OF THE ROGER N. CLARK

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meridian at midnight. Since there are twelve months in a year and 24 hours of right ascension, the constellations move 2 hours per month. Counting 2 hours backwards from 0 hours (the same as 24 hours), we find that on August 20, right ascension 22 hours should be approximately on the meridian at midnight. Three hours earlier the right ascension would be three hours less, so at 9 p.m., right ascension 19 hours should be on the meridian. A star map shows that constellations near R.A. 19 hours include Lyra, Aquila, Sagittarius, Corona Australis, Telescopium, and Pavo. Precession The Earth is spinning like an off-balance top in that the direction of its axis is very gradually changing. Thus the stars are gradually changing their declination and right ascension. The direction of the Earth's axis follows a slow cyclical motion, with a 26,000 year period, called precession. Thus, star charts need to be updated with a new coordinate grid about once every 50 years. Since precession is very predictable, the coordinates for a given year can be changed to those for another year by putting them through mathematical formulae. Whenever a coordinate is given for an object, the year is specified when that coordinate is correct. This time is referred to as the epoch or, more precisely, the equinox. Most coordinates are currently given for equinox 2000 if the object is "fixed" like a star, or for the actual date (e.g. 1988.5) if the object is a short-term phenomenon like a comet. The following simplified formulae for precession are reasonably accurate. If RA and Dec are an object's right ascension and declination (RA expressed in decimal hours and Dec in decimal degrees), the yearly changes i1RA and i1Dec in these coordinates are given by: i1RA = 0.051 + 0.022 sin(15R.A.) (equation 3. lOa) i1Dec = 0.33 cos(15R.A.) (equation 3.10b) where i1RA is expressed in minutes '-0f time VISUAL ASTRONOMY OF THE DEEP SKY and i1Dec in minutes of arc. Multiply by total number of years desired, negati going back in time, and apply the to RA and Dec. More complex formulas be used for stars near the poles, but the given here work well in most of the sky. Many modern observatories have precise positioning capabilities, and coordinates to the epoch of the date and the observation is made. By adding corrections such as for the flexure in telescope mount and atmospheric ref the telescope can be aimed to within a of arc-seconds of an object. Such modern technology has led to generation of professional astronomers do not need to know where something is the sky. If they know its coordinates, computer tells when it is observable points the telescope right at it. astronomers are now beginning to use technology too. But even a totally telescope must first be accurately aligned the celestial pole for the system to work, that can only be done with a knowledge where things are in the sky. Furthermore, "flying on ins takes much of the fun out of sky watching, leaves the viewer helpless in the face problems. Most amateurs against it for beginners, who need to learn sky for themselves. Once the brighter and major constellations can be ". lUJ'U1J "" the fainter objects that cannot be seen the unaided eye can be found by using one the following methods. 1. Star hopping Star hopping should be learned by amateur astronomers. It is the method starting at a known naked-eye star and a chart to move carefully from star to until the desired object is reached. The best way to plan such a search is draw a circle on a piece of clear plastic senting the field of view of the finder at the correct scale for the star chart. A ring the right size works just as well. search route can be tested by sliding the cle across the chart. The route is then cated on the sky. Examples of star are illustrated in Figures 3.lOa through e. -- 2. Using setting circles . ·rdes are graduated scales on a tele- Setting , Cl quatorial mount. Th ey a 11 ow t h e scope s e to be positioned on an astronomical object . 1 . d d . shou - learning to star-hop. . . . . telscoe y "dialing in" the object's coordinates. This is not as sl' d P e d as It sou b n . s, an 1": It Id not be consl ere a su stltute lor The first step in usmg settm . g Clrcls IS to a Ign . ) . . e . . h . 1 1 Ha . h · h ' 1 I· the mount's right-ascensIOn aXIs (also II d the polar aXIs so It pomts to t e ce es- . I pole. One method IS to msert a specỊa fi der telescope ng t mto t e mount s po ar axis. Such finders have a speCla r . etlc e t at In . 1 . I h is a simple matter to align the star pattern on shoWS the star pattern at the celestial pole. It the re tide with that on the sky. A few manufacturers offer such finders inside their mountings. A second method of alignment is to chose two stars at the same declination about 6 telescope is aimed at one star and then swept hours apart, with one on the meridian. The to the other; the mount is adjusted until the telescope can be made to point to both stars Figure 3.10a. How to find the open cluster M4 1 by star hopping from Sirius in the con . stellation Canis Major. The circles show a typical finder telescope's 6° field of view. First center Sirius in the finder, then sweep 2/3 of a finder field south and a bit east, keeping track THE EYE AND THE TELESCOPE by only moving in right ascension (that is, wi th the declination axis locked). For example, consider one star on the meridian and one 6 hours further west. If you