[P-CG21] Planetary Magnetosphere, Ionosphere, and Atmosphere

[PCG21-P04] Development of the near-infrared high-resolution echelle spectrograph ESPRIT for observations of Jupiter's aurora and Io’s volcanic activity

*Ryota Kinoshita¹, Takeshi Sakanoi¹, Masato Kagitani¹, Aya Otomo¹, Yasuhiro Hirahara² (1.Tohoku University, 2.Nagoya University)

Keywords:Io, Jupiter's aurora, echelle grating, spectrograph

Jupiter is the largest giant planet in the solar system, and its plasma originating from Io volcanoes is dominant and accelerated in the co-rotating magnetosphere which causes auroral emission at high-latitudes through ionosphere-thermosphere coupling processes. Therefore, understanding the relationship between Jupiter's aurora, plasma supplied from Io, and plasma transport in the magnetosphere is essential for understanding Jupiter's magnetospheric variability. We are developing a near-infrared high-resolution echelle spectrograph ESPRIT(R~20,000) to be installed on the T60 planetary telescope at the Haleakala summit observatory in Hawaii (3,055 m), and also planned to be installed on the PLANETS 1.8 m off-axis telescope. ESPRIT covers a wide-wavelength range of 1-4µm. Mounting on the T60 telescope (F12), the plate scale is 0.3"/pixel and the slit length is 50". With this long slit and wavelength resolution, the entire Jupiter will be captured to derive the Doppler velocity of the H3+ auroral emission lines simultaneously in the northern and southern hemispheres. The echelle grating is Newport #53*453 (braze angle 71 degrees, groove 31.6/mm, effective size W=130mm x D=23mm). The vacuum chamber consists of the Raytheon InSb 256x256 array detector (30-35 K), collimator and camera mirrors, and several driving mechanisms, such as, a two-stage filter turret (16 positions, including order sorters), a slit exchange, an image-spectral exchange, and a grating angle adjustment mechanism. Only the picomotor of grating angle adjustment mechanism is installed outside the radiation shield because of its specifications, and others are driven by stepping motors in the radiation shield. This grating angle adjustment mechanism can change the angle of the echelle with respect to the incident light from 60 to 75 degrees. The required accuracy for this angular stability is determined by the accuracy of the Doppler velocity measurement (0.5 km RMS) of the observation target (Jupiter's aurora), with an absolute repeatability of less than 0.4 degrees and stability of less than 1" (assuming 5 min exposure time). The stability of the physical length in the push/pull direction in this case is 10 um, which corresponds to a plate scale of 1/10 pixels, or 3 um. A slit viewer camera, a gas lamp for wavelength calibration, and a blackbody for intensity calibration are mounted outside the vacuum chamber. The size of ESPRIT is ~800x500x400 mm and its weighs is about 150 kg. The vacuum cooling test has been conducted by previous developments (Kambara, 2020, Master's thesis), and both the radiation shield (90 K) and the detector box (30 K) have been confirmed to be sufficiently cooled. The detector and driving circuit are the same design as the TOPICS near-infrared camera, of which performance was already verified at the Nayoro telescope of Hokkaido University. In this presentation, we report the test results of the driving mechanisms (filter turrets, slit exchange, image-spectral switching, and grating angle adjustment). First, tests of the mechanical parts were conducted under normal temperature and atmospheric conditions, and all the mechanisms were confirmed to operate normally. In particular, the grating angle adjustment mechanism was confirmed to have an absolute repeatability of less than 0.06 degrees and stability within 2.4 um (lss than a plate scale) for 5 min. After this, a mechanism operation test was conducted under a vacuum cooling test. Next, we confirmed that the other mechanisms worked correctly at room temperature with an positioning error better than 1%. We are now preparing the installation of InSb detector and driving circuit from TOPICS, and we will carry out electrical and imaging tests with calibration light sources. We plan to install this instrument on the Haleakala T60 telescope before Jupiter's opposition in November 2024.