Modeling and verification of the diffraction-limited visible light telescope aboard the solar observing satellite HINODE

HINODE, Japanese for "sunrise", is a spacecraft dedicated for observations of the Sun, and was launched in 2006 to study the Sun's magnetic fields and how their explosive energies propagate through the different atmospheric layers. The spacecraft carries the Solar Optical Telescope (SOT), which has a 50 cm diameter clear aperture and provides a continuous series of diffraction-limited visible light images from space. The telescope was developed through international collaboration between Japan and US. In order to achieve the diffraction-limited performance, thermal and structural modeling of the telescope was extensively used in its development phase to predict how the optical performance changes dependent on the thermal condition in orbit. Not only the modeling, we devoted many efforts to verify the optical performance in ground tests before the launch. The verification in the ground tests helped us to find many issues, such as temperature dependent focus shifts, which were not identified only through the thermal-structural modeling. Another critical issue was micro-vibrations induced by internal disturbances of mechanical gyroscopes and momentum wheels for attitude control of the spacecraft. Because the structural modeling was not accurate enough to predict how much the image quality was degraded by the micro-vibrations, we measured their transmission in a spacecraft-level test.