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Rendering of the TMT adaptive optics system and MODHIS Front End Instrument. On top of the adaptive optics enclosure (blue box) is the hexagonal shaped MODHIS Front End Instrument where light from exoplanets, stars and other objects being studied is fed into optical fibers. Image credit: TMT International Observatory

Insights from the MODHIS Midterm Conceptual Design Review

The Multi-Objective Diffraction-limited High-resolution Infrared Spectrograph (MODHIS) team recently conducted a midterm conceptual design review at the TMT Project Office in Pasadena on 30 January 2024. Led by Principal Investigator Dimitri Mawet (Caltech), co-Principal Investigator Michael Fitzgerald (UCLA), and Project Scientist Quinn Konopacky (UCSD), the MODHIS team showcased significant updates regarding MODHIS's design and expected performance.

As a pivotal instrument for the Thirty Meter Telescope (TMT) International Observatory (TIO), MODHIS stands as the second TMT science facility to be located behind the state-of-the-art adaptive optics  (AO) system, NFIRAOS. MODHIS uses single mode fibers to channel the AO-corrected light to two spectrographs situated on the Nasmyth platform. These spectrographs will have extremely high spectral resolution and stability, enabling astronomers to explore the composition of planetary atmospheres within and beyond our solar system. Additionally, MODHIS will facilitate precise measurements of extrasolar planet masses based on their orbital dynamics around the host stars. MODHIS will be able to measure motions with an astonishing precision of better than 50 cm/s (a little over 1 mile per hour).

The review included presentations on MODHIS’s performance budgets, notably the detailed precision radial velocity budget, while additional discussions centered around the interfaces between MODHIS, NFIRAOS and the Structure framework, to help ensure optimal functionality and integration. Other discussion included the prospect of integrating a polarimetry mode into MODHIS, presenting unique scientific avenues for exploration.

A few words about Polarimetry:

Stars generally emit unpolarized light, meaning the electric field of the emanating light wave can oscillate in any direction perpendicular to the propagation of the light. However, when light is reflected off a surface, refracted or scattered through an atmosphere or a dusty environment, or affected by a magnetic field, it can become polarized, with its electric field oscillating in a preferential direction.

The type and the degree of polarization of the light can provide valuable insights into the nature of the element(s) involved in its polarization as well as the presence of a magnetic field, offering complementary information to what is obtained from imaging or spectroscopy alone.

Recognizing the potential significance of polarimetric measurements, the MODHIS team is considering incorporating this capability into the instrument. Several science cases supporting the use of polarimetry were presented during the review, including the detection of clouds in exoplanets atmospheres, analysis of exoplanet atmosphere compositions, the search for biosignatures, identification of magnetic fields, searching for exomoons (moons orbiting exoplanets), and the study and characterization of active galactic nuclei and the black holes at their centers.

In preparation for the upcoming final Conceptual Design Review Phase, the team will conduct a feasibility study to assess the possibility of integrating polarimetry into MODHIS.

Polarimetry Proxima Cen

Potential polarimetric science cases for MODHIS. This figure shows the magnetic field around Proxima Cen using HARPS (Klein et al. 2021, MNRAS, 500, 1844) as an example of the work that could be done with MODHIS. Magnetic field structures can be mapped using the polarized light that probes spot geometries, and then correlated with potentially habitable planet locations. Image credit: TMT International Observatory

Dave Andersen (TIO instrument group leader) said “The MODHIS Midterm Conceptual Design Review demonstrated significant strides in advancing this groundbreaking instrument, promising transformative insights into the cosmos and enhancing the capabilities of the TMT Project.” Andersen extended sincere gratitude to all the reviewers who dedicated their time and provided thoughtful inputs during the MODHIS review process. Additionally, Andersen acknowledged the tireless efforts of the entire MODHIS team, whose hard work has been instrumental in advancing the project. Special appreciation is also extended to Hiroshi Terada (TIO) and Larry Lingvay (Caltech) for their exceptional project management skills in orchestrating the coordination and execution of the project's different aspects.


TMT’s MODHIS team and reviewers during the MODHIS Midterm Conceptual Design Review, Pasadena 30 January 2024. Image credit: TMT International Observatory

The MODHIS instrument

The MODHIS instrument. Exploded CAD rendering of subsystems that will mount onto the top port of NFIRAOS, including the Structure, Rotator, and OIWFS (SRO) and Front End Instrument. Image credit: TMT International Observatory

MODHIS Spectrographs and Calibration System Design

MODHIS Spectrographs and Calibration System Design. Light from stars or planets gets fed into fibers at the MODHIS top end. The fibers run to 2 spectrographs located on the Nasmyth platform that cover the entire near-infrared spectrum. Image credit: TMT International Observatory





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