The infrared imaging spectrograph (IRIS) for TMT: spectrograph design

Abstract: The Infra-Red Imaging Spectrograph (IRIS) is one of the three first light instruments for the Thirty Meter Telescope (TMT) and is the only one to directly sample the diffraction limit. The instrument consists of a parallel imager and off-axis Integral Field Spectrograph (IFS) for optimum use of the near infrared (0.84um-2.4um) Adaptive Optics corrected focal surface. We present an overview of the IRIS spectrograph that is designed to probe a range of scientific targets from the dynamics and morphology of high-z galaxies to studying the atmospheres and surfaces of solar system objects, the latter requiring a narrow field and high Strehl performance. The IRIS spectrograph is a hybrid system consisting of two state of the art IFS technologies providing four plate scales (4mas, 9mas, 25mas, 50mas spaxel sizes). We present the design of the unique hybrid system that combines the power of a lenslet spectrograph and image slicer spectrograph in a configuration where major hardware is shared. The result is a powerful yet economical solution to what would otherwise require two separate 30m-class instruments.

• The paper presents a hybrid design integrating lenslet arrays and image slicers to achieve high-resolution, adaptable near-infrared spectroscopy.
• It details precise optical and mechanical strategies, including cryogenic mechanisms and sub-pixel alignment techniques, to optimize spectral imaging.
• The spectrograph is engineered to advance astronomical studies from exoplanet characterization to black hole mass measurement using TMT adaptive optics.

Introduction

The paper "The Infrared Imaging Spectrograph (IRIS) for TMT: Spectrograph Design" (1007.1978) presents the design and implementation details of the IRIS spectrograph, a crucial component of the Thirty Meter Telescope (TMT). As one of the primary instruments, IRIS is engineered to operate at the diffraction limit within the near infrared range (0.84μm-2.4μm), benefiting from the TMT's adaptive optics corrections. The spectrograph is distinguished by its hybrid design, integrating two cutting-edge integral field spectrograph (IFS) technologies—lenslet arrays and image slicers—to deliver optimized performance across various scientific applications.

Scientific Objectives and Requirements

The IRIS spectrograph aims to advance several astronomical investigational fronts. Key objectives include the detailed study of solar system bodies' atmospheres and surfaces, characterization of exoplanets, and discerning the stellar content in galaxies like M31 and M33. It also addresses the challenge of determining black hole masses in distant galaxies, and performing spatial dissections of galaxies during pivotal star formation epochs (z~1-4). These demanding scientific goals impose stringent specifications on the spectrograph, particularly in terms of spatial resolution, field of view (FOV), and spectral sensitivity.

Hybrid Design Approach

The IRIS spectrograph adopts a hybrid design that synergizes lenslet and image slicer technologies. The design capitalizes on the strengths of each technology at different spaxel scales:

• Lenslet Array: Utilized at the fine scales (4mas, 9mas), the lenslet-based spectrograph excels by directly sampling the PSF, providing high image quality and sufficient FOV to capture extensive PSF halos.
• Image Slicer: Employed at the coarse scales (25mas, 50mas), it efficaciously utilizes pixels for bandwidth, a critical parameter at larger scales where lenslets would otherwise waste spaxels on sky projection.

Shared hardware components, such as the cryogenic grating turret, IR detector, and spectrograph camera, enhance the economic viability of the IRIS spectrograph, negating the necessity for separate instruments to achieve similar capabilities.

Optical and Mechanical Design

Integral Field Spectrograph Channels

IRIS facilitates four selectable spaxel scales via its IFS channels, which allow for multifunctional observational modes that include parallel imaging and spectroscopy. The design employs cryogenic stage mechanisms for channel selection, optimized for minimal wavefront errors.

Pre-Optics and Collimators

Shared pre-optics are critically designed with collimators built from BaF2/S-TIH11 and similar materials, optimized for chromatic performance and aligned with the IFS's rigorous imaging requirements. This configuration supports high fidelity in light capture and transmission, essential for benchmarking faint astronomical targets.

Camera and Detector Alignment

Design concepts underpinning the lenslet and slicer collimators, alongside the camera triple mirror anastigmat (TMA), stress precision and alignment robustness. They incorporate off-axis paraboloidal mirrors and multiple aspheric surfaces to mitigate alignment errors. Stability tolerances across components are calculated to ensure sub-pixel accuracy, vital for the IRIS's performance.

Spectrograph Performance

Notable is the IRIS's capacity to facilitate a diverse range of spectral resolutions and bandpasses, tailored through a complex arrangement of filters and gratings. Moreover, the integration of an Atmospheric Dispersion Corrector (ADC) and a precise Lyot stop ensures minimized atmospheric and thermal interferences, bolstering observation fidelity.

Conclusions

The architecting of the IRIS spectrograph reflects a meticulously calibrated blend of modern IFS technologies, leveraging both lenslet and image slicer advantages to accommodate diverse scientific requirements. The hybrid spectrograph design maximizes resource utilization and cost-effectiveness, paving the way for significant advancements in near-infrared astronomical research. This approach enables the IRIS instrument to explore heretofore inaccessible regions of the universe, with implications for both observational and theoretical astrophysics. Future developments in AI and instrumentation technology could further refine these systems, potentially enhancing automated calibration and error-correction methodologies.