OF THE ROGER N. CLARK

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-- --- VISUAL ASTRONOMY OF THE DEEP SKY -- A VISUAL ATLAS OF DEEP-SKY OBJECTS • • • • ------- 5' --- Photograph of N GC 6946. South is up. (Courtesy National Optical Astronomy Observatories.) Drawing of NGC 6946. Scale: 0.75 arc-min/cm 8-inch filLS Cassegrain 20mm Erfle (117x) 12.4mm Erfle (188X) Viewing Distance (cm) 25X: 183 200X:23 50 X: 92 300 X : 15 100 X: 46 400 X : II air mass: 1.30, faintest star: 14.5 at zenith, 188x; no tracking 8/12/83 10:20-10:35 UT at Waianae ranch, Hawaii; R. Clark 216 217
-- VISUAL ASTRONOMY OF THE DEEP SKY NGC 6960 AND NGC 6992-5 THE VEIL NEBULA IN CYGNUS NGC 6960: R.A. 20h 45.7m, Dec. 30° 43' NGC 6992-5: R.A. 20 h 56.4m, Dec. 3]0 42' (2000.0) Technical. The Veil Nebula is a spectacular supernova remnant showing beautiful, intricate detail. Observers have given it many names in attempts to convey its delicate appearance: the Cirrus, Bridal Veil, Filamentary, Network, and Loop Nebula. Each is appropriate. The nebula, discovered by William Herschel in 1784, is fully 2.7° in diameter. Modern investigations have shown it to be expanding by 0.06 arc-second per year. Spectroscopic analysis shows its present physical expansion rate to be 45 kilometers per second. These two values allow us to compute a distance of I 300 light-years. The original expansion rate after the supernova explosion must have been well over 1000 kilometers per second. The expansion has slowed because the gas has been plowing into interstellar matter. Using a reasonable estimate of the deceleration rate, the time since the supernova can be calculated as about 30 000 years. No remnant star has been identified, and none may exist. The energy to light the Veil probably comes from its ongoing collision with interstellar matter, a process known as shock excitation. - Visual. The Veil Nebula is a p : ized object ng amateur astronomers and IS sure to be arnO . a ta . A VISUAL ATLAS OF DEEP-SKY OBJECTS rget for medlUm to large telescopes at any (N rthern Hemisphere) summer star party. NC 6960 and 6992-5 are arcs each with a total magnitude of about . NGC 6960 has a length of about O arc-mlllutes, an average 'dth of .6 arc-mlllutes, and a mean surface WI . b ightness of23.2 magmtu d es per square arcs;cond. NGC 6992-5 has a size of 78 by 8 arc-minutes and a mean surface brightness of 23.6 magnitudes per square arc-second. Bo h appear brighter than the Merope Nebula III . the Pleiades or the outer loop of the Onon Nebula, M42. They are fainter than the Trifid Nebula, M20. The Veil is divided into three parts. The brightest is NGC 6992-5, which is centered 2.7° northeast of the 4th-magnitude star 52 Cygni. The nex brightest part is NG 6960, which passes nght next to 52 Cygm. The th ird and faintest is an unnamed triangular patch between the brighter two. There are other, smaller and fainter fragments of the nebula that have not been reported visually by amateurs in recent years. How hard it is to detect the Veil is somewhat controversial. Some observing manuals say that NGC 6960 and NGC 6992-5 require a 6- or 8-inch telescope at low powers under an excellent sky, and that the triangular patch cannot be seen visually in any amateur telescope. Others say the brighter parts are visible in 7 X 50 binoculars. The reason for this disparity is unclear. I suspect it lies in particular observers' experiences trying to find the objects. Under only moderate skies, the nebula may indeed be difficult to detect in an 8-inch. But slight improvements in sky quality show the Veil's beauty in smaller telescopes. The drawings of August 12, 1983, illustrate the view through the 8-inch tele Scope under good skies. Only one magnification, 117 x, was tried because clouds interrupted the observing session. Even so considerable detail could be seen. Of NGC 6960, the part north of 52 Cygni was easy. The edges were sharp, and the width slowly decreased to a fine point at the northernmost extreme. The bend in the nebula 12 arc-minutes north of 52 Cygni was easily detected, as was a small fork that extended an arc-minute or so to the northeast. The part south of the star (actually curving to the southeast) could not be detected. That portion becomes very wide and dim. The glare from 52 Cygni makes the observation difficult by adding stray light to the field. NGC 6992-5, the brighter arc, showed considerable detail through the 8-inch under good skies. The brightest portion, near the northern extreme, is NGC 6992. The southern end becomes broad and looked forked in the 8-inch. That end is NGC 6995. The surface brightness varied along the length of the arc, an indication that if the sky were slightly better, the individual filaments that make up the nebula could be resolved. Under good skies, none of the Veil was visible in the 2.4-inch, 7.9x finder. But under skies showing stars only about 0.3 magnitude fainter, the Veil showed much better. The change resulting from this slight increase in sky quality was amazing. Now NGC 6992-5 could be seen through the 2.4-inch finder, and in the 3-inch finder at 31 X, all three portions of the Veil were visible! This observation, on August 26, 1984, was the first time I had seen the triangular