3:30 PM - 5:00 PM
[PCG19-P10] An adaptive optics system of Tohoku 60cm telescope at Haleakala observatory for daytime monitoring of Mercury's sodium exosphere
Keywords:Mercury, adaptive optics, ground observation
This talk will report on the development of a visible adaptive optics (AO) instrument for Tohoku 60cm telescope (T60) at Haleakala Observatory in Hawaii. The current goal is to join the ground-based support observation for the ESA-JAXA joint Mercury mission BepiColombo on the orbit in 2025~2028.
The summit of Haleakala on the Maui island (height: 3,040 m) is one of the suitable site for monitoring the variable solar-system bodies due to clear sky and good seeing (0.7" in average at λ=500nm). T60 has continuously contributed to the solar-system studies with its remote control capability.
Mercury, one of the targets of this development, has an alkali metal exospheric atmosphere which is observable in sodium line emissions at 589.0 and 589.6 nm. Continuous observations over several tens of minutes to several months is requested for the support to BepiColombo. Since it is an inner planet with a maximum separation angle of only about 20deg (diameter: 5-12"), observable time after sunset or before sunrise is limited up to about an hour. Daytime observation is mandatory of monitoring observation of Mercury’s sodium atmosphere though, the sunlight heating of the ground causes large atmospheric turbulence, which degrades the seeing to ~2". Therefore, adaptive optics is essential to compensate for such degraded seeing.
Our visible AO system for T60 consists of a 140-element MEMS deformable mirror (Boston Micromachine) and a Shack-Hartmann wavefront sensor (TIS DMK33UX287 and Thorlabs MLA150-7AR). The small aperture of T60 allows the AO to achieve its diffraction limit (0.21") in visible band: The size of each AO element is 6 cm on the primary mirror, which is large enough to cover the size of atmospheric fluctuations at Haleakala (15 cm at night and 8 cm during the day at λ= 500 nm). The test system has been installed at the Cassegrain focus of T60 in March 2022, and we developed and evaluated the AO control software by remote control. There are two methods for wavefront compensation calculation: (1) Singular value decomposition of the calibration matrix with appropriate censoring accuracy, and (2) mode-specific control by expanding the wavefront into Zernike modes. In (1), stable closed-loop AO control at 300 Hz was achieved for natural stars up to 3.5 mag. In a nighttime test observation of a 2.5-mag star, an FWHM of 0.69" was achieved under natural seeing of 3.2“.
In February 2023, we visited the Hawaii site again for refurbishment and testing to realize the followings: (1) To install a viewing aperture in front of the wavefront sensor for stable wavefront sensing during daytime observations, in order to improve the contrast between the bright background and the stars; (2) Conversion of the pitch of the microlens array in the wavefront sensor from 150μm to 300μm, in order to increase the amount of available light for target objects fainter than 3.5 mag (Mercury: -1.4 to 5 mag); (3) To compare the Zernike mode expansion control (more suitable for high-speed and stable control) with the singular value decomposition method. In this presentation, we will report the status, results of evaluation tests, and future plans. We also aim to apply the system for the Galilean satellites with about 5 mag. Therefore, the goal is to make AO which works effectively for targets with brightness less than them. Observations of these Jovian system targets will be linked with the ESA Jovian mission JUICE (on Jovian orbit:2031~2037). This development will also be applied to the 1.8-m off-axis telescope PLANETS, which is under development for the deployment at the summit of Haleakala. It will enable disk-resolved observations of objects with a visual diameter of about 1".
The summit of Haleakala on the Maui island (height: 3,040 m) is one of the suitable site for monitoring the variable solar-system bodies due to clear sky and good seeing (0.7" in average at λ=500nm). T60 has continuously contributed to the solar-system studies with its remote control capability.
Mercury, one of the targets of this development, has an alkali metal exospheric atmosphere which is observable in sodium line emissions at 589.0 and 589.6 nm. Continuous observations over several tens of minutes to several months is requested for the support to BepiColombo. Since it is an inner planet with a maximum separation angle of only about 20deg (diameter: 5-12"), observable time after sunset or before sunrise is limited up to about an hour. Daytime observation is mandatory of monitoring observation of Mercury’s sodium atmosphere though, the sunlight heating of the ground causes large atmospheric turbulence, which degrades the seeing to ~2". Therefore, adaptive optics is essential to compensate for such degraded seeing.
Our visible AO system for T60 consists of a 140-element MEMS deformable mirror (Boston Micromachine) and a Shack-Hartmann wavefront sensor (TIS DMK33UX287 and Thorlabs MLA150-7AR). The small aperture of T60 allows the AO to achieve its diffraction limit (0.21") in visible band: The size of each AO element is 6 cm on the primary mirror, which is large enough to cover the size of atmospheric fluctuations at Haleakala (15 cm at night and 8 cm during the day at λ= 500 nm). The test system has been installed at the Cassegrain focus of T60 in March 2022, and we developed and evaluated the AO control software by remote control. There are two methods for wavefront compensation calculation: (1) Singular value decomposition of the calibration matrix with appropriate censoring accuracy, and (2) mode-specific control by expanding the wavefront into Zernike modes. In (1), stable closed-loop AO control at 300 Hz was achieved for natural stars up to 3.5 mag. In a nighttime test observation of a 2.5-mag star, an FWHM of 0.69" was achieved under natural seeing of 3.2“.
In February 2023, we visited the Hawaii site again for refurbishment and testing to realize the followings: (1) To install a viewing aperture in front of the wavefront sensor for stable wavefront sensing during daytime observations, in order to improve the contrast between the bright background and the stars; (2) Conversion of the pitch of the microlens array in the wavefront sensor from 150μm to 300μm, in order to increase the amount of available light for target objects fainter than 3.5 mag (Mercury: -1.4 to 5 mag); (3) To compare the Zernike mode expansion control (more suitable for high-speed and stable control) with the singular value decomposition method. In this presentation, we will report the status, results of evaluation tests, and future plans. We also aim to apply the system for the Galilean satellites with about 5 mag. Therefore, the goal is to make AO which works effectively for targets with brightness less than them. Observations of these Jovian system targets will be linked with the ESA Jovian mission JUICE (on Jovian orbit:2031~2037). This development will also be applied to the 1.8-m off-axis telescope PLANETS, which is under development for the deployment at the summit of Haleakala. It will enable disk-resolved observations of objects with a visual diameter of about 1".