CONSIDERATIONS ON COMA IN SKYSHOOTING - By Chris Schur
     This lecture will cover a topic which is not usually stressed 
as important in skyshooting as say choice of film, but key
astrophotographers will tell you it has played a major role in
their work.  What I am referring to is todays fast lenses and
mirrors.  
     In recent times, the fast newtonian has revolutionized visual
astronomy, the big mirrors with their daringly short tubes offer
new portability to dark sky sites with average vehicles.  For
example, Dan Wards 17 1/2 inch dobsonian will just pack in to his
minivan for remote observing outings.  Only a few decades back,
the typical newtonian was about f/8, and a ten inch was maximum
for such a vehicle.  The new breed of thin mirror f/4.5 scopes
has also prompted a new type of astrophotography -- ultra short
exposure, low resolution shots that nearly anyone could master in
a short while. 
     Take for example the rave of the Meade DS10 and DS16 scopes.  
A quick look at their adds show right away what their trying to
sell:  five minute shots of deep space objects that record more
than the most experienced sky shooters were getting on film only
a decade or so back.  Load that Konica 3200 film in the old 35mm
and blast away, Galaxies, bright nebula, comets, you name it as
long as its fuzzy and bright, it will be there on the film.
     But my friends, a new trend is developing, and hitting hard. 
You'll see it now in every astronomy magazine, and its spreading
fast.  Its called High Resolution Astrophotography.  What is
happening is well outlined in Wallis and Provins book on
astrophotography, after the thrill of fast mirrors quick results
is gone, the COMA remains!
     I have been doing prime focus sky shooting with newtonians for
ten years now, and have seen radical changes in focal ratios in
astrographs.  The bottom line is this, with fast optics, the coma
at the edge of the field will ruin a perfectly guided and focused
astrophoto, unless something is done.  Where does the magic
number lie, the fine mix between fast mirror and sharp images at
the edges of the frame?  Talking with the top astrophotographers
you'll get the idea real fast, f/5 or slower is the secret
formula.  Indeed, for 35mm compare these results:  At f/4, the
coma starts looking pretty bad on the short dimension of the
frame.  No good.  F/4.5 your short dimension is Ok, but is very
bad on the long dimension.  To get the sharpest results right to
the corners, I mean microdots to the edges, f/5 is needed.  Most
of us feel that f/6 is getting a bit too slow.  Even at f/5, it
will take 90 minutes at a dark site to reach sky fog limit with
hypered 2415.  
     Its better to have a crisp shot, right to the edge on a faster
and grainier film, than use a slower finer grained emulsion and
use an f/4 mirror.  A guy showed up at one of our astronomy club
meeting recently with a whole batch of 8x10s taken with his f/4
newtonian.  The guiding was OK, but it reminded me of the Star
Wars hyperdrive scene, with all the stars in the periphery of the
field stretched out from the center.  Alls he had to do, was put
as cardboard stop in front of his newtonian, and make it f/5. 
His ten minute shot would have become twenty, but the results
would have been an order of magnitude better.
     No doubt many of you have such fast systems, and can easily
convert them to f/5 for the sharpest results.  IF you don't think
you can guide for twenty minutes, but ten is OK, then read my
lecture on Chained Guiding, and you will discover that a twenty
minute shot is merely two ten minute exposures strung end to end. 
You can do it, and that order of magnitude of improvement you
have been seeking might be as close as a pair of scissors and a
cardboard box away.