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Wide-Field Optical Spectrometer (WFOS)


Field of View: 25.5 (8.3x3) arcmin2

Wavelength range: 0.31 – 1.0 µm (full spectral range @R=1500 in a single exposure)

Spatial Resolution: Seeing limited (GLAO compatible)

Spatial Sampling: 0.05 arcsec per pixel

Total Slit Length: >500 arcseconds (8” slits and 0.5” gap provides for maximum 58 targets)

Spectral Resolution: R = 1500-3500 @0.75" slits, higher resolution possible with narrower slits

Throughput: >25% from 0.31 – 1.0µm, >45% from 0.35 - 0.9 µm, not including the telescope and atmosphere

WFOS Throughput R=1500

Above: Throughput curve for R~1500 and R~3000 (click to enlarge)

WFOS Throughput R=5000

Above: Throughput curve for R~5000 (click to enlarge)


WFOS Signal to Noise Ratio. M(ab)=21 Above: SNR curve for M(ab)=21 (click for full sensitivity information in all modes)

Status (January 2023): WFOS is currently in Prelminary Design Phase.  A Conceptual Design Phase was completed in February 2022.

WFOS IFU Upgrade Investigation


GLAO Upgrade Investigation


 Documents and Tools


WFOS Information Flyer



Principal Investigator
Chuck Steidel (Caltech)

Project Scientist
Eric Peng (NOIRLab)

Project Manager
Alastair Heptonstall

TMT Science Instrumentation Group Lead
Dave Andersen

Wide Field Optical Spectrograph (WFOS): Slit mask based imaging spectrometer
WFOS Render 1

Above: Rendering of WFOS viewed from near the M3 mirror

WFOS will provide highly efficient imaging and multi-slit spectroscopy over the wavelength range 0.31-1.0 µm and a wide field of view of 8.3x3 arcminute, centered at the telescope optical axis.

Using precision cut focal plane slitmasks, WFOS will enable multi-slit observations of ~50 to 80 objects simultaneously, depending on the chosen slit length and gap, limited by the total slit length of 8.3 arcmin. The full 0.3-1.0 µm wavelength range can be captured in a single exposure at R=1500 with a 0.75 arcsec slit, and with two (three) exposures at R=3500 (5000). The spectrograph channels include remote control over the camera articulation (central wavelength) and the grating angle of incidence (diffraction efficiency vs. wavelength). This ensures that the system throughput is always optimized for the science program. 

WFOS Render 2

Above: Rendering of WFOS on the Nasmyth platform

WFOS has a gravity invariant vertical rotation axis to eliminate rotation-dependent flexure. The instrument incorporates a robust structure that not only supports and provide optical stability to the various components of the instrument, but also meets seismic resilience requirements. In addition, it will incorporate an enclosure to provide a light-tight and thermally-stable environment for the optical elements.


WFOS on the Nasmyth Platform

Above: The orientation of WFOS in the Nasmyth platform showing the incoming beam, ADC and M4 fold mirror to bring the beam down the vertical axis.


WFOS main 002


Above (left): Cutaway of WFOS on the Nasmyth platform showing the gravity invarient design of the rotating structure for Red and Blue Spectrograph channels. Above (right): Detailed view of rotating structure with Slitmask, guiding and wavefront sensors, and Red/Blue channels


Multi-slit masks are designed by the user(s), fabricated by the observatory, and then loaded into the instrument prior to observing. Users generate the list of fields and science, wavefront sensing and guiding objects to be observed, objects, calibration sources, filters, resolution modes, and exposure times according to observing plans for specific atmospheric conditions and other variables. Data reduction pipeline modules for quick look and quality control include a full instrument simulator to support instrument design and development of the data processing, archiving and distribution system. Full system configuration, guide star and science target acquisition occurs in less than 5 minutes. Calibrations can be performed during the day thanks to an internal calibration source.

The instrument is designed for use in natural-seeing conditions, but it is upgradable to take advantage of possible future ground layer adaptive optics. Possible future additional functionality include an integral field unit and supplemental gratings.

WFOS optical design

Above: The WFOS optical layout. The right shows the camera positions needed to cover the full wavelength range. At R=1500, the full range in each arm is covered in a single exposure, for R=3500, two exposures are needed in each arm.

