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How to watch the transit of Venus without blinding yourself (+video)

The transit of Venus across the solar disk won't make it okay to stare into the sun. Here's how to watch this rare astronomical event safely. 

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By far, the safest way to view the transit is to construct a so-called pinhole camera. A pinhole, or small opening, is used to form an image of the sun on a screen that is placed about three feet behind the opening. [Video: How to Make a Solar Eclipse Viewer]

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Binoculars or a small telescope mounted on a tripod can also be used to project a magnified image of the sun onto a white card. Just be sure not to look through the binoculars or telescope when they are pointed directly at the sun! Venus should appear as a distinct, albeit tiny dot on the projected image.

A variation of the pinhole theme is the "pinhole mirror." To make one, cover a pocket mirror with a piece of paper that has a quarter-inch hole punched in it. Open a sun-facing window and place the covered mirror on the sunlit sill so that it reflects a disk of light onto the far wall inside. 

The disk of light is an image of the sun's face. The farther away from the wall you place it the better; the image will be only one inch across for every 9 feet (2.7 meters) from the mirror. Modeling clay works well to hold the mirror in place. 

Experiment with different-size holes in the paper. Again, a large hole makes the image bright, but fuzzy, and a small one makes it dim but sharp. Darken the room as much as possible for the best effect. Be sure to try this out beforehand to make sure the mirror's optical quality is good enough to project a clean, round image. Of course, don't let anyone look at the sun in the mirror.

Acceptable filters for unaided visual solar observations include aluminized Mylar.  Some astronomy dealers carry Mylar filter material specially designed for solar observing. Also acceptable is shade 13 or 14 arc-welder's glass, which is available for just a few of dollars at welding supply shops. 

Unacceptable filters include sunglasses, old color film negatives, photographic neutral-density filters, and polarizing filters. Although these materials have very low visible-light transmittance levels, they transmit an unacceptably high level of near-infrared radiation that can cause a thermal retinal burn. The fact that the sun appears dim, or that you feel no discomfort when looking at the sun through the filter, is no guarantee that your eyes are safe.

Using a Telescope

Projecting the sun’s magnified image through binoculars, or better yet a telescope, on to a white card or screen is relatively safe and can be used for group viewing. For serious transit observing, a telescope with a full-aperture solar filter is much better. These filters are attached to the side of the telescope that faces the sun, not the side facing your eye. This will cause most of the sunlight to be filtered out before entering your telescope. [How to Safely Photograph the Venus Transit (Photo Guide)]

The transit should be watched only with an appropriate solar filter — a solar filter that is sold by a reputable outlet of astronomical equipment. If your telescope comes with a filter that screws into the eyepiece, discard it immediately! These filters have been known to crack under the intense heat of the sun's magnified image. 

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