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Here comes Halley's. Some tips on Halley's comet for the casual sky-watcher

ALTHOUGH professional astronomers may pursue Halley's comet with giant telescope and spacecraft, the once-in-a-lifetime visitor is pre-eminently an object for casual sky watchers as well. ``You're going to see Halley's comet best if you're looking at it just casually and without doing any serious scientific observation,'' says Britain's veteran amateur astronomer Patrick Moore. And expert photographer Dennis di Cicco of Sky & Telescope magazine expects ``that some of the most esthetically appealing pictures of Halley's comet will probably be done by amateurs at this return. . . . The average person who owns a 35-millimeter camera should be able to obtain a permanent record of the comet f or his keepsake.''

The powerful but special instruments that professionals use present data in arcane forms with few direct glimpses of the visual majesty of the sky. That's why Mr. Moore says, ``You're going to see it [the comet] best either with the naked eye or with binoculars.'' Moonlight may make viewing difficult in early December. But by Dec. 8 or so, the comet should be visible about half way up the southeastern sky in the evening. It may begin to show a tail that should be even more evident in January.

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Right now, Halley doesn't seem very impressive. Even when seen through a 16-inch telescope at Kitt Peak National Observatory recently, it looked to reporters like a fuzzy gray star or, perhaps, a celestial dandelion puff whose seed -- the comet nucleus -- is hidden by the fluff. This fluff is the comet's atmosphere of dust and gases called the coma. It now surrounds Halley's nucleus out to distances of several hundred thousand miles. Any tail that was beginning to form was hidden behind Halley's head as

the comet came straight at us.

That nucleus -- mainly water ice mixed with cosmic dirt -- has a mass roughly estimated at 1,000 billion tons, give or take 10 percent. It's equivalent to a good-size mountain on Earth. That's small as astronomical objects go. But this little snowball is about to release clouds of dust explosively as it nears the sun. This should build up the visibility of the coma and the tail.

Newspaper star charts, which now often show the comet's location, will tell you where to look for Halley. Binoculars with specifications in the range of 7 x 50 (magnification of 7, objective lens 50 mm. across) to 12 x 70, should be most useful for most people. To see Halley well you need to get well away from the glow of city lights and allow about 20 minutes for your eyes to adapt to the dark.

As for photography, the International Halley Watch offers this advice. Mount the camera securely on a tripod or other support. It is best to lock the camera shutter open. Most cameras have a ``B'' shutter setting for this purpose, although automatic models that can be set for long-duration exposures may also work. Use a cable release to open the shutter to minimize vibration. Exposures of 10 seconds to 10 minutes should show the comet on so-called fast color or black-and-white film, those with ASA ratin gs of 800 or higher. However, Earth's rotation will likely show trailing of both comet and background stars. If you have access to a small amateur telescope mount with a mechanism for star tracking, you can eliminate the trailing.

The International Halley Watch recommends using lenses with focal lengths in the range of 28 mm to 200 mm. The lens should be set to its lowest possible f-stop -- that is, it should be wide open.

Halley swings past the sun on Feb. 9 and can't be seen for a few weeks either side of that time. Then it will reappear as seen from Earth as it heads back toward the outer edge of the planetary system. It will be better seen from the Southern Hemisphere, then. You need to be well below latitude 42 degrees north to get it above your local horizon at all, although some northern-temperate-zone viewers may get a glimpse of the comet's tail.

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Sky Publishing Corporation -- 49 Bay State Road, Cambridge, Mass. 02238-1290 -- will publish a detailed article on Halley photography in the January issue of Sky & Telescope. You can write to it for more photo information. The magazine also maintains a pre-recorded updated sky position for Halley as part of a general astronomical update message. Call 617-497-4168. Arizona State University also has a Halley Hotline, at 602-965-2345. Both are toll calls.