Solar Observing Safely: Eclipse Glasses, White-Light Filters, And Sunspots

Solar observing is the part of amateur astronomy a senior can do at 10 AM with a hot coffee instead of at midnight with a blanket. The Sun shows sunspots, granulation, prominences, and full-disk transits. A small refractor with a proper filter resolves spots the size of three Earths drifting across the disk over a few days. The trade-off is that the Sun is the only target in the sky that can permanently blind an observer in seconds. The rules for safe solar observing are short, strict, and non-negotiable.

There are two safe ways to look at the Sun. The first is with eclipse glasses or a handheld solar viewer that meets the ISO 12312-2:2015 standard. Reputable brands sold in the United States include Rainbow Symphony, American Paper Optics, Thousand Oaks Optical, and the Celestron EclipSmart line. A pair costs 2 to 8 dollars. Eclipse glasses are for looking at the Sun directly with the unaided eye. They are not for use over a camera or a telescope. The second way is with a white-light solar filter that fits over the front of the telescope, called an aperture filter. The two leading materials are Baader AstroSolar Safety Film and Thousand Oaks Solarlite. Both filter out 99.999 percent of the sunlight, leaving a safe yellow-white image with sunspots and granulation visible. A pre-made aperture filter for a small refractor or Schmidt-Cassegrain costs 60 to 200 dollars. Larger filters for an 8-inch scope run 200 to 400 dollars. Both materials block 99.999 percent of UV, visible, and infrared light. Anything less, including welders glass below shade 14, smoked glass, or stacked sunglasses, is not safe.

The two filters NOT to use are eyepiece solar filters that screw into the bottom of an eyepiece, and any filter that is cracked, scratched, or shows any pinholes. Eyepiece solar filters were sold on cheap import telescopes decades ago. They sit at the focal point of all the suns concentrated light and can crack or melt in seconds. If a telescope kit ships with one, throw it away. A safe aperture filter sits at the front of the telescope and blocks the light before it enters the tube. The third safe option is hydrogen-alpha solar observing through a dedicated H-alpha telescope like the Lunt LS50THa or the Coronado PST, in the 700 to 2000 dollar range, which shows surface details and prominences invisible to white-light filters. Sources include the American Astronomical Society Solar Eclipse Task Force at eclipse.aas.org, NASA at science.nasa.gov, the Baader Planetarium store at baader-planetarium.com, Thousand Oaks Optical at thousandoaksoptical.com, and the Lunt Solar Systems site at luntsolarsystems.com.