Max Planck Institute for Astronomy - Annual Report 2005

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94 IV. Instrumental Development L3 Sun Fig. IV.1.8: Locations of the Lagrangian Points L1 to L5 in the Sun – Earth system (not to scale). At L2 a satellite orbits the Sun with the same angular velocity as the Earth. Since L2 is a metastable point, JWST will circle it on extended Lissajous orbits. The destination of the journey is the Lagrangian Point L2 (Fig. IV.1.8). There, 1.5 million km from Earth on the prolongation of the line Sun – Earth, the satellite will »feel« the joint attractive forces of Sun and Earth, and despite its larger distance to the Sun it will orbit the Sun with the same angular velocity as the Earth. So for ten years, we will see JWST from Earth always in anti-solar direction. Although the point L2 is a solution of the three-body problem of celestial mechanics, JWST will not be stationed exactly at that point. There are at least three arguments against it: 1. L2 is a metastable point, that is, smallest perturbations will drive a satellite away from there (comparable to a pencil standing »stably« on a fingertip …), 2. for stationing braking manoeuvres would have to be carried out directly at L2, 3. the satellite would experience solar eclipses because the Earth is standing in between, interrupting its energy supply. Therefore loop-shaped orbits around L2 are chosen in practice (Fig. IV.1.9). These Lissajous orbits around L2 can have very large diameters: some hundred thousand km in the ecliptic and perpendicular to it. The orbital period around L2 can be half a year and the deviation from L2 as seen from Earth up to � 30°. The larger the diameter of these loops around L2 the easier are L4 Earth L5 L1 the orbital manoeuvres during closing-in and later orbit corrections. A limit is set, however, by the scattered-light requirements of the tube-less JWST. At large angular distances from L2 Sun and Earth would no longer be occulted simultaneously. Seen from L2, the Earth has the same angular size as the Sun, and in the mid-infrared range the Earth is bright! Passive and active cooling JWST-Orbit 150 Mio. km 1.5 Mio. km Already during the approach to L2 the unfolding process of JWST, which is tightly folded into the payload nose cone of the ariaNe 5, will be started (Fig. IV.1.10). More than 100 mechanisms (hinges, motors, sensors, … ) have to be activated in order to open the tennis-court sized multilayered radiation shield and the 6.5 m-telescope. The radiation shield reduces the thermal radiation of the Sun by a factor of millions: of the 300 kilowatt incident on JWST, less than 0.1 watt will be left on the telescope side. In this way, the primary mirror can passively cool to – 240 °C, sufficiently low for sensitive observations with all instruments on board. The radiation shield consists of five layers of Kapton foil each of which is vapor-coated with aluminium on the Sun-facing side. This way as much radiation as possible will be reflected back into space. The side of the foil turned away from the Sun is coated with silicon, which acts as a blackbody radiator in the infrared range, thus cooling the foil. Thermal radiation escapes through the 15 cm wide gaps between the foils (Fig. IV.1.2). The foil package will have to resist L2
Earth = Launch (L) Trajectory correction maneuver 1 L + 15 hrs Sunshield deployment L + 2 days 384 000 km Telescope deployment L + 4 days Observatory available for ISIM activities L + 70 days Observatory »first light« (ISIM at safe operating temp) L + 59 days Moon Fig. IV.1.9: Timetable for the flight of JWST to its orbit around the Lagrangian Point L2. After 119 days (launch � 109 days) the final orbit is reached. A few days later the test operation of the instruments will begin. � 1500000 km impacts of micro-meteorites, the solar wind, cosmic rays, extreme temperature variations and manifold mechanical stresses in order to ensure the millionfold radiation reduction over the entire duration of the mission. The cameras of Miri, however, have to be cooled to – 268 °C (about 5 K). Here a decisive modification in the development took place in 2005. Up to then work on a cryostat with solid hydrogen was in progress. With this simple and well-proven technique a just sufficient operating temperature of – 267 °C could be achieved. But in view of the desired weight cuts the cryostat soon appeared to be too heavy. Therefore a mechanical cooling machine was chosen. The motor-driven first compressor stage will be placed on the satellite part of JWST and linked to the cooling head in Miri by flexible pipes. In principle this technique has advantages: lower temperatures, lower masses, almost unlimited operation times … . On the other hand there are risks concerning time, costs for development and testing, and the use of this new technique for a flagship mission without any possibility for maintenance or replacement. If this active cooling proves a success with JWST it probably will be used in many future missions. The involvement in the JWST mission offers the MPIA the chance to carry on in its tradition of interesting technological developments in infrared instrumentation, and guarantees participation in the most exciting scientific IV.1 Instruments for the James Webb Space Telescope 95 L2 orbit achieved L + 109 days Trajectory correction maneuver 2 (if required) L + 25 days L2 Initiate ISIM testing and certification L + 113 days � 374000 km Lissajous-loops Fig. IV.1.10: For launch, the 6.5 m-telescope and its multi-layered radiation shield more than 30 m across have to be folded many times in order to be stowed within the 5 m wide nose cone of the ariaNe 5 rocket. Two days after launch, during the journey to L2, the unfolding process is started.
