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TMT’s Multi-objective Diffraction-limited High-resolution Infrared Spectrograph (MODHIS) - MODHIS is an Adaptive Optics-fed diffraction-limited, single-mode fiber-fed high-resolution spectrograph that is part of TMT’s suite of first-light instruments. (Left): Concept of MODHIS support structure, rotator, On-Instrument Wavefront Sensor (OIWFS) and Front-End Instrument (FEI). (Right): By studying the shape of spectral lines at high spectral resolution, MODHIS will be used to measure the atmospheric composition, spin, and orbit of exoplanets and will also target other astronomical sources from planets in our solar system, to brown dwarfs, to far-away Active Galactic Nuclei. Image credit: TMT International Observatory

MODHIS Completes First Phase of Conceptual Design Review

Pasadena, CA –  TMT’s Multi-Objective Diffraction-limited High-resolution Infrared Spectrograph (MODHIS), a first-light instrument, has successfully completed the first phase of its conceptual design. MODHIS will take advantage of the large collecting area and TMT’s unprecedented angular resolution to dramatically advance a broad range of science programs, such as the search and characterization of exoplanets, the study of the environment of supermassive black holes at the center of the Milky Way and more distant galaxies, and the composition and dynamics of large solar system bodies.

The MODHIS instrument is a collaboration of the California Institute of Technology (Caltech), University of California, Los Angeles (UCLA), University of California, San Diego (UCSD), NASA Jet Propulsion Laboratory (JPL), and the TMT Project Office in Pasadena. Dimitri Mawet (Caltech), MODHIS’ Principal Investigator; Quinn Konopacky (UCSD), MODHIS’ Project Scientist; and Michael Fitzgerald (UCLA), MODHIS’ co-Principal Investigator, presented the details of the instrument science cases and associated science requirements.

“MODHIS is an infrared high-precision spectrograph. The instrument will undertake studies of exoplanet atmospheres using transit/close-in planet spectroscopy, direct spectroscopy of imaged companions, as well as radial velocity measurements. Operating in the diffraction-limited regime over a 0.98 µm - 2.46 µm wavelength range, TMT uses adaptive optics to efficiently inject starlight directly into single-mode fibers that illuminate a high spectral resolution (R 100,000 spectra), cryogenic, diffraction-limited spectrograph. To maximize performance, the spectrograph uses a compact design combined with high internal stability (<0.5 m/s). This combination will enable unique TMT capabilities for exoplanet and general astrophysics studies,” said Mawet.


MODHIS Key Science Goals and Objectives

One of the top scientific priorities for the upcoming decades is to develop capabilities for exploring other planetary systems and determine if they can support the emergence of life. TMT’s MODHIS will be a powerful instrument capable of studying the composition and physical characteristics of other worlds while searching for subtle biosignatures in their spectra that could indicate the presence of life.

The instrument will be sensitive to a large fraction of the exoplanets that have been detected by transit surveys (including the TESS and Kepler missions), augmenting greatly the range of exoplanet studies that can be enabled by TMT first-light instruments. MODHIS offers four major techniques to detect and characterize exoplanets:

1- Transit Spectroscopy:

Astronomers can study planets as they transit in front of their host stars, by capturing starlight that filters through the planet’s atmosphere. MODHIS transit observations of close-in exoplanets, from hot Jupiter to mini-Neptune, super-Earth and Earth-sized planets, will provide a unique tool to determine the composition of their atmosphere, the nature of high-altitude clouds and hazes, and explore exoplanets diversity.

2- Direct Spectroscopy:

MODHIS will use high-dispersion coronagraphy to precisely characterize the composition of exoplanet atmospheres, measure their orbital velocity and spin, and produce Doppler imaging to map atmospheric and/or surface features. MODHIS will also enable a detailed characterization of the compositional and physical properties of brown dwarfs, astronomical objects intermediate between a giant planet and a star.

3- Nulling Interferometry

For exoplanets and circumstellar disks separated from their host star at – or within – the diffraction limit of TMT, fiber nulling will be used to make their detection and spectroscopic characterization possible. MODHIS will enable studying exoplanets at various key stages in their formation and evolution:

  • newborn (1 Myr) protoplanets residing in nearby star-forming regions to witness planet formation in action;
  • adolescent (~30 Myr) planets that recently finished forming and are still radiating away the signatures of their formation process; and
  • mature (> 1 Gyr) planets.

4- Precision Radial Velocity (PRV)

With a radial velocity precision better than 30cm/s, MODHIS will greatly improve our understanding of exoplanet demographics and the physical parameters that may influence planet formation and evolution, including the measurement of the planet mass and properties of the host stars (mass, radius, temperature and more). Additionally, MODHIS will enable huge improvements in radial velocity measurements for stars in the immediate vicinity of the Galactic center.


MODHIS Design Concepts

The interim Conceptual Design Review introduced the general architectural concept of MODHIS, and addressed aspects of operational flexibility, efficiency and risks. Trade studies, sensitivity calculations and simulated spectra were presented to demonstrate that the adopted design will successfully enable the MODHIS science cases.

The review focused on a description of the Support Structure, Rotator and On-Instrument Wavefront Sensor (SRO) and the Front-End Instrument (FEI). The MODHIS SRO and FEI will be installed on the top port of the TMT facility AO system. NFIRAOS will deliver an adaptive optics corrected field to MODHIS. The MODHIS SRO and FEI will work together to efficiently couple the target light to single mode fibers, which will run down the telescope structure and through the cable wrap to reach the spectrograph and calibration modules placed in a thermally and vibrationally-isolated room in the telescope pier.

