Eclipse Observing and Vision Safety
Solar observing. The sun emits intense
radiation in the infrared, visible and ultraviolet bands of the
electromagnetic spectrum. We protect ourselves, at least partially,
against the infrared and ultraviolet wavelengths of light with hats,
sunscreen lotions, or by seeking shelter out of the sun’s direct rays.
We deal with the visible wavelengths of light, reflected from bright
surfaces, with sunglasses. Sunglasses, however, do not provide
protection to the eyes from looking directly into the sun.
Figure 3-1 The Electromagnetic Spectrum showing the location of visible
light with ultraviolet and infrared rays on opposite wavelengths to the
visible spectrum. Illustration by David Frantz.
Our eyes are very sensitive to infrared, ultraviolet and intense visual
solar radiation, and can easily sustain temporary or permanent
damage—even blindness—from staring at the sun. The condition is
called solar retinopathy (i.e., burns on the retina) and symptoms may
not appear for hours after staring at the sun. In mild cases, symptoms
may disappear; in moderate or severe cases, permanent loss of vision or
portions thereof result.
This condition is not unique to a solar eclipse,
contrary to what many are led to believe by well–meaning astronomers
and medical professionals, but is caused by looking at the sun. The
emphasis on safety for the impending solar eclipse results in
unfortunate wording in media interviews that so convince people not to
look at the sun “during the eclipse” that they mistakenly believe it
is safe to do so once the eclipse is over.
The sun at totality. During—and only
during—totality, it is perfectly safe to look at the totally eclipsed
sun, without any filtering devices, even through telescopes. Obviously,
during totality it is the new moon which is being observed in silhouette
against the sun’s outer atmosphere, and it is safe to view the moon
without special filters.
However, great care must be taken to insure that
such viewing does not begin until the last bead of direct sunlight winks
out, and even greater care must be taken to insure that observations end
before the edge of the sun reappears from behind the moon’s limb at
the end of totality. This latter task is best left to a reliable
volunteer timekeeper, or tape–recorded countdown, to warn of the
approach of third contact at the observer’s site, than to the
preoccupied observer’s personal judgment.
Lunar observing. The moon, whether in or out of
eclipse, presents no danger to the eyes. Thoughtful amateur astronomers
should not assume that lay persons or the media know this, and should be
prepared to field their concerns and inquiries at times of lunar
eclipses with assurances and explanations rather than ridicule. The
moon, which is full at times of lunar eclipse, can be uncomfortably
bright during the early and late stages of an eclipse, however, and
filters can enhance the observing experience.
Photograph 3-2 Nature provides an excellent pinhole
camera projector: the overlapping leaves of a tree! 10 May 1994
annular, Baja California, Mexico. Photograph taken by Mike
safe and what isn’t? It is never safe to stare directly at the sun
with the unaided eye. At times this seems to be unavoidable. Driving to
and from work on east–west highways can expose commuters to the rising
and setting sun for extended periods of time. Tinted windshields and
sunglasses may handle the casual glances, but are poor substitutes for
avoidance. Effective use of sun visors or similar blocking devices is
Many believe that viewing the sun’s reflection
off a body of water or piece of dark glass or metal automobile hood is
safe; it is not, for the sun’s reflected light can also cause eye
The safest way to observe the sun with the unaided
eye is to project an image of the sun onto another surface; this
projection method guarantees that at no time is the eye exposed directly
to solar radiation.
The next–safest way is to obtain approved solar
viewing filters from a reputable science supply house or astronomical
company. Inspect them carefully for flaws before each use, then discard
and replace them after a reasonable period of time.
It is never safe to observe the sun through any
kind of optical instrument without suitably approved astronomical
filtering devices securely attached. This includes binoculars, small
spotter scopes, and camera viewfinders and lenses.
Projection method. The use of projection to view the sun’s image indirectly is by
far the safest approach to solar and solar eclipse observing. This
method may be applied either with or without optical aid, although the
former presents a much larger image and more rewarding appearance. Both
approaches will be discussed, along with their advantages and
Pinhole projectors. A simple pinhole projector can
be made from a cardboard shoebox. Cut a small opening about 2.5 cm (1
in) square in the center of one end of the shoebox, and tape a piece of
aluminum foil over the opening. Make a small opening in the center of
the aluminum foil with a pin. The sun’s image will be projected onto
the opposite end of the shoebox, where it can be viewed by an observer.
Viewing may be enhanced by leaving the lid on the
shoebox and cutting a small viewing slit in the side of the box near the
end where the sun’s image is being projected, and placing the eye near
this slit. The sun’s image may then be seen against a relatively dark
background. The best view is obtained with the observer’s back to the
sun and the aluminum pinhole held over the shoulder, guaranteeing that
the observer is not tempted to glance at the sun while viewing the
A similar device can be made by replacing the
cardboard shoebox with a large mailing tube. The pinhole can be made in
an aluminum foil cap taped to one end. A paper viewing screen, made
translucent by a drop or two of cooking oil, may be taped to the other
end in lieu of cutting a viewing slit in the side of the tube.
