How to observe Solar Eclipses safely

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How to observe Solar Eclipses safely

Postby abuali » 12 Jan 2010, 13:42

Observing Solar Eclipses Safely
Adapted from Chapter 11 of Totality: Eclipses of the Sun
Copyright ©1999 by Mark Littmann, Ken Willcox and Fred Espenak

Index to Observing Eclipses Safely
* Introduction
* The Pinhole Projection Method
* Solar Filters
* Welders' Goggles
* Camera and Telescope Solar Filters
* Fully Exposed and Developed Black-and-White Film
* Eye Suicide
* Observing with Binoculars
* Observing with a Telescope
* A Final Thought


Permanent eye damage can result from looking at the disk of the Sun directly, or through a camera viewfinder, or with binoculars or a telescope even when only a thin crescent of the Sun or Baily's Beads remain. The 1 percent of the Sun's surface still visible is about 10,000 times brighter than the full moon. Staring at the Sun under such circumstances is like using a magnifying glass to focus sunlight onto tinder. The retina is delicate and irreplaceable. There is little or nothing a retinal surgeon will be able to do to help you. Never look at the Sun outside of the total phase of an eclipse unless you have adequate eye protection.

Once the Sun is entirely eclipsed, however, its bright surface is hidden from view and it is completely safe to look directly at the totally eclipsed Sun without any filters. In fact, it is one of the greatest sights in nature.

There are five basic ways to observe the partial phases of a solar eclipse without damage to your eyes. We will describe each of them below. We'll also explain how to safely watch an eclipse with binoculars or a telescope.

The Pinhole Projection Method

One safe way of enjoying the Sun during a partial eclipse--or anytime--is a "pinhole camera," which allows you to view a projected image of the Sun. There are fancy pinhole cameras you can make out of cardboard boxes, but a perfectly adequate (and portable) version can be made out of two thin but stiff pieces of white cardboard. Punch a small clean pinhole in one piece of cardboard and let the sunlight fall through that hole onto the second piece of cardboard, which serves as a screen, held below it. An inverted image of the Sun is formed. To make the image larger, move the screen farther from the pinhole. To make the image brighter, move the screen closer to the pinhole. Do not make the pinhole wide or you will only have a shaft of sunlight rather than an image of the crescent Sun. Remember, this instrument is used with your back to the Sun. The sunlight passes over your shoulder, through the pinhole, and forms an image on the cardboard screen beneath it. Do not look through the pinhole at the Sun.

Solar Filters

A second technique for viewing the Sun safely is by looking at it directly through a specially designed solar filter. Such filters permit only a miniscule fraction of the Sun's light to pass through them. Advertisements for solar filters may be found in popular astronomy magazines.

One such type of filter is made of an aluminized polyester. Beware, though, that polyester, a plastic, comes in various thicknesses and with various coatings. You need a metal coating to save your eyesight and you need to examine the Polyester for small holes that could allow unfiltered sunlight to reach your eyes and damage them. A good solar filter will allow you to look comfortably at the filament of a high-intensity electric lamp.

A second type of solar filter is made from a black polymer which gives a yellow/orange tint to the Sun which is more pleasing than the bluish color seen with aluminized polyester filters. Either filter type is completely safe provided that it has an optical density of 5.0 or more. This means that only 0.01% of the Sun's light can pass through the filter.

When using any kind of filter, however, do not stare for long periods at the Sun. Look through the filter briefly and then look away. In this way, a tiny hole that you miss is not likely to cause you any harm. You know from your ignorant childhood days that it is possible to glance at the Sun and immediately look away without damaging your eyes. Just remember that your eyes can be damaged without you feeling any pain.

A list of solar filter retailers can be found in Totality, Appendix C . For more information, see: Solar Filters, by B. Ralph Chou. Another excellent reference on safe solar filters, is: B. Ralph Chou: "Solar Filter Safety," Sky & Telescope, volume 95, February 1998, pages 36-40.

Welders' Goggles

Welders' goggles or the filters for welder's goggles with a rating of 14 or higher are safe to use for looking directly at the Sun. They are also relatively inexpensive.

