NINJA : Subaru Optical-to-NIR spectrographoptimized for LTAO

Chihiro Tokoku, Shinobu Ozaki, Takashi Moriya, Kenshi Yanagisawa, Kentaro Motohara, Masami Ouchi, Nozomu Tominaga,
Masayuki Tanaka (National Astronomical Observatory of Japan), Yoshito Ono, Yosuke Minowa, Yutaka Hayano,Yusei Koyama,
Sadman Ali (Subaru Telescope), Masaomi Tanaka, Masayuki Akiyama (Tohoku University), Tohru Nagao, Yoshiki Matsuoka (Ehime University),
Kosuke Kushibiki, Shogo Homan, and Akino Yasuda (University of Tokyo), and Michitoshi Yoshida ((National Astronomical Observatory of Japan),
720 mm1480 mm

INTRODUCTIONLTAO PERFORMANCES IN NIR

Near-INfrared and optical Joint spectrograph with Adaptive optics (NINJA)

Figure 6 shows a simulated performance of the LTAO mode in NIR at the Subaru telescope
as a function of wavelength. The LTAO mode can realize almost diffraction-limited
resolution in the wide wavelength range from the visible to NIR. The LTAO performancehighly depends on off-axis separation of NGS and decreases with larger separation (Figure5). However, we can achieve FWHM < 0.1" at wavelength > 0.6 μm even with a NGS of 60"
off-axis.

is a new wide-band spectrograph for the Laser Tomography Adaptive Optics (LTAO),
being proposed as a visiting instrument for the Subaru Telescope.
In the ULTIMATE-Subaru project [1], in addition to the key priority function of the wide-field
Ground Layer AO (GLAO), LTAO capability is also planned to be developed to achieve
narrow field observations with near diffraction-limited resolution not only in the NIR bands
but also in the visible (> 0.6 μm). Taking full advantage of LTAO, NINJA realizes a highly
efficient, broadband, medium-resolution spectroscopy for point sources.
In the first phase, a LTAO mode will be implemented to the Subaru as a part of theupgrades of the existing AO system, AO188 [4][5], by adding a new LTAO wavefront
sensor (WFS) system which will be installed into behind AO188 and upgrading of theexisting laser guide star (LGS) system to add four-beam configuration to generate 4 LGSs
for the LTAO mode. Those development for the LTAO mode is being proceeded in theframework of the ULTIMATE-Subaru Tomography Adaptive optics Research experimenT(ULTIMATE-START) project [6][7]. At the same time, Subaru Nasmyth Beam Switcher(SNBS)[8] will also be newly introduced to switch the light downstream of AO188 andLTAO to multiple instruments and to allow multiple instruments work simultaneously. In thesecond phase, when the ULTIMATE- Subaru GLAO system is available and AO188 isdecommissioned, the LTAO system will be upgraded to be compatible with Adaptive
Secondary Mirror (ASM).

Simulation Conditions:- Median condition with 7-layers model- On-axis performance- 1x1 visible NGS WFS※ the LGS focus drift should be monitoredwith a different camera.
- NGS separation = 40″- NGS magnitude = 16 mag in R
- No instrumental PSF is included- Optimal tomographic reconstructor with assuming aperfect turbulence profile information

OPTICAL DESIGN OFNIR SPECTROGRAPH

Figure 1 : Conceptual optical diagram of NINJA, the telescope (M1, M2, and M3 represent the primary
mirror secondary mirror, and tertiary mirror, respectively), and the AO systems in two phases, before
and after installation of the ASM. Solid arrows show the optical paths and dash arrows show thefeedback control lines of the AO system.

Figure 6 : Simulated performance of LTAO (FWHM vs Wavelength, EE vs Wavelength)

The optical layout of the NINJA NIR spectrograph is based on white pupil layout
proposed by Baranne [11] (Figure 3). A grating is used in a quasi-Littrow configuration. A
collimator and cross-dispersing prisms are used in double path. A concave field mirror
transfers a pupil image around the first element of a camera lens system. Efficiency ofthe spectrograph is estimated to be ~43% not including the detector.

Collimator

SlitFigure 5 : Field configuration of LTAO.

DEVELOPMENT PLAN
The first light of NINJA NIR spectrograph is set at early 2025.
NOWCross disperser

SCIENCE OBJECTIVES
Grating

The main scientific objective of NINJA is to reveal origins of elements through observations of kilonovae
as well as supernovae. Especially, through the observations of the kilonova AT2017gfo followed by aConcave field mirror

double neutron star merger discovered by the gravitational wave event GW170817, it has been confirmedthat r-process elements are synthesized in double neutron star mergers [9]. However, AT2017gfo
remains to be the only kilonova identified with gravitational waves. Many more observations of kilonovae
are required to understand the general nucleosynthesis properties of double neutron-star mergers. In

ACKNOWLEDGMENTS

560 mm1260 mm

Cameraorder to estimate how much r-process elements are synthesized in a double neutron star merger, it isDetector

required to conduct spectroscopic observations in the optical and NIR of a kilonova at least for a week
after its merger. Indeed, VLT/X-Shooter spectra of AT2017gfo that covered the optical and NIR
simultaneously played essential roles in revealing its nucleosynthesis signatures [10][11]. During the O5

225 mmof the gravitational wave observatories starting in 2027, about 50 neutron star mergers out to 200 Mpc
are expected to be discovered by gravitational wave events in a year. To conduct spectroscopic
follow-up observations of kilonovae at 200 Mpc for a week after the mergers, we require aspectrograph with a limiting magnitude of 22 mag in the NIR (Figure 2). By reaching 22 mag withNINJA, we can significantly increase the observational constraints on the nucleosynthesis in neutron-starFigure 3 : Optical layout (top) and opt-mechanical design (bottom).
Table 1 : NIR spectrograph parameters

mergers during O5 and obtain general nucleosynthesis properties in neutron star mergers.

Figure 2 : Expected J -band light curves of kilonovae at 130 Mpc and 200 Mpc. In order to observe akilonova at 200 Mpc for one week after its merger, limiting magnitude deeper than 22mag is necessary.

Figure 4 : Simulated Echelle format.
The black solid square shows thedetector area of the HAWAII-2RG.

Figure 7 : Development plan of NINJA NIR spectrograph.

This work was supported by Japan Society for the Promotion of Science (JSPS) KAKENHI, Grant-in-Aidfor Scientific Research (S), Grant Number 21H04997. Part of the development is supported by theAdvanced Technology Center, National astronomical observatory of Japan.

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