Comet shooting - photographing Comet Hyakutake

PSA Journal, Dec, 1996 by James R. Roy

There were 900 cars and maybe 2500 people at Sky Meadows State Park near Paris, Virginia, on Saturday, March 23, 1996, but it was not to see the authentic 1840s house or the lovely meadows. Instead, we had a look at the best comet in at least 20 years, Hyakutake. Not only that, it was wonderfully clear, and the head of the comet was five times bigger than the soon-to-set Moon. It was fully visible with the naked eye, and low power binoculars were helpful, while long focal length telescopes were generally not. The last really bright comet was West, in 1976. Halley, in 1985-86, required at least binoculars to get a decent view, because it was not as inherently bright or as close to Earth as it was in 1910, when Halley was so close that Earth passed through its tail.

Photographing Hyakutake was something of a challenge. I was one of only four students of Geoff Chester of the National Air and Space Museum Planetarium at his 1988 astrophotography class. With such a clear night sky at Sky Meadows, it was possible to try long exposures with a fast normal or wide angle lens on a regular tripod, using fast film. I tried Fujichrome 200 with a 50mm f/ 1.4 for 5, 10, and 30 seconds and TMax 3200 for a few seconds to 4 minutes. Bracketing is essential. However, my clock drive battery died, and with such a large crowd at Sky Meadows, there seemed little chance to plug my clock drive into 110 volt current. On the other hand, PSA and Greater Washington Council of Camera Clubs members Scott and Andrea Blair (she's council president) invited me to stay over at their home in rural Linden, Virginia, which has a sky at least as dark as Sky Meadows, and without 900 cars (and their distracting headlights) running up and down the road. I followed Scott home, plugged into their porch outlet, and soon heard the reassuring hum of my clock drive smoothly following the stars as the Earth rotated out from under the comet and star field.

I spent more time than usual getting polar alignment on my telescope because without it you will get star trails and a too-fuzzy comet. Then it was not too hard to take bracketed exposures of 5 seconds to 5 minutes with the gear above, plus a 200mm f/3 lens for isolating the head of the comet and a 20mm f/3.5 for most of the sky in one exposure.

Astrophotography is not especially difficult, but it is time consuming and unpredictable. With a fuzzy, extended object like a nebula, galaxy, or comet it mainly is a fight to get enough contrast to show the object before the base fog of the film overpowers the all-too-few photons sent out by the subject. For that reason, maximum lens and film contrast and lack of reciprocity failure are most important. For the astronomical professional, this can mean 14-inch square glass plate negatives, Schmidt cameras weighing 5 tons, and 4-hour guided exposures, but an amateur can buy or make gas-hypersensitized Kodak Tech Pan and use almost any clock-driven telescope with a camera and lens riding it "piggyback" to take good pictures. I happened to have some TMax 3200 frozen for just such a contingency, so I defrosted and used it. The first pictures, taken at Sky Meadows, have some severe star trailing because of the long exposures and lack of clock drive. The pictures from Linden are much better because of the working clock drive. Star trailing is worst with 1) long lenses and 2) long exposures. The angular size of the comet, much bigger than the Moon and only about 10,000,000 miles away, suggested that anything longer than a 200mm would not get much of the tail in, because the tail was about 30 degrees long.

While black-and-white films have mainly a base fog problem with long exposures, color films change color with overlong exposures. Traditionally, Ektachrome goes blue, Kodachrome and some Fujichromes green, and older Agfachrome brown. You can try a filter, but it makes the exposure even longer and adds another two surfaces of glass to reduce contrast. Nearly all color slide and print film is formulated to have no noticeable color shift for exposures of about 1 second to 1/10,000, but watch out for very long times. Color print film also has a color shift, but often this can be minimized by taking consecutive negatives and printing through both of them, a technique called "stacking," which enhances contrast and color at a small cost in sharpness. Special astrophotography films can go 20 minutes to 4 hours without too much loss of color sensitivity or reciprocity failure, but they are expensive, hard to find, and dependent on moisture in the air, local pollutants, temperature, and other factors. For instance, for years telescope maker Celestron sold a dry-ice loaded "Cold Camera" that got slide film down 100+ degrees below zero to increase speed about 5 times. However, it took only one frame at a time of 35mm. You then had to hand-develop the individual chips, and reloading was a tough job in complete darkness. Today, 800 to 3200 speed film makes exposures of under a minute quite viable with a fast lens on an SLR. It is preferable to lock up the mirror to avoid vibration, too.