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TMT’s Wide-Field Optical Spectrometer Instrument Passes Conceptual Design Review

TMT’s Wide-Field Optical Spectrometer (WFOS), one of its three first-light instruments, successfully completed its Conceptual Design Review in February 2022. This review completed the conceptual design phase and ushered WFOS into its Preliminary Design Phase.

The successful Conceptual Design Review was the result of an international collaboration involving teams from the U.S., China, India, and Japan led by Chuck Steidel (PI; CIT), Reston Nash (Mechanical Lead; CIT), Jason Fucik (Optical Lead; CIT), Eric Peng (PS; PKU), and Davide Lasi (PM; TIO)

WFOS is the largest UV/visible monolithic optical imaging spectrograph ever designed for construction in the field of astronomy. WFOS will provide near-UV, visible and near-IR multi-object and single-slit spectroscopic capabilities and will be the seeing-limited, visible-light workhorse instrument available during the first years of TMT.

WFOS Science Impact

The breadth and depth of the WFOS science cases, its high throughput across the entire optical spectrum (0.31 – 1.0µm), and the wide range of available spectral resolutions and grating configurations will make WFOS a highly compelling workhorse instrument for TMT.

Among the main science programs to be carried out with WFOS are the study of the formation and evolution of galaxies, the origin and properties of stellar populations in nearby galaxies, and the nature of transient astrophysical phenomena. WFOS will also help answer some important questions about the structure and state of the intergalactic medium during early epochs of the Universe, the properties of high-redshift galaxies during the era of peak star formation, the nature of dark energy, and the study of exoplanet atmospheres.

Thanks to its very high throughput and wide spectral coverage coupled with a 30-meter telescope, WFOS will make previously inaccessible phenomena and objects available for highly sensitive spectroscopic studies, opening new windows of investigations and boosting the study of the faint universe at higher spatial, spectral and temporal resolution.

WFOS System Engineering

WFOS requirements were found to be well-defined, comprehensive and unambiguous, leading to a straightforward and powerful design that will not call for extensive technology development. The reviewers praised the brilliant optical design and the impressive amount of mechanical design work, which is well- developed for a conceptual design, for balancing functionality and complexity to meet key performance requirements.

One of the main requirements, which dictates the architecture of the whole instrument, is to cover an entire spectral channel in one exposure at a resolving power of 1500. Higher spectral resolutions up to 15,000 are available over narrower spectral band passes. Such requirements drive the collimator focal length, the beam size and the camera field of view. The review panel found that the flow-down from science requirements to instrument architecture was exceptional, giving the WFOS team confidence that the right direction has been found for the instrument design.

WFOS Mechanical Design

WFOS is a large 8.3m-tall, 5.5m-wide, and 7.5m-long instrument, with a gravity invariant vertical rotation axis. It is composed of 16 main subassemblies, including about 50 motors, and almost all structures, mechanisms and mounts are made of steel. The instrument weighs 42 tons and will operate at ambient temperature.

WFOS key-subsystems are:

- The Static Structure (SSTR) and Rotating Structure (RSTR) that comprise the structure of the instrument;

- The Atmospheric Dispersion Corrector (ADC) and Fold Mirror (M4) that direct the telescope beam vertically, thus making the instrument gravity invariant with respect to field de-rotation;

- The Slitmask Exchanger (SMX) and Slitmask Fabrication Facility;

- The Acquisition, Guiding, and Wavefont Sensors (AGWFS);

- The Collimator (COL), including a dichroic and static folds that split the beam in a Blue and a Red Channel (BCHAN and RCHAN) and present a clear pupil at the dispersers in the spectrograph channels;

- Transmission gratings, a camera barrel, and CCD detector cryostat, as well as mechanisms for the exchange of gratings and filters and the rotation of the cameras within each spectrograph channel; and

-  A dedicated Calibration System (CAL) that provide additional functionality to the core spectrometer.