center on the first and try to move to the second, any error will be largely due to an alignment error in azimuth (the polar axis is pointed to one side of the pole). If the telescope ends up pointing south of the second star, then turn the entire mount until the star is centered. Now move back to the star near the meridian. This move is sensitive to an alignment error in altitude (the polar axis points too high or low). If the star is not centered, then move the mount in altitude to correct the position. Theoretically the mount should now be aligned, but repeat the procedure to check. The accuracy of polar alignment you need depends on how accurate your setting circles are, what your field of view is when searching for objects, and how closely you want them centered. The accuracy requirement for visual observing is much less severe than for astrophotography. If your field of view is about 1°, and the setting circles are graduated 20 • • .- .. , of star patterns along the way. (East is always the direction of increasing right ascension.) M41 should be visible as a faint, sparkling haze. Chart reproduced from Sky Atlas 2000.0 by W. Tirion. 40 41
every 1 ° in declination and 4 minutes in right ascension, then the polar alignment should be better than 1°. But it probably does not need to be better than about 112°. If, on the other hand, you wish to center small planetary nebulae at high power in a 10 arc-minute field of view, you need big, observatoryquality setting circles that can be read to Om ou, • • • . . .. . . . ." . 0578 • 0R • 049/ 20m Figure 3.10b. Finding faint NGC 253 in the constellation Sculptor is more difficult than finding M 41. The galaxy is in a s tar-poor region only about two degrees from the south galactic pole. The brightest star in the region is Beta Ceti, 7° to the north and slightly west ofNGC 253. Start by placing Beta Ceti in the center of the field of view. (You must know the constellations well enough to locate Beta Ceti!) Just outside the southern edge of the 5° field is a pair of 5th-magnitude stars. Find VISUAL ASTRONOMY OF THE DEEP SKY lh about 2 arc-minutes, and the should be better than about 5 "r.r_,;_ . Such alignment will take time and methods than described here, and is not feasible with a portable instrument. Once the mount is aligned, the setting cles must be set to read correctly. This easily done by centering the telescope Om • . , . 'sw . • "'- 20m ., .6·! • . . • them and center them. Now NGC 253 is south of the field of view and a little east. Move south until the two 5thstars are just outside the north 253 should be just east of center. fraction of a degree east should put in the center of the field of the main as well as the finder. The correct spot can estimated by comparing the positions of faint stars in the finder with those on chart. Chart reproduced from Sky Atlas by W. Tirion. oordinates are known and then star . w e setting circles until they read the movlllg . ' reet pOSItiOn. t a star in the regIOn and turn the set- cor f h' bl h . . aim a ung . h . ' Cl " b ' I yOU rnlg t ht have difficulty finding objects. h polar alignment was not adequate, . g in a locahze regIOn 0 t e s y IS to One way to circurr:ve d nt t . IS pr f o h em k ",,: en observlll 'rdes fo read t at star s coor mates. Th' way any error m pOlntmg near y IS minim · . ' 1 Id d to e . to h Th ' e . 1 k fi . I I 'z ed When another area of the sky is b observed, the settmg Clrc es wou nee b reset using a star m t at area. e time t resetting the Clrc es ta es away rom spen . pre . . cious observmg time. ' Note: the right ascensIOn Clrc e must 20m 14h I,()m , • e • • T7.:ui •• z/·/ 'e · ' . e. • IJe : THE EYE AND THE TELESCOPE I . . d' b e 20m URSA MAIOR 547S0S . . ." 80'9.79.' ... .. ." .' . "". 0"" 53710 95350 • 535J-So: 05297 Figure 3.lOc. M51, the Whirlpool Galaxy, is a favonte of amateur astronomers. Finding it is not too difficult as it lies near the end star of he Bi Dipper (Eta Ursa Majoris). Start by entenng the finder on that star. Move west nearly half a finder field to center the h-magnitude star 24 Canum Venaticorum. ow move south about a third of a field and .. • carried along with the telescope by a clock drive, or it will give false readings just a minute or two after being adjusted to the coordinates of a known star. In this author's opinion, setting circles should not be used except on permanent mountings by very experienced astronomers. The beginning amateur needs to learn the sky, and the learning will be impeded if setting circles are used. Many amateurs find that they remember object's positions among the stars (but not their coordinates) after only a year or two of active observing, and can position a telescope on one of those objects before most people would have time even to look up the coordinates. While examining a 5111 13h I,()m 20m 05055, ,,63 0'U . . " :800 . . +:,00 ., @, " west a little. M51 is just west of a 7th-magnitude star. Note the many other galaxies in this region, and how one would star hop to them. The brightest galaxies are the Messier objects: M51, M53, M94, and MlO1. Chart reproduced from Sky Atlas 2000.0 by W. Tirion. 42 43
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 25 and 26: controlled depends on how many laye
Page 27: The Lumicon H-Beta Filter (Figure 3
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:
-- VISUAL ASTRONOMY OF THE DEEP SKY
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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