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You are here: Home > What's Up > Celestial Events > Transit of Mercury 2006 > How to Observe
How to Safely Observe
the Transit of Mercury
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CAUTION! DANGER!
Never look directly at the Sun. Always use a proper solar filter. Only the following filters are recommended:
- #14 welder's glass, available at most hardware stores;
- filters specifically designed for solar observation — such as aluminized mylar filters or black polymer filters — available at astronomy stores.
The Sun's intense radiation (visible light, ultraviolet and infrared) always presents a serious danger to the eye, especially to the central part of the retina known as the macula. The highly sensitive cells in this area can easily be burned since they absorb a maximum quantity of solar radiation which is then converted to heat. Retinal burns do not produce pain so there is no warning, and once damaged, these cells never regenerate. This page shows examples of retinal damage caused by observing the Sun without adequate eye protection.
Adequate solar filters should reduce the Sun’s infrared and ultraviolet radiation to within accepted safety thresholds: They should allow no more than 0.001% of the Sun’s visible light to pass (equivalent to an optical density of 5). While some filters or “systems” claim to cut visible light to acceptable levels, they allow nearly all the dangerous infrared and ultraviolet radiation to pass through!
Do not use the following:
- sunglasses (even expensive ones);
- color negative film exposed to light and developed;
- slide film unexposed to light and developed;
- glass plates covered with soot;
- crossed polarizing filters;
- color acetates;
- all other filters not specifically designed for solar observing.
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Solar observation requires the utmost care: The Sun’s intense radiation can cause instantaneous burns to the retinas and damage them permanently. If you want to view the Sun directly, either with the naked eye or an optical instrument, it is vital to use adequate filters that are specially designed for solar observation. Unless you are familiar with these filters, the Montreal Planetarium suggests that you project the Sun’s image onto a screen instead. This method also offers a considerable advantage: It allows several people to see the transit of Mercury simultaneously. However, no matter which method you choose, children should only observe the Sun under an adult’s supervision.
Projection with an optical instrument
Projection with binoculars
Using ordinary binoculars, it’s an easy matter to project the Sun’s image onto a screen made of white cardboard. Obviously, you can’t look through the binoculars to find the Sun, however, you can use the shadow cast by your binoculars to centre the Sun. Once the Sun’s image appears on the screen you can sharpen it using your binoculars’ focussing wheel. It is preferable to block one of the two objective lenses; otherwise two overlapping images will form.
The longer the projection distance, the larger the image will be, but it will also appear softer. You should experiment to find the ideal distance for the most satisfying image. A projection distance of between 1 and 2 metres usually produces good results.
Image stability is also important. Mount your binoculars on a camera tripod. If your binoculars don’t have a mounting socket, they can be attached to a tripod using strong elastic bands or tape. Avoid last minute trials: Test your set-up beforehand!
Lastly, you can construct a sunshade but cutting two holes in a large sheet of cardboard and slipping the objective lenses through. This will cast a shadow on the screen, thereby improving the contrast of the projected image.
Never leave this apparatus unattended: No one must ever look through the eyepiece while the instrument is pointed at the Sun.
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Projection with a small telescope (refractor or reflector)
A small telescope provides a larger solar image than can be obtained with binoculars, and a much shorter projection distance. Place an eyepiece in the instrument: Chose a low power ocular. In the case of a refractor, you have the option of inserting a star-diagonal between the focuser and the eyepiece itself: This arrangement will project the image at right angles to the telescope. Don’t forget to cover your finder scope, or better yet, remove it completely.
Centre the Sun by moving the telescope until its shadow appears the smallest. Project the Sun’s image onto a white cardboard placed about 20 cm from the ocular. Focus the image using the telescope’s focussing knob. You can reduce ambient light reaching the screen by placing it inside a box: The Sun’s image will have improved contrast. Some commercial telescopes come equipped with a projection screen and mounting bracket. These are practical but not essential.
Important note: The telescope’s opening should not exceed 5 cm (2") to avoid overheating and damaging the eyepiece. If necessary, construct a diaphragm by covering the opening with cardboard that has a 4 to 5 cm hole cut into it. The dust caps of some instruments are already fitted with a small hole covered by a cap. These are ideal. Make sure the dust cap, or cardboard diaphragm, is securely taped in place to avoid accidental removal.
Never leave this apparatus unattended: No one must ever look through the eyepiece while the instrument is pointed at the Sun.
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Direct observation with a solar filter
Before using a solar filter inspect it thoroughly to ensure it is not damaged. Look for tears, creases, scratches and perforations, no matter how small. In case of doubt, play it safe and don’t use the filter.
Optical instruments with a solar filter (binoculars and telescopes)
Mercury will be too small to be visible against the sun without some sort of optical aid: in fact, a magnification of 50 to 100 power is the suggested minimum to see the tiny planet during the transit. The most interesting and dramatic views of the transit of Mercury will therefore be through an optical instrument equipped with a standard solar filter. This combination provides the sharpest and most detailed image, however, only one person at a time can see it.
Standard solar filters are made of aluminized Mylar film; or optical glass covered with a thin metallic layer deposited by vaporization in a vacuum. Glass filters are usually mounted in aluminium rings that can be slid over the instrument’s opening. They come in different sizes to suit a range of telescopes and binoculars. Mylar filters are available in similar mounts, and also come in large sheets that can be cut to fit any instrument. Both types are sold at stores that specialize in astronomical equipment.
Solar filters should always be placed over the opening of the instrument to reduce the amount of light entering. In the case of binoculars, both lenses should obviously be covered. Make sure the filters are well secured with tape to avoid accidental removal. (Caution: Some telescopes still come with tiny solar filters that thread into the eyepiece. These filters are extremely dangerous and should never be used: They can crack without warning due to intense solar heat. These filters should be discarded!)
Don’t forget to cover your finder scope, or better yet, remove it completely.
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