There are basically two types of comets: faint ones and bright ones. The bright ones certainly get all the glory, and names like Halley, West, Hyakutake, and Hale-Bopp are all familiar to most amateur astronomers. Occasionally a fainter comet will take the spotlight such as when Comet Shoemaker-Levy 9 impacted Jupiter in 1994. Every few years there is typically a relatively bright comet, just barely visible to the unaided eye, such as the recent Machholz in the winter of 2004/2005. These are by far the most photogenic comets, but their apparent size and proximity to Earth can make them tricky targets for the CCD imager.
The fainter comets, while not nearly as spectacular as the big ones, are still well within reach of amateur telescopes and CCDs. These comets often show much detail in CCD images, occasionally glide past deep-sky objects making for pretty celestial conjunctions, and valuable scientific data can be gathered on the comet's activity and brightness.
Wide Field Comet Images
Comet Hale-Bopp, the most-viewed comet of the 20th century, was as bright as the brightest star in the sky, and its magnificent tail spanned more than 10 degrees of sky. By bright comet standards, this is actually a fairly short tail. Comet Ikeya-Zhang in 2002, despite being 100 times fainter than Hale-Bopp, still showed a 6-degree tail, and Comet Hyakutake, in 1996, had a tail stretching an incredible 100 degrees, or more than halfway across the night sky! In order to capture the entirety (or even most) of a large comet, a wide-angle lens is needed. Often a telescope will prove to have too narrow a field of view to fit a whole bright comet. Another technique for capturing a lengthy comet tail is to mosaic a number of smaller images.
Most CCD cameras can easily be attached to a wide-angle camera lens. This is good technique to use for imaging large deep-sky objects as well. A camera that provides a field of view of less than half a degree on an 8" SCT will give a huge 28-degree field with a 28mm wide-angle camera lens, enough for all but the very largest comets. Even a small CCD with a common 50mm camera lens will give greater than a 5-degree field, comparable to a 300m lens used with film, a common comet-photographing setup in the days of film astrophotography.
Comets on the Go
Part of what makes comet imaging tricky is the fact that comets are moving relative to the background stars. Telescope mounts are set up to track the stars as Earth rotates; anything moving with a significantly different motion may not be tracked properly during an exposure. With very bright comets this is often not much of a problem. Since the exposures are so short, the motion of the comet relative to the stars is not noticed. However, with dimmer comets, the motion against the stars can become apparent. Often this shows up in tri-color images because of the longer overall time required. Even if each individual frame does not show the comet's motion, there might be movement between each image. If the final RGB image is combined by matching the stars in the field, the comet will be blurred. Instead, aligning the comet in each image will result in a sharp image of the comet itself, but there will be three images of every star, one red, one green, and one blue. This is usually a compromise that must be made, although with one-shot color CCD cameras becoming more popular this is less of a problem.
If a comet is faint, it may be necessary to guide on the comet itself, as you would guide on a star for a long exposure of a deep-sky object. Using a self-guiding CCD (such as those from SBIG) will not work in this case as the comet cannot be seen by the guiding chip when it is centered on the imaging CCD. Off-axis guiding is also not possible. The solutions include the use of a separate guidescope, using a camera such as the Starlight Xpress series cameras with STAR2000 feature (which allows self-guiding on a target within the main chip), moving the telescope a predetermined direction and distance at given intervals throughout the exposure (a math-intensive solution which is mediocre at best), or the use of a high-precision programmable mount which can use the known orbit of a comet to track it accurately. Short of spending $10,000 on a fancy new mount, the guidescope solution is probably best if guiding on the comet is necessary with a non-STAR2000 camera, but taking many short exposures and combining them later might well be the simplest way to go.
The techniques for making a comet mosaic are essentially identical to creating a mosaic of a deep-sky object (see the Mosaics page). The one additional consideration is again the movement of the comet itself. If enough time elapses between the first image and the last, they may not properly align due to the comet's motion.
Above: An example of a comet mosaic. This image shows a spectacular disconnection event in Comet Ikeya-Zhang's ion tail on the evening of March 11, 2002. The image is a composite of six separate pictures taken with an 8" SCT. The mosaic spans about 2.5 degrees of the sky. Image by Gil Jones.