SKYSHOOTING COMETS - A CHALLENGE FOR ASTROPHOTOGRAPHERS II, By Chris Schur In my last lecture, we discussed why average comets pose difficult challenges to astrophotographers to get a really clear shot. Although they may be fairly bright, they move quickly against the background starfield. Therfore conventional tracking methods cannot be used efficiently. Here we will discuss another aspect of comet photography, pushing your scope to its limits to capture the less significant members of this group of objects as well as the brightest in their class. SMALL AND FAINT COMETS Lets start with the most common type of comet, the small and faint (Mv<9) ones. These occur at a rate of up to a dozen or more per year, and this more frequent appearance allows a good deal of practice for improving techniques and equipment. If you keep this in mind, you'll be ready for the brighter and more detailed objects which occur much less frequently. Large apertures such as ten inches or more are essential, along with long exposures and good guiding. But this is where we encounter the first dilemma of would be comet photographers, comets don't sit still during your exposure, and short exposures such as five minutes or less will either not record them at all, or very faintly. But the longer exposures, fifteen to thirty minutes that are required to show them well, also trail them onto the film as a short streak, and thus blur all the fine details. Also the small size of these objects requires a fine grained but slower film, thus aggravating the problem all together. The situation is not hopeless however, because once we know the comets daily motion, and direction of its movement by simply plotting up its current position on a suitable atlas, we can easily compensate for its movement and hold it stationary on the film. This we covered in some detail in the previous lecture. Assuming that we track the comet properly, what can we expect on the film once a long exposure is printed up? Generally, a small round diffuse glow will be recorded, usually much brighter in the center, and possibly a short fan-like extension of the tail. Don't expect the faint comets to always have a tail, most don't. Because of thier low contrast and overall lack of color, color films have no advantage with these dim objects. Most photograph pure white, or at best with a slightly bluish cast. BRIGHT COMETS If you've kept up your practice at tracking and film processing, you'll be ready when the really bright comets make their appearance. For some people, these are reasons for living. And nothing can awaken public interest like a brilliant comet. While a bright comet - about third or fourth magnitude can occur every year or two, a really brilliant comet, say second magnitude or brighter won't come by more often than a decade or two. Most of the bright and brilliant comets have tails ranging from two degrees in length on past twenty degrees or more. Some, such as Comet IRAS-ARAKI-ALCOCK had no tail at all! The coma will vary in size too, anywhere from a quarter degree up to two degrees or more in diameter. We can see here that a variety of focal lengths must be used. The prime focus of the telescope will reveal only the head and inner coma, but since the internal details of bright comets are greater, one should consider a wide range of exposures on color film to record the comets heart. Look carefully at the color contrast in the coma. Several comets in recent years revealed on color emulsions a beautiful sky blue coma, with the leading edge a saturated emerald green color. Really active comets will have an overall green or turquoise colored coma that of course would never have been noticed on black and white films. The tails are usually long and bright enough to record superbly with short to medium telephotos, making them suitable for less skilled astrophotographers to achieve some really spectacular photographs. You will find however, that comet tails are really low surface brightness objects, and while color films will record some blue or yellow tints to the gas and dust tails on the really brilliant comets, most tails will be recorded best in the higher contrast black and white. One final point with bright comets, is that with fast 50mm or 135mm lenses, we often can get the comet setting against a distant scene such as a mountain or distant trees. Exposures under five minutes guided on the stars will not significantly blur the foreground and such scenes always make the most inspiring astrophotos. Magazines are always on the look out for such shots, especially ones with recognizable picturesque foregrounds such as a palm tree or famous land form. FILMS FOR COMETS Below is a listing of B&W as well as color films we have tried with good results. Your selection will be based on equipment and your specific needs. 1. The best color film we have tried so far is Kokak Pro400 ppf negative film. It has extremely fine grain, is fast, and a huge dynamic range for recording the large variations in brightness across the comets face. Its color response is remarkable; soft sky blues, emerald greens and straw colored yellows are recorded very well, with good color separation. 2. Fuji 400 and 800 Super G Plus films are a second choice, but these faster films have much less color saturation and differentiation than the much higher resolution Kokak 400. 3. The best B&W film is hypered Kodak 2415, developed in D19. Its high contrast (gamma = 3.5 - 4) brings out tail structures like no other film. The combination of this film with a red filter such as the Wratten #29 allows good penetration of twilight to pick out the maximum details in the sungrazers. For small comets, it can't be beat. Its ultra fine grain, and good speed make it a perfect match when used with a large newtonian and prime focus.