Solar filters are typically made with a specially coated Mylar or glass substrate mounted in a cell that fits snugly over the front of the telescope. Such filters offer safe white-light views of the Sun.
Sky & Telescope photo by Craig Michael Utter.
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.
Welder's filters of shades 12 through 14 are popular and safe solar filters easily obtained at welding-supply outlets. Most observers prefer shades 13 or 14; the solar image through a shade-12 filter is uncomfortably bright.
Sky & Telescope photo by Chuck Baker.
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.