Mirror projectors. It is possible to view the sun
from indoors with a simple mirror device. Select a window facing the
sun. Tape a piece of paper, with a 2.5 cm (1 in) hole cut in its center,
onto the window, then cover the rest of the window with dark cloth or
sheets of newspaper.
Next, place a mirror against an opposite wall so
that the sun’s light coming through the opening in the window hits it.
Reflect the sun’s light from the mirror onto a piece of paper or white
cardboard attached to the wall beneath the window.
Finally, cut a 2.5 cm (1 in) hole in another piece
of stiff cardboard, tape a piece of aluminum foil over the hole, and
make a pinhole in the center of the aluminum foil. When this pinhole
card is moved between the mirror and the cardboard beneath the window,
an image of the sun should come into focus on the wall beneath the
The mirror will have to be moved to follow the
sun’s motion across the sky as the eclipse progresses, but this method
assures comfortable indoor viewing of the partial phases as long as the
sun is shining through the window opening.
A similar method, recommended by the Royal
Astronomical Society of Canada in their Observer’s Handbook 1994, is
to cover a small pocket mirror, except for a small opening about 6 mm
(1/4 in) square, and position it where the sun’s rays can reflect off
the opening and into a darkened room. The reflected spot will be a
pinhole image of the sun’s disk. Try varying the size of the opening,
and the distance you project the image, to maximize image size,
sharpness and brightness.
Photograph 3-3 A number of options can be
considered for optical projection. Sun Spotter II, a commercially
available product, produced excellent images of the 10 May 1994 annular
from Baja California, Mexico. Photograph taken by Mike
Optical projectors. A pair of binoculars or a small telescope can be used
without filters to project an image of the sun onto a screen, a
hand–held square of cardboard or posterboard. The instrument being
used should be securely mounted and carefully monitored at all times to
make certain that curious onlookers do not attempt to look directly
through the instrument and damage their vision. For public observing
sessions, amateur astronomers are urged to rope off the instrument and
the screen to keep spectators from placing their eyes between the two.
A round cereal box, such as an oatmeal container,
can also be used. Simply slip one end of the box over the eyepiece end
of a telescope and view the projected image at the other end through a
viewing flap or hatchway cut in the side near the end opposite the
Viewing filters. Whenever an observer intends to
use filters to view the sun or a solar eclipse directly, there are
inherent risks in the procedure even when the filters themselves are
capable of blocking all of the sun’s harmful rays and dimming the
visible light to a comfortable level.
Filters which are used in conjunction with
telescopes or other optical devices present the greatest risk to
observers, for the sun’s light is being intensified and magnified by
the optics of the instrument. Anyone who has ever focused the sun’s
light through a magnifying glass and ignited a leaf or a piece of paper
has personal knowledge of the sun’s ability to damage the eyes.
The two approaches to filtering the sun’s rays
for direct viewing are the rear–mounted and front–mounted methods,
so named for their placement with respect to the optical path of an
Photograph 3-4 The best thing to do with
rear-mounted eyepiece solar filters: throw them away!
Rear–mounted filters. The most dangerous type of filter is one that is placed at the
eyepiece end of binoculars or telescopes, for these are taking the full
force of the magnified image of the sun.
Even though they are capable of safely filtering
out the sun’s harmful rays, their rear–mounted placement subjects
them to intense heat from the sun. As they heat up, their glass expands
within their mounting cells. If they have internal flaws or if their
mounting cells are too tight to allow for expansion as they heat up,
they will crack; many will crack even if well made and properly mounted.
An observer looking through one when it cracks is unlikely to react
quickly enough to withdraw the eye before sustaining serious injury.
Unfortunately, they tend to be the most common type among beginning
amateur astronomers, since such filters frequently come with the popular
and inexpensive imported 50–mm to 60–mm (2–in to 2.4–in)
refracting or 76 mm (3–in) reflecting telescopes distributed through
popular department–store chains. The best advice is to throw them out
and obtain a safer, front–mounted filter.
Front–mounted filters. These filters are placed
over the front of binoculars, telescopes and camera lenses and filter
the sun’s light before it ever enters the optical system. Therefore,
the heat stays away from the instrument and the observer’s eyes. For
small instruments, they can be full–aperture filters, meaning they
have the same surface area as the telescope lens or mirror; for bigger
instruments it is usually desirable to mount a smaller filter into a
cell which fits over the end of the telescope, effectively reducing the
aperture and saving money on the filter as well.
