Viewing the Sun provides an enjoyable way to supplement the usual nighttime observing activities, but you should be aware of the potential for serious injury and take precautions to ensure your safety and the safety of others. (See "Solar Filter Safety.") Viewing the Sun also demands extra vigilance when it comes to equipment. Never leave a telescope or binoculars unattended, especially when children are about. It takes only a moment of inattentiveness to create a dangerous situation. So whether you want to see a transit of Venus, a partial eclipse of the Sun, or the appearance of a giant spot on the solar surface, here's a summary of how to turn your gaze toward the Sun safely.
You don't need a telescope to observe the Sun; all that's required is an appropriate filter, and a piece of No. 14 arcwelder's glass is the traditional choice. This safe filter material is available at any welding-supply store (check your yellow pages for a local dealer) in convenient 4-inch-wide pieces that allow viewing the Sun with both eyes. Although welder's glass imparts a green hue to the Sun, one of these economical filters might be all you ever need for casual observing. But while welder's glass provides satisfactory naked-eye views of the Sun, its poor optical quality makes it unsuitable for use with binoculars and telescopes.
Filters that are not safe, though sometimes recommended in error, include smoked glass, stacked sunglasses, crossed polarizing filters, neutral-density camera filters, metallized candy wrappers, and compact discs. While these may greatly dim the Sun’s glare, invisible radiation may get through and damage your eyes. And don’t use a camera with a telephoto lens, even if the lens has photographic filters on it that appear to darken the Sun.
One method of solar observing that requires no filtration whatsoever is solar projection. An eyepiece is placed in the telescope's focuser and used to project an image of the Sun onto a convenient flat surface. Telescopes with folded light paths, such as Newtonians or Schmidt-Cassegrains, are not recommended since the converging beam of light can produce enough heat to damage internal components. A small refractor is best — a long skinny tube with a lens at one end and a focuser and an eyepiece at the other. Start by attaching a cardboard shield around the tube near the front lens, then cover (or remove althogether) the small finderscope that's attached to the larger tube. Binoculars can also be used; mount them firmly on a camera tripod, place a cap over one side’s front lens, and put a cardboard shield around the other one’s tube.
To aim the instrument safely, point the tube toward the Sun and move it around until sunlight begins to pour out of the eyepiece.
Don’t look into the eyepiece! Place the white card behind the eyepiece (in the shadow cast by the cardboard shield) and focus the image. Experiment with the card’s distance from the eyepiece to get the most pleasing combination of size and brightness of the Sun’s projected image.
This projection method has one advantage over all the others: many people can observe the solar surface at the same time. But if you’re viewing the Sun with a group, never leave a telescope or binoculars unattended — especially when children are about.
For more information about this method of Sun watching, see "Observing The Sun By Projection."
Sun and Telescope
When it comes to outfitting optical instruments for solar viewing, a number of excellent options are available. However, there is one type of filter that is very dangerous: the eyepiece Sun filter. These were once commonly supplied with imported telescopes and consist of a piece of dark glass mounted in a cell that screws into the bottom of an eyepiece. The heat from the Sun concentrated by a telescope can shatter these filters without warning. Today's advice is to destroy these filters to ensure they can cause no harm.
For safe viewing, most observers choose either a glass or Mylar solar filter mounted in a cell that fits securely over the front aperture of a telescope. Such filters are made with light-rejection coatings that allow only a fraction of a percent of the Sun's light to pass. This style of filter protects not only your eyes but your equipment too, since the potentially harmful heat of the Sun never enters the telescope.
Glass solar filters generally produce a yellow or orange Sun, while Mylar filters usually yield a blue image. Aesthetics aside, there are other differences to consider. Mylar filters tend to offer better contrast between the solar disk and bright faculae surrounding active regions. However, Mylar's blue-tinted image also suffers more from scattered light and atmospheric dispersion than the orange image produced by a glass filter.
Telescopic solar observing is pretty straightforward since vendors make filters sized to fit most popular instruments. Simply attach your filter to the front of the tube so that it cannot fall off, and you're in business. Don't forget to make sure that your telescope's finderscope is capped at the objective end or, better yet, removed completely. Aiming the telescope without a finder might seem problematic but it is quite simple. Just move the telescope around until its shadow is minimized, at which point the Sun should be within the field of a low-power eyepiece.
For most visual astronomy, bigger is better — the larger the scope, the more light collected and the greater the theoretical resolution. However, when it comes to solar observing, the playing field is tipped in favor of smaller scopes. Light-gathering is not an issue since we are trying to dim the Sun's intense glare, but what about resolution? Here again, the advantages of a large instrument are essentially neutralized by atmospheric turbulence. Daytime seeing is rarely steady enough to permit the maximum resolution of even a 4-inch telescope.
Glass and Mylar filters can also be used with binoculars. You can purchase filters for many binocular sizes, and you can even make your own from Mylar solar-filter material available from several venders. Make sure filters are firmly affixed so that they will not fall off or blow away in a gust of wind.