Eclipse Watching: How to Make a Solar Filter

(adapted from Observe: Eclipses, published in 1979 by the Astronomical League and "Safe Solar Filters," by B. Ralph Chou, Sky and Telescope, August 1981)

Materials:

• a roll of Kodak Pan-X black and white film
• two pieces of cardboard (about 8" square or larger) to act as a holder

Open the roll of film, unravel it completely, and hold it for 30 seconds under a bright light. After exposing the film, have it developed normally. If you send it off for processing, include a note saying that the film is fully exposed and to be used for solar observation; request it to be developed and returned uncut. (It is important to stress this because many labs may think the film was sent in by mistake.)

Once the negatives are returned, cut them into pieces about 5 cm by 5 cm. Cut a slightly smaller square opening in the middle of both pieces of cardboard. Tape TWO pieces of film together into a sandwich, and tape this over the opening in the cardboard. Position the second piece of cardboard over the first, and tape them together. The cardboard frame will act as a shade from the bright Sun, while you safely observe through the film sandwich.

Note: A single layer of black and white film is not safe-it reduces the amount of light hitting the retina, but it does not eliminate the danger from prolonged observation of the Sun. Two layers of film effectively reduce the incoming sunlight to a safe level. Black and white film is safe because it contains silver, which absorbs sunlight, including ultraviolet and infrared light. Color film does not contain silver and is not safe. Undeveloped film is not safe.

Activity on the way. The Sun is approaching another level of high activity-called solar maximum. This period is characterized by greater numbers of sunspots, an increase in coronal activity, and, because of the relentless solar wind (a coronal product), subsequent increases in auroral activity in Earth's atmosphere. This plot of the average number of sunspots per month for the past 45 years clearly shows the Sun's 11-year sunspot cycle. Photo courtesy of National Solar Observatory/Sacramento Peak.

Classroom Activity: Pinhole Camera

• large cardboard box
• small square of aluminum foil
• white paper
• tape
• straight pin

Cut a 5-cm hole in one end of the cardboard box. Tape the aluminum foil over the hole, and use a straight pin to put a very small, even hole in the foil. On the inside and at the opposite end of the box, tape a piece of white paper. Voila! You have a pinhole camera!

Sunlight enters the pinhole and a small image of the Sun will be projected on the sheet of white paper. To use your camera, stand with the Sun behind you, and hold the box over your head, the pinhole side facing the Sun. Do not look through the pinhole at the Sun. Instead, look at the image of the Sun projected on the white-paper screen. Larger boxes are going to give you larger Sun images; in addition, a larger box is going to be a more comfortable fit for your head!

As an alternate camera design, use the cardboard tubes from paper towel rolls. The tubes from hefty towels are a bit larger than those from standard towels; get one of each, and slide the smaller one into the larger. At the end of one of the tubes, tape the aluminum foil, and make a tiny pinhole. At the other end of the two-tube assembly, tape a piece of wax paper (your screen). Aim the tube at the Sun, and notice how sliding the tubes in and out changes the size of the Sun image projected on the wax paper.

Another design will provide larger images, and larger images make sunspot observations possible. Cover a flat mirror with a sheet of thick paper that has a 0.5-cm hole in it. Secure the mirror with tape or putty such that it reflects the Sun's image onto a wall or projection screen.

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