SKYSHOOTING COMETS - A CHALLENGE FOR ASTROPHOTOGRAPHERS By Chris Schur
     The appearance of a bright comet in the evening sky can stir 
the sense and wonder in even the most unresponsive individuals. 
Astronomy clubs really come alive, and the "dormant" armchair
individuals dust off their old scopes in their closets.  But for
the astrophotographer, the ability to capture the shortlived
beauty of these transient visitors on film and done right taxes
the skills of even the most persistent astrophotographers.  In
this first part of a series of lectures, we shall present various
methods which we have found to be successful to capture these
ghostly visitors  on the emulsion, along with tips and
suggestions for using your equipment to obtain the best results.
     Comets offer a special challenge to the deep sky photographer.
Their tremendous variability in size, brightness, and internal
structure make it impossible to shoot them with only one camera
lens or telescope or even a single type of film.  Often times the
brightness difference between the tail and coma is so great as to
make printing them in the darkroom a compromise situation.  These
difficulties can be for the most part overcome, and with special
tracking procedures outlined later, you will find that we can
push our telescopes to their limits and record every bit of
detail the comet will offer. 
AVERAGE COMETS
     These range in magnitude from about five to nine and typically
two or three per year appear.  Many can be seen to varying
degrees in a pair of 11x80's, but the best views are  always
through the telescope.  Consequently,  shots through the main
optics at prime focus reveal the most details.  Average comets
can have up to a degree or two of tail, or maybe none at all.  If
a tail is suspected visually, it is best to frame up the comet in
the viewfinder such that the possible tail will stretch the
diagonal in the frame with the head nearly in the corner. 
Stopping down your fast  newtonian to f/5 or f/6 will reduce the
vignetting in the corners to a negligible amount such that good
detail and brightness is retained across the entire field.
     As far as movement during the exposure, comets can provide a
special challenge.  Brighter comets are closer to the Sun or
Earth, and hence their daily motion across the sky is more than
more distant comets.  Either special tracking procedures, or
possibly the use of fast hypered films to make possible short
exposures (5-10mins) can record them as nearly stationary
objects.  The good news here is that these brighter objects
usually show some degree of color on the film, usually bluish or
greenish, and are well worth shooting on color emulsions.
     The details you will record will vary greatly with the object,
but in general, the inner coma and bright nuclear region are best
recorded on a finer grained color emulsion.  This is because of
the wide latitude of brightness densities that can be recorded on
a color negative film  which will show more than a burned out
core. Look for distinct brightness zones in the coma, color
variations, and jets from the nucleus.  The faint tail,if any, is
always recorded with a higher contrast black and white emulsion. 
This will reveal any rays, knots and dark zones behind the head
that might be present.
TRACKING ON A COMETS MOTION.
     Most comets will require a long enough exposure such that you
must allow for it while taking the exposure.  The two easiest
ways to do this, with the least amount of equipment
modifications, is to guide on the comets nucleus, or offset guide
on an available nearby bright star with a transit eyepiece. 
Since nearly all comets nuclei are too faint for high
magnification guiding, the offset method offers the best hope. 
We constructed a reticle on 1/16 inch plexiglass with the
pattern of evenly spaced scribes on one axis of the crosshair
moving away from the center.  The Idea is to find the scale of
the spaces of the tick marks on the reticle by timing a stars
drift across the pattern with the drive turned off.  A new comets
motion is calculated in degrees per day and the position angle by
measuring the movement  of the angle directly off the Tiron
chart.  Next we calculate the comets motion during the exposure
and advance the guide star in the opposite direction, during the
exposure by the same amount the comet will move.  The end result
is the comets sits still on the film, and the stars trail an
amount the comet would have moved on a standard tracked shot.
     Skyshooting comets offers a challenge to astrophotographers
unlike that of the usual deep sky targets.  The fact that they
all move about against the background stars at different rates
and in curving paths should not discourage the determined
amateur.