Here comes the Sun

For amateur astronomers who hunt at night for deep-sky prey, the Sun is a nuisance. Sunrise allows nebulae, clusters, and galaxies to go into hiding, forcing these night owls to give up the chase. It’s their loss because the Sun is arguably the ultimate in cosmic big game. There is a real element of danger to this daytime hunt. Veteran Sun-stalkers know that, like the game animals that inhabit plain, forest, and jungle, the Sun can strike down the unwary pursuer — not with fang and claw, but with a blinding flash of light.

The time-honored way to view the Sun in relative safety is by using the telescope to project a solar image onto a white cardboard sheet. Not only is the method safe, but it’s also simple and inexpensive. The Sun’s overwhelming light and heat mandate a minimal amount of aperture; large scopes should be downsized with a 2- to 3-inch-diameter aperture mask. Even then, enough heat can accumulate to “cook” an expensive multi-element eyepiece. Opt for something simple and inexpensive (the Huygens or Ramsden designs are ideal), and make sure it’s low power. The entire solar disk and relevant detail can be captured with magnifications of just 20–30x.

When aiming a telescope toward the Sun, never use the finder! Keep it covered, especially if you’re hosting a solar star party where a curious attendee might try to sneak a peek (it happened to me!). Instead, rely on the shadow of your scope’s tube. When it’s as compact as can be and the eyepiece is brightly illuminated, you’re on target. Position the cardboard sheet about a foot from the eyepiece, and focus until the bright circle of light (the Sun’s projected image) is sharp at the edges.

This circle is the Sun’s photosphere, or visible surface. If there is any solar activity, you should see sunspots — gray blotches with dark centers. Dark only because they’re cooler than the surrounding surface, sunspots change appearance from hour to hour and day to day as they evolve and drift in and out of view with the Sun’s rotation. Higher magnifications allow a detailed look at the photosphere, revealing its mottled, grainy appearance. This is granulation, an effect produced by masses of hot gas rising, cooling, and then sinking on the Sun’s seething surface.

Since high magnification isn’t necessary for casual solar observing, will binoculars suffice? I’d say yes, but I worry about the risk to the optics should binoculars be used for solar projection. If you’d like to observe the Sun with binoculars or simply prefer a direct telescopic view, invest in an aperture solar filter. Made of black polymer, Mylar, or specially coated glass, it affixes to the front of the binocular barrel or telescope, blocking out all but a safe amount of incoming light. Costs depend on type and telescope aperture, ranging from $20 for a black polymer filter suitable for standard binoculars or a 2-inch refractor to over $200 for a glass filter designed for a 16-inch scope. Do-it-yourselfers who wish to construct an aperture solar filter should look into a tutorial posted on the website of the Springfield Telescope Makers. Log on to http://tiny.cc/stellafanesun.

Small-aperture telescopes manufactured during the latter half of the 1900s were often accessorized with Sun filters that screw into the eyepiece. Don’t use them! Concentrated heat at the eyepiece has caused screw-in Sun filters to shatter without warning. Don’t believe me? My January 2010 column, “Screw-in solar filter hazards,” relates several accounts from readers who were momentarily blinded when theirs suddenly splintered.

Solar projection and aperture Sun filters deliver what is called a “white light” view of the Sun. If you’d like to see our star in a whole new light (literally!), look into a specialized filter or telescope that allows only a narrow bandwidth of light (in this case, the Hydrogen-alpha line of the solar spectrum) to reach the eyepiece. They’re a lot more money — a few hundred dollars to as much as a few thousand — but the visual reward is worth every penny. What could be more breathtaking than to view a solar prominence arching high above the Sun’s limb or to capture a solar flare erupting from the vicinity of a sunspot?

If you’re a dedicated backyard astronomer, don’t limit your quest to the denizens of the night sky. The most magnificent prey of all roams the daytime sky.

Questions, comments, or suggestions? Email me at gchaple@hotmail.com. Next month: We see the unseen! Clear skies!