OF THE ROGER N. CLARK

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INTRODUCTION The eye i a remarkably adaptable light detector, able to function in bright sunlight and faint starlight, an intensity range of more than ten million. This is a considerably greater range of brightness than can be handled by man-made instruments such as photographic film or television cameras. The eye consists of the cornea, which provides most of the lens action; the lens, which provides the focusing ability; the iris, which restricts the aperture and the amount of light entering the eye; and the retina, which detects the light and sends the signal to the brain. The structure of the eye is shown in Figure 2.1. In the retina, light is recorded by a photochemical reaction: when light impinges on a photochemical substance, a reaction sends an electrical signal to the brain. The sensitivity of the eye depends on the amount of chemicals present. It is primarily by varying the amount of photochemical material, not by opening the iris, that the eye can adapt to an amazing range of light levels. RODS AND CONES The light-detection devices in the retina are of two types: rod cells and cone cells. The cones are concentrated in the fo vea, where visual acuity is greatest. This small area, appearing less than a degree in diameter, is the center of vision. We normally aim the fovea directly at an object to see maximum detail. However, the cones are not as sensitive to light as the rods. At the center of the fovea (on the visual axis), there are no rods at all. The rods increase in density from zero at the center of the fovea to a maximum at about 18 degrees off-axis. The density of rods and con- 2 The human eye es is shown in Figure 2.2. There are no rods or cones at all on the spot where the optic nerve leaves the eye. This is called the blind spot. The rods and cones respond best to different colors of light, with the rods slightly more blue-sensitive than the average of the cones. This is shown in Figure 2.3. The cones are entirely responsible for color vision, and since they require bright illumination one sees no color in dim light, when only the rods are working. The rods and cones are connected to nerves via ganglion cells. In the fovea, a ganglion may serve a single cone, but in the periphery of the retina, where rods are prevalent, one ganglion may be connected to 100 rods. It is via the ganglion cells that the electrical signals from the rods and cones are transferred to the optic nerve and the brain. The detection of faint light apparently depends on how many ganglion cells are involved. Those in the periphery each serve rods covering an area about 20 minutes of arc in diameter. The detection and contrastdiscrimination capabilities of the eye involve the summation effects of several ganglion cells. These are the capabilities that at low light levels are of interest to the visual astronomer. UNITS OF BRIGHTNESS Two factors govern the eye's detection of light. One is the total brightness of an object, and the other is its surface brightness. Surface brightness is the total amount oflight divided by the area over which it spread. A physicist might describe an object's total brightness by units of power (that is, energy flow), such as watts or number of photons per second. However, more specialized terms have been introduced for isual perceptins. The term illuminance descnbes the total hght utput of an object in the wavelengths seen y a typical human eye. One of the earliest units of illuminance measure was a candle. Requirements for precision led to the unit called the candela, the total visual light emitted in all directions by a standard candle made of a specific material and of a certain size. The lumen (lm) is another common unit of measure, and is equal to a candle divided by 4 Jt (=12.5664). Surface brightness, intensity per unit area, is described by another term, luminance. Note the subtle, but important, difference between Vitreous humor OptiC nerve THE HUMAN EYE Horizontal Section of the Right Eye Figure 2.1. The human eye. (From Sky & Telescope April, 1984 luminance and illuminance. Common units of luminance are candelas per "square meter, or lumens per square meter. Astronomers use their own brightness unit: the stellar magnitude. And instead of linear measurements of distance on a surface, they use angular measurements of distance on the sky. The magnitude was invented because the eye responds to light approximately logarithmically. One magnitude corresponds to a change in brightness by a factor of 100 1/5 , which is about 2.51. Five magnitudes is a factor of 100 in brightness and 10 magnitudes is a factor of 100 times 100 or 10 000. Cornea Aqueous humor Pupil 4 5
