WIDEFIELD ASTROPHOTOGRAPHY - 5

In this fifth and final lecture on skyshooting widefield systems
we will discuss some of the mechanical problems which can cause
difficulties with your efforts, and the question of guiding
tolerances.  We will also cover the best filters for this type of
work and last, examining the final results to get the most out of
your adventures in this type of astrophotography.

Some mechanical problems.

Despite careful polar alignment and accurate tracking, the flexing
of a poorly designed camera mount or improperly balanced tube
assembly can ruin a potentially fine shot.  Most astrophotographers
tend to overlook balance as a cause for poor results.  It has been
demonstrated that most mountings will track better if the balance
is shifted on the counterweighting such that the scope slowly leans
Eastward, when the RA clamp is loosened.  In other words, it is
much better to have the gear pull the telescope, than the telescope
pulling forward on the gear.  Tracking can be up to ten times
better one way than the other, so be certain to check the balance
just before each shot, or at least when moving to a new part of the
sky.  

Flexure can ruin a potentially well guided shot too.  All to often,
poor guiding is blamed, when in reality the tracking was good, but
the camera flexed a bit relative to the guiding telescope.  To
eliminate this, as discussed earlier, a sturdy mount must be
devised to insure a rigid connection to the back of the telescope
tube.  A wooden saddle may have to be installed on the tube itself
with straps if the material the tube is made from bends with the
weight of the camera.  Cardboard tubes, or those made from
fiberglass should be suspect.  
     
Guiding tolerances.

Even if you are guiding the best you can, and have eliminated both
flexure and mechanical problems, you may wish to know just how
accurate you will need to track to obtain best results.  By using
the simple rule of thumb that a 50mm. lens with todays fine grained
films can just resolve a tracking error of about an arcminute (60
seconds), you can scale your tracking tolerance accordingly.  For
example, a 135mm. lens, which is 2.7 times longer focus than the
50mm. lens will resolve an error 2.7 times smaller, or 60
seconds/2.7 = 22 seconds.  Your tracking must be such that the
guide star is kept within an imaginary boundary of this size.  Take
a look at several double stars of known separation to determine
what your tolerance will look like in the guiding eyepiece.  For
the 35mm. lens in this example, the double 61 Ophiuchi would fill
the bill nicely.  

Filters for wide field photography.

For use in either combatting light pollution or improving the
contrast in emission nebula photography, filters are becoming an
indispensable part of the skyshooter's accessories.  There are
basically two filters that are useful for wide field work.  The
first type, are deep red filters such as the Wratten 25 or the
Lumicon H-Alpha Pass filter, will not only cut natural or
artificial sky fog substantially, but provide a big increase of
the contrast in recording the highly sought after and dim emission
nebulosities.  Objects of this class include the North America
Nebula and the California nebula.  Of course, we must use B&W film
with these filters.  The only one that has the red sensitivity to
work well being hypered 2415.

For color shots which is what most individuals will be using, there
is a filter that will drastically cut sky fog at light polluted
sites, and at dark sites bring out the emission nebula at a much
higher contrast and detail.  This special filter is available at
Lumicon as the Deep Sky Filter, and has proven to be the best
photographic filter on the market.  We do not recommend this filter
for ultra wide angle lenses because of the restricted acceptance
angle of the multi-layer construction.  But for 50mm. or longer
focal lengths it works very well.  Exposures can be lengthened
about 3 times the sky fog limit without the filter, and not
sacrifice any details in star clouds or non emission objects in the
field.  

Studying your results.

With some care and patience you could end up with a beautiful
collection of constellation slides or prints, along with close ups
of favorite individual objects and regions.  At this point the real
pleasure begins, the study of the results.  The best way is to get
out the Sky Atlas 2000 or Skalnate Pleso charts, and try to
identify as many deep sky objects as possible on your shots.  You
will discover many additional objects not plotted on the charts. 
This will be especially true in areas near the Milky Way, and
portions of the sky shot through nebular filters.  It should be
strongly emphasized that the "plates" you obtain can be used as
tools for observing.  Since the star charts lack the fine details
that many of your exposures will reveal, you will be able to go
back out to the telescope, and examine many new deep sky wonders
that you may not have been previously aware.  

Conclusion.

There is a facet of astrophotography for everyone.  Wide field
skyshooting allows us to see much more of the universe than is
possible by vision alone.  The rewards of capturing a small portion
of the Galaxy permanently on film for everyone to enjoy is
something anyone can be proud of.