WFOS on-axis Field of View

Above: A possible target configuration of 62 targets in the 8.3'x3' WFOS instrument field of view, shown within the full TMT field of view (dashed circle).


WFOS Science

The science cases that motivate the need for WFOS and dictate its design range from solar system programs to the most distant objects in the universe. Some of the most challenging science areas include;

WFOS Science Case - IGM and CGM Tomography

WFOS will have the sensitivity to use more numerous faint galaxies (~200 x the spatial density of quasars) as background sources for absorption forest mapping of the IGM and CGM characteristics. The spatial sampling will mean that there will be multiple sight-lines through foreground galaxy haloes.

Tomography of the high-redshift intergalactic medium: A flagship science case for WFOS is the characterization of circumgalactic and intergalactic material. The spatial density of faint galaxies is much higher than that of quasars and faint galaxies can be used as background sources for high line-of-sight density Lyman alpha forest mapping of the comsic web and for probing the chemistry, temperature, density and kinematics of circumgalactic material.

Rest-frame UV properties of high-redshift galaxies: The internal processes affecting galaxy evolution, such as star formation and AGN activity and the interplay with the local environment are complex topics that require a highly sensitive instrument to advance observational studies into unexplored periods of cosmic history when the universe was very different to what it is today. 

Gamma-ray bursts, supernovae, tidal flares and other transients: Rapid spectrophotometric followup of transient sources, especially those of low brightness, e.g. high redshift supernovae, will revolutionise many areas of astrophysics from Cosmology to Fundamental Physics.

Dark Energy: Deep multi-object spectroscopy of faint galaxies will secure accurate redshifts and spectral energy distributions, allowing accurate calibration of photometric redshifts obtained with 8-m class telescopes such as the Rubin Observatory. This will allow the determination of how galaxy SEDs depend on environment. Also related to observations with Rubin Observatory, weak lensing cosmology will be improved by spectroscopic studies of galaxies to determine their kinematic orientations. For more details, see

WFOS Partners (listed aphabetically)

California Institute of Technology (Caltech)
India TMT Coordination Center (ITCC)
National Astronomical Observatories of Japan (NAOJ)

WFOS Team  (by discipline, listed alphabetically)
Instrument Leads
Chuck Steidel (Principal Investigator, Caltech), Alastair Heptonstall (WFOS Project Manager, TMT), Eric Peng (Project Scientist, NOIRLab), Jason Fucik (Lead Optical Engineer, Caltech), Reston Nash (Lead Mechanical Engineer, Caltech)

WFOS Science Team
Chuck Steidel (Caltech), Eric Peng (NOIRLab), Erica Nelson (Colorado, USA), John O’Meara (Keck, USA), Crystal Martin (UCSB), Khee-Ghan Lee (IPMU, Japan), Norio Narita (Tokyo, Japan), Kimihiko Nakajima (NAOJ, Japan), G.C. Anupama (IIA, India), Vivek M (IIA, India), Roberto Abraham (Toronto, Canada), Ting Li (Toronto, Canada), Michael Balogh (Waterloo, Canada), Evan Kirby (Caltech), Mansi Kasliwal (Caltech),  Karen Meech (Hawaii), Casey Papovich (Texas A&M, GMACS PS)

Institutional Team Leads
Ramya Sethuram (ITCC), Sivarani T. (ITCC), Shinobu Ozaki (NAOJ)

Optical Designers
Bernard Delabre (TMT), Devika Divakar (ITCC), Jason Fucik (Caltech), Shinobu Ozaki (NAOJ), Sriram S. (IIA)

Mechanical Designers
Ajin Prakash (ITCC), Fumihiro Uraguchi (NAOJ), Govinda K.V. (IIA), Hari Mohan Varshney (IIA), Reston Nash (Caltech), Risa Shimizu (NAOJ), Sudharsan Kambhala (ITCC), Viswanatha N. (ITCC), Yutaka Komiyama (NAOJ)

Detector Designers
Roger Smith (Caltech)

Electrical and Software Designers
Kumar T. S. (ARIES), Ramya Sethuram (ITCC), Sivarani T. (IIA), Vaishaly Nigam (IIA)

David Andersen (TMT Science Instruments Group Leader), Alastair Heptonstall (TMT WFOS Project Manager), George Jacoby (TMT Science Instrument Consultant), John Miles (TMT Instrumentation Senior Systems Engineer), Warren Skidmore (System Scientist)