Piloting a telescope to our nearest star isn’t much safer. A sticker on most telescopes provides an ominous, helpful warning: “CAUTION! VIEWING THE SUN DIRECTLY WITH THIS TELESCOPE MAY CAUSE PERMANENT EYE DAMAGE!”
|The Sun up close|
| Whether observed by projection or through a filter, the Sun is a fascinating sight. Its visible disk, the photosphere, appears slightly darker near the edge — a phenomenon known as limb darkening. Under steady atmospheric conditions, you’ll notice a grainy appearance — granulation — to the photosphere. Granulation results from the rise and fall of huge masses of gas on that seething cauldron that is the Sun’s surface.
The obvious features are the sunspots — relatively cool regions on the photosphere that appear as dark blotches. A large sunspot will have a dark center, the umbra, surrounded by a grayish penumbra.
It’s fun to view the Sun each day and watch sunspots develop, then gradually fade, all the time drifting across the face of the rotating Sun. Bright patches of light — plages — often accompany sunspots located near the dark solar limb.
Each day, I view the Sun with anticipation. I’m never quite sure what I’ll see. — G. C.
Words like this would scare away even our intrepid solar astronaut. The good news is you can explore the Sun with a telescope, as long as you exercise caution and common sense. The safest and least expensive way to view the Sun is by solar projection — using your telescope to project the Sun’s image onto a white cardboard screen. This method allows many to view the Sun at the same time. The common 2.4-inch (60mm) refractor is ideal for solar projection. A large-aperture telescope collects too much sunlight and needs to be “stopped down” with an aperture mask, a cardboard sheet with a 2- to 3-inch circular hole, taped to the front of the tube.
High magnification isn’t necessary for a target as big as the Sun — 25x–35x will do the trick. The eyepiece I normally use for solar projection is a 20mm Huygenian. While lacking the sophistication of modern, pricier eyepieces, the Huygenian’s simple design — a pair of air-spaced convex lenses — makes it a good choice for solar observing. The sunlight collected and concentrated by a telescope’s objective lens or mirror could “cook” the cemented optics of more complex eyepieces. Don’t risk a $400 wide-field eyepiece for solar projection.
Don’t use the finder scope when aiming your telescope at the Sun — even that little scope collects a blinding amount of sunlight. If you’re viewing the Sun with curious onlookers — especially children — leave the lens cap on your finder. Target the Sun by studying your telescope’s shadow. When the shadow’s as compact as it can get, and you see bright light illuminating the eyepiece, you’re on target. Project the Sun’s image onto a white cardboard sheet facing the eyepiece about a foot away. The bright circle you see is the Sun’s disk. Focus until the circle’s edge is sharp.
What if you’re going solo or want a detailed view with that $400 wide-field eyepiece? This is where solar filters come in. Beware of filters that come with inexpensive telescopes and screw into the eyepiece. Just as concentrated sunlight can cook an expensive eyepiece, it can heat up and shatter such a filter. I shudder when I think of those occasions years ago when I used such a filter to show schoolchildren the Sun!
For direct solar-viewing, you need an aperture filter that attaches to the front of the telescope. It intercepts incoming sunlight, allowing a safe amount to reach the eyepiece. Aperture filters can be pricey — costs run from $50, depending on your scope’s size and the filter’s quality — but they are well worth the cost.
If you want to compare your view of the Sun with those obtained by professional astronomers, I recommend three web sites: umbra.nascom.nasa.gov/images/latest.html (from the scientists of the orbiting SOHO satellite); www.bbso.njit.edu/cgi-bin/LatestImages (from the Big Bear Solar Observatory); www.spaceweather.com (Dr. Tony Phillips’ web site on the Sun-Earth environment).
Astronomy readers can monitor auroral activity by visiting Astronomy.com’s “space weather” page.