5. Implementation of Satellite

Main events during the implementation of ASTRO-D are already shown. As an example of the efforts in this period, the development of the extendible optical bench is briefly reviewed below.

X-rays are electromagnetic waves with extremely short wavelengths. Total reflection takes place for X-rays only when incident rays graze the reflector surface. Because of the small grazing angle of incident X-rays, the X-ray telescope needs a long focal length to have a large X-ray collecting area. The X-ray telescope on board ASTRO-D has a focal length of 3.5m. The nose fairing of the ISAS launch vehicle, M-3SII, cannot, however, accommodate an optical bench with a 3.5m focal length. In order to achieve the required focal length, we employed an extendible optical bench (EOB) that was folded short during the launch and extended in orbit.

A working team comprising ISAS staff from both science and the engineering sides and engineers from a manufacturer was organized and began to design the basic structure of the optical bench and the extension mechanism in 1987. The optical bench was designed to be constructed by a truss structure composed of CFRP tubes in order to maintain the required accuracy despite temperature changes in orbit. The optical bench was divided into two parts, a moving and a fixed part. The moving part needed to change its position in orbit by means of a sliding mechanism, and to be latched precisely to satisfy the requirement for the focal length to be 3.5 m. Then, the first engineering model was constructed and the basic concept was verified in this year.

Subsequent to the engineering model, the prototype model was constructed in 1988-1989. At this stage, the interface of the EOB with the spacecraft was carefully and inclusively discussed by involving engineers responsible for the spacecraft system and structure in the EOB design meeting. In early 1990, a vibration test was done for the EOB PM model. In spite of the long and careful preparations, a part of the EOB was distorted during the test. It was simply due to a deficiency in the mechanical strength of part of the structure, and was soon reinforced. The EOB model was integrated in the spacecraft mechanical model and another vibration test performed. The mechanical model was also used as the thermal model after some modifications and the thermal design was verified in a thermal-vacuum test in the ISAS space chamber.

The flight model of the EOB was constructed in 1990-1991. The EOB was integrated in the spacecraft flight model in October, 1991, and the extension test performed. The clamping mechanism, however, was kept unreleased. The moving part of the EOB must be tightly clamped to the fixed structure during the vibration phase at the launch but must be released before the extension in orbit. The clamping mechanism was repaired. There were no more problems in the extension test, even in the thermal vacuum test simulating the space environment.

As a result of these long efforts, the EOB was successfully extended to the required focal length in orbit on March 2, 1993, 10 days after the launch.