attempt to view or photograph the sun with a telescope or
camera will require the use of an effective solar filter and
this must fully cover the objective of the instrument. Modern
solar filters consist of glass or mylar sheets on which is
deposited a reflective metal coating. They are available in
frames which fit over telescope objectives or screw onto camera
lenses. Some filters are designed for both viewing and photography
while others are suitable for photography only. Users should
double check with their suppliers that the filters they buy
or borrow will properly protect their eyesight. Other types
of filters are suspect and definitely not recommended for
instrument work. Obviously the telescope or camera must not,
under any circumstances, be pointed in the direction of the
sun without filters being fitted. On a telescope the finder
must be covered or, preferably, removed altogether. The only
camera type recommended is that with a through-the-lens viewfinder.
A solar filter fitted on the lens of such a camera will provide
with a telescope with tracking capability is straightforward,
providing the user has the attachments to fit a 35mm camera
to it. A rough polar alignment is all that is required. Finding
the sun may require a little patience, and a shade to shield
the user's eyes would be helpful. Focusing on the edge of
the moon is fairly easy, particularly if the camera has a
fine focusing screen, and refocusing between shots will certainly
produce at least some crisp shots. A cable release for shutter
operation is essential, and shutter vibration can be reduced
further if the camera has a mirror which can be locked in
the up position. As a rough guide an exposure of 1/60 sec
for an f10 instrument with a solar filter using ISO 100 film
at prime focus (or its equivalent with other parameters) is
near the mark and a bracket of exposures around that will
produce a satisfying result.
a 35 mm camera with its own lens is not quite so simple. Assuming
there is no telescope available to carry it, it must be mounted
on a firm tripod which has clamps to enable the camera to
be moved to find the target and for incremental adjustments
over time. A screen on the camera or tripod should be set
up to shade the eyes while searching and focusing, though
a right angle viewer would obviously be desirable. A standard
50mm lens will produce a solar (or lunar) image of only 0.5mm
on a 35mm negative. It may be possible to enlarge this to
as much as 7mm, but that is hardly satisfactory. A 400mm lens
will produce an image size of 3.6mm. with possible enlargement
to about 55mm - the longer the focal length the better. Correct
focusing is critical in telephoto lenses; once again, a fine
focusing screen is desirable with refocusing between shots.
Anti-vibration measures as previously mentioned should be
eclipse 17 September 1997. There are not, of course, the same
hazards in this type of photography as in solar work. In this
eclipse the lateness of the hour is the difficulty; the moon
starts to enter the penumbra at 00.11am and clears it at 5.22am.
This is a total eclipse and the moon will be completely within
the umbra from 2.15am until 3.17am (all WAST). Because the
moon is full at eclipse time, it could be painfully bright
to observe it through an instrument until it is well into
the penumbra. A light reduction filter could be useful up
to this time, but can be removed as the eclipse progresses
and replaced later if required. It is worth noting that, while
an eclipse of the sun is visible in varying degrees of disc
coverage depending on geographical distance from its centreline,
in a lunar eclipse the entry of the moon into the earth's
shadow can be seen from all locations which would normally
be in moonlight at that time.
the sun and moon subtend about 0.5 degrees at the earth so
previous comments on photographic image size apply. The following
yardsticks for exposure are recommended to an f10 instrument
using 100 ISO film with no filter:
full moon - 1/250 sec.
moon in penumbra - 1/30 sec.
partial (umbra and penumbra together) - 10 secs.
total (completely in umbra) - from 90 secs to 5 mins (+).
The reason for the wide range in d. is that the light being
reflected from the moon, even in totality, varies enormously
with differing atmospheric conditions on earth. Sometimes
the moon can be seen with the naked eye while in the umbra
while at other times it is difficult to see even with a telescope.
Readers who attempt this shot should use colour film as the
moon can glow in a rich red to copper colour. They should
also use a tracking device set, if possible, at the lunar
rate of movement.
fixed camera which permits multiple exposures on a single
frame can be used for both types of eclipses to produce interesting
shots showing the sequence of development of the eclipse.
Anyone trying this would have to be very confident of exposure
times and, at least for the moon, would have to change exposures
between shots. The beginning and end of shooting would have
to carefully planned so that the sequence fits onto the frame
and the camera is properly oriented. The timing between exposures
should be selected using the rates of apparent movement; for
the sun and the shadow cone of the earth it is 1 degree in
4 minutes westward while the moon's westward movement is less
than this by 1 degree eastwards in 2 hours. This is a fairly
ambitious project and a rehearsal using the uneclipsed sun
or moon is recommended to those considering it.
conclusion it should be stated that this article merely offers
basic notes on eclipse photography, a subject on which highly
competent practitioners have written copiously. Much of this
material is available in our library. It is hoped, however,
that some of our members who have expressed an interest in
astrophotography are able to "have a go". Those who do should
keep a record of their endeavours for future eclipses - lunar
events are fairly frequent and there is another solar eclipse
visible in WA in February 1999.
ABOUT THE AUTHOR
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.