1. Wavelength coverage: 0.95-2.4 μm.
2. Temperature stability: 1 mK.
3. Spectral resolution: R≃100,000.
4. Doppler velocimetry precision: 30 cm/s.
5. Throughput: ~10% (including Strehl ratio).
6. Spatial sampling: diffraction limit (6 mas in y, 15 mas in K), sampled by single-mode fibers.
7. Field of view (patrol) for MOS: 4".
MODHIS is a diffraction-limited high-resolution infrared facility for TMT-NFIRAOS. The notional concept features a very compact and stable spectrograph and cost-effective design built to fully exploit the existing NFIRAOS infrastructure and boost the scientific reach of TMT after first light. MODHIS is based on the latest diffraction-limited single-mode fiber injection, detector, multiplexing, and calibration (Laser Frequency Comb) technologies, and will provide unprecedented capabilities to the TMT community, potentially soon after the commissioning and science verification of NFIRAOS+IRIS, which it perfectly complements. MODHIS will take R ≃100,000 spectra from 0.95 to 2.4 μm simultaneously, from anywhere in a r=4” field of view sampled at the diffraction limit.
Fig. 1: HISPEC/MODHIS Front-End Instrument (FEI) notional concept.
Fig. 2: Compact, diffraction-limited (single-mode) fiber injection unit and pyramid wavefront sensor unit of the Keck Planet Imager and Characterizer linking the Keck II adaptive optics system and NIRSPEC.
NFIRAOS will provide a high quality (Strehl ratio greater than 50% at J, H and K), high throughput, low background beam to three output ports, one of which is currently not allocated (Fig. 1). MODHIS will use the free output science port to install a fiber injection unit (FIU) similar to those recently installed, at the Subaru and Keck telescopes (Fig. 2). Single-mode fiber technology transfers the light from the science focal plane to the spectrograph with little loss. The fiber output is reconfigured in a permanent pseudo entrance slit. In essence, the advantages of diffraction limited single-mode fiber-feeding the spectrograph are:
High dispersion, i.e. R ≃ 100,000 is provided by a red/blue two arm Echelle type design (Fig. 3). The diffraction-limited input greatly simplifies the design and procurement of dispersive elements. Our design fits in a small suitcase (Fig. 4), and uses gratings available off-the-shelf. At the core of each spectrograph arm is a H4RG detector, whose large format is sufficient to satisfy our wavelength coverage and multiplexing requirements.
Fig. 3: The MODHIS red-arm opto-mechanical layout incorporating a TMA collimator, reflection grating, cross-disperser and a TMA camera giving a fixed spectral format on the detector array in an overall compact design form. The blue-arm has the same components and is almost identical.
Fig. 4: The MODHIS spectrograph conceptual mechanical layout showing the two spectrographs (red arm and blue arm) on the optical bench.
We recently built a fiber injection unit to link the Keck II Tele- scope adaptive optics bench to NIRSPEC, the current R ≃ 37,500 workhorse infrared spectrograph of the Keck Telescope (KPIC; Mawet et al., 2017). The target light is injected into a single-mode fiber and sent with minimal losses to NIRSPEC. The first assembly of the FIU (Fig. 2) focuses starlight from the AO system into single-mode fibers while the second formats the outputs of the fibers into the slit plane of NIRSPEC. MODHIS will leverage this recent development in fiber feeds. Integral field and multi-object capabilities could be provided by image slicing techniques and independent field steering mechanisms per slice.
The critical step in achieving high precision wavelength solution is the simultaneous observation of a laser frequency comb (LFC). This technology has now been demonstrated at the IRTF, Keck and Subaru observatories down to ≃ 0.3 m/s long-term precision (Yi et al., 2016). The laser comb stamps each stellar spectrum with a highly uniform and precise wavelength ruler against which to measure the small shifts due to the Doppler motions. A future upgrade to an octave-spanning f-2f comb will drop the long-term Doppler precision to < 1 cm/s. Finally, single-mode spectrometers present an efficient way to inject a comb, which itself is single mode. For wavelengths where LFC technology is not yet available, we will use a Fabry-Perot Etalon.
Potential exoplanet discovery and characterization space for TMT-MODHIS, simulated planet populations within 27 pc (Kopparapu et al. 2018). Diagonal line is for 20 cm/s RV precision. Red markers; giant planets > 6 RE (Earth radius), orange; Neptunes (6 > RE > 3.5), yellow; mini-Neptunes (3.5 > RE > 1.75), dark green; hot super-Earth and Earth-size planets (1.75 > RE > 0.5), and light green; temperate super-Earth and Earth-size planets.
Transit spectroscopy. The RV technique has pioneered the field of exoplanets, but is currently reaching a bottleneck at ≃ 0.5 m/s due to our limited understanding of the influence of stellar activity and variability at optical wavelengths. Moreover, most late-type stars, due to their red colors, are currently out of reach of optical RV powerhouses such as HARPS and Keck/HIRES, and will continue to be so for next-generation instruments such as VLT/EXPRESSO and Keck/KPF. Infrared RV will provide wavelength leverage on both issues simultaneously. Taking full advantage of its innovative diffraction-limited design, MODHIS will be designed to reach sub-m/s (requirement: 30 cm/s, goal: 10cm/s) velocity precision. Joint operation with Keck/KPF and other contemporaneous optical RV spectrographs will provide unique opportunities to better model and remove spurious stellar noise sources.
Direct spectroscopy. Exoplanet searches at very small angles using high dispersion coronagraphy with simple coronagraphs. Molecule mapping in directly imaged exoplanets, exoplanet rotation measurements (length of day), exoplanet radial velocities, Doppler mapping (cloud dynamics, global circulation, winds, weather).
Precision RV. Masses, density and bulk composition of small transiting planets (Kepler, K2, Gaia, TESS, CHEOPS and PLATO). Planets searches for very cool stars, young stars and binary stars.
Hydrogen emission line profiles probing disk accretion physics. Atomic line widths indicating age. Zeeman splitting measuring stellar magnetic fields. Molecular emission lines (water, CO) revealing disk kinematics, abundances, and temperature structure.
Composition of planets, moon, small bodies (e.g. comets, asteroids, KBOs). Isotopes as key markers of physical markers of physical formation processes in our solar system.
Stellar convection, differential rotation, stellar magnetic activity, mass loss. Chemical enrichment in Galaxy disk and bulge. Sub-giants and giants in the outer Galactic halo and in the neighboring dwarf galaxies. Chemical mapping to probe the origin and formation history of ancient stars.
Population III stars, reionization, intergalactic medium, massive galaxies evolution, supermassive black holes.
Extreme Precision RV (≲ 10 cm/s) over multi-decade baselines with Laser Frequency Comb can potentially probe cosmological constants and the Hubble expansion in real time (Sandage-Loeb test).
