Photographing Comet Hale-Bopp
(and other comets)

by Ralph Paramor

There just may be the opportunity to capture comet Hale-Bopp on film in May and our Editor has suggested that a few notes for would-be photographers may be useful. Actually, conditions at that time are far from ideal for photography as the comet will be low in the north-west shortly after sunset and setting earlier every evening. The appearance of the comet before dawn around August may present better prospects, though its magnitude will have diminished considerably. However, astrophotographers are ever optimistic and we should be prepared to make the most of any opportunity which presents itself.

Firstly, it should be recognised that the most basic photographic gear can produce excellent pictures of comets if they are sufficiently bright to be seen with the naked eye. Exposures can be brief if fast film is used and guiding the camera during exposure is not essential, though this will be necessary for longer exposures as discussed below. The minimum requirements are a camera which can be set for time exposures, a lens of a focal length to provide the required field of view and fast aperture, a tripod to mount them on, a cable release to obviate camera shake as the shutter is operated and suitable film. If these requirements are met there is no reason why satisfying photographs of a comet which is visible to the naked eye cannot be taken. To exemplify this I have recently seen a splendid photograph of Comet Hyakutake taken by Peter Edwards at Chittering in March 1996 with a 35mm Pentax SLR with a lens of focal length 50mm. Exposure was for 2 minutes at an aperture setting of f1.7 using Kodak Royal Gold ISO 1000.

The occurrence of a naked eye comet is a special event and it is worth while to expend effort and money to obtain a satisfying record. I would recommend that interested members should try a range of exposures and aperture settings to produce at least one photograph of which they can be proud - the cost of extra film would be money well spent.

The length of the Hyakutake tail in the photograph mentioned above was approximately 35 degrees and this fitted nicely into the frame of a 50 mm focal length lens (field size on 35mm film is about 40 x 25 degrees). A longer tail may go better on the field of a 35mm lens (52 x 38 degrees) while a shorter one could perhaps fit on the field of a 135mm lens (15 x 10 degrees). Changes in lens will result in changes to f ratios (aperture settings) and exposure times. Experimentation is the keyword.

Composition of the photograph is also important. Wide-angled shots (for this purpose those using lenses with a focal length of less than, say, 135mm) can usually include the outline of natural features such as trees, hills or rock outcrops, or even artificial ones like buildings, towers etc. These can add a pleasing aspect to the photograph, but it is advisable to ensure that the camera is oriented so that the bottom of the picture will be seen to be horizontal.

I should mention that attempts to photograph through city light pollution have less chance of success, though it may be possible to record a comet, over a city landscape, if it is sufficiently bright. Unfortunately, skyglow can cause overexposure of a film of 400 ISO rating after about 30 seconds. Dark skies call!

As far as film selection is concerned I would recommend a colour film with an ISO rating of 400 to 1000; faster films are available but their coarse grain may not suit. Slide film at ISO 400 would be suitable. Experts suggest aperture settings for the lens should be one or two stops less than its widest one to eliminate distortion at the field edges, but don't be afraid to experiment.

Twilight will certainly affect the situation in May but will not necessarily preclude a useful result. I understand that a bright yellow or orange filter (Wratten #12, #15 or #21) will assist in overcoming background light such as this. Try this out if you own or have access to them.

One feature of any comet which presents a difficulty for the photographer is the fact that it is moving relative to the background stars and this movement can be quite rapid while the comet is near perihelion. This movement is ignored without real penalty in the basic method mentioned above. For example, the Hyakutake photograph mentioned does show discernible star trailing, but it is so slight as to be insignificant and does not detract from the picture. If, however, we want to use a longer focal length lens for greater magnification (and smaller field) this will inevitably entail a higher f ratio and longer exposure. If we attempt an exposure long enough to require guiding our dilemma is whether to guide on the comet or a star. In the former case our photograph will show star trails and in the latter, while the star fields are crisp, the comet image will, theoretically, be smudged. In practice, because comets are by nature fuzzy objects, this smudging may be difficult to detect by eye and may not present a problem. If, however, we are seeking to record detail, such as structures in the coma or tail, we should guide on the comet and accept the star trails. Of course it is not a problem to try both methods and select the photograph which looks best to you. It is appropriate to mention here that guiding on the comet presents its own difficulty in that the object to guide on is extensive. If the edge of the coma is reasonably well defined it may be possible to place it in one quadrant of the guiding eyepiece so that the two crosshairs are tangential to the edge and guide so that they remain so. The practical alternative is simply to estimate the centre of the head of the comet and guide on that.

It should be noted that guiding on the comet can only be achieved with the camera "piggybacked" on a properly aligned and equipped telescope, or by the use of a separate guide scope rigidly fixed to the photographing instrument. Off axis guiders cannot be used to guide on the comet as they work by taking light from a source outside the photographic field.

There is a whole field of study of the positions of comets generally and changes which can occur within their nuclei and tails. Most of these objects are small and faint and advanced methods are used to track and monitor them. The special photographic techniques used may be of interest to the advanced amateur but are beyond the ken of the author and the scope of this article.

We have, at this time, the prospect of a naked eye comet appearing in our evening skies and, with any luck, we will be able to capture it on film. I would recommend that you select a site away from citylights and on the coast so as to obtain a clear view to the horizon. Good hunting!

ABOUT THE AUTHOR

Ralph Paramor's interest in astronomy started when he was required to complete a unit which included astronomical observations in surveying as part of an engineering course. His interest remained dormant until he joined ASWA about 16 years ago, and since that time he has made several presentations at Society meetings and written various articles for The Sidereal Times. He is interested in all aspects of astronomy but is currently wrestling with the art of astrophotography.