Secrets of High-Power Comet Observing
Here's what to look for before going to the eyepiece.
Yet the tail was only a part albeit a big part of the comet's spectacular close approach to Earth. Indeed, observers who found time to glimpse Hyakutake's bright head under high magnification were rewarded by dynamic displays of Sunward jets and a tailward spine. Then, all too soon, the comet and all it had offered faded into the twilight glow. Who really had time to fathom the dynamic action going on close to the comet's nucleus when so grand a sight was before us?
Fortunately, another bright comet, Hale-Bopp (C/1995 O1), arrived, and remained visible to the naked eye for months, not days. Amateurs with even small telescopes had an opportunity to probe this comet's nuclear regions and see some of its most secretive details.
A comet is a whimsical creature. Its behavior mimics that of a cat, being largely independent of our will and wishes. Yet Hale-Bopp was a splendid telescopic sight during the summer of 1996, some nine months before the peak of its display. By then a vast array of jets spewed dust and gas from a seemingly pulsating nucleus. Multiple parabolic hoods surrounded the inner coma, and a needlelike spine occasionally appeared in the antisolar direction. Such features are sometimes fleeting, sometimes dramatic. They go through cycles of brightness and periodically have odd appearances.
The clarity of the features in Hale-Bopp's head was outstanding by historic standards. Comet expert John Bortle notes that only a handful of bright comets in the last century have displayed such distinct, near-nuclear structure particularly when more than 0.75 astronomical unit (110 million kilometers) from the Sun. Yet Hale-Bopp even displayed conspicuous structure when more than 4 a.u. out!
Probing the Inner Sanctum
I always work from high power to low power, building a central foundation upon which ever-larger details can be incorporated. High magnification is also the most important aid for picking out fine details in the comet's innermost regions. Using it reduces the surface brightness of the surrounding coma, causing subtle features to stand out from their otherwise bright background. So once you've located the comet with low power, examine it briefly for any obvious details. Then center the comet and replace the low-power eyepiece with one having a shorter focal length to yield higher power, preferably one that magnifies 50 times the inches of your scope's aperture. Even higher magnifications can be used if the atmospheric conditions and your telescope's optics allow.
Alternating quickly between these eyepieces should reveal a dramatic effect: at low power the pseudonucleus usually looks slightly swollen, while high power shows it as a fainter starlike point surrounded by one or more round envelopes of deep inner coma. (This is not the comet's true nucleus, which is much too small to be resolved.) On rare occasions the pseudonucleus occults or passes arcseconds from a brighter star. The latter's glare further diminishes the brightness of the inner coma and allows you to penetrate deeper into this inner sanctum.