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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

Spatial Sampling: 0.05 arcsec per pixel

Image quality:

Imaging mode - <0.45 arcsec FWHM (including contributions from the ADC at z = 60°

Spectroscopic modes -  0.25 arcsec >80% encircled energy at every wavelength

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, >30% from 0.35 - 0.9 µm, not including the telescope

Sensitivity: texp=5x900s, S/N=150 per element, R=3500 V=20.5

Status (August 2021): Conducting conceptual Design Phase including refreshment of Science Requirements . Successful interim CoDR June 2020, planned final CoD review Q4 2021. Updated designs for cameras, ADC system, slit-mask exchanger and mask manufacturing facility, static instrument structure, collimator, coatings and calibration unit.

WFOS IFU Investigation


 Documents and Tools


WFOS Information Flyer



Principal Investigator
Chuck Steidel (Caltech)

Project Scientist
Eric Peng (KIAA-PKU)

Project Manager
Davide Lasi

TMT Science Instrumentation Group Lead
Eric Chisholm 


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

WFOS will provide near-ultraviolet and optical (0.31 – 1.0 μm) imaging and spectroscopy over a 8.3x3 arcminute field-of-view. Using precision cut focal plane masks, WFOS will enable short-slit observations of ~50 to 80 objects simultaneously (depending on the chosen slit length and gap, limited by the total slit length which is required to be >500 arcsec). WFOS will be able to be used in natural seeing. The full wavelength range can be captured in a single exposure at R=1500 with a 0.75 arcsec slit. Between 1 and 4 separate observations using articulating cameras will be needed to cover the full wavelength range, depending on the desired resolution. 

WFOS has a gravity invariant vertical rotation axis to minimize instrument flexure and will incorporate a robust structure to support the various components of the instrument. In addition, it will incorporate an enclosure to protect its components and provide a light tight environment for the optical elements. Possible upgrades and additional functionality include an integral field unit, image slicing spectroscopy and options for gratings if funding allows.

WFOS opto-mechanical layout

Above: WFOS opto-mechanical layout showing the vertical axis and large slewing bearing, and location of the slit masks.

WFOS on the Nasmyth Platform

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

Multi-slitmasks 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. Observer/operator Graphical User Interfaces are developed according to TMT standards for all instruments and subsystems. 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.

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 Team 

California Institute of Technology:
Chuck Steidel - (Principal Investigator)
Reston Nash - (Lead Mechanical Engineer)
Jason Fucik - (Lead Optical Engineer)
Stephen Kaye - (Electrical/Detector Engineer)
Roger Smith - (Electrical/Detector Engineer)

Indian Institute of Astrophysics:
Govinda KV - (Senior Mechanical Engineer)
Viswanatha N - (Senior Mechanical Engineer)
Sudarsan K - (Mechanical Engineer)
Hari Mohan Varshney - (Mechanical Engineer)
Devika Divakar - (Optical design analysis & ZEMAX simulations)
Amirul Hasan (Optical Designer)
Ramya Sethuram - (TMT India WFOS Project Scientist)
Sivarani Thirupathi (Astronomer)

National Astronomical Observatory of Japan:
Yutaka Komiyama - (Mechanical Engineer)
Fumihiro Uraguchi - (Mechanical Engineer)
Satoshi Miyazaki - (Astronomer/Optical Engineer)

European Southern Observatory:
Bernard Delabre - (Optical Engineer)

Hangxin Ji - (Optical Engineer - NIAOT)
Eric Peng - (WFOS Project Scientist - KIAA-PKU)

Thirty Meter Telescope Organization:
Davide Lasi - (Project Manager)
John Miles - (Lead Systems Engineer)
Eric Chisholm - (Instrument Technical Manager)