irror system. The design team studied a bearing system for the linearly moving mirror which had an expected lifetime of three years, and selected to develop a magnetic suspension/drive system in order to avoid problems caused by lubrication of the mechanism in vacuum.
Since the IMG adopted a pair of moving and fixed flat mirrors for the interferometer, the magnetic suspension system not only had to suspend the 10 cm diameter mirror attached to the armature, but also enable alignment control in the x- and y-directions of the armature moving in the z-direction to maintain accurate parallelism. The magnetic circuit of the moving mirror armature was composed of an inner yoke, a drive coil fixed to the optics bench, inner permanent magnets at the end of the armature, and the outer armature itself. Eight other suspension electromagnets with eight eddy current type detection gap sensors were used to suspend and control the armature alignment.
Figure 21: Schematic view of IMG optical system (image credit: CRIEPI, JAROS)
To satisfy the wavelength accuracy requirement of less than 3 x 10-7 in the ratio at 25 ±5ºC for three years, the newly designed HeNe laser made of a glass block with an ultralow thermal expansion coefficient (less than ± 5 x 10-8/K) was used. The laser beam was expanded and injected into the interferometer, and light interference was detected with four Si detectors positioned circumferentially in order to detect the position and the inclination of the moving mirror.
The IMG used three bands with two PV-type InSb detectors for the shorter wavelength range and one PC-type HgCdTe (MCT) detector for the longer wavelength range to cover the required wavelengths between 3.3 µm and 14 µm. The MCT detector is well known to have a nonlinear relation between the input power and the output signal. The ground test showed that nonlinearity arose when the sensor target temperature was higher than 280 K. This effect was compensated for by a quadratic equation at the processing stage of the interferograms in the ground system. The coefficients of the quadratic equation were defined from the results of ground tests.
The cold components, detector focal plane and the shield attached for eliminating the background radiation from the warm optical system, were placed in the vacuum vessel, and were thermally combined with the cryogenic cooler displacer. The field of view (FOV) was defined by the size of the detectors as 8 km x 8 km at the ground. Figure 22 shows the outline of the detector component.
Figure 22: Outline of the detector component (image credit: CRIEPI, JAROS)
Figure 23 shows the moving mirror alignment and suspension control system diagram. The control system maintained the parallelism of the two mirrors to within 1.9 x 10-4 degrees (0 = peak) throughout the 10 cm length of the moving mirror path. An additional 3 seconds were required for the mirror to return. The alignment control system had double control loops. The basic control loop was controlled by the gap sensors to sustain the initial alignment bias obtained during the startup alignment modes. The second control loop supports the dynamic alignment mechanism that maintains the defined parallelism throughout each observation. The control system was also designed to accept the predefined alignment bias which could be modified by the ground command. This alignment bias defined the offset angle of the moving mirror.
Figure 23: Moving mirror alignment and control system diagram (image credit: CRIEPI, JAROS)
Figure 24: Overview of the IMG FTS optical system (image credit: JAXA)
Figure 25: The operation pattern of the observation/calibration cycle of the IMG in the observation (image credit: CRIEPI, JAROS)
Figure 26: Illustration of one observation cycle and corresponding footprint (image credit: CRIEPI, JAROS)
Operational experience of IMG: The IMG operated continuously day and night, but, was under the basic schedule of 4 days operation and 10 days standby. Before the start of the System Initial Checkout, the launch locks for movable parts of the IMG, such as the trav