Photographing Iridium Flares
The most interesting flares are those that become at least as bright as magnitude –2; those brighter than –4 generally elicit a "Wow!" from observers. When the sky is in deep twilight or darker, a point of light this bright can be captured with a "snapshot," but all you'll have in the resulting photograph is a starlike point of light and a dark background — not exactly a prize-winning composition.
But if you place the camera on a sturdy support and keep the shutter open during the 10-second or so duration of most flares, you'll record the event as a streak of light. Any 35-mm camera loaded with ISO 100 film and fitted with a 50-mm lens set to f/2.8 or faster will easily capture the streak of a magnitude –2 flare. Most digital cameras have a basic speed setting that is equivalent to a film speed of ISO 100, but the actual focal length of the "normal" lens will probably be less than 50 mm. Otherwise the use of film and digital cameras is essentially the same for this type of photography.
The only challenge in photographing the streak of an Iridium flare is pointing your camera at the right spot on the sky. Experience has shown that predictions on the Heavens Above Web site of a flare's location in the sky, and especially its time, are very accurate. The position, however, is given in angles of azimuth and altitude. The azimuth is measured around the horizon starting at true north (the direction of Polaris) rather than magnetic north indicated by a compass. The azimuth angle increases as you sweep along the horizon toward east (90°), south (180°), west (270°) and back to north (0°). Altitude simply goes from 0° at the horizon to 90° directly overhead.
Getting the timing right is important, too. Accurate time signals for setting your watch are available from many sources, including the U.S. Naval Observatory (telephone number +1-202-762-1401).
If you're good at estimating angles, you can pinpoint the location on the sky where you should aim your camera. For example, if a prediction gives the azimuth and altitude as 135° and 20°, respectively, you'll find the flare 20° above the point on the horizon exactly halfway between east and south. Your fist, held at the end of an outstretched arm, subtends an angle of about 10° and is a useful tool for estimating angles. Flares leave streaks on film that are usually less than 10° long, so you'll have plenty of room for small pointing errors if you're using a normal or wide-angle camera lens.
Conditions vary by location, but here are some starting points. In the suburbs of large metropolitan areas where light pollution is visible but not overwhelming (with 4th-magnitude stars remaining visible to the unaided eye), try using a 5- to 10-minute exposure at f/2.8 with ISO 100 film. Moonlight can actually help illuminate the foreground and be used to advantage. With the Moon's phase around first or last quarter, exposures of 1 to 2 minutes at f/2.8 with ISO 100 film will be enough to illuminate the foreground but still keep the sky dark enough to show stars well. Exposures half that long should work around the time of full Moon. Such long exposures are often not possible with digital cameras, but check with the camera's manual to be sure.