OF THE ROGER N. CLARK

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VISUAL ASTRONOMY OF THE DEEP SKY THE EYE AND THE 'TELESCOPE Table 3.3. Typical emission lines ofnebulae -- Table 3.4. Natural and manmade light pollution Color Wavelength (angstroms) Atom or lon Notes Strongest lines: Violet 3727 Oxygen 11 Forbidden line, often strong 3869 Neon III Forbidden line 4340 Hydrogen I H gamma, 40% as strong as H beta line Blue-Green 4861 Hydrogen I H beta, 30% as strong as H alpha line Green Green Red 4959 5007 6548 Oxygen III Oxygen III Nitrogen 11 Forbidden, 30% as strong as 5007 line Forbidden, usually strongest of all Forbidden, 30% as strong as 6584 line 6563 Hydrogen I Halpha Red 6584 Nitrogen 11 Forbidden line Weaker lines: Violet 3798 3835 3888 3969 Blue Blue Yellow Red Red 4102 4471 4686 5876 6300 6364 6717 6731 Hydrogen I Hydrogen I Hydrogen and Helium I Hydrogen I and Neon III Forbidden Hydrogen I Hydrogen I Helium 11 Helium I Oxygen I Oxygen I Sulfur 11 Sulfur 11 appear better. If city lights emitted only a few wavelengths, they too might be filtered from the observer's view. Fortunately, this can be done - at least partially. To understand how, we'll need to examine the nature of light from celestial objects. The light from a star consists of many wavelengths or colors. Such light is called continuum radiation, since its spectrum appears nearly continuous. Some nebulae also give off continuum radiation. But many of the best and brightest emit light at only a few, specific wavelengths. These two types are reflection and emission nebulae, respectively. Reflection nebulae shine by reflected starlight, similar to the way a cloud in the Earth's sky reflects sunlight or the way the molecules in the Earth's atmosphere scatter it. Because the molecules and some dust particles in our atmosphere are smaller than the wavelengths of visible light, blue light is scattered more Forbidden line Forbidden line Forbidden line Forbidden line efficiently than red, and thus the sky appears blue. This type of scattering is called Rayleigh scattering after the scientist who first described the effect. Reflection nebulae "shine" in part by Rayleigh scattering of starlight, so the spectrum is continuous, much like that of a star but usually bluer. Emission nebulae, on the other hand, shine in only a few colors. They come in two types: planetary and diffuse nebulae. Both shine because starlight excites specific types of atoms, which re-emit this energy only at certain wavelengths. The brightest spectral lines (colors) emission nebulae are listed in Table 3.3. The atom that emits the radiation is listed, fo l lowed by a Roman numeral that indicates its ionization state. (I is un-ionized, II means the atom is missing one electron, III means two electrons are missing, and so on.) A fo rbidden line is not really fo rbidden to happen; it just cannot be observed in labor- =- - Manmade: Source Mercury Vapor Wavelength (angstroms) 3660 Mercury Vapor 4050 Mercury Vapor 4360 Mercury Vapor 5460 5750 Mercury Vapor Mercury Vapor 3200-7300 Lucalox Mercury Vapor: 5500-7000 Lucalox Mercury Vapor: 5000 Lucalox Mercury Vapor: 4000-7500 Low Pressure Sodium 5893 High Pressure Sodium 3500-7000 Incandescent 4000-7000 Natural: Airglow 5577 Airglow 5893 Airglow 6300 Moon 3500-7500 atories because the gas density must be lower than in the best artificial vacuums. Normally when an atom is excited, it emits a photon within about 10- 8 second. But the excited states that result in fo rbidden lines can last minutes to hours. In the laboratory, atoms cannot remain undisturbed that long because they collide with each other or the walls of the container. Because less energy is needed to excite the fo rbidden lines, they are much stronger than ordinary lines when they can occur at all. Hydrogen is the most abundant element in emission nebulae, but the forbidden lines of oxygen emit the most light. Just as light from deep-sky objects comes in two types, continuum and discrete wavelengths, so does light pollution. For example, moonlight and light from incandescent bulbs is of the continuum type. Fortunately, the light from airglow and most streetlights is at discrete wavelengths in different parts of the spectrum than most of the light from nebulae. So the two can be separated with an appropriate filter. Such filters are known as nebula filters or . bght-pollution filters. The term "nebula filter" Comments Strongest mercury line 40% as strong as 3660 line 75% as strong as 3660 line 98% as strong as 3660 line 96% as strong as 3660 line Continuum; 3% as strong as 3660 line Peak is 5700-6200 angstroms 15% of Peak Continuum; 5% of Peak N early monochromatic Partly continuum; brighter in red Continuum; brighter in red Oxygen Sodium Oxygen Continuum; similar to sunlight is more appropriate, Since VIews of nebulae are improved most. The usefulness of such a filter depends on the nature of the interfering light. Table 3.4 lists common sources. In examining Tables 3.3 and 3.4, we see that most light pollution is at wavelengths different from the light of nebulae. This is illustrated graphically at the tops of Figures 3.5 to 3.8. Referring back to Figure 2.3, recall that the peak response of the human eye's night vision is near 5000 angstroms (at the color green), or the same wavelength as the strongest nebular line at 5007 angstroms. But there are strong light pollution lines close by, near 4400 and 5400 angstroms. A filter must cut the spectrum pretty finely to reject these wavelengths and still have a high transmission at 5000 angstroms. This can only be achieved with modern interference-filter technology. Interference filters have very thin layers of partially reflecting material separated by thin transparent layers. Depending on a layer's thicknesses, different wavelengths of light will undergo constructive or destructive interference. The number of wavelengths that can be 32 33