Warning! Do not attempt to use these filters behind a pair of binoculars or telescope (that is, between your eyes and the binoculars or telescope). The magnifying optics of these devices will focus the full power of the Sun onto the welder's filter which could crack and shatter from the intense heat after only a few minutes. If you wish to observe the eclipse with binoculars or a telescope, you must use a specially designed solar filter on the front end (or Sun-side) of the instrument. These filters are discussed in the next section. (2002/09/03)

Camera and Telescope Solar Filters

Many telescope and camera companies provide metal-coated filters that are safe for viewing the Sun. They are more expensive than common Mylar, but observers generally like them better because they are available in various colors, such as a chromium filter through which the Sun looks orange. Through aluminized Mylar, the Sun is blue-gray. As with the Mylar, you can look directly at the Sun through these filters.

Caution: Do not confuse these filters, which are designed to fit over the lens of a camera or the aperture of a telescope, with a so-called solar eyepiece for a telescope. Solar eyepieces are still sometimes sold with small amateur telescopes. They are not safe because of their tendency to absorb heat and crack, allowing the sunlight concentrated by the telescope's full aperture to enter your eye.

Fully Exposed and Developed Black-and-White Film

You can make your own filter out of black-and-white film, but only true black-and-white film (such as Kodak Tri-X or Pan-X). Such films have a layer of silver within them after they are developed. It is this layer of silver that protects your eyes.

To make your own solar filter, proceed as follows. Open up a roll of black-and-white film and expose it to the Sun for a minute. Have it developed to provide you with negatives. Use the negatives for your filter. It is best to use two layers. With this filter, you can look directly at the Sun with safety.

Remember, however, that if you are planning to use black-and-white film as a solar filter, you need to prepare it at least several days in advance.

Caution: Do not use color film or chromogenic black-and-white film (which is actually a color film). Developed color film, no matter how dark, contains only colored dyes, which do not protect your vision. It is the metallic silver that remains in black-and-white film after development that makes it a safe solar filter.

Eye Suicide

Standard or polaroid sunglasses are not solar filters. They may afford some eye relief if you are outside on a bright day, but you would never think of using them to stare at the Sun. So you cannot use sunglasses, even crossed polaroids, to stare at the Sun during the partial phases of an eclipse. They provide little or no eye protection for this purpose.

Observing with Binoculars

Binoculars were astronomy writer George Lovi's favorite instrument for observing total eclipses. Any size will do. He used 7 x 50 (magnification of 7 times with 50-millimeter [2-inch] objective lenses). "Even the best photographs do not do justice to the detail and color of the Sun in eclipse, and especially the very fine structure of the corona, with its exceedingly delicate contrasts that no film can capture the way the eye can." The people who did the best job of capturing the true appearance of the eclipsed Sun, he felt, were the nineteenth-century artists who photographed totality with their eyes and minds and developed their memories on canvas.

For people who plan to use binoculars on an eclipse, Lovi cautioned common sense. Totality can and should be observed without a filter, whether with the eyes alone or with binoculars or telescopes. But the partial phases of the eclipse, right up through the Diamond Ring Effect, must be observed with filters over the objective lenses of the binoculars. Only when the Diamond Ring has faded is it safe to remove the filter. And it is crucial to return to filtered viewing as totality is ending and the western edge of the Moon's silhouette begins to brighten. After all, binoculars are really two small telescopes mounted side by side. If observing the Sun outside of eclipse totality without a filter is quickly damaging to the unaided eyes, it is far quicker and even more damaging to look at even a sliver of the uneclipsed Sun with binoculars that lack a filter.

Observing with a Telescope

Some observers, including astronomy historian Ruth Freitag, prefer to watch the progress of the eclipse through a small portable telescope, which offers stability and is much less tiring to use for extended periods than binoculars. A telescope also provides more detail at higher powers, if this is desired. The solar filter is removed at totality. When she wants to see a wider view of the corona, she switches to the finder scope.

A Final Thought

Just remember, Lovi said, "Don't try to do too much. Look at the eclipse visually. Don't be so busy operating a camera that you don't see the eclipse. And don't set off for the eclipse so burdened down by baggage and equipment that you are tired and stressed and too nervous to enjoy the event."

Astronomer Isabel Martin Lewis also warned of the dangers of too many things to do: "A noted astronomer who had been on a number of eclipse expeditions once remarked that he had never SEEN a total solar eclipse."

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