I SPENT MY FORMATIVE years in what was then rural San Diego County. Our front porch opened onto an expanse of foothills so wild that we often startled coyotes away from our front yard when we retrieved the morning paper. The night sky was absolutely dazzling, and the millions of stars overhead enticed me out to enjoy them in secret.

Throughout the summer that separated second from third grade, my head never hit the pillow before I was scheming that night's escape. I had recently heard about Halley's comet--of its famous appearance of 1910 and of its imminent return. So, armed with a noisy party favor to scare off the coyotes, I posted scores of predawn hours atop a nearby hill, fully expecting to be the first person to see the comet on its return journey. By the time autumn's chill finally drove me off my hill, I felt utterly defeated. Only years later did I discover that I had misheard the year of Halley's return as 1968 instead of 1986.

In the decades since that disappointment, I've continued my love affair with the night sky, but I've never seen a cometary display to match the description of Halley's 1910 appearance. Halley's pathetic showing in 1986 left that stiff-lipped little boy within me still waiting for the heavens to redeem themselves.

Well, it looks like they finally might. Comet Hale-Bopp (C/1995 01) looks even more impressive than it did just a month ago. The comet's nucleus is at least three times larger than Comet Halley's, and it has been ejecting bright jets of dust for many months. Some astronomers now believe Hale-Bopp may be the brightest comet this century. Here's how to make a detailed photographic record of the comet's passage.

If you think that so many pictures are going to be taken of this comet that yours can't be scientifically useful, think again. Professional astronomers want your help. A comet's tail changes rapidly, and the professionals need an army of observers to track these changes. For example, every few days, when the magnetic field created by the solar wind changes direction, the gas tail can separate from the comet's head. The comet can repair these "disconnection events" in just 30 minutes; you could be the only person who records it.

With a good 35-millimeter camera (single-lens reflex), a few lens attachments and a little practice, you can take vivid portraits of Hale-Bopp. Manual cameras are better for this work than the newer electronic automatic models. They perform better in cold weather, and you never risk a power failure during a long exposure. Try to find one that will let you lock up the internal mirror so that it won't jar the camera when you trip the shutter. If, while prowling around a used camera shop, you find a much coveted Olympus OM-1, Nikon F series or Pentax LX, buy it! Finally, because any vibration can spoil your image, you'll also need a cable release (available through camera supply companies for about $20) to activate the shutter without touching the camera.

When it comes to film, there is a trade-off between speed and grain size. Faster films require shorter exposures to catch the same detail, but they have a larger grain size and so give poorer resolution. As a general rule, you should use the slowest film your subject will allow, but nothing slower than ISO 400. Many astronomers prefer black-and-white film for its superior resolution. Kodak's T-Max 400 gives excellent results. If you decide to take color pictures, Fuji's Super G 800 and Kodak's Royal Gold 1000 get high marks for prints, and for slides Kodak's Ektachrome P1600 nudges out Fuji's Provia 1600 in side-by-side comparisons.

The correct exposure time depends on too many factors to guess accurately. The best advice is to shoot the comet at several exposures ranging from 10 seconds to 10 minutes. In general, a wide-angle lens requires shorter exposures than a telephoto lens of the same aperture, and the larger the aperture of the lens, the shorter the required exposure time. After reviewing the results of a roll or two, you should be able to narrow the range of exposure times.

The earth's rotation steadily shifts the sky, causing stars (and comets) to create curved streaks on any extended-exposure picture taken by a stationary camera. To compensate, you'll need to shift the camera with the sky throughout the exposure for any exposure longer than one minute. If you have a telescope with a sidereal drive, you can buy a piggyback mount to attach your camera to the telescope. (Lumicon in Livermore, Calif., sells one for $60; telephone: 510-447-9570.) Battery-operated drives for cameras are also available but very expensive. (Pocono Mountain Optics in Daleville, Pa., sells one for $295; telephone: 800-569-4323 or 717-842-1500.) Or you can build a hand guider for less than $20. This design comes compliments of Dennis Mammana, resident astronomer of the Fleet Space Theater and Science Center in San Diego, and is the product of the collective cleverness of Dennis and other gifted astrophotographers. By manually twisting the adjusting screw one quarter turn every 15 seconds, you can eliminate star trails for up to 10 minutes on wide-field shots. Or use a high-torque DC motor powered by a car battery to turn the screw, then sit back and enjoy the comet.

Many astrophotographers take beautiful wide-angle portraits of comets that contrast the feathery tails against a stark landscape. Though often stunning, these images fail to capture many of the tail's most interesting features. Better science requires a closer view. The ideal field of view for recording tail features is about five degrees, which requires a lens with a 400-millimeter focal length. Because the tail may extend 20 degrees or more, you'll need to make a mosaic of images to catch the whole comet.

Remember, the longer the shutter is open, the more bad things can happen to your picture, so keep exposures as short as possible. This means you want a "fast"--that is, wide aperture--lens. The highest-quality 400-millimeter lenses can cost up to $6,000. Don't fret if you have to go to a shorter lens and a wider angle. A standard 135-millimeter telephoto is still scientifically useful. You will have to do some market research and balance your budget against the science you wish to accomplish. If possible, avoid mirror telephoto lenses and zoom lenses, whose optics are generally unsuitable for the sharp contrasts of astrophotography.

A few things to keep in mind: To help avoid darkroom catastrophes, begin each roll of film with a well-lit picture of something--anything. Without a reference image, darkroom technicians often can't identify the edges of a starry frame and sometimes miscut the film (to eliminate the risk entirely, many astrophotographers have their slides or negatives returned uncut). Also, carefully log the date, time and sky conditions for every observation you make. Most important, you must commit yourself to a regular observing schedule. A series of consistent observations made from the same site over many nights is much more valuable than a scattering of disjointed observations taken around the world. To do the job right, you've got to be out there on every clear night possible.

To find out more about observing comets or to learn how to contribute your observations, contact the Harvard-Smithsonian Center for Astrophysics via e-mail (Also see the January column for details about comets' tails.) I gratefully acknowledge informative conversations with Daniel W. E. Green of the Smithsonian Astrophysical Observatory and Dennis Mammana.

Check out the Society for Amateur Scientists's Web page or call 1-877-527-0382.

Further Reading

Astrophotography II. Patrick Martinez. Willmann-Bell Publishers, 1987.

Guide to Observing Comets. Smithsonian Astrophysical Observatory. Available for $15 from International Comet Quarterly, 60 Garden St., Cambridge, MA 02138.

Astrophotography: An Introduction. H.J.P. Arnold. Sky Publishing, 1995.

The Society for Amateur Scientists (SAS) is a nonprofit research and educational organization dedicated to helping people enrich their lives by following their passion to take part in scientific adventures of all kinds.