Terry Lovejoy, an Australian amateur astronomer, recently become the first person to discover a comet using an off-the-shelf digital camera. He made his find on March 15, while examining images he had taken with a Canon 350D (Digital Rebel XT) as part of an ongoing search for comets. (The photo at left is a composite of his discovery images. Used by permission.)

Lovejoy started using digital SLRs for hunting comets in late 2004. He uses the Digital Rebel XT, equipped with a 200mm telephoto lens, as well as another Canon DSLR, both secured to an equatorial mount that tracks the stars, allowing for long exposures. (No telescope is involved.) He had examined about 1,000 different star fields over the course of the search, taking multiple shots of the same region of sky, processing them using IRIS, and then "blinking" between them, looking for unusual objects, particularly ones that appear to move against the starry background from frame to frame or that show visible characteristics of comets.

He took a series of 90-second exposures of star fields in the predawn sky of March 15, and was preparing to process them when he noticed a fuzzy, greenish patch that appeared in nearly the same location in a number of the images. It looked suspiciously comet-like to him--comets often look green in photos due to the fluorescence of two molecules, cyanogen (CN) and diatomic carbon (C2) when exposed to sunlight. A small but clear motion over the course of the image set indicated that the object was "nearby" (in the inner solar system). The following night, a New Zealand amateur confirmed his discovery, and it was announced to the world shortly thereafter.

Though this is the first comet to bear Lovejoy's name, it isn't his first discovery, and not even his first to employ new digital detection methods. In 1999, he was the first amateur to find a comet in online images--uploaded to a NASA site from the SOHO (Solar and Heliospheric Observatory) spacecraft--soon after such images became publicly available. In the following year, he found 10 more SOHO "sungrazing" comets. These tiny comets are named for the SOHO spacecraft rather than the person who first detects them. Since then, he has continued as an active observer, photographer, and hunter of comets in the night sky. He came very close to finding several comets in the night sky without any luck--until now.

These days, the vast majority of cometary discoveries are made by professional sky surveys tasked to identify near-Earth asteroids, using extremely sensitive, automated CCD cameras. Amateur comet discoveries have become increasingly rare, especially discoveries made visually (staring through a telescope's eyepiece). Some amateurs have availed themselves of high-quality (and very expensive) astronomical CCD cameras coupled with telescopes, with measured success.

Comet Lovejoy--officially C/2007 E2 (Lovejoy)--was in the constellation Indus, deep in the southern sky, when Terry discovered it. At the time, it was fairly faint, beyond the range of standard binoculars from a dark-sky site. It has begun a long northward trek towards a not-so-close encounter with Earth (about 40 million miles away, at its closest) in late April. Around then, it should be easily visible in binoculars in the morning sky if you know where to look. (By early April, a finder chart for the comet should be available on the Comet Chasing Web site). Official projections have it reaching a peak brightness of just below naked-eye visibility, but the performance of individual comets is hard to predict. As noted comet discoverer David Levy once said, "Comets are like cats. They have tails, and they do precisely what they want."

Terry Lovejoy's setup, with two digital SLRs on an equatorial mount, with which he discovered his comet.

Post by Tony Hoffman