VISUAL ASTRONOMY OF THE DEEP SKY THE HUMAN EYE Since astronomical objects cover an area of sky, their surface brightnesses are described in magnitudes per square arc-second. The full Moon, fo r example, is a half degree (1800 arc-seconds) in diameter, so it covers 2.5 million square arc-seconds of sky. Dividing its brightness by its area gives it a surface brightness of 3.6 magnitudes per square arcsecond. Astronomical objects differ vastly in both total brightness and surface brightness. The full Moon and the planet Mars have nearly the same surface brightness; their total amounts of light are so different only because the Moon covers a much larger area of sky. The Moon and the Sun, on the other hand, have nearly the same apparent size. Here it's a difference in surface brightness that causes such dissimilar amounts of light. An object of a certain total brightness (such as the Moon) also illuminates the surface of the Earth with a certain number of lumens per square meter. Any astronomical object illuminates the Earth's surface in such a manner. Examples are in Table 2.1. The illumination an object causes on the Earth's surface is directly relevant to astronomy. A telescope objective has a given area on which light from the object falls. The illumination per unit area times the area of the ;:-- E 160 E 140 .. Cl) Q.. 120 (I) "0 c:: ctI (I) ::3 0 ..c::: - 100 80 60 0: 40 LU CO 20 0
Page 1 and 2: OF THE ROGER N. CLARK
Page 3 and 4: Published by Sky Publishing Corpora
Page 5 and 6: CONTENTS M78 (NGC 2068) NGC 207 1,
Page 7 and 8: PREFACE During research for this bo
Page 9: VISUAL ASTRONOMY OF THE DEEP SKY AB
Page 13 and 14: .. .. .. .. VISUAL ASTRONOMY OF THE
Page 15 and 16: VISUAL ASTRONOMY OF THE DEEP SKY TH
Page 17 and 18: e viewed with enough magnification
Page 21 and 22: lens and a piano-convex eye lens. A
Page 25 and 26: controlled depends on how many laye
Page 27 and 28: The Lumicon H-Beta Filter (Figure 3
Page 29 and 30: every 1 ° in declination and 4 min
Page 31 and 32: error of only one quarter the field
Page 33 and 34: Table 4.1. Ideal limiting magnitude
Page 35 and 36: 5 Making drawings and keeping recor
Page 37 and 38: more detail in the future. They add
Page 39 and 40: .. .. .. ... Contrast (C) C Log(C)
Page 41 and 42: VISUAL ASTRONOMY OF THE DEEP SKY A
Page 47 and 48: --- VISUAL ASTRONOMY OF THE DEEP SK
Page 49 and 50: --- M76 (NGC 650-651), PLANETARY NE
Page 53 and 54: -- -- M45, THE PLEIADES OPEN CLUSTE
Page 55 and 56: -- Brightest stars of the Pleiades
Page 57 and 58: M42 (NGC 1976), M43 (NGC 1982), THE
Page 59 and 60: --- --- --- VISUAL ASTRONOMY OF THE
Page 61 and 62:
-- --- NGC 2023 NGC 2024, IC 434 (T
Page 63 and 64:
--- --- M78 (NGC 2068), NGC 2071, D
Page 65 and 66:
VISUAL ASTRONOMY OF THE DEEP SKY --
Page 67 and 68:
--- M67 (NGC 2682), OPEN CLUSTER IN
Page 69 and 70:
-- VISUAL ASTRONOMY OF THE DEEP SKY
Page 71 and 72:
--- -- M82 (NGC 3034), PECULIAR GAL
Page 73 and 74:
-- --- --- MI05 (NGe 3379) NGe 3384
Page 75 and 76:
Page 77 and 78:
MI09 (NGe 3992), GALAXY IN URSA MAJ
Page 79 and 80:
-- --- M99 (NGC 4254), GALAXY IN CO
Page 81 and 82:
== -- VISUAL ASTRONOMY OF THE DEEP
Page 83 and 84:
--- --- -- NGC 4449, GALAXY IN CANE
Page 85 and 86:
-- -- --- VISUAL ASTRONOMY OF THE D
Page 87 and 88:
--- -- M90 (NGC 4569), GALAXY IN VI
Page 89 and 90:
--- -- M94 (NGC 4736), GALAXY IN CA
Page 91 and 92:
Page 93 and 94:
-- VISUAL ASTRON0MY OF THE DEEP SKY
Page 95 and 96:
-- --- -- VISUAL ASTRONOMY OF THE D
Page 97 and 98:
-- -- VISUAL ASTRONOMY OF THE DEEP
Page 99 and 100:
--- -- M83 (NGC 5236), GALAXY IN HY
Page 101 and 102:
Page 103 and 104:
VISUAL ASTRONOMY OF THE DEEP SKY A
Page 105 and 106:
Page 107 and 108:
--- VISUAL ASTRONOMY OF THE DEEP SK
Page 109 and 110:
--- -- VISUAL ASTRONOMY OF THE DEEP
Page 111 and 112:
---------------- 5 / --------______
Page 113 and 114:
Page 115 and 116:
VISUAL ASTRONOMY OF THE DEEP SKY --
Page 117 and 118:
Page 119 and 120:
Page 121 and 122:
Page 123 and 124:
-- MI5 (NGC 7078), GLOBULAR CLUSTER
Page 125 and 126:
Page 127 and 128:
Page 129 and 130:
Page 131 and 132:
Page 133 and 134:
-- ·rl E I u L ro 5 +J Q) (f) 0 Y
Page 135 and 136:
-- --- VISUAL ASTRONOMY OF THE DEEP
Page 137 and 138:
VISUAL ASTRONOMY OF THE DEEP SKY NG
Page 139 and 140:
'2 5 -rl E I U L CO '---' -t-J ill
Page 141 and 142:
VISUAL ASTRONOMY OF THE DEEP SKY AP
Page 143 and 144:
Page 145 and 146:
VISUAL ASTRONOMY OF THE DEEP SKY Ta
Page 147 and 148:
Page 149 and 150:
-- Appendix E A catalog of deep-sky
Page 151 and 152:
Page 153 and 154:
-- -- --- VISUAL ASTRONOMY OF THE D
Page 155 and 156:
--- --- VISUAL ASTRONOMY OF THE DEE
Page 157 and 158:
Page 159 and 160:
Page 161 and 162:
Page 163 and 164:
Page 165 and 166:
Page 167 and 168:
-- APPENDIX F: OPTIMUM DETECTION MA
Page 169 and 170:
------- VISUAL ASTRONOMY OF THE DEE
Page 171 and 172:
Page 173 and 174:
VISUAL ASTRONOMY OF THE DEEP SKY -
Page 175 and 176:
Page 177 and 178:
VISUAL ASTRONOMY OF THE DEEP SKY -
Page 179 and 180:
Page 181 and 182:
Page 183 and 184:
G VISUAL ASTRONOMY OF THE DEEP SKY
Page 185 and 186:
INDEX INDEX M70 (NGC 6681), 312, 31
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OF THE ROGER N. CLARK

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