MODHIS subsystem location concepts

MODHIS subsystem location concepts - The MODHIS Support Structure, Rotator and On-Instrument Wavefront Sensor (SRO) and Front-End Instrument (FEI) are located on the top port of the TMT facility AO system NFIRAOS. The MODHIS Fiber Delivery System (FIB) transports the light to the Spectrograph (SPEC). SPEC and Calibration system (CAL) will be placed in a TMT lab located in the pier in order to provide a stable environment for measuring Precision Radial Velocities - Image credit: TMT International Observatory

The light delivered by NFIRAOS will pass through MODHIS’ key optical sub-systems:

  • Structure, Rotator and On-Instrument Wavefront Sensor (SRO) provides the mechanical and thermal interface to NFIRAOS, rotates the instrument, and senses tip, tilt and focus when MODHIS is used in Laser Guide Star mode,
  • Front-End Instrument (FEI) relays the light from NFIRAOS and couples that light efficiently into the single mode optical fibers. The FEI includes an Acquisition and Pupil Imaging Camera (APIC) that is essential for this function,
  • Fiber Delivery System (FIB) transports the light and links the FEI to the backend spectrometers SPEC through the telescope and cable wrap (estimated length ~ 110m),
  • Spectrographs (SPEC) composed of two separate spectrographs for the “blue” (0.98-1.33 micron wavelengths) and “red” (1.49-2.46 micron wavelengths). The spectrographs contain no moving parts to maximize their stability and precision,
  • Calibration system (CAL) provides the various calibration signals that can be injected into NFIRAOS, FEI or SPEC directly.
Exploded CAD rendering of MODHIS instrument subsystems on the telescope

Exploded CAD rendering of MODHIS instrument subsystems on the telescope - MODHIS subsystems that are mounted on the top port of NFIRAOS, include the Structure, Rotator, and On-Instrument Wavefront Sensor (SRO) and Front-End Instrument (FEI) devices - Image credit: TMT International Observatory

 The FEI design is extremely compact and enclosed on top of the On-Instrument Wavefront Sensor (OIWFS) design allowing the two subsystems to share a common thermally-controlled environment (cooled to -30°C) with NFIRAOS.

TMT Structure, Rotator and On-Instrument Wavefront Sensor (SRO) mechanical design concept

TMT Structure, Rotator and On-Instrument Wavefront Sensor (SRO) mechanical design concept - SRO is shown on top port of NFIRAOS - Image credit: TMT International Observatory

The supporting structure connects the OIWFS to NFIRAOS via three mounting locations on NFIRAOS that are located three meters away from the optical axis.

TMT MODHIS Spectrometer Mechanical Design

TMT MODHIS Spectrometer Mechanical Design - The Blue Spectrometer and Red Spectrometer are enclosed in Dual cryostats mounted side by side on a common optical bench placed in a lab located in the TMT pier - Image credit: TMT International Observatory

The RED (resp. BLUE) spectroscopy channel is the red (resp. blue) arm of a dual spectrometer configuration, where each spectrometer channel is housed in its own separate cryostat. The red spectrograph covers wavelengths from 1.49µm to 2.46µm, while the blue spectroscopy channel operates over the 0.98µm – 1.33µm range.

The MODHIS spectrograph subsystem design will be largely inherited from the High-resolution Infrared Spectrograph for Exoplanet Characterization (HISPEC) instrument, which is currently being developed for the W.M. Keck Observatory and is scheduled for first light in 2026.

The MODHIS Fiber Management System and Front-End Instrument concepts share some design elements with HISPEC but also include TMT-specific components.


MODHIS Operational Concepts

MODHIS operational concepts were clearly developed for a conceptual design-level instrument, and will be guided by several high-level requirements:

Stability: The spectrograph will be housed in a lab located in the TMT pier, far from the telescope, in a stable undisturbed location, with minimal to no moving parts. The combination of minimal interaction or motion within the spectrograph, and the ability to measure and calibrate any possible changes in the entire system, will ensure a high-level of stability and reproducibility of MODHIS measurements.

Ease of use: All observing interfaces and observing tools, such as the telescope and AO status, the guider software, etc., will be readily accessible to all MODHIS users, regardless of their level of observing experience.

Reliability: MODHIS design and operations will depend on demonstrated reliability and repeatability, minimizing potential hardware and software crashes. To maintain peak reliability and stability, the MODHIS spectrographs will be isolated and minimally disturbed while the front-end components will be designed with the need for few technical adjustments.

Flexibility: While MODHIS will offer fixed resolution and wavelength coverage, the goal is for the front-end system to provide as much configuration flexibility as possible to enable a large range of science cases.

Rapid deployment and configuration: MODHIS will also be designed to allow for rapid changes in configuration, both during and between observations. The time to deploy or re-configure MODHIS will be kept to a minimum. The time needed to start a MODHIS observation will be less than 10 minutes if an instrument switch is involved, and less than 5 minutes between MODHIS observations of any targets in the sky.

Screenshot of the participants of TMT’s MODHIS Conceptual Design review phase 1 held on August 12, 2022

Screenshot of the participants of TMT’s MODHIS Conceptual Design review phase 1 held on August 12, 2022 - Nearly 30 engineers and scientists from Caltech, UCLA, UCSD, JPL and TMTPO attended the remote review and assessed the progress of MODHIS conceptual design - Image credit: TMT International Observatory

David Andersen, TMT’s Science Instrument Group Leader, said “Thank you to the instrument team who performed a lot of work and provided the TMT community with its first detailed look at MODHIS. TMT also appreciates the reviewers who gave of their time to make MODHIS a better instrument going forward. MODHIS will provide unprecedented capabilities to the TMT community, in particular towards the characterization of exoplanets in their habitable zone. MODHIS will start operations soon after the commissioning and science verification of IRIS, both instruments being fed by NFIRAOS and complementing each other perfectly!  

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