OF THE ROGER N. CLARK

VISUAL ASTRONOMY OF THE DEEP SKY
ABOUT THIS BOOK
be beneficial for all readers, during their first
encounter with the book, to read the chapter
summaries before the technical portions.
Chapters 3, 5 and 7 contain little math and
need not be skipped by the beginner.
The capabilities of the eye are presented in
Chapter 2. We will see that the eye is more
sensitive to faint, extended light sources if the
light is spread out over about 5 to 10 degrees.
Because the ability to see low-contrast features
in faint objects depends on their angular
size, the visual observer will detect different
amounts of detail at different magnifications.
The implications of this fact for observing
faint astronomical objects are discussed
in Chapter 3, which also presents an introduction
to telescopes, mountings, eyepieces,
and how to find celestial objects.
Chapter 4 analyzes the faintest star observable
in a telescope and shows how this is a
function of both the magnification and sky
brightness, as well as the more commonly
known factor of telescope aperture.
In Chapter 5 the techniques of making and
keeping good records and drawings are discussed.
We analyze the visibility of the galaxy
Messier 51 in Chapter 6 to illustrate the detection
of low-contrast features in deep-sky
objects. In this instance, the visibility of a
galaxy's spiral arms is addressed, and we see
that there is an optimum magnification for
viewing faint detail.
Chapter 7 describes the visual appearance
of more than 90 deep-sky objects. A catalog of
photographs and drawings is presented to
illustrate what can be seen with a typical
amateur telescope. This chapter is laid out so
that ·a photograph and drawing are reproduced
on facing pages at the same scale, to
show exactly what in the photograph can be
viewed in the telescope. Viewing distances
from the drawing to the eye are given so the
drawing can be seen at the same apparent
size as at various. magnifications in a telescope.
The appendices include charts of star clusters
for use in determining the faintest star
visible in a telescope, a catalog of more than
600 deep-sky objects and the optimum magnifications
to use on each, and equations for
determining rise and set times of an object, as
well as its altitude and azimuth.
* * *
This book presents many new concepts,
some of which may be difficult for the experienced
amateur to accept. The most revolutionary
is the "optimum magnified visual
angle." This is the magnification at which an
object should be viewed for best detection.
Research on how the eye detects faint objects
contradicts several basic pieces of conventional
wisdom. One is the belief that low
magnification should be used to concentrate
a faint nebulosity on a small area of the eye's
retina. This would be true if the retina worked
passively, like photographic film. But it
doesn't. The visual system has a great deal of
active computing power and combines the
signals from many receptors to detect a faint
extended object. Increasing the magnification
spreads the light over more receptors,
and the brain's processing power can then
bring into view fainter objects having lower
contrast.
Another interesting concept is how the
optimum magnification for a given object
varies with the size of the telescope. Since the
surface brightness is less in a telescope of
smaller aperture, the optimum power is higher
than in a bigger telescope! This applies only
to deep-sky objects, and is illustrated nicely
in Appendix F. Brighter objects, such as details
on planets, fall on a different part of the
eye's detection curves. In that case, about the
same magnification should be used on all
telescopes. Planetary observing is not discussed
in this book, but this conclusion is very
interesting nonetheless.
Another new concept is the highest magnification
that may ever be usefully employed
on a telescope. It is normally accepted that
the highest power is about 50 to 60 times the
objective in inches. This limit is correct only
for bright objects such as the Moon and
planets. For fainter objects the eye has less
resolution and needs to see things larger, so
higher powers are called for. At the limit of
the eye's detection ability, the highest useful
magnification is on the order of 330 per inch
of objective! These extremely high magnifications
are useful in specialized cases such as
detection of detail in a small planetary nebula.
For the drawing of NGC 7662 in Chapter
7, a magnification of nearly 600 was needed
with an 8-inch telescope.
These results are based on an elaborate
study of the eye's performance carried out
during World War 11 and published in 1946
by Blackwell (see the bibliography) . .
This information
has been around for qUIte some
time and is occasionally presented in some
fo rms (e.g. Roach and Jamnick, 1958).
However, it has not been fully understood. It
took considerable computer processing to
convert Blackwell's original data into a form
useful in an astronomical context. Thus, it is
not surprising that these concepts have not
been previously discovered.
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