OF THE ROGER N. CLARK

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VISUAL ASTRONOMY OF THE DEEP SKY
THE HUMAN EYE
Table 2.3. Approximate suiface brightnesses of commonly observed objects
-
N
(J) 0
(J) Q)
Q) (J)
c: I
+-0 0
.J::
Cl ca
.. 20
m
(J)
Q) Q)
0 "0
ca ::::I
- +-0
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(J)
c:
Cl
ca
E
-
25
0
Dark Adaptation at Different
Positions on the Retina
10 20 30
Time in Dark (minutes)
Figure 2.4. a) Dark adaptation measured with a 2° diameter test object placed at various angular
distances from the fovea. Derived from data in Middleton, 1958.
ID
(,)
c
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-
(/)
Cl
00
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ID
>
0
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E
0
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-
40
Object
Sun (magnitude -26.7, d = 1800")
VenuS (at greatest elongation,
-4.3, d,= 24", illum. = 50%)
Clear daytime sky (at horizon)
Full Moon (-12.5, d = 1800")
Mars at nearest opposition (-2.8, d = 25.1")
Overcast daytime sky (at horizon)
Jupiter at opposton (-2.5, d = 49.8)
Saturn at OpposItIOn (-0.4, d = 20.5 )
Heavy daytime overcast (at horizon)
UranuS (S.7, d = 4.2")
Neptune (7.6, d = 2.5")
Sunset at horizon, overcast
Clear sky 15 minutes after sunset (at horizon)
Clear sky 30 minutes after sunset (at horizon)
Fairly bright moonlight (at horizon)
Moonless, clear night sky (at horizon)
Moonless, overcast night sky (at horizon)
Dark country sky between stars (at zenith)
Candelas per
sq. meter
3000000000
15000
10000
6000
4000
1000
800
700
100
60
30
10
1
0.1
0.01
0.00 1
0.0001
0.00003
Magnitudes per
sq. arc-sec
-10.7
l.9
3
3.6
3.9
5
5.7
5.9
8
8.6
9.3
10
13
15
18
20
23
24
(J)
(J)
Q)
c:
+-0
.J::
Cl
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m
Q)
0
ca
-
..
::::I
(J)
-
N
0
Q)
(J)
I
0
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ca
20
(J)
Q)
"0
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+-0
c:
Cl
ca
E
-
25
0
Dark Adaptation:
Different -Sized Objects at
Center of Vision
20
Time in Dark (minutes)
b) Dark adaptation as measured with test objects of different angular sizes located at the
center of vision. The sizes are the diameter in degrees at which the objects could just be
detected. Derived from data in Hecht et al. (1935) after Bartley (1951).
30
Note: Sky brightness values at the horizon can vary by a factor of ten and are adapted from
Middleton (1958).
For the Sun, Moon, and planets, the total stellar magnitude and diameter (d) in arcseconds
(") for which the computation was performed are listed in parentheses.
becomes larger for objects that are just detectable.
In other words, the eye's resolution
or ability to see detail is much coarser in the
dark .
The eye's ability to see a point source, such
as a star, increases as the background luminance
gets dimmer. This is shown in Figure
8. Dark country skies are better than city
skies for seeing faint stars because the background
is fainter - not because the country
skies are significantly more transparent.
Astronomers often refer to sky darkness as
"transparency," but this is a misnomer.
A low-contrast object is more easily detected
if it is larger. For an extended object
such as a galaxy viewed in a telescope, magnification
does not change the contrast with
the background, because both the sky's and
the object's surface brightnesses are affected
equally. Some visual observers have stated
that a dim object's contrast with the sky
background increases with higher magnifica-
tion, but this is clearly wrong. The contrast
merely looks greater because of the increased
detection capabilities of the eye.
This facet of human vision has not been
described in print to my knowledge, so I will
coin a name for the maximum magnification
that will help detection: the "optimum magnified
visual angle". This angle is shown in
Figure 2.6 and also Figure 2.7b. For those
readers familiar with calculus and the slope
of a curve, the optimum magnified visual
angle occurs when the first derivative (the
slope) of each curve in Figure 2.6 is equal to
.
-1.
If an object is at the threshold of detection
and smaller than the optimum angle, more
magnification will make it easier to see.
When the object is magnified beyond the
optimum angle, its surface brightness decreases
faster than the eye's contrast detection
threshold, and the object will become
harder to detect. Remember that even for an
10
II