[PCG25-11] High-contrast imaging of planetary and satellite atmospheres by the 1.8 m off-axis PLANETS telescope
Keywords:telescope, PLANETS, development, high contrst, planetary atmosphere, satellite
Our major scientific goal is to detect faint emission surrounding planet and satellite in the solar system as well as exoplanets, such as Jovian satellite Europa’s plume and Martian ionosphere. These emissions are so faint (10-3 to 10-6 to the brightness of planetary or satellite disk) close to the main disk (less than a few arcsec) that the measurement of these distributions and time variabilities are difficult. PLANETS is appropriate to observe these targets by taking advantages of high-contrast imaging and spectroscopic capability and monitoring operation optimized for the targets.
The telescope optics has a Gregorian focus with a FOV of 6' (F/13). The main mirror is Clearceram Z-HS with a diameter of 1850 mm and thickness of 100 mm. So far, the glass ceramic blank of main mirror was made by Ohara Inc. in 2010, the rough grinding was carried out by Harris/Excelis in 2012. After that, the main mirror has been stored at the University of Hawaii. Last December, the mirror was shipped to Japan to carry out the final polishing. We glued 36 metal adapters on the backside of mirror to connect the mirror support. The mirror support adopts the whiffletree with warping harness system which is similar to TMT and the Seimei telescope. We made the elemental test of the whiffletree system, and confirmed that the performance for stress input is as expected by the structure model with a finite element method (FEM), and the repeatability (hysteresis) for stress change is in the acceptable range. We are now developing the whiffletree support, and will complete by the end of March 2020. After that, we will carry out the final polishing using a dragging three probe method with a robot-arm system at Logist Lab./Astro-Aerospace within a year. Compared with a traditional CGH-type interferometric metrology, the dragging three probe method and polishing with the robot arm are characterized by the free-form metrology with three-probe. We expect to obtain the accuracy of main mirror better than 20 RMS nm by the final polishing. In addition, we will fabricate the telescope mount and structures using the proto-type mount Seimei telescope now stored at Nagoya University. We will assemble the whole PLANETS telescope, and achieve the first light and technical demonstration, particularly on the high-contrast and low-scattering capability, in Japan within a few years. Further, we already have the construction permit with conservation district use application (CDUA) at the summit of Haleakala, Hawaii, and we plan to install PLANETS there as soon as we get the funding for the observatory construction.