2 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 (right) of the 2004 transit of Venus at http://www2.eng.cam.ac.uk/~hemh/transit.htm.  Additional instructions for pinhole projectors are at http://www.exploratorium.edu/eclipse/how.html; from the Exploratorium.

3 You may project a magnified view of the sun through a reflector telescope or binoculars onto a white surface, which conveniently allows a larger number of people to watch concurrently. See http://casa.colorado.edu/~dduncan/wp/?page_id=261 for video instructions for projecting the sun, by Dr. Doug Duncan.

The projection technique sometimes 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. (See Unattended Equipment Hazards, left column.) 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, SCT or 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.  Additional pages at his Ask Mr. Science web page indicate how to take sunspot data and analyze them.

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 A 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 now-improved Sun Funnel.  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 written by AAS Press Officer Richard Tresch Fienberg.

 

B. Ralph Chou, MSc, OD
Associate Professor, School of Optometry, University of Waterloo

[See also http://youtu.be/4RGr9FcBrSM for talk at 2012 Symposium at University of Toronto.]

Watching the profile of Venus as it passes across the Sun during a transit is a wonderful demonstration of the way the solar system works.  Over the course of several hours, Venus traces a path across the disk of the Sun, then leaves, in what can be thought of as an extreme example of an annular eclipse of the Sun.

Observing the Sun, however, can be dangerous if the proper precautions are not taken. The solar radiation that reaches the surface of Earth ranges from ultraviolet (UV) radiation at wavelengths longer than 290 nm, to radio waves in the metre range. The tissues in the eye transmit a substantial part of the radiation between 380–400 nm to the light-sensitive retina at the back of the eye. While environmental exposure to UV radiation is known to contribute to the accelerated aging of the outer layers of the eye and the development of cataracts, the primary concern over improper viewing of the Sun during the transit is the development of “solar retinopathy” or retinal burns.

Exposure of the retina to intense visible light causes damage to its light-sensitive rod and cone cells. The light triggers a series of complex chemical reactions within the cells which damages their ability to respond to a visual stimulus, and in extreme cases, can destroy them. The result is a loss of visual function, which may be either temporary or permanent depending on the severity of the damage. When a person looks repeatedly, or for a long time, at the Sun without proper eye protection, this photochemical retinal damage may be accompanied by a thermal injury—the high level of visible and near-infrared radiation causes heating that literally cooks the exposed tissue. This thermal injury or photocoagulation destroys the rods and cones, creating a small blind area. The danger to vision is significant because photic retinal injuries occur without any feeling of pain (the retina has no pain receptors), and the visual effects do not become apparent for at least several hours after the damage is done (Pitts 1993). Viewing the Sun through binoculars, a telescope, or other optical devices without proper protective filters can result in immediate thermal retinal injury because of the high irradiance level in the magnified image.

Because the apparent diameter of Venus is only 1/30 that of the Sun, there is never a time during the transit when it is safe to look at it without proper eye protection.   Failure to use proper observing methods may result in permanent eye damage and severe visual loss. This can have important adverse effects on career choices and earning potential, because it has been shown that most individuals who sustain solar retinopathy eye injuries are children and young adults (Penner and McNair 1966, Chou and Krailo 1981, and Michaelides et al. 2001).

The same techniques for observing the Sun outside of eclipses are used to view and photograph the transit (Sherrod 1981, Pasachoff 2000, Pasachoff and Covington 1993, and Reynolds and Sweetsir 1995). The safest and most inexpensive method is by projection. A pinhole or small opening is used to form an image of the Sun on a screen placed about a metre behind the opening. 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. All of these methods can be used to provide a safe view of the transit to a group of observers, but care must be taken to ensure that no one looks through the device. The main advantage of the projection methods is that nobody is looking directly at the Sun. The disadvantage of the pinhole method is that the screen must be placed at least a metre behind the opening to get a solar image with a silhouetted disk of Venus that is large enough to be easily seen.

The Sun can only be viewed directly when filters specially designed to protect the eyes are used. Most of these filters have a thin layer of chromium alloy or aluminum deposited on their surfaces that attenuates both visible and near-infrared radiation. A safe solar filter should transmit less than 0.003% (density ~4.5) of visible light and no more than 0.5% (density ~2.3) of the near-infrared radiation from 780–1400 nm. (In addition to the term transmittance [in percent], the energy transmission of a filter can also be described by the term density [unitless] where density, d, is the common logarithm of the reciprocal of transmittance, t, or d=log10[1/ t]. A density of ‘0’ corresponds to a transmittance of 100%; a density of ‘1’ corresponds to a transmittance of 10%; a density of ‘2’ corresponds to a transmittance of 1%, etc.). Figure 1 shows transmittance curves for a selection of safe solar filters (Chou 1981, 1998).  The “safe” zones of the plot are transmittance levels less than 0.0001 between 200 and 780 nm (above the 1E-04 line in the graph) and transmittance levels less than 0.001between 780 and 1400 nm (above the 1E-03 line in the graph).  The longer infrared between 1400 and 2500 nm does not get past the tears and front of the eyeball, so is not a problem.

Read more: Viewing the Transit & Eye Safety

Fine Print:

• Construcción de un Embudo Solar para observar manchas solares y el Tránsito de Venus
• Embudo Solar para observar manchas solares y el Tránsito de Venus

Read more: Galileoscope Solar Filter