OF THE ROGER N. CLARK

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--- NGC 2261, HUBBLE'S VARIABLE NEBULA IN MONOCEROS R.A. 06h 39.1m, Dec. 08° 43 ' (2000.0) Technical. NGC 2261 was discovered by Sir William Herschel in 1783, but it has been known as Hubble's Variable Nebula ever since Edwin Hubble found, on photographs taken between 1900 and 1916, that it had changed shape. NGC 2261 is a small reflection nebula that appears rather like a comet. The variable star R Monocerotis is at its apex, and appears as if it were the "comet's" nucleus. R Mon varies by up to 4 magnitudes in an irregular way. Its variations are responsible VISUAL ASTRONOMY OF THE DEEP SKY for the changing appearance of the which it illuminates. However, the and star do not vary together. It seems dust clouds orbit close to the star, blocking light to a greater or lesser degree and their moving shadows on the nebula. Some changes have been seen to occur ' as little as one day, and shadows have by up to I arc-second in four days. ''''''ll!elI that might be seen in large amateur scopes can occur on timescales as short as month. On nights of very good seeing, large telescopes R Mon does not appear stellar. Apparently dust close to the forms a very small, brilliant reflection - . . Size telescopes even under only moderate d ' h'l d I Ifig the winter skies an Immediately thought It . as a comet. A chec of several star charts Bu rnham 's Celestial Handbook set me :aight. At magnitude 10 with a size of2 by 2 arc-minuts, it has a mean surface brightness of 20.1 magnitudes per square arc-second. This is quite bright compared to most nebu I ae. Although no detail was seen in the 8-inch nder moderate skies, it should be possible to ee detail under excellen t skies if the shadows at the time are high in contrast. Since the surface brightness is high, and the nebula is a Visual. NGC 2261 is quite easy in medium skies. I first fou It w d .1 e ran . om y sweep- k A VISUAL ATLAS OF DEEP-SKY OBJECTS reflection from dust grains, it may appear bluish in large telescopes under dark skies. • Photograph ojNGC 2261 . South is up. (Courtesy National Optical Astronomy Observatories.) 112 Scale: 0.8 arc-min/cm 8·inch f! 11.5 Cassegrain 12.4mm Erfle (188X) 9mm KelIner (260X) Drawing ojNGC 2261. Viewing Distance (cm) 25x:l72 200X:21 50x: 86 300x: 14 100x: 43 400X;11 113 -- 5' -- air mass; 1 .07, faintest star: 13.5 at zenith, 188x; no tracking 1/13/83 10:35-1 1 :05 UT at Hawaii Kai, Hawaii; R. Clark
VISUAL ASTRONOMY OF THE DEEP SKY --- P M46 (NGC 2437), OPEN CLUSTER IN A VISUAL ATLAS OF DEEP-SKY OBJECTS UPPIS WITH NGC 2438, A PLANETARY M46 ' NEBULA R.A. 07h 41.9m, Dec. _14° 49' NGC ' 2438: R.A. 07h 41 .9m, Dec. _]40 43 ' (2000.0) Technical. 46 was discover . ed in 1 771 by Charles Messier. The cluser IS abouṭ 54. 00 light-years distant and 30 lIght-years In diameter. It contains between 200 and 500 stars, 'th about 150 between . magnitudes 10 to 13. h . M46 is unusual In t at It appears to contain a planetary nebula, NGe 2438. But the WI nebula is not a member of the star cluster, since it is moving through space at a different velocity. The nebula appears 65 arc-seconds about 3000 light years from us, and has a is 16th magnitude. in diameter. It is probably in the foreground diameter of about 1 light-year. Its central star Visual. M46 is a pretty cluster at any magnification, where it all fits within the field of view. The cluster is round, with a diameter of 25 arc-minutes. The total visual magnitude is 8, and in small instruments and low power, where the cluster is not resolved, the mean surface brightness is 23.6 magnitudes per square arc-second. The planetary nebula is best seen at medium to high powers. At 150x or 200x with a wide-field eyepiece, the whole cluster and the planetary can be seen simultaneously. The nebula is 11 th magnitude, with a mean surface brightness of 19.8. In mediumsized telescopes, a star slightly northwest of center is often mistaken for the central star, which is fainter. In the 8-inch under moderate skies, the nebula appeared as a doughnut noticeably fainter on the west side. 114 115
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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
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: --- --- M78 (NGC 2068), NGC 2071, D
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 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: --- -- VISUAL ASTRONOMY OF THE DEEP
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VISUAL ASTRONOMY OF THE DEEP SKY --
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-- VISUAL ASTRONOMY OF THE DEEP SKY
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-- -- VISUAL ASTRONOMY OF THE DEEP
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VISUAL ASTRONOMY OF THE DEEP SKY A
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-- MI5 (NGC 7078), GLOBULAR CLUSTER
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--- VISUAL ASTRONOMY OF THE DEEP SK
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-- VISUAL ASTRONOMY OF THE DEEP SKY
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-- ·rl E I u L ro 5 +J Q) (f) 0 Y
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-- --- VISUAL ASTRONOMY OF THE DEEP
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VISUAL ASTRONOMY OF THE DEEP SKY NG
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'2 5 -rl E I U L CO '---' -t-J ill
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VISUAL ASTRONOMY OF THE DEEP SKY AP
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VISUAL ASTRONOMY OF THE DEEP SKY Ta
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-- Appendix E A catalog of deep-sky
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-- -- --- VISUAL ASTRONOMY OF THE D
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--- --- 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 -
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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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