most
of you, but sure signs of eye fatigue - and LOSS of acuity for astronomical viewing (yes,
the two go hand-in-hand more than any other factor) include:
a) dry itching feeling behind the eyelids;
b) a seeming inability to keep the eyelids "propped open;"
c) excessive tearing or - oddly - excessive dryness;
d) matting of this mucous in the corners of the eye;
e) headaches "behind" the eye sockets; and,
f) difficulty focusing for long periods and/or difficulty concentrating.
I have always told my astronomy students to get a good rest before an observing project,
particularly if it is a visual one. This is not to rest the body so much as it is to rest
the mind and the eyes - remember, THEY are an essential part of your telescope!
Try to practice this rule, particularly if you have had a hard day and physically feel
tired (yet you STILL want to observe): we know that our telescope MUST equalize to the
outside air in both summer and winter, so we plan ahead and put them out about one- to
two-hour PRIOR to viewing, depending on the size of the primary lens or mirror. You may as
well NOT waste your time if you do not allow your telescope to "cool down;" so, knowing
that you HAVE to do that....why not set up the telescope outdoors, come back in and relax.
DO NOT READ A BOOK! Don't read the paper, and minimize the evening news; simply let your
eyes REST. You'll be surprised just how this technique not only rests your eyes for a
night's viewing....it rests YOUR MIND and attitude as well. Try it...you'll be surprised at
Eye fatigue also sets in after a hour or so of strenuous viewing through the telescope, or
particularly "guiding" a long exposure photograph; your eyes are really taxed to the limit
and so is your concentration after about one hour, so be sure to give yourself some
Remember that we use our eyes constantly, every waking minute. And like other organs of
the body they require rest over and above merely sleeping at night.
There are two big problems that are paramount in telescope viewing ability and pleasure
that you experience when looking through your telescope. Both become exaggerated over
prolonged periods of using your eyes.
a) DRY EYES - some people, myself included, have a condition in which the eye does not
produce enough natural lubricant (or "tears"); in some cases, no tears whatsoever can be
produced. In other people with "normal" tearing, the production of tears can slow or even
cease after prolonged stress on the eye. This condition is known as "dry eye," and is more
serious for you observing than may first appear.
Tears are a necessary part of the eye's ability to FOCUS; the LENS of the eye of course
does most of the focusing, but it is designed to depend on tears to do a small fraction of
the focus; if there are no tears to coat the eye, then your ultimate focus will most
definitely be "off."
There are two methods to replenish tears for any eye: 1) "artificial tears", drops that
are available at any pharmacy; use as much as necessary and as often as needed; and, 2)
"rolling your eyes," a technique that will also be used in the following discussion
concerning "eye floaters;" merely close your eyelids and roll the eyes gently one way, and
then the other.....repeat and leave eyes closed for a brief moment. Then return to
You will surprised what a difference having a lubricating film across the eye will do for
visual acuity and your ability to simply focus like the pros!
b) EYE "FLOATERS" - Just like in a crime scene at a lake, a "floater" with the eye is a
dead body; only the "body" is a detached cone or rod (see discussion above) that has come
loose from the retina and floats amidst the thick AQUEOUS HUMOR, the liquid filling the
As gross as it sounds, we ALL have floaters but some have more - and larger - than others.
Typically, unless two or more floaters become entangled which they very often do
permanently (I've had two sets since I was 17 years old that are like old friends to me
now), you will not notice them. However, large floater masses are clearly visible when
1) looking at bright sky or bright objects on a sunlit day;
2) when reading a book on your lap under a bright light; and,
3) when tee-ing off a golf ball on a bright day.
Notice I did not include looking at a computer screen; seems a lot like reading the book,
doesn't it? Not with floaters as can be clearly seen in the attached diagram.
Floaters are an absolute nuisance for visual astronomy; you will be looking for a tiny
festoon on Jupiter and here comes this horrible looking, amoebae-shaped creature floating
across the field of view! And then it stops....dead in its tracks right in front of your
image of Jupiter!
You can clearly see why this happens in the diagram: floaters are suspended in the aqueous
humor and move only according to gravity and rapid motions of the eyeball. When you are
looking at the computer screen, the television, or at a ball game, the floaters respond to
gravity and settle like pond sediment at the bottom of the eye cavity. On the other hand,
looking at the lower drawing, you can see that if your are reading a book on your lap,
looking down to drive a golf ball, or - LOOKING IN YOUR TELESCOPE, the floaters respond
again to gravity, only this time right between your eye lens and the retina! You SEE them!
To get rid of pesky floaters, move back away from the telescope and rotate your eyes in
their sockets rapidly back and forth while holding your eyes skyward (of course, leave the
eyes in your head for safety). This will shuffle them up and they will simply move out of
They WILL, of course return in time. Merely do the same procedure again. Rest
assured....they WILL NOT go away. Like I said....in time they will become easily
recognized "old friends."
5) OPTICAL DEFECT AND DISEASE
No eye is perfect and we all suffer from some degree of optical defect and/or disease.
Indeed, we ALL have floaters....that is "optical defect." Another optical defect that will
interfere with your focusing and acuity is a corneal scratch or scar.
People who work outdoors, such as construction workers, lifeguards and lumberjacks
frequently suffer from another optical defect in which the "yellow spot" immediately at
the center of the retina has become burned and scarred from exposure to sunlight over very
long periods of time. This is particularly common for persons living on or near bright
beaches and those who enjoy boating as a hobby.
Eye disease unfortunately can take many forms and affect very difficult viewing, such as
that necessary for astronomy. Eyesight correction requirements is a form of eye disease,
requiring only the fitting of proper glasses.
IF YOU WEAR GLASSES, take them off to observe; do not attempt to focus with them on for
three very important reasons:
1) you cannot enjoy the full field of view provided by your telescope/eyepiece combination;
2) your visual acuity is much better without the glasses,simply focus for your eyes using the telescope focus to compensate for your eyeglasses;
3) your optics of your multi-hundred/thousand-dollar telescope are NOT BETTER than
the cheap lenses of you glasses!
REGARDING DISEASES OF THE EYE which can affect your telescopic viewing:
the many minor/major infections on the outside (cornea) of the eye will greatly hamper your ability
to see the faintest star or the finest detail; drops help greatly, but in many cases,
physicians recommend complete REST of the eye, and observing with your telescope is NO
EXCEPTION to this "prescription."
Perhaps the most serious and detrimental to astronomical use of you eyes is the widespread
internal eye disease of GLAUCOMA, resulting in considerably increased internal pressure of
the eye; this has an end result of focusing difficulty, poor retinal perception, loss of
peripheral vision, and many other adverse conditions. However, ophthalmologists can
prescribe eye drops that, if taken properly, can maintain a near-normal eye performance
both in astronomy use and in everyday life.
I hope this helps you to better understand how important the eye is in astronomy; there is
not a day that goes by that I don't look in my accessory case and remember WHY I have been
given the privilege to even look there in the first place! I have the greatest accessory
available - my eyesight!
Treat YOUR precious accessory with the best of care and there will never be any need to
"upgrade" or trade in on the latest version....they just don't get any better!
Dr. P. Clay Sherrod
Arkansas Sky Observatories