Naruhisa Takato

Subaru Adaptive Optics Program

The system overview and the current status of an adaptive optics system for the Cassegrain focus of Subaru 8.2 m telescope under construction atop Mauna Kea is presented. The system is composed of a wavefront curvature sensor with 36 elements photon-counting APD modules and a 36-element bimorph deformable mirror. We aim to get the Strehl ratio of greater than 0.6 at the K band (2.2 micron) using natural guide stars as wavefront reference under the average seeing condition (approximately 0.45 arcsec) at Mauna Kea. It is scheduled to be in operation in 1998. Expected performance, especially the sky coverage when employing natural guide stars are also presented. currently we are testing prototype system with basically identical specifications as those of the final system. We present here the optical system, deformable mirror, wavefront sensor, control system of the final system, and simple introduction and experimental results of the prototype system.

In-situ measurement of the Subaru Telescope primary mirror reflectivity

The reflectivity of telescope primary mirror is one of the fundamental parameters that shows the telescope performance. However, it has been difficult to obtain absolute value, especially the wide range spectroscopic performance measured in-situ on the primary mirror due to the lack of suitable measuring instrument. To overcome this challenge, we developed a portable spectrophotometer to measure the absolute spectroscopic reflectivity of telescope primary mirror. Its small dimension and light weight enable in-situ measurement on the primary mirror. This spectrophotometer covers the spectral range from 380 nm to 1000 nm with 2 nm resolution. The incident angle to the measuring surface is 12 degrees. The measurement beam size is about 12 mm in diameter. To obtain the absolute value, we adopted the principle of V-N method for the spectrophotometer. A sequential measurement also enables us to cancel the instability of the instrument. The Subaru Telescope primary mirror was recoated with Aluminum on October 20, 2017. It was the eighth coating work from its arrival at Maunakea, Hawaii in 1998 and was about four years from the previous recoating. Before the recoating work, the reflectivity measured with the spectrophotometer was 70~76 % (@400 nm), 75~80 % (@600 nm), and 73~78 % (@800 nm). The large dispersion of the reflectivity is from non-uniform contamination of the surface, especially from the accumulation of dust particles on the mirror. After the fresh coating of Aluminum, the values returned to 92.1 % (@400 nm), 90.5 % (@600 nm), and 85.8 % (@800 nm) with standard deviation less than 0.6 %. There were the data taken at the outside of the vacuum chamber right after the recoating. The great advantage of our spectrophotometer is its capability of getting absolute spectroscopic reflectivity of the primary mirror in-situ. We can continue to monitor the reflectivity of the primary mirror in-situ using this spectrophotometer, even after the primary mirror is mounted on the telescope. This helps us better understanding of the long-term reflectivity degradation.

Minimum Redundant Aperture Masking Interferometry with Tip-Tilt Wavefront Correction

We made aperture masking optical interferometry experiments using up to 30 apertures and with a tip-tilt correction of wavefront error. We examined the performance of minimum redundant configurations of 11-30 sub-apertures on the pupil plane mask. These configurations have two advantages; the redundancy noise is as small as realized in non-redundant masking method, and the uv-coverage is as high as in speckle interferometry enabling to get reconstructed images without mask exchange. We also examined the effect of tip-tilt wavefront correction within a telescope pupil in front of the aperture masking optics. The light coherency between sub-apertures was shown to increase by the correction.

Performance of Japan TI CCD housed in a microminiature refrigerator

This paper describes the structural design and the performance of a CCD camera system developed for use at the prime focus of the 105-cm Schmidt telescope at Kiso Observatory. In this system, the CCD chip is housed in a compact vacuum microchamber holding a Joule-Thomson cryogenic refrigerator, mounted on the plate holder adaptor located at the prime focus of the telescope. A computer controls the CCD driver electronics. Digital image data can be displayed on a CRT and can be saved on a magnetic tape.