17:15 〜 18:45
[PPS05-P05] Adaptive optics system of Tohoku 60cm telescope at Haleakala observatory for daytime monitoring of Mercury's sodium exosphere
キーワード：水星、補償光学、昼間
We report on the development of a visible adaptive optics (AO) system 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 through 2028.
Mercury has tenuous surface-bounded exosphere containing H, He, O, Na, Ca, and K. The resonant scattering emission of the Na D-lines (589.0 nm and 589.6 nm) is bright enough to be observable from Earth. The typical time scales for variability expected from the interaction with the solar wind are on the order of a few minutes. Previous ground-based observations have shown that the north-south ratio of brightness in the Na exosphere changes on a time scale of several tens of minutes, which is consistent with the brightness response due to magnetospheric particle sputtering. There are also some global brightness patterns in Na exosphere. However, previous observations using the slit scanning technique took about an hour to get Mercury's global Na brightness distribution. Using a high-resolution spectrograph (R=50,000) equipped with an integral field unit (IFU) installed on the T60, our objective is to obtain 2-dimensional spectroscopic observation with a cadence of 5 minutes.
Mercury's maximum elongation from the Sun is only about 20 degrees. Therefore, using a regular telescope, the observation time after sunset or before sunrise is less than an hour typically. To monitor the Mercury’s exospheric Na brightness through the coordinated observation with the BepiColombo mission, it is necessary to carry out continuous observations not only during short periods after sunset or before sunrise, but also during the daytime when the atmospheric seeing condition is poor (2 to 5” typically). Our visible AO system installed on T60 (T60-AO) aims to stably observe Mercury with a spatial resolution of 1 arc-seconds even during daytime with poor seeing conditions.
T60-AO consists of a 12x12 140-element MEMS deformable mirror (Boston Micromachine) and a Shack-Hartmann wavefront sensor (TIS DMK33UX287 with a microlens array). Field-of-view and number of pupil division for the wavefront sensor are 22” and 14.2, respectively. Closed-loop AO control at 400 Hz was successfully achieved for stars with brightness up to 4.4-mag and for Saturn (visual diameter is 18”) during night-time, as well as for Mercury (visual diameter is 8”) during daytime.
Prior to the actual AO test observation, we measured seeing conditions with the T60 and its AO wavefront sensor at Haleakala Observatory over nine nights from September to October 2023. Fried parameters (R0) at 500 nm are 16.1 cm during night and 9.9 cm during day, typically. According to our AO-loop simulation and observed seeing condition, the AO compensated wavefront error is supposed to 159 nm, achieving the full-width at half-maximum (FWHM) of point-spread function (PSF) < 1.0” even under poor nighttime seeing conditions (probability density > 25%, R0=11.7 cm) and daytime typical seeing conditions (probability density > 50%). Then we conducted AO test observation over five nights from July to October 2023. The result shows FWHMs of PSFs with T60-AO were < 1.0” (0.7” typically) under seeing conditions of 2-3” (2.1” typically).
In addition, we successfully observed Mercury’s Na exosphere during daytime on 8, 9, and 13 December 2023 with T60-AO and the high-resolution spectrograph. The preliminary results indicate that the observed brightness at high-latitude on Mercury changed during 16-data acquisition over 30-minutes period.
Mercury has tenuous surface-bounded exosphere containing H, He, O, Na, Ca, and K. The resonant scattering emission of the Na D-lines (589.0 nm and 589.6 nm) is bright enough to be observable from Earth. The typical time scales for variability expected from the interaction with the solar wind are on the order of a few minutes. Previous ground-based observations have shown that the north-south ratio of brightness in the Na exosphere changes on a time scale of several tens of minutes, which is consistent with the brightness response due to magnetospheric particle sputtering. There are also some global brightness patterns in Na exosphere. However, previous observations using the slit scanning technique took about an hour to get Mercury's global Na brightness distribution. Using a high-resolution spectrograph (R=50,000) equipped with an integral field unit (IFU) installed on the T60, our objective is to obtain 2-dimensional spectroscopic observation with a cadence of 5 minutes.
Mercury's maximum elongation from the Sun is only about 20 degrees. Therefore, using a regular telescope, the observation time after sunset or before sunrise is less than an hour typically. To monitor the Mercury’s exospheric Na brightness through the coordinated observation with the BepiColombo mission, it is necessary to carry out continuous observations not only during short periods after sunset or before sunrise, but also during the daytime when the atmospheric seeing condition is poor (2 to 5” typically). Our visible AO system installed on T60 (T60-AO) aims to stably observe Mercury with a spatial resolution of 1 arc-seconds even during daytime with poor seeing conditions.
T60-AO consists of a 12x12 140-element MEMS deformable mirror (Boston Micromachine) and a Shack-Hartmann wavefront sensor (TIS DMK33UX287 with a microlens array). Field-of-view and number of pupil division for the wavefront sensor are 22” and 14.2, respectively. Closed-loop AO control at 400 Hz was successfully achieved for stars with brightness up to 4.4-mag and for Saturn (visual diameter is 18”) during night-time, as well as for Mercury (visual diameter is 8”) during daytime.
Prior to the actual AO test observation, we measured seeing conditions with the T60 and its AO wavefront sensor at Haleakala Observatory over nine nights from September to October 2023. Fried parameters (R0) at 500 nm are 16.1 cm during night and 9.9 cm during day, typically. According to our AO-loop simulation and observed seeing condition, the AO compensated wavefront error is supposed to 159 nm, achieving the full-width at half-maximum (FWHM) of point-spread function (PSF) < 1.0” even under poor nighttime seeing conditions (probability density > 25%, R0=11.7 cm) and daytime typical seeing conditions (probability density > 50%). Then we conducted AO test observation over five nights from July to October 2023. The result shows FWHMs of PSFs with T60-AO were < 1.0” (0.7” typically) under seeing conditions of 2-3” (2.1” typically).
In addition, we successfully observed Mercury’s Na exosphere during daytime on 8, 9, and 13 December 2023 with T60-AO and the high-resolution spectrograph. The preliminary results indicate that the observed brightness at high-latitude on Mercury changed during 16-data acquisition over 30-minutes period.