Max Planck Institute for Astronomy - Annual Report 2005

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92 IV. Instrumental Development The mechanisms are provided by MPIA Within the European Miri consortium – comprising 21 institutes – the MPIA is responsible for the development of both grating wheels and the filter wheel since 2002. The demands on these »mechanisms« are high, including: • Maximum reliability and most precise positioning for hundreds of thousands of movements • Low electric driving power to avoid heating • Vibration resistance during launch (� 40 g) • Operation both under cryo-vacuum and laboratory conditions • Operation times of at least 10 years, reliable re-start even after a long rest • Rapid and cost-effective availability The working group »Infrared Space Telescopes« at MPIA has gained longstanding experience in building such cryo-mechanisms. The Institute, together with C. ZeiSS and Dornier, had built three optical exchange wheels for the iSopHot instrument onboard the European satellite iSo. During the mission lasting 29 months these wheels performed hundreds of thousands of high-precision movements at T � – 270 °C. For iSopHot and the paCS instrument of the European HerSCHel Space Telescope focal-plane choppers were developed that could perform hundreds of millions of movements at – 270 °C with high reliability. This development and testing experience will be used for Miri and NirSpeC, although the challenges will increase: the wheels for the JWST are many times larger and heavier, and the precision requirements even higher. In particular, because of the »warm« launch of the JWST the wheels have to be operable in the entire temperature range from � 20 °C to – 265 °C and from standard pressure to high vacuum. All wheels of Miri and NirSpeC will have drives based on our detent concept that has proven its worth in iSopHot. As an example, Fig. IV.1.5 shows the filter wheel for Miri. At the periphery 18 small ball bearings are mounted, according to the number of position settings. A wedge-shaped element on a moving lever latches between two ball bearings, thus locating the position of the wheel with a repeatability of � 1 arcsecond. The central motor is a direct drive (torque motor) without transmission. It is moved by an electric pulse by about 15 ° so that the tip of the detent moves over the neighbouring ball bearing. The exact positioning can then be carried out mechanically without electric power by the spring tension of the detent or even slowly decelerated electrically. This drive concept avoids feedback from an electrical position indicator and holding currents since positioning is always carried out mechanically with great reliability. Depending on the wheel size, small fractions of a Joule are applied for each step. Since no electric current flows after the � 1 second-long driving step, and the astronomical observations take 1000 to 10 000 seconds, the mean electric power loss of this drive concept is virtually negligible. To increase the precision the duplex ball bearings are engineered as integrated bearings, that is, one of the rings also serves as the bearing axis. The raceways are covered with a thin layer of molybdenite (MoS 2 ). It prevents cold welding in high vacuum and provides good lubricating properties at low temperatures. As an additional protection all ball-bearings have a thin titanium-carbide coat. Sophisticated model philosophy According to the current time schedule, filter and grating wheels of highest quality and reliability will have to be delivered in 2008. In order to meet this time schedule the development is done step by step – a method that always has paid off with space experiments. As early as 2004, prototypes of the wheels had been set up at MPIA, the design models followed in the year under report. In 2006 and 2007, qualification models will be built for which the full space-flight qualification (vibrations, endurance, … ) will have to be verified. In 2008 the final flight models of all wheels will be delivered while at the same time the qualification models will be upgraded to serve as spare flight units (installation of new ball bearings and so on). Only this sophisticated model philosophy ensures that matured, highly reliable technology is used in the expensive space missions. For months, and under all environmental conditions required, each of the models mentioned above will be tested, refined, tested again and in the next step be equipped with components of ever-increasing quality (and cost). The design models (Fig. IV.1.6) set up in the year under report at MPIA were already able to meet essential requirements during cryo-vacuum tests: high positioning accuracy, low power consumption… But unexpected weak points were found, too: 1) the integrated duplex ball bearings used were easily soiled by microparticles, increasing their friction coefficient, 2) a thermal decoupling disk between the steel components (ball bearings, motor) and the aluminum structure showed cold embrittlement of its flexible elements at – 260 °C, 3) the TiC-coats of the balls did not withstand the high mechanical and thermal stresses. As far as the time schedule permits, these problems are eliminated in the same model, in particular if the improvements can be made in the technical workshops at MPIA. Retrofittings that take longer can only be used in the next model. So, e.g., the manufacturer of the ball bearings needs almost one year to fit in suitable uncoated balls and to design dust covers. After a tendering procedure in the middle of 2005, the MPIA selected the offer by C. ZeiSS for the building of the qualification and flight models of the wheels for Miri. On November 29 th , 2005, the relevant contract was signed in Oberkochen (Fig. IV.1.7). MPIA was granted the funds for this industrial fixed-price contract by the German Aerospace Center (DLR), Bonn. With
the iSopHot instrument (iSo) and the paCS chopper (HerSCHel) the C. ZeiSS company has proven its high competence for building opto-mechanical instruments to be used in the cryo-vacuum of outer space. Abb. IV.1.6: Mounting of the Miri grating wheel in the test cryostat for tests at – 265 °C by C. Schwab. Extreme cleanliness requirements have to be fulfilled. (MPIA) Abb. IV.1.7: Signing of the JWST contracts in November 2005 at C. ZeiSS in Oberkochen. From left to right: Prof. Henning (MPIA), Prof. Lemke (MPIA), Dr. Kötter (C. ZeiSS), Dr. Wiemer (C. ZeiSS). Together with the contract for Miri, a contract for NirSpeC, the European spectrometer for the JWST, could also be signed. Here the roles are reversed: MPIA contributes as a subcontractor to the development of the grating and filter wheels for NirSpeC. In the consortium led by C. ZeiSS, MPIA is responsible for the development of the electric components (motors, position sensors, …). Although the order placed with MPIA by C. ZeiSS is smaller than the contract for Miri, we and C. ZeiSS are expecting a high synergy because of the many common goals in the development of both instruments. Location: L2 IV.1 Instruments for the James Webb Space Telescope 93 According to the original time schedule the launch of JWST was forseen in 2011. Because of JWST’s huge size and mass of about six tons this can only be accomplished by the European ariaNe 5 rocket. Shortly before and immediately after launch critical phases will begin for the Heidelberg mechanisms in the instruments. During the last tests of all instruments on the launch pad the optical wheels will be turned too, but now at a temperature of � 30°C and damp external air. This will require very high operating currents and may damage the sensitive MoS 2 layers of the ball bearings. After take-off of the rocket all mechanisms built delicately for weight reasons are exposed to strong vibrations, with accelerations of up to 45 times the gravitational acceleration on Earth. Only after surviving these dangerous hours the quiet phase of the flight through space will begin. Gradually the »feel-good conditions« for the mechanisms will be reached: vacuum and low temperatures. Now ground control can order the careful running-in of the wheels. As a result the humidity trapped in the MoS2 layers of the ball bearings will be slowly removed and the bearing friction will decrease by a factor of 3.
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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 �
Page 44 and 45: 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 74 and 75: 72 III.3 The Galaxy-Dark Matter Con
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Page 84 and 85: 82 III. Scientific Work DDO 53 HO I
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Page 90 and 91: 88 IV. Instrumental Development All
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Page 112 and 113: 110 V. People and Events 1� 10�
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142 V. People and Events against sp
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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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