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.
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