Kotaro Akutsu

Abstract The Hyper Suprime-Cam (HSC) is an 870 megapixel prime focus optical imaging camera for the 8.2 m Subaru telescope. The wide-field corrector delivers sharp images of 0${^{\prime\prime}_{.}}$2 (FWHM) in the HSC-i band over the entire 1${^{\circ}_{.}}$5 diameter field of view. The collimation of the camera with respect to the optical axis of the primary mirror is done with hexapod actuators, the mechanical accuracy of which is a few microns. Analysis of the remaining wavefront error in off-focus stellar images reveals that the collimation of the optical components meets design specifications. While there is a flexure of mechanical components, it also is within the design specification. As a result, the camera achieves its seeing-limited imaging on Maunakea during most of the time; the median seeing over several years of observing is 0${^{\prime\prime}_{.}}$67 (FWHM) in the i band. The sensors use p-channel, fully depleted CCDs of 200 μm thickness (2048 × 4176 15 μm square pixels) and we employ 116 of them to pave the 50 cm diameter focal plane. The minimum interval between exposures is 34 s, including the time to read out arrays, to transfer data to the control computer, and to save them to the hard drive. HSC on Subaru uniquely features a combination of a large aperture, a wide field of view, sharp images and a high sensitivity especially at longer wavelengths, which makes the HSC one of the most powerful observing facilities in the world.

In order to realize straight motion at the highest level of accuracy, a system is introduced in which five degrees of freedom of a moving table; vertical and horizontal positions, and pitch, roll and yaw angles, are controlled precisely. The table is supported by active air bearings composed of PZT actuators and air pads. A PID-PD controller is designed achieving the table motion resolution of 0.02 μm and 0.02 arc sec. Errors of two degrees of freedom; horizontal position and yaw angle, were measured by a straight edge master, profile error of which was eliminated using the reversal method. Absolute accuracies of controlled straight motion were 0.14 μm in horizontal position and 0.14 arc sec in yaw angle over 600 mm travel.

The Thirty Meter Telescope is a next-generation optical/infrared telescope to be constructed on Mauna Kea, Hawaii toward the end of this decade, as an international project. Its 30 m primary mirror consists of 492 off-axis aspheric segmented mirrors. High volume production of hundreds of segments has started in 2013 based on the contract between National Astronomical Observatory of Japan and Canon Inc.. This paper describes the achievements of the high volume production trials. The Stressed Mirror Figuring technique which is established by Keck Telescope engineers is arranged and adopted. To measure the segment surface figure, a novel stitching algorithm is evaluated by experiment. The integration procedure is checked with prototype segment.

The Thirty Meter Telescope (TMT) is a next-generation optical/infrared telescope to be constructed on Mauna Kea, Hawaii toward the end of this decade, as an international project. Its 30 m primary mirror consists of 492 off-axis aspheric segmented mirrors. This paper describes the progress of the test fabrication of an outermost mirror segment for the TMT as a joint R&D program between National Astronomical Observatory and Canon. A zero-expansion glass CLEARCERAM™ blank was polished by a computer-controlled small-tool polishing machine (CSSP, Canon) and its surface shape was measured by a touch-probe measuring machine(A-Ruler, Canon). Residuals of lower Zernike terms of the surface shape were 11 nmRMS, clearing the original specifications based on the structure function. There remains, however, a need to fulfill latest revised specifications. Possible solutions to improve and achieve the new specifications and a plan for revising the process for mass production are also described.

For the mass production of large, severe aspherical optics, we have developed a new stitch algorithm named ARSA (Approximated Reference Shape Algorithm) which can be used for free-form measurement machines. The stitching measurement, which combines plural partial measurements into a whole shape, has been developed based on interferometry and is now widely used. The well known algorithm calculates the stitching parameters so as to minimize the mismatch in the overlapping area. However, with a free-form measurement machine using a scanning probe, the measured data are sparse and contain 3D points, causing significant calculation errors such as interpolation errors in the overlapping area. ARSA is based on minimizing the error of estimating the approximated reference shape. This algorithm has the unique feature that the calculation error is smaller in the case of severer asphericity and the interpolation error is also small for the same reason as in the case of interferometry. We classify the workpiece's shape into three spatial frequency domains and assess the performance for each case by simulation. The simulated measurement data are three sets of 3D points in fan-shaped partial measurements, and the workpiece's shape is a convex hyperboloid whose asphericity is 1.282mm for the full aperture of 1200mm. In all three domains the calculation results coincide with the theoretical values to within nanometer level. These results show the validity of this new algorithm.