Observe Eclipses! 
Excerpts from book by Dr. Michael D. Reynolds and Richard A. Sweetsir

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 Reynolds.

What’s 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 best.

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 damage.

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 disadvantages.

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 eclipse.

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 window.

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 Reynolds.

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 telescope.

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 instrument.

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 telescope entirely.

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.

Unsafe 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.