PRIME FOCUS ASTROPHOTOGRAPHY 3 LECTURE

In this third lecture in a series on prime focus skyshooting. 
Here we will discuss guiding methods, drive systems, films.

A prime consideration when guiding is comfort.  You cannot guide
for very long and your best when standing, or bending over(3)
the tube.  A thirty minute exposure can seem like forever if you
are not comfortably seated.  A tall bar stool, or a seat that
moves up and down on the rungs of a ladder is ideal.  In a seated
position, one can usually guide for an hour or more, without  too
much difficulty especially if the guidestar is suitably bright.

You also must estimate the amount of tube travel during the
exposure.  This is not usually a problem with smaller scopes, but
with a large newtonian the eyepiece end of the tube can traverse
several feet during a long exposure!  You may reach a point where
you will have to discontinue the exposure, or if you have an
internal shutter at least interrupt the exposure, and move your
seat.

Drives.


Obtaining a well tracked exposure will also depend on the
attributes of your drive corrector.  For example, if the erratic
errors in the gear are faster than the correction capability of
your drive corrector, then an inadequately tracked exposure will
result.  Some drive correctors we've seen are so slow to react,
that they couldn't even keep up with the average periodic errors
found in most schmidt cassegrain telescopes!  The point is, check
the maximum errors present in your system, and make certain your
drive corrector can handle them.  You may even have to replace
your present model to achieve this or at worst, install a higher
quality gear on the scope.

Even the best of drives have occasional glitches.  Wind gusts
come up, or airplanes fly through the area. (Usually in front of
your subject)  We have installed on every one of our large
newtonians, an internal shutter mechanism to interrupt the
exposure when needed.  The idea is simple - only allow the
shutter to be open when the guidestar is exactly on the
crosshair.  In this way, it is nearly impossible to produce a
negative that shows detectable guiding errors.  After changing
over to this system, the improvement in the quality of our work
was instantly discernible.

We used to consider that if 50% of our astrophotos came out with
perfect tracking, then we had a reasonably successful evening. 
Not anymore!  Since then, many of our club members have installed
the internal shutters and that success rate has climbed to nearly
100%.  In addition there is an added bonus that has enabled many
of our astophotographers to take much longer exposures than ever
before.  Because our group employs the Arizona On/Off axis system
for guiding, the advantage of the guide star still being visible
in the second focuser when the light is blocked off to the
camera.  The idea is to guide for fifteen minutes or so, then
close down the internal shutter.

At this point, one takes a five or ten minute coffee break and
goes back to the telescope. After recentering the guidestar, the
shutter is reopened to resume the exposure. The routine is
repeated after another fifteen minutes.  In this fashion a series
of fifteen minute exposures is chained together to form an
equivalent exposure of an hour or more.  We call this "chain
guiding."  It was discovered that guiding in short bursts(4)
resulted in noticeably better tracking than when we tried to go
for a hour long stretch.

A few notes on films.

Just because you may have a large and well made newtonian, don't
get the idea that you will be guaranteed great astrophotos with
just any film.  With the correct choice of film the potential for
a great shot is there.  But the wrong choice will lead to a
wasted effort and frustration.  With the increasing availability
of gas hypered films, the requirement for impossibly long
exposures to achieve satisfying results will be no longer
necessary.

For most functions, hypered technical pan emulsion - Kodak 2415,
is the undisputed foremost preference.  This film combines good
speed and contrast, and ultra fine grain with superb red speed. 
This film will not disappoint you.  For those who insist on color
emulsions, the hypered Kodak Pro 400 ppf is the best choice.  Although
the contrast is much lower than the 2415, it still has favorable
color response over a wide spectral range including the essential
H - Alpha line.  Its fine grain, and good speed make it a fine
choice to produce colorful high resolution astrophotos of the
brighter objects.

As for color slides, hypering is not a practical option because
of the severe color balance shift in the background.  The
unhypered Agfa 1000 slide film however has been received very
well, and produces beautiful rich colors on a variety of deep sky
objects.