controlled depends on how many layers are deposited. Ordinary colored filters are added to block wavelengths far from the primary transmittance region. Interference filters can have very high transmittance at very narrow wavelengths. They are ideal for isolating the light of nebulae and have uses in many other areas of science. In the early 1970's the technology to make them was quite expensive, but now they can be mass produced and cost no more than a high quality eyepiece. Light-Pollution Lines VISUAL ASTRONOMY OF THE DEEP SKY I What about continuum light poll This is harder to deal with. It can be only by making the filter absorb all light at the wavelengths of the nebula. wavelength band pass of an interference can indeed be made extremely narrow. example, filters used by amateurs for ing solar prominences have a band pass only 0.7 angstrom at the wavelength of rogen alpha, 6563 angstroms. At wa as fa r away as 6500 angstroms, the filter transmit less than one thousandth as much - . - - - - I I - - - . - -. - - - - I - - -: - - - - - - - - - - - - . - Ne bu la r Lin e s ;==:;==r=;==r=::!:r=:;==r==;II=r==r=;=::;==;=:;==rr=::;==;:=r==r=, - 0 en Q) "0 :::J +-c -2 Cl as E '-' Q) -4 en / I C I 0 a. ... _ - - - en Q) a: Q) > +-as Q) a: -6 -8 Figure 3.5. The spectral transmISSIOn of the Lumicon UHC filter is shown for light arriving face-on (solid line) and at an angle of about 20° (dashed line) . Note how the band pass (the central peak) shifts to shorter wavelengths as the filter is tilted, and how the blocking of the light outside the bandpass becomes less effective. A relative response of -2.0 magnitudes is a transmittance of 16%, -5.0 magnitudes is 1 %, and -8.0 magnitudes I I I I I I , Lumicon UHC Filter 4000 5000 6000 7000 Wavelength (angstroms) . - .. - - IS 0.06%. The positions of major lines from Table 3.3 and the light-pollution lines from Table 3.4 shown above the graph. The pollution lines are natural airglow lines oxygen. The broad dashed line that around 5800 to 6000 angstroms Lucalox streetlamp. Continuum sources are not shown because they affect wavelengths to some degree. -:- k Similarly, a nebula filter could be at Its pea . constrUC e 5007 angstroms. THE EYE AND THE TELESCOPE d I h I· h t d that transmitte on y t e Ig t at owev , . . H er interference filters have some cl un eSlra . they wo ble charactenstlcs. In partlcu ar, . I h I ·f rk at the correct wavelengt on y I I S making up the filter are exact y t e the ayer . . h corr - t thickness. Two thmgs dIsturb t e h· k ss of these layers: temperature an t !C ne tilting of the filter. . As the temperature changes, the layers . wIll expan elength changes accordmgly. e narw d or contract and the transmItted ' . Th d · row-band solar filters are m unte m a sma oven to precisely control theIr temperature. A lar filter's wavelength can actually be scanso . necl slightly by changmg t e oven temperarure. . When a filter is tilted, the transmItted wavelength becomes shorter, the bandpass Light- Pollution Lines _ _'_ _ _ -'- _ -'- h I h d II broadens, and more unwanted light is transmitted. If you have access to a nebula filter, try tilting it and you will see its color change from greenish to purple. These effects limit the design of nebula filters for practical purposes. All nebula filters are made with bandpasses broad enough so temperature changes have no effect. Tilting the filter, however, does alter its characteristics. Because stars emit continuum light, when any wavelengths at all are blocked, stars will appear fainter. The narrower the band pass, the more starlight will be lost. Furthermore, many stars are bright enough to stimulate color perception, so if the filter transmitted only the nebular line at 5007 angstroms (green), all stars would appear green. Thus some manufacturers have made filters that transmit some blue and red light so the color balance is closer to normal. Light pollution at these wavelengths is not very great. ""=_"""'_'"'""'"'O:":';'....L...I_'_ _ .L.... _ __' _ _ ..:..:.:."'"'" '"'_'_ _ _ _ Ne bular Lines r=:;==r=i=::;=::!:r=r==r==;I==r==r=;=::;==;=:;==ri=::;==;:=r==r=j - 0 en Q) "0 :::J +-c -2 Cl as E '-' Q) -4 en c 0 a. en Q) -6 a: Q) > +-- as Q) a: -8 , , , , , 4000 5000 6000 Wavelength (angstroms) , \ , , I , , , , , ,'- Edmund Deep Sky Filter on-axis ' Lumicon Deep Sky Filter on-axis 7000 Figure 3.6. Transmittance curves of the Edmund Deep-Sky Filter (solid line) and Lumicon Deep Sky Filter (dashed line). 34 35
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 and 10: VISUAL ASTRONOMY OF THE DEEP SKY AB
Page 11 and 12: VISUAL ASTRONOMY OF THE DEEP SKY TH
Page 13 and 14: .. .. .. .. VISUAL ASTRONOMY OF THE
Page 17 and 18: e viewed with enough magnification
Page 21 and 22: lens and a piano-convex eye lens. A
Page 23: VISUAL ASTRONOMY OF THE DEEP SKY TH
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:
-- VISUAL ASTRONOMY OF THE DEEP SKY
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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