PRIME FOCUS ASTROPHOTOGRAPHY
WITH LARGE NEWTONIANS

This is the first part in a series on skyshooting with large
amateur instruments.  They provide special challenges for both
the telescope maker as well as the astrophotographer.  In this
lecture, we will discuss the mechanical aspects of the instrument
itself, which must often be transported to a dark site, and some
physical constraints on telescope design.

INTRODUCTION.

Now is a golden era for the prime focus astrophotographer.  A
great deal has been written about this form of skyshooting, but
little has been said about taming the major sized amateur scopes. 
With the increasing availability of large mirror sets at
affordable prices, we can now build scopes that rival the power
and capability of many professional observatories. It is easier
too, with simplified darkroom chemistries, gas hypering, and new
high resolution films. Today amateurs can do many types of
astronomical research in many fields for the very first time. 
Modern techniques can allow us to bridal the enormous potential
of today's larger newtonians.  Although you may have to upgrade
your telescope for astrophotography, the rewards are well worth
the effort.  


Mechanical aspects.

A well built substantial newtonian that is capable of research
quality work, will have three basic characteristics.  First, the
base must be rock solid, have a firm footing on the ground, and
not bend or flex beyond certain strict limits.  Second, the
mounting must move easily, and the drive must track smoothly. 
There should be no play or wobble, and it should be capable of
carrying the tube without strain.  Third, the tube must be rigid
and well sealed against the elements.  In addition, the optics
should be held securely, with no shifting of internal components
as the tube is pivoted across the sky.  

One of the primary considerations will be transporting your large
newtonian to a dark sky site.  For example, just loading your
instrument into the family station wagon or truck, can wear you
out.  The best solution, is to break the scope into smaller but
more manageable sections.  However, a compromise must be reached: 
transportability versus parts count.  A critical component may be
accidentally left at home if there are too many small pieces.  On
the other hand, larger components can be awkward and heavy to
lift.  A good compromise, is about four to six pieces.  Although
it seems like a contradiction to stability, keeping the weight
down of all the individual parts can make an important difference
in how you will transport your behemoth.

The tube is a special problem.  Its long length, and sizable
diameter can be difficult to pack along with the other
components.  One solution that is workable is to split the tube
into two sections in the middle, each half locking together with
guide pins and latches.  

Once you have your scope assembled at the observing site, 
you may soon discover that the wind has a much greater effect on
the greater cross sectional area of its tube, compared with other
smaller scopes set up near by.  The tube can act like a sail, and
the stars in the eyepiece may be doing figure eights form the
shaking!  Here we can see that when a larger scope is used for
astrophotography it must be much more solid than a strictly
visual instrument.  Some scopes may require a more massive tube
and  a beefing up of the thrust surfaces in the mount.  A
standard test to determine just how solid your telescope is, and
weather or not it is suitable for astrophotography has been the
"thump test."  To do this install a medium power eyepiece in the
scope, and center a star in the field.  Thump the tube with your
fist at the eyepiece end, and observe just how long it takes for
all motion to cease in the field.  If the movement stops in five
seconds or less, the telescope is suitable for prime focus work. 
Otherwise, consider the above modifications to bring it within
this acceptable limit.