By Chuck Bueter
To observe the transit of Venus directly you must protect your eyes at all times with proper solar filters. However, do not let the requisite warnings scare you away from witnessing this rare spectacle. You can experience the transit of Venus safely, provided you use proper eye protection.
1. Direct viewing
The use of eclipse shades will permit a large number of people who do not have specialized equipment to observe this event. However, as the planet approaches the limb of the sun, subtleties like the ‘black drop’ effect will not be discernible. At one minute of arc in size, Venus is near the visual limit of most people’s eyes. It’s tiny compared to the sun, which is about 32 arcminutes in diameter.
Yes! Eclipse Shades or Solar Shades appear similar to sunglasses, but they have a special filter that permits safe viewing if the filter is in new condition. Eclipse/solar shades are available through Astronomers Without Borders. Before looking at the sun, inspect the material to make sure the lenses are not scratched or compromised in any way. If so, discard the shades.
No! Do not be lulled into thinking that you can look safely at the sun while wearing sunglasses, for sunglasses do not protect your eyes sufficiently. So don’t try it! Do not try to view the sun directly with the naked eye or through any questionable medium. These children, depicted on the April 28, 1883, cover of Harper’s Weekly, are at risk of serious eye injury. They are using smoked glass, which is not sufficient.
2. Pinhole projectors
Pinhole projectors are a safe, indirect viewing technique for observing an image of the sun. While popular for viewing solar eclipses, pinhole projectors suffer from the same shortcomings as unmagnified views when Venus approaches the edges of the sun. Small features like the ‘black drop’ effect will not be discernible. Dr. Hugh Hunt demonstrates a successful pinhole projection of the 2004 transit of Venus and additional instructions for pinhole projectors are at www.exploratium.edu.
3. Projecting a magnified view
You may project a magnified view of the sun through a telescope or a pair of binoculars onto a white surface, which conveniently allows a larger number of people to watch concurrently. See www.popastro.com for instructions for projecting the sun with a telescope, along with solar activities like sunspot counting.
The projection technique often has its own limitations. Because magnified projections usually have an exposed focal point beyond the eyepiece, a bystander can inadvertently put her eye or body in the sight line of the sun. Hence, a projecting telescope must not be left unattended. Large reflector telescopes can generate too much heat by concentrating a lot of the sun’s energy on the secondary mirror and eyepiece, so the incoming light must be attenuated first. ‘Stop down’ the aperture. Likewise, Schmidt-Cassegrain telescopes can experience too much heat build-up as the light bounces internally.
Hubert van Hecke provides the design and instructions for making his sunspotter. The Exploratorium demonstrates how to view a planet in transit safely by projecting the image of the sun with binoculars Important: do not look at the sun through binoculars without solar filters on the large ends of both the barrels. Do not leave this rig unattended.
4. Closed-loop device
The safest method for allowing a large crowd to witness the transit of Venus concurrently is to project a magnified image through a closed-loop device. A popular projection device used during the 2004 transit of Venus was the Must-See TV (Transit Venus) Screen. Made from simple materials (a plastic funnel, a clamp, an eyepiece, and some projection fabric), the device fits in your telescope like an eyepiece with an appendage. A clear image of Venus transiting the sun appears on the screen. Because the entire light path is enclosed, observers are not at risk. A larger version of the screen uses a bucket to yield a larger image. Download simple instructions and supplies list, from the 2003 GLPA Annual Conference workshop.
Bruce Hegerberg’s design for a Sun Gun is online at his website. The Other closed-loop devices are commercially available from Science First and Solarscope. They provide a surface on which you can safely trace the sun’s outline and sunspots onto a piece of paper.
5. Through a telescope
The transit of Venus is perhaps best viewed directly when magnified, which demands an appropriate solar filter over the large end of the telescope. Often made of glass or Mylar, these “white light” filters block about 99.99% of the incoming sunlight, which allows the eyepiece then to magnify the image. A filtered, magnified view will show the sun (either blue or orange), the planet Venus, the ‘black drop’ effect, and sunspots. See www.skyandtelescope.com for a list of retailers.
Note #1: The sun’s immense energy must be drastically reduced before it enters the telescope. Do not use small filters that fit over the eyepiece (as found in some older, cheaper telescopes), for the concentrated sunlight can shatter them.
Note #2: Remove unfiltered finder scopes so they are not inadvertently accessed. Do not rely on a lens cap – even if it is taped on – to keep the eyes of a prying person at bay.
Special telescopes with built-in hydrogen-alpha filters show additional solar features, such as the sun’s surface granulation and prominences extending outward into space.
6. Live webcast
Transit not visible from your location? Watch the live webcast from the fun team at NASA EDGE.