OF THE ROGER N. CLARK

7
A visual atlas of deep-sky objects
This chapter illustrates many of the best
galaxies, star clusters and nebulae as they
appear in a modest amateur telescope. Most
of the observations were made by the author
with a homemade 8-inch fll1.5 Cassegrain
reflector from observing sites no better than
those near any large American city. The
drawings have good, uniform quality control,
so an observer should be able to tell, after a
few trials, whether he or she can expect to see
less or more in any object illustrated, given
his or her particular telescope and sky
quality.
Facing each drawing is a photograph at the
same scale and orientation. Thus the viewer
can readily determine which features in
photographs can and cannot be seen. Also
given are the distances from which to view
each drawing so the object looks the same
size as at various magnifications in a telescope.
Full data are given for each observation,
including how long it took to make the preliminary
drawing at the telescope. Much
additional time was spent preparing each final
drawing, as discussed in Chapter 5. All
drawings were made by either method 1 or 2
described in that chapter.
THE PERSONAL EQUATION
Astronomers haye long spoken of the "personal
equation" to account for differing results
by visual observers. The personal equation
is a correction to be applied to. an individual's
data to bring it to some impersonal
standard. The differences in what people see
probably depend more on their experience
than on actual differences in their eyes. The
fundamental capacities of the eye are about
the same for most people. For example, in
controlled tests of the faintest visible star, the
difference from one person to the next is
probably less than one magnitude, and even
this may be due largely to how well someone
has learned to use averted vision.
Focusing a telescope corrects for nearsightedness
and farsightedness. Other eye problems
aside, visual acuity depends on the density
of rods and cones in the retina; sensitivity
depends on their photochemical action and
links to the brain. This neural architecture is
probably much the same in most people. On
the other hand, years of practice can make
great difference in fine tuning the techniques
of visual observing.
Hopefully the information in this book
greatly shorten that time. My own growth in
observing ability is interesting in this regard.
Starting as a very active amateur in 1968, I
had observed all the Messier objects
many NGC objects by 1971. I located
supposedly difficult ones that turned out
be not very hard. Even the notoriously
sive Horsehead Nebula was easy in a d
country sky. By early 1982 I was
detailed drawings of everything 0
and that summer I decided to write this
That autumn I did most of the research
analysis for the previous chapters.
While doing so, I realized I had not
reaching the fundamental limits of the eye.
had not known the concept of the
.
magnified visual angle: how to match
telescope power to the eye's detection ch
teristics. The result of this increased
standing can be seen in the drawings of
Orion Nebula (M42) made in January 1
and J an uary 1983, on pages 101 and 1
The second drawing shows much more
tail. Although sky conditions were
better, most of the improvement
from using the eye and telescope
more effectively. A greater range of
-
Ica I , . .
t eO
• . . •
N
when examining M42, these detaIls are
quickly seen. .
Previously each observer had to dIscover
suC h techniques by hit or miss, which often
1'["
toO k many years or a I eHme. . W' It h an un d er-
standing-of the matena 1 presente d' m t h' IS
book the time should be shortened to
erhps a year or so. It does take consider­
fi t'ons and spending more time studying
h biect, resulted m such features as famt
J
arCS of nebulosIty commg mto vIew :
A VISUAL ATLAS OF DEEP-SKY OBJECTS
ow,
ble practice to develop good techniques. After
all, one can read a book on how to drive a
car but learning to drive happens behind the
whel. With these thoughts in mind we will
now explore the variety and beauty that can
be seen through small amateur telescopes.
AVERTED VS. DIRECT VISION
The appearance of deep-sky objects depends
strongly on whether direct or averted vision is
used. Direct vision has sharper resolution but
lacks sensitivity. Thus, looking straight at an
object will show its brighter parts in detail,
while the fainter parts may be totally lost.
Changing from direct to averted vision and
back can produce some interesting blinking
effects. For example, when looking directly at
a globular star cluster, the bright central
mass of stars may be partially resolved into
individual pinpoints, but the fainter outer regions
are invisible. Averted vision will show a
fuzzy, unresolved (or less resolved) central
region, but the outer parts come into view.
Some open clusters show similar effects.
While using averted vision, fainter stars may
be seen in the center of the cluster but they
will not be clearly resolved. Thus they give
the impression of a faint nebulosity in the
cluster. This effect is shown in some of the
drawings, such as of Mll and M67, and it
crops up in many old descriptions of clusters
by early visual observers.
.
In the case of diffuse nebulae and galaxies,
If you can see the object with direct vision at
all, you can probably increase the power to
see more detail. Low powers should be used if
tḥe. nebula is very large and already at the
lImIt of averted vision. If all you want is to
detect the object's existence, then a power
sould be used that magnifies its apparent
SIze to about 3° or 4°.
But usually the nebula is brighter than the
detection limit and powers many times more
than that required for simple detection can be
used in an effort to see detail- perhaps swirls
and dark spots in a nebula, or faint mottlings,
spiral arms, or dark lanes in a galaxy. The
range of visual experiences is actually quite
large with modest amateur telescopes.
In some cases, stars coincide with the
nebul or galaxy in view. A good example is
the planetary nebula M27, which is in a rich
region of the Milky Way with many foreground
and background stars plus a 13thmagnitude
central star. These stars can be
hard to detect against the nebula's light. The
central star, for instance, appears fainter than
13th-magnitude field stars; it is difficult in an
8-inch telescope at low powers or with
mediocre sky conditions. But high powers
spread out the nebula's light while stars remain
point sources so far as the eye is concerned.
So the central star becomes much
easier.
One of the hardest aspects of averted vision
to master is holding the eye motionless on one
point for six seconds or more while trying to
grasp detail in the periphery of your vision.
The eye tends to jerk, especially if fatigued.
On the other hand, in some conditions moving
the eye (or gently jiggling the telescope)
helps bring an object into view, because
peripheral vision is highly sensitive to anything
moving.
THE OBSERVATIONS AND DRAWINGS
The drawings in this chapter are the product
of great effort to detect all the detail that
could possibly be seen. This detail is necessarily
portrayed more prominently than it
actually appeared. If a true representation
were drawn, it would take the reader similar
time and effort to discern it on the printed
page. This would amount to several minutes
for simple objects, and hours for something
complex like the Orion Nebula.
It's worth remembering, however, that the
true detail and contrast in deep-sky objects is
not subtle at all - as the camera proves. Only
the detection limits of the eye make them
seem so. After all, our eyes were not designed
for astronomy but for the very different job of
day-to-day survival on Earth.
Since you are probably reading this book
m adequate room light, the detail in each
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