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Max Planck Institute for Astronomy
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Max Planck Institute for Astronomy
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Contents Preface ..................
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6 I. General I.1 Scientific Goals U
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8 I. General massive protostars and
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10 I. General achieve higher angula
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12 I. General The institute is co-l
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14 I. General Point Source sensitiv
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16 I. General Edinburgh Groningen D
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18 II. Highlights II.1 Light Echoes
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20 II. Highlights 10 arcsec Fig. II
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22 II. Highlights AB Dor C AB Dor A
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24 II. Highlights II.3 The First He
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26 II. Highlights II.4 Planetesimal
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28 II. Highlights azimuthal directi
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30 II. Highlights ticles fall faste
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32 II. Highlights ANTU Seyfert 2 ga
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34 II. Highlights cretion disk, one
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36 II. Highlights 8.5 �m 2.3 ly 0
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38 II. Highlights II.6 Massive Star
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40 II. Highlights log M� /pc2 �
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42 II. Highlights II.7 Observations
Page 46 and 47: 44 II. Highlights log IR luminosity
Page 48 and 49: 46 II.8 Dynamics, Dust, and Young S
Page 50 and 51: 48 II. Highlights and we see a temp
Page 52 and 53: 50 III. Selected Research Areas III
Page 54 and 55: 52 III. Scientific Work Fig. III.1.
Page 56 and 57: 54 III. Scientific Work the Data Ar
Page 58 and 59: 56 III. Scientific Work The Field o
Page 60 and 61: 58 III. Scientific Work lower than
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Page 68 and 69: 66 III. Scientific Work line observ
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Page 72 and 73: 70 III. Scientific Work Column dens
Page 74 and 75: 72 III.3 The Galaxy-Dark Matter Con
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Page 80 and 81: 78 III. Scientific Work tion rate o
Page 82 and 83: 80 III. Scientific Work observed is
Page 84 and 85: 82 III. Scientific Work DDO 53 HO I
Page 88 and 89: 86 III. Scientific Work tribution o
Page 90 and 91: 88 IV. Instrumental Development All
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Page 112 and 113: 110 V. People and Events 1� 10�
Page 114 and 115: 112 V. People and Events V.2 Open H
Page 116 and 117: 114 V. People and Events V.3 Furthe
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Page 120 and 121: 118 V. People and Events The IMPRS
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Page 126 and 127: 124 V. People and Events These oppo
Page 132 and 133: 130 V. People and Events V.10 The P
Page 134 and 135: 132 V. People and Events Abb. V.11.
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Page 142 and 143: 140 V. People and Events V.14 Where
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144 Staff Directors: Henning (Actin
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146 Working Groups Rainer Lenzen, H
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148 Cooperation with industrial Fir
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150 Conferences Conferences and Mee
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152 Conferences D. Lemke: JWST-miri
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154 Conferences/Service in Comitees
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156 Publications Szalay, I. Szapudi
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158 Publications Hamilton, C. M., W
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160 Publications Brinkmann, D. G. Y
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162 Publications Zheng, X. Z., F. H
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164 Publications of the wheel mecha
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166 Publications ASP Conf. Ser. 331
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The Max Planck Society The Max Plan
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Max Planck Institute for Astronomy - Annual Report 2005

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