The Herschel 3.5 m Silicon Carbide Telescope Final Qualification Campaign
- Paper number
IAC-07-A3.I.A.22
- Author
Dr. Charles Koeck, EADS Astrium, France
- Coauthor
Mr. Yves Toulemont, EADS Astrium, France
- Coauthor
Mr. Thomas Passvogel, European Space Agency (ESA)/ESTEC, The Netherlands
- Coauthor
Dr. Göran Pilbratt, European Space Agency (ESA)/ESTEC, The Netherlands
- Coauthor
Dr. Dominic Doyle, European Space Agency (ESA)/ESTEC, The Netherlands
- Coauthor
Mr. Dominique Pierot, EADS Astrium, France
- Year
2007
- Abstract
In 2008, the European far infra-red and millimetre wave space observatory Herschel will be launched to the L2 Earth-Sun Lagrangian point, from which it will observe the cold Universe in the spectral range between 80 and 670 µm wavelength. The Herschel spacecraft is equipped with the largest telescope ever built up to now for space applications. This telescope was developed by Astrium Satellites of Toulouse for ESA, and has recently terminated its qualification and acceptance campaign. It is now stored in Astrium premises, and is ready for the final integration onto the Hershel spacecraft. The Herschel telescope features a 3.5m diameter main mirror, and is based on a Cassegrain optical design. The whole telescope makes use of the Silicon Carbide technology. The major challenge was the manufacturing of the monolithic primary mirror. Thanks to the silicon carbide outstanding performances, the total weight of the telescope is only 320kg (corresponding to 33kg/m2). The main performances specified to the telescope for the scientific observation are the focus position, whose knowledge accuracy has to be better than ±3mm, and the wave front error which has to be kept below 6µrms under the particularly severe environments of launch mechanical accelerations of 10g and operational temperature of 70K. This operating temperature (220K below the integration temperature), combined with the high sensitivity of the telescope optical combination, makes the tests definition and operation very challenging. Several innovations had to be developed in the domain of test engineering and optical ground support equipment. Among them, the gravity compensation policy, the Hartmann optical tests method under vacuum, and the test data processing method. The paper will recall the telescope design features, and in particular will explain the material trade-off (among silicon carbide, CFRP and aluminium) that was performed at beginning. Then the paper will present the telescope manufacturing process, driven by the SiC technology and the unusual size of the primary mirror. Lastly, a special highlight will be given about the telescope integration and optical qualification under cryogenic condition.
- Abstract document