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.