it in Focus
It is surprisingly difficult to achieve the focusing accuracy
required to produce satisfactory results when using prime
focus or eyepiece projection astrophotography. Comments and
suggestions on the following paragraphs apply when these methods
are employed. One phenomenon which should be recognised is
that once the correct focus is achieved, it cannot be assumed
that it will last indefinitely. Small changes in viewing conditions
or temperature of the instrument may degrade the sharpness
of focus during a session and it is good practice to check
it from time to time.
Sadly, split image focusing screens do not work for f ratios
slower than about f/4, so the best method is to achieve focus
by using a matte screen. Unfortunately the standard screen
on most cameras is too coarse to achieve good results. Some
cameras have interchangeable screens and one with a very fine
matte would be best for astrophotography. If the object being
photographed is suitable, focus should be attempted on fine,
low contrast detail which disapppears if the focus is less
than perfect. Often it is more practical to focus on a nearby
star and then move to the object. Some cameras can accomodate
a magnifier which is focused on the camera screen and greatly
assist in achieving overall focus. They are usually right-angled
viewer types which make the photographer's job much more comfortable.
This is an extremely accurate, if somewhat fiddly method of
focusing, and some photographers use it in preference to other
methods. It works on the fact that, unless they are interrupted,
the rays of light from a point source (a star is used in this
method) will leave the telescope in a converging cone, cross
over at the apex, and continue on in a diverging cone. The
precise focus is at the point where the rays cross over, ie
at the apexes of the cones. A tool with a straight sharp edge
is located in the film plane and is moved across at right
angles to the axis of the cones. When it cuts through the
light path either in front of or behind focus, the edge can
be seen to be extinguishing the image slowly from left or
right. When it cuts through at the apexes, the image will
be extinguished instantaneously, indicating precise focus.
I have a small piece of perspex cut so that it can be held
and slid along the film guide while the camera back is open.
A small piece of film with one straight and bevelled is glued
to it. The "knife edge" is the edge of the film which is located
precisely where the film will be in the camera. One problem
with this system is that the camera back must be open to set
the focus initially. Once the film is loaded, it cannot be
opened for checking the focus for later shots. A solution
to the problem is to make a device which has an edge set permanently
at precisely the same distance from the telescope as the film
plane when it is in focus. With the camera removed, this is
held against the telescope, focusing is carried out and then
the loaded camera is placed in position.
This involves placing a solid diaphragm with two diametrically
opposed holes cut in it in front of the objective. The idea
is that the two images which can be seen would merge when
true focus is achieved. I have not used this type, though
I believe they perform better on bright objects because of
the inevitable light loss. Devices such as this can be obtained
from commercial sources or made by DIY enthusiasts.
of a Calibrator
One firm with which I am familiar produces an astrophotography
system which incorporates a fairly unusual method of focusing.
It involves setting a 10mm eyepiece in a frame on a calibrator
remote from the telescope and adjusting its focus onto a preset
target within the calibrator. The frame with the eyepiece
held in it is then placed behind an open box fitted to the
telescope eyepiece holder and the telescope focus adjusted
onto the desired object through the 10mm eyepiece. The frame
is then removed and the system camera placed in its position.
The film plane is then at the precise focus position. This
sounds rather complicated but is relatively simple in practice.
method is used the attainment of precise focus is an essential
ingredient for the production of the quality photgraphs.
Ralph Paramor's interest in astronomy started when he was
required to complete a unit which included astronomical observations
in surveying as part of an engineering course. His interest
remained dormant until he joined ASWA about 16 years ago,
and since that time he has made several presentations at Society
meetings and written various articles for The Sidereal
Times. He is interested in all aspects of astronomy but
is currently wrestling with the art of astrophotography.