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, revealing sunspots, faculae, limb darkening, and a host of lesser features.
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.
Welder's glass 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 number-12 filter is uncomfortably bright.
Sky & Telescope photo by Chuck Baker.
The Sun is one of the few objects that display a rewarding amount of detail without a telescope or even binoculars. The only equipment you need is an appropriate filter a piece of No. 14 arcwelder's glass is the traditional choice. This safe filter material is available at any welding-supply store 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.
The part of the Sun that we actually see is a layer called the photosphere. It is the nearest thing the Sun has to a "surface." The first features you are likely to notice on the photosphere are large sunspots.
The solar surface, seen in white light, includes sunspots, faculae, prominences, and other features associated with the magnetic fields that result from the motion of ionized matter in the Sun's interior.
Courtesy Don Davis.
Spots of this size are fairly common when the Sun is active, and occasionally several groups are visible at the same time. Tracking the visibility of large spots provides an interesting project for naked-eye observers. In addition to sunspots, look for decreasing brightness toward the edge of the Sun's disk. This limb darkening is the result of looking through a progressively thicker cross section of the darker, cooler upper photosphere near the Sun's limb.
Sun and Telescope
Closeup of a sunspot. This image, taken by the Swedish 1-meter Solar Telescope on La Palma in the Canary Islands, resolves the filaments making up the spot's penumbra with a resolution of 0.1 arcsecond, courtesy of an advanced adaptive-optics system. The black-and-white images have been colorized here to emphasize contrasts. The smallest resolved structures are about 90 kilometers across.
Courtesy Royal Swedish Academy of Sciences.
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. Somewhat better sunspot detail is seen with a glass filter, but faculae are usually rendered all but invisible. Although these filters are more alike than different, it is probably best to give some thought to what you most want to see before purchasing a filter for your telescope.
Various types of solar filters are helping increase the popularity of solar observing. Here, Sky & Telescope staff members demonstrate several different ways of safely studying the Sun.
Sky & Telescope: Craig Michael Utter.
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. In addition to making it possible to view small sunspots, binoculars will show the limb darkening with greater ease than the naked eye alone. You can purchase filters for many binocular sizes, and you can even make your own from Mylar solar-filter material available from several vendors. Make sure filters are firmly affixed so that they will not fall off or blow away in a gust of wind.