These filters may be made from metal–coated
glass, mylar or plastic material, and provide adequate safety and
pleasing views. However, they are not without safety concerns which
should be constantly addressed.
The coatings on glass filters can deteriorate with
time or become scratched; the best of these have the coated surfaces
sandwiched between two pieces of glass, protecting them from the
elements and from damage due to handling. The mylar and plastic filters
can also suffer surface degradation, but are more prone to pin pricks
due to handling in use. Some mylar and plastic filters also sandwich
their coated surfaces for added protection and longevity, but these
types are generally inexpensive enough to warrant replacement after a
reasonable amount of use.
Front–mounted filters present an additional risk
to the observer intent on direct solar viewing. They are only as safe as
the method used to securely mount them to their instrument. Observers
who rely on masking tape are risking their vision at every observing
session. Even filter mounts which seem to fit snugly over the optical
tube have been known to fall or be knocked off.
The best approach, and one that is absolutely
essential around the general public or playful school students, is to
fashion a mount that attaches snugly and is secured in a way that only
you can quickly release at totality. The finder scope should also be
equipped with a securely–mounted filter or be removed from the
Distributors of safe front–mounted filters are
listed in Appendix 5, but sources and addresses frequently change;
potential purchasers should consult periodicals such as Sky &
Telescope and Astronomy magazines for up–to–date listings and
prices. They are advised to plan such purchases at least several months
prior to an eclipse to guarantee delivery in ample time to practice
observations and photography on the uneclipsed sun.
Photograph 3-5a, b Front-mounted filters.
Above, a Thousand Oaks coated glass filter. Below, a selection of
Tuthill mylar filters.
Other approaches. The classic system of telescopic solar observing employed a
combination of an unsilvered glass secondary or diagonal mirror, called
a Herschel Wedge, which ejected all but a tiny fraction of the sun’s
light from the telescope, and a rear–mounted sun filter of Number 4
density dark glass mounted between the eye and eyepiece. While still a
reliable approach, the availability of inexpensive full–aperture
front–mounted filters makes changing secondaries and mounting sun
filters unnecessarily time–consuming unless you have a telescope
dedicated solely to solar observing.
For the naked–eye,
the use of a Number 14 welder’s glass provides adequate protection
from infrared as well as visible light to allow for safe viewing. They
are not recommended for use with telescopes or binoculars.
The Eastman Kodak Company quotes medical
authorities as recommending neutral–density filters of metallic silver
and having at least a 6.0 density for naked–eye use. To make these,
unroll a newly–opened roll of black–and–white panchromatic
photographic film containing silver, such as Kodak Plus–X or Tri–X,
to direct sunlight. Then roll it back onto its spool or into its
cassette and have it developed for maximum density according to the
manufacturer’s recommendations. When it has been processed, cut it
into equal lengths long enough to cover one or both eyes, and tape two
thicknesses of the film together for viewing. Mounting the film in a
cardboard frame makes a convenient holder and protects against
fingerprints. The National Society to Prevent Blindness correctly points
out that non–professionals frequently misunderstand and misinterpret
these critical instructions for making photographic film filters. Color
films and newer black–and–white films which do not contain silver
are not safe to use, nor are undeveloped film or developed negatives
with photographic images on them. Do not use this approach unless you
know what you are doing!
Unfortunately, film filters are generally too dark
to use as a photographic filter, and less–dense photographic filters
do not provide adequate protection to the eyes for even the briefest of
glances at the sun through a camera viewfinder. Also, photo processors
rarely develop black–and–white film in house anymore, necessitating
use of more expensive labs, lengthy shipping delays, or, if you are so
equipped, developing the film yourself. Again, the best approach is to
use the front–mounted professional sun filters instead, which are safe
for visual use and yield pleasing photographs. Planetaria and museums
often sell filters especially made for eclipse viewing.
Photograph 3-6 The importance of safe solar
viewing as all times, whether during a solar eclipse or solar
observing in general, cannot be overemphasized. Mike Martinez
(left) and Jeremy Reynolds demonstrate the use of naked-eye mylar and
film filters at the 10 May 1994 annular eclipse.
methods reemphasized. Never view the sun, either in or out of eclipse,
with the unprotected eye. The only exception is totality, when none of
the sun’s bright surface is visible. Sunglasses, crossed polarizing
filters, color negatives, color transparencies, color films,
black–and–white film containing no silver, undeveloped film, bottles
of colored or dyed water, reflections of the sun in dark glass or
standing water, and smoked glass do not provide adequate protection and
are unsafe solar viewing options. Serious eye damage can accompany the
use of any of these methods. Shun all rear–mounted filters on optical
instruments. Finally, even approved–safe direct–viewing filters
should be inspected carefully for flaws, scratches and damage before
risking eye injury by their use.