SKYSHOOTING COMETS - A CHALLENGE FOR ASTROPHOTOGRAPHERS III By Chris Schur This is the third part of the series of lectures on photographing comets. In this lecture, we will discuss the more unusual cases, comets that bear strange appendages, or ones that move very quickly across the evening sky. And of course we couldn't forget the best comets of all, the Sungrazers! SPECIAL CASES Fast movers. Every once in awhile a comet will swing close to the earth at such a rate that even short exposures blur their motion. About once or twice per decade you'd encounter one of these mavericks. Comet Iras - Araki - Alcock was one such comet; large, bright and moving at one point 35 degrees per day! We tried to guide on the nucleus but even with the drive corrector fast button held continuously down, we couldn't keep up with it. Admittedly such comets are quite rare, but you can allow for their occasional appearance by putting a switch on your drive corrector that will make the fast button really fast, and slow button almost stall the motor. Most of the time you would not use such an option but it will be there, ready just in case. A more common option is the short focal length short exposure approach. Here we use a moderately fast film in a fast lens or mirror system such as a 135mm f/2.8 or Schmidt camera and keep the exposure under 5 minutes. This usually works, and that's how we managed to keep up with IRAS. Antitails. Sometimes, but not always, the earth will pass through the orbital plane of a comet while it is still bright and not during full moon, and allows us to look at the image of the comet with the orbital plane superimposed upon it. The flat thin sheet of dusty material is then sometimes seen projected onto the comets coma as a thin band of material seldom extending more than a degree or two from the coma, while the tail ward spike is often invisible on top of the tail itself. The sunward spike has a dark backdrop and can then be detected. Anti tails are only visible for a few days at best, centered on orbital plane crossing, and are composed mainly of yellowish dust. They are seldom seen visually and color films don't have the contrast to record them. We suggest you shoot a comet during orbital plane crossing (CRAS notices predict the date) even if none is apparent visually. Use a very high contrast Black and White film with a gamma of at least 2. You will able to significantly boost the contrast of the elusive phenomenon by using a yellow filter since it is a dust phenomenon. We had a 50 cc yellow filter for the schmidt camera, but some of the Wratten series maybe more suitable for your set up. We also found to our amusement that while the Schmidt did a great job recording the anti tail of comet Bradfield, the prime focus shot with a 10" scope showed no trace of it. It is important to have significant contrast to detect the feeble anti tail. Sungrazers. Many bright comets cannot be readily photographed properly due to the fact that they are immersed in the bright twilight, despite a brilliant nucleus and coma. Comet West back in 1976 was such an object at first, and to get a really good overall shot of the comet, we had to wait about a week for it to be in a darker sky. To avoid burned out shots from twilight fog, here are some suggestions. We keep around one of those 400mm f/6.3 "Girl watcher" telephoto for the purpose. Its slower f ratio means less sky fog, and long focal lengths gets us in close to where the comets brightest parts are. Generally speaking, deep red filters will suppress the blue sky background for black and white shots, and is a good idea anyway to boost contrast. For color work with Comet Halley, we had good success with this lens and some yellow filters which also doubled the contrast without a severely discolored shot. Split Tail Comets. Comets are composed of primarily two constituents, dust and ice. Most of the time, the two are indistinguishable in normal comets, or perhaps a by a yellow or bluish tint in a small part of the tail. The recent Comet Wilson was such a comet, rather pale in color and appearing mainly white in color photographs. Comets Halley and West were good examples of a clear separation of the two tail types. The dust in a comet shines by mainly reflected sunlight, and appears yellowish with the characteristic dark absorption bands seen in typical solar spectra. The gas however emits light as its molecules become excited by the solar ultraviolet radiation, and numerous bright spectral lines appear, mainly in the blue-green portions of its spectrum. These spectral lines give the gas tail a bluish tint, and under favorable conditions where the excitation is very high especially around the comets head and coma, a turquoise or emerald green color can be seen. In Comet West for example, the pale golden dust fan spread diffusely over the inner tail, while a series of blue rays emanated from the dust fan, and resembled an auroral curtain. What a comet West was! Obviously, only the brightest comets can have split tails, but they are always best photographed on color emulsions, preferably one with a good broad panchromatic response.