Next month's solar eclipse is this year's big news in amateur astronomy. This month we continue preparations for the eclipse with a discussion of available equipment that allows safe eclipse viewing.
During a solar eclipse, safe observing techniques must be used during the partial eclipse phases. Only during the brief minutes when the sun is 100 percent covered by the moon ("totality") is it safe to view the eclipse directly and unfiltered. However, on August 21 Maryland will not see the total eclipse; only its partial phases will be visible everywhere in the state, weather permitting. Therefore, everyone observing from Maryland needs protection at all times during the eclipse.
Again, at no time in Maryland will it be safe to view the eclipse directly without protection or other safe means of observing.
Adequate equipment when used responsibly will permit safe viewing of the sun. However, if misused or used improperly even safe equipment can result in damage to a person's eyesight. This article will describe safe equipment, how to use it, how not to use it, and in some cases how to construct or where to obtain.
The simplest method is solar projection. Almost anything can be used to cast tiny images of the sun on the ground, such as the dappled sunlight beneath a tree. During the eclipse the ground will be covered with tiny eclipsed suns resulting from "pinhole" projection caused by the leaves.
You can also make a simple solar eclipse projection "theater" with these simple materials: Shoe box, tin foil, white card, tape and a needle. Cut a small hole in one end of the box and tape the tin foil over the hole. Tape the white card inside the opposite end of the box. Finally, carefully poke a very small hole in the foil with the needle.
What to do? Hold the box with the foil end facing the sun and turn it until the sun's image can be seen on the card. During the partial eclipse the tiny image of the sun should have a "bite" out of it as the moon passes in front of it. Refer to the illustration for more information.
What not to do? Putting the box on your head and viewing the sun through the pinhole would be a very dumb thing to do.
Another way to perform solar projection is using an unfiltered telescope or binocular. The telescope can be aimed at the sun and its unfiltered image projected onto a white card or piece of paper several feet away. This method carries some risk. The danger with this is a passerby could mistakenly take a peek in the eyepiece and receive instant eye damage. Thus, constant supervision is required, as well as reminding those around you to not look into the telescope.
What to do: The hardest part may be finding the sun in the telescope. Even if the telescope is equipped with a small "finder scope" for locating objects, you cannot safely look directly through it either. So start by keeping the finder capped. I use the shadow method. With your back to the sun, look at the shadow that the telescope tube casts on the ground. Adjust the telescope until its tube's shadow is a perfect circle which means you are pointed near the sun. Hold the card or paper about a foot from the eyepiece. When the sun's image appears on the paper, focus the telescope and move the paper closer and farther from the eyepiece to create a solar image whose size and brightness is pleasing.
What not to do: Do not look through the unfiltered telescope pointed at the sun at any time or let anyone else. Do not hold the card or paper too close to the eyepiece. Doing so may cause the paper to catch on fire due to the solar heat. If the paper does catch on fire, drop it and be thankful that it wasn't someone's eye.
The same technique may be used with tripod-mounted binoculars. However, only use one of its two tubes and keep the other capped at all times.
Beware that projection can cause heating of the telescope and its optics. It is not recommended for moderately large telescopes. I know of at least one case where a telescope caught fire because the unfiltered sun's image was magnified internally on the inside of the tube.
Perhaps the best safety feature of any projection method is they work best when the observer's back is safely facing the sun. If you are viewing a projected image and your back is not to the sun, are you doing something wrong? The worst part is, the sun is unfiltered and so there is the possibility of a rapid eye injury occurring.
The other technique involves filtered views of the sun. Filters must be adequate in reducing not only visible light, but also harmful infrared and ultraviolet light. If you do not know the quality of the filter material you are considering, it's best not to take a chance by using them.
Perhaps the easiest to obtain material is welder's glass. But not just any density. Ask an expert, they will say use nothing but No. 14, the densest shade. Of course, back when No. 13 was the densest, they said to use nothing but it. Same thing before that when No. 12 was densest. So over the years I've collected all three shades in my kit. But be safe, and stick with No. 14, available where welding supplies are sold.
What to do: Standing with your back to the sun, hold the No. 14 welder's glass up to your eyes. Then turn around and look up at the sun. It will look green.
What not to do: Do not use any of the lighter shades. Do not walk up to an unfiltered telescope used for projection and try looking into the eyepiece through the welder's glass at the sun. The magnified projected solar image can heat up the glass and crack it, thus allowing unfiltered sunlight to potentially cause blindness.
Light weight solar filter materials include aluminized Mylar and black polymer. These are commonly used in "eclipse glasses" which I like to call "true" sun glasses. Such glasses are usually made of cardboard with a piece of filter material for each eye.
Eclipse glasses may be ordered online. The Westminster Astronomical Society (WASI) is another source for these. They are available at their public events for a token donation (typically $1) per pair. See the end of this article for a list of upcoming WASI events for July.
What to do: Same as with the welder's glass except the sun image will appear whitish-blue with aluminized Mylar or yellow-orange with the black polymer. Eclipse glasses are only for nearly naked eye viewing.
What not to do: Same as with the welder's glass. Do not look through an unfiltered telescope or binoculars with your eclipse glasses because the sun will burn a hole through them and damage your eyes.
Solar filters on telescopes are popular with amateur astronomers. But there is one kind that should be avoided. Those are the small ones that screw into an eyepiece barrel. If you have one of these, don't use it with your telescope.
What to do: Use it like a welder's glass. Hold it in front of your eye and look up at the sun.
What not to do: Do not screw it into an eyepiece. Don't use it to look through an unfiltered telescope or binoculars.
Most solar filters for telescopes are either the two types mentioned before — aluminized Mylar or black polymer — or glass with an evaporated metal coating. They are sold by specialty telescope stores and online, and come in various sizes to fit different telescopes. The raw filter material can also be bought and attached to homemade adapters made to fit the telescope, but that is beyond the scope of this article.
These filters are for "white light" viewing, meaning visible light. There are specialized filters and telescopes for viewing different wavelengths of the sun's light. But in this discussion we are only concerned with white light filters used for visual use. (There are also photographic filters, but because they are used with cameras may not block out all of the sun's harmful rays.)
Any white light solar filter should be attached to the "field" end of the telescope. That is, not the eyepiece end. They go over the main opening covering it entirely. Depending on the kind of material and the brand the solar image may appear white, blue, yellow or orange.
The use of filtered telescopes for viewing the sun is highly specialized with many exceptions, special cases and cautions. Most people that might use it already own one. If you don't and are interested, there is much information available on the web. Just google "telescope solar filter."
This month we talked about the safety equipment you can use to view the upcoming solar eclipse. Next month we will discuss what to expect when viewing the eclipse and maybe a couple of tips about photographing it.
In the months leading up to the eclipse, WASI has made safe solar eclipse glasses available to visitors at its public events. Following is a list of events for July. More information and other events may be found at WestminsterAstro.org by clicking on the "Events" tab.
•Bear Branch Nature Center, "Planetarium Show and Star Party," 7-10:30 p.m. Saturday, July 8 — planetarium requires seat reservations ($5), 410-386-2103
•Winfield VFD Carnival, booth, 6-10 p.m., July 10-15
•Bear Branch Nature Center, WASI monthly meeting, 7:30 p.m. Wednesday, July 12
•Black Ankle Vineyards, "Wine Under the Stars," 7-9:30 p.m. Friday July 14
•Carroll County 4-H Fair, booth, 10 a.m.-10 p.m., July 29-August 4
Be prepared, safe and wise by taking care of your eyes during the solar eclipse.