patch. Under these improved conditions, the 8- inch showed the faint part of NGC 6960 south of 52 Cygni easily at 117x. At 188X, the narrow tip at the northern end had sharper edges than when seen the year before. NGC 6992-5 contained a wealth of detail. At the northern end of this arc, in NGC 6992, individual filaments appeared. Throughout the Veil, the filaments range from only I to 5 arc-seconds in width. Resolving them demands excellent skies and at least an 8-inch telescope. For the drawing made that night, the best view was at 188x. At that power only portions of NGC 6992-5 could be seen at one time. I had the impression that the filaments coincided with star chains. In fact, about two-thirds of the way toward the southern end I drew a nearly circular loop. This is not nebulosity but many faint stars that gave that impression. Farther south, where the nebula's width increases, several additional arcs could be seen. Some amateurs using large telescopes have reported seeing colors of individual filaments. The Veil is truly a beautiful structure that will reveal its intricate splendor to medium size instruments under good to excellent skies. 218 219
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OF THE ROGER N. CLARK
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Published by Sky Publishing Corpora
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CONTENTS M78 (NGC 2068) NGC 207 1,
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PREFACE During research for this bo
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VISUAL ASTRONOMY OF THE DEEP SKY AB
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VISUAL ASTRONOMY OF THE DEEP SKY TH
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.. .. .. .. VISUAL ASTRONOMY OF THE
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e viewed with enough magnification
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lens and a piano-convex eye lens. A
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controlled depends on how many laye
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The Lumicon H-Beta Filter (Figure 3
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every 1 ° in declination and 4 min
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error of only one quarter the field
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Table 4.1. Ideal limiting magnitude
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5 Making drawings and keeping recor
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more detail in the future. They add
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.. .. .. ... Contrast (C) C Log(C)
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VISUAL ASTRONOMY OF THE DEEP SKY A
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--- VISUAL ASTRONOMY OF THE DEEP SK
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--- M76 (NGC 650-651), PLANETARY NE
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-- -- M45, THE PLEIADES OPEN CLUSTE
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-- Brightest stars of the Pleiades
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M42 (NGC 1976), M43 (NGC 1982), THE
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--- --- --- VISUAL ASTRONOMY OF THE
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-- --- NGC 2023 NGC 2024, IC 434 (T
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--- --- M78 (NGC 2068), NGC 2071, D
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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 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 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 103 and 104: VISUAL ASTRONOMY OF THE DEEP SKY A
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 115: VISUAL ASTRONOMY OF THE DEEP SKY --
Page 123 and 124: -- MI5 (NGC 7078), GLOBULAR CLUSTER
Page 127 and 128: --- VISUAL ASTRONOMY OF THE DEEP SK
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 145 and 146: VISUAL ASTRONOMY OF THE DEEP SKY Ta
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Page 149 and 150: -- Appendix E A catalog of deep-sky
Page 153 and 154: -- -- --- VISUAL ASTRONOMY OF THE D
Page 155 and 156: --- --- VISUAL ASTRONOMY OF THE DEE
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-- APPENDIX F: OPTIMUM DETECTION MA
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------- VISUAL ASTRONOMY OF THE DEE
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--- VISUAL ASTRONOMY OF THE DEEP SK
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VISUAL ASTRONOMY OF THE DEEP SKY -
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VISUAL ASTRONOMY OF THE DEEP SKY AP
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VISUAL ASTRONOMY OF THE DEEP SKY -
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VISUAL ASTRONOMY OF THE DEEP SKY Ta
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VISUAL ASTRONOMY OF THE DEEP SKY Ta
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G VISUAL ASTRONOMY OF THE DEEP SKY
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INDEX INDEX M70 (NGC 6681), 312, 31
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OF THE ROGER N. CLARK

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