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.