Key developments since the 2020’s interim review include:

- An improved static structure mechanical design, including an integrated cable wrap, informed by preliminary mechanical analysis;

- A new mechanical design of the ADC, which allows it to independently move both prisms and retract the front prism during TMT’s primary mirror maintenance;

- A new mechanism concept to tip-tilt M4 for pupil centering, and flip it for daytime calibration;

- A simpler, new slitmask exchange system design that optimizes mask replacement time, and a first concept for the mask fabrication facility and its concept of operation;

- A concept of the blue and red cameras and detector cryostat, as well as the detailing of the design of all the spectrograph mechanisms; and

- A detailed trade-study to identify viable future options for CCD science detectors;

- A simpler optical design of the calibration system, now located to the rear of the instrument.

WFOS Optical Design

The WFOS front-end incorporates a linear Atmospheric Dispersion Corrector (ADC) and controlled fold mirror (M4) that direct the beam to the slit mask and guidance system. The ADC is composed of two prisms ~1.4 m in diameter with an apex angle of 7.6° mounted on a translation and rotation drive mechanisms. The prisms have a maximum separation of 2 m where the spacing of the prisms is set by the elevation angle of the telescope. The ADC rotates based on the azimuth position of the telescope. 

The optical design also includes an on-axis two-mirror collimator, and nearly identical blue and red imaging spectrograph channels that enable the simultaneous wavelength coverage from the bluest to the reddest ends of the optical spectrum. For daytime calibration, the output of an integrating sphere can be directed into the instrument by tilting M4.

The WFOS team also presented improvements in the optical design, including updated optical prescriptions for the ADC, detailed performance and tolerance budgets, new designs of the blue and red cameras with fewer elements and optimized for manufacturability, and a new calibration system optical design with a single lens and no moving parts. Small-scale prototyping of Volume Binary Gratings and multi-layer dielectric coatings for the large ADC prisms have also been carried out.

Moreover, significant effort was put into maximizing the throughput of WFOS and technological advances in available glasses, coatings and gratings have been combined to yield an instrument with a total system throughput greater than 50% over most of the bandpass.

WFOS Operational Concepts

WFOS operational concepts were clearly developed and demonstrated an extensive amount of research. WFOS supports three main modes of operation: direct imaging, long-slit (single object) spectroscopy, and multi-slit spectroscopy. The instrument will support a wide range of spectral resolutions ranging from 1,000 to 15,000 via different combinations of gratings and slit widths.

Direct imaging mode uses no slit mask or dispersing element and the instrument re-images the 8.3’ x 3.0’ FOV onto a detector array with a plate scale of 52 mas per pixel. Direct imaging can be used with broadband filters (> 1000 Å) or narrowband filters (< 100 Å).

Long-slit spectroscopy will be used for observations of single targets while multi-slit spectroscopy can acquire up to ~60 targets in a single exposure assuming 8” long slit-lets and 0.3” gaps between slits in the spatial direction. The focal plane layout allows space for the guiding cameras, acquisition cameras, wavefront sensors, and someday, possibly, Laser Guide Star wave front sensors for Ground-Layer Adaptive Optics correction. The patrolling guide cameras are needed for target acquisition and guiding, but are also used for the fine alignment of multi-slit masks and single slit targets.

The review also featured the conceptual design of WFOS software, which will enable remote control of all instrument functions, sensors, and detectors, and will provide access to the status and performance of all subsystems and components. The WFOS instrument control software complies with the overall TMT software architecture, and the team received constructive feedback from the reviewers to improve the definition of the interfaces between the WFOS software and other telescope subsystems.

Comments from WFOS Review Participants

The review board congratulated and acknowledged the WFOS team on the quality of the presentations and documentation delivered, recognizing the excellent work undertaken by the instrument team to complete the conceptual design phase: “The documentation and review material were of the highest quality. WFOS team has done an outstanding work and showed us they are working very well together to pass the Conceptual design milestone. The effort deployed on this design was found very inspiring and the whole team deserves a big round of applause!”

“We are thankful to the reviewers’ engagement and effort to support TMT. Their recommendations will definitely make WFOS a more successful instrument,” said Davide Lasi, WFOS Instrument Project Manager.

“This is a major achievement for TIO’s instruments,” said Dave Andersen who recently took the position of TMT Science Instruments Group Leader. “It has been a great pleasure to begin my work with TMT by sitting in this review and hearing all the fantastic work done by the teams.”