OF THE ROGER N. CLARK

VISUAL ASTRONOMY OF THE DEEP SKY
A CASE STUDY: THE WHIRLPOOL GALAXY, MESSIER 51
Table 6.1. Radial brightness profile if M51 for different sky conditions
-
Distance
from
Brightness in magnitudes/sq. arc-second
nucleus Darkest sky
(arc-sec.) A B C D E
0 18.00 17.98 17.98 17.94 17.84
25 20.56 20.48 20.33 20.03 19.5 1
31 20.73 20.64 20.47 20.13 19.57
38 21.07 20.95 20.73 20.31 19.67
46 21.39 21.23 20.95 20.46 19.75
55 21.32 21.17 20.90 20.43 19.73
65 Mo 21.40 21.24 20.96 20.46 19.75
77 M 20.48 20.41 20.27 19.98 19.47
92 21.37 21.22 20.94 20.45 19.75
110 22.39 22.03 21.51 20.78 19.91
130 22.60 22.18 21.60 20.82 19.93
150 21.98 21.72 21.31 20.67 19.86
170 22.11 21.82 21.38 20.71 19.87
190 23.50 22.70 21.87 20.95 19.98
210 24.25 23.00 22.00 21.00 20.00
Contrast M,Mo 1.33 1.15 0.89 0.56 0.29
Log contrast 0.124 0.061 -0.051 -0.252 -0.538
Data in column A are from Schweizer (1976), Astrophysicaljoumal Supplement 31, pp. 31
from his Figure 5b at position angle 1350 from the galaxy's nucleus. Data are B3 magnitudes,
which are reasonably close to visual magnitudes.
If we boost the power to 120 x, the apparent
width of the spiral arm swells to 60 arcminutes.
Its surface brightness is now fainter,
25.0 magnitudes per sq. arc-second. But the
eye is much more sensitive both to faint objects
and low-contrast objects if they are large.
Under sky condition A, the eye could see
something 60 arc minutes wide even as faint
as 25.7 magnitudes per sq. arc-second. So the
arm is now quite visible in an 8-inch scope. It
could even be detected in a sky as poor as
condition D.
Interestingly, the graph shows that boosting
the power from 120 X to 200 X would
bring the spiral arm just into detectability in
a 6-inch scope under sky A. Even though the
arm would be more apparent in the 8-inch
than the 6-inch at all powers, it is less apparent
at 200x than it was at 120x.
Note that a faint object becomes harder to
detect at both very high and very low powers.
This is why the thick arcs for various telescopes
curve up at both ends. Detection is
easiest when the object is at the bottom
each arc - at the optimum magnified visual
discussed in Chapter 2. The optimum
fied visual angle is shown by the
wobbly line climbing up the graph from
er left to upper right.
DISCUSSION AND SUMMARY
The sizes and contrasts of deep-sky
vary greatly. So does the size and contrast
detail within a single object. So a large
of magnifications is needed to see all
detail. For any feature, there is an
magnified visual angle at which it can best
detected. Fortunately, the decrease of
tability on each side of the optimum is
so fine gradations of magnification are
needed.
The eye's response to light, like the
sponses of our other senses, is logarithmic.
the sequence of magnifications used
follow a logarithmic trend. For example,
Figure 6.1. Messier 51. The line shows where the photometric brightness measurements analyzed
in this chapter were made. (Courtesy Palomar Observatory.)
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