Gaia is the European Space Agency's cornerstone mission for global space astrometry. Its goal is to make the largest, most precise three-dimensional map of our Galaxy by surveying an unprecedented number of stars.
This paper gives an overview of the mechanical system engineering and verification of the payload module. This development includes several technical challenges. First of all, the very high stability performance as required for the mission is a key driver for the design, which incurs a high degree of stability. This is achieved through the extensive use of Silicon Carbide (Boostec® SiC) for both structures and mirrors, a high mechanical and thermal decoupling between payload and service modules, and the use of high-performance engineering tools. Compliance of payload mass and volume with launcher capability is another key challenge, as well as the development and manufacturing of the 3.2-meter diameter toroidal primary structure. The spacecraft mechanical verification follows an innovative approach, with direct testing on the flight model, without any dedicated structural model.
Placed on the Sun-Earth L2 Lagrange point, SPICA will operate in the 5 to 210 μm wavelength range. Astrium has been
contracted by ESA/ JAXA to update the study of the SPICA telescope from a 3.5 m design (compatible to the Japanese
HIIB launcher) to a 3.2 m design (compatible to the HII-A with the short 5S fairing): despite a similar fairing diameter,
the shorter length of the fairing envelope results in a reduction of the M1-M2 distance and an associated diameter
reduction of M1. Maximization of the M1-M2 distance within the constraints is important to maintain a reasonable
polishing criteria of the main reflector. Therefore the M2 assembly sizing and the back focal length become main
parameters for the telescope optical design. The main constraints are driven by the telescope requirements such as focal
length, maximizing the diameter of M1 (3.2 m) and, M1 f-number (critical for the manufacturing aspects). The WFE
must be below 350 nm rms, and operational temperature below 6K. .
The main issues addressed in this paper are:
- an improved telescope design based on the Astrium background in Silicon Carbide technology which has been tried-an-tested
for mirrors and structural parts on several space projects, including HERSCHEL and Gaia (brazing, polishing,
assembling, iso-static mountings).
- performances which are taking advantage of the SiC properties ,such as homogeneity of the single-phase material
inside the structure, and structural stability from ambient to the operational temperature range. Our study shows that the
SiC telescope design can fulfil all the mechanical and optical requirements for SPICA.
- the verification and optical tests definition which will be key elements in the qualification of the telescope to be
incorporated in the logic of the satellite verification activity to be conducted in Japan.
Placed on the L2 Lagrangian point, the Space Infrared Telescope for Cosmology and Astrophysics (SPICA) will operate in the 5 to 200 μm wavelength range, at 4.5K. The large aperture telescope (3.5m diameter in a single piece) requires a strong manufacturing mastering, associated with high technical performances. The background acquired by EADS-Astrium (France) on the 3.5m Silicone Carbide Herschel Telescope is a key for the success of the SPICA development. EADS-Astrium has been awarded by the Japan Aerospace Exploration Agency (JAXA) and Sumitomo Heavy Industries to assess the feasibility of the 3.5m all SiC telescope through a design phase contract. The Telescope driving requirements are the large diameter of 3.5m especially critical for the manufacturing aspects, and the Wave Front Error which has to be kept below 350nm rms over a large temperature range from ambient to the operational temperature of 4.5K which requires a strong mastering of the distortions.
Large Space based IR are presently under development. These telescopes are placed on the L2 Lagrangian point and will operate in far infrared range. EADS-ASTRIUM is manugacturing HERSCHEL telescope and will extend its technology to the SPICA Telescope.
HERSCHEL operates in the spectral range between 80 and 670 μm wavelength and is devoted to astronomical investigations in the far-infrared, sub-millimetre and millimetre wavelength range. ASTRIUM has been awarded by ESA to manufacture tgeh 3,5m all SiC telescope. The concept for the HERSCEL telescope is based on an axisymetric, 3,5-m-diameter Cassegrain design. The driving requirements are the large diameter (3,5m) especially for the manufacturing aspects, the WFE which has to be kept below 6μrms, the operational temperatuer (70k) which brings distortionas wrt ambient environment, and finally the mass to keep below 300kg. This Development is part of the ESA HERSCHEL PLANK program.
SPICA Telescope driving requirements are also the large diameter (3,5m) especially critical for the manufacturing aspects, the WFE which has to be kept below 350nmrms, and the operational temperature (4,5K) which requires to master the distortions wrt ambient environment. Telescope will operate in the 5 to 200 μm wavelength range. ASTRIUM has been awarded by Sumitomo and ISAS to study the faisability of teh 3,5m all SiC telescope.
The main features developed in this paper are:
The final design and the recent manufacturing developments of the HERSHEL telescope and the expected performances of such a telescope in space environment
The preliminary design of the SPICA telescope and teh predicted performances which are taking advantage from the Silicone Carbide properties developed for HERSCHEL telescope, especially considering the homogeneity inside the structure its stability from abient to the operational temperature range (4,5K). The study shows that the Silicone Carbide Telescope design can fulfil the mechanical and optical requirements, in a passive way without actuators.
Since ten years ASTRIUM has developed sintered Silicon Carbide (SiC) technology for space applications. Its unique thermo-mechanical properties, associated with its polishing capability, make SiC an ideal material for building ultra-stable lightweight space based telescopes or mirrors. SiC is a cost effective alternative to Beryllium and the ultra-lighweighted ULE. In Complememt to the material manufacturing process, ASTRIUM has developed several assembly techniques (bolting, brazing, bonding) for manufacturing large and complex SiC assemblies. This technology is now perfectly mature and mastered. SiC is baselined for most of the telescopes that are developed by ASTRIUM. SiC has been identified as the most suitable material for manufacturing very large crygenic telescopes. In this paper we present the development of Φ 3.5 m telescope for Herschel Mission. Herschel main goal is to study how the first stars and galaxies were formed and evolved. The Herschel Space telescope, using silicon carbide technology will be the largest space imagery telescope ever launched. The Herschel telescope will weight 300 kg rather than the 1.5 tons required with standard technology. The Herschel telescope is to be delivered in 2005 for a launch planned for 2007.