Tmt comp background

Wide-Field Optical Spectrometer (WFOS)


Status (September 2018): Downselecting between different Conceptual Designs

A downselection process is being followed to identify either a slit-mask based instrument concept of a fiber based instrument concept for continued development as a TMT First Light Instrument. As a result, all previous design requirements for WFOS below are subject to change.

This page will be updated when the decision to downselect one instrument architecture has been made (expected late October 2018).

Science with WFOS
  • Tomography of the high-redshift intergalactic medium
  • Gamma-ray bursts, supernovae, tidal flares and other transients
  • Rest-frame UV properties of high-redshift galaxies
Documents & Tools

WFOS project webpage


Principal Investigator
Kevin Bundy (UCSC)

Original Design Requirements (prior to downselect)

Field of View
40.5 arcmin2 (need not be continuous)

Wavelength Range
0.31 – 1.0 µm

Spatial Resolution 
Seeing limited (GLAO ready)

Image quality: imaging
0.2 arcsec FWHM over any 0.1µm wavelength interval (including contributions from the telescope and the ADC at z = 60°

Image quality: spectroscopy 0.2 arcsec FWHM at every wavelength

Total Slit Length
500 arcseconds

Spatial Sampling
0.15 arcsec per pixel, goal < 0.1 arcsec

Spectral Resolution
R = 500-5000 for a 0.75 arcsec slit, 150-7500 (goal)

30% from 0.31 – 1.0µm, or at least as good as that of the best existing spectrometers

Spectra should be photon noise limited for all exposure times > 60 sec. Background subtraction systematics must be negligible compared to photon noise for total exposure times as long as 100 Ksec. Nod and shuffle capability in the detectors may be desirable

Wavelength Stability
Flexure at a level of less than 0.15 arcsec at the detector is required

Xchange-WFOS: Slit mask based imaging spectrometer

The Xchange-WFOS will provide near-ultraviolet and optical (0.31 – 1.0 μm) imaging and spectroscopy over a 20 to 25 square arcminute field-of-view. Using precision cut focal plane masks, WFOS will enable short-slit observations of ~50 to 60 objects simultaneously. Xchange-WFOS will be able to be used in natural seeing or with GLAO correction. Between 1 and 4 seperate observations using articulating cameras will be needed to cover the full wavelength range, depending on the desired resolution.

Xchange-WFOS will incorporate a robust structure to support the various components of the instrument and minimize instrument flexure. 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, increased field of view and options for gratings if funding allows.

Xchange-WFOS Field of View

Xchange-WFOS Field of View

Xchange-WFOS instrument concept and camera optical concept

Xchange-WFOS instrument concept and camera optical concept


Fiber-WFOS: Deployable fiber fed multi-object spectrometer

Fiber-WFOS will provide simultaneous coverage from 0.31 – 1.0 μm at a fixed resolution of R~3500 for 126 sky-target fiber-bundle pairs (using 3 spectrographs) over a 50 square arcminute field. Each fiber will be 0.33" on sky and arranged in bundles of 7 fibers (see below). Fiber-bundles will be mounted on dual-axis zonal positioners with a patrol fields of ~22" and 100% overlap.

A near-ultraviolet/optical imaging channel will provided via one of the guide cameras with a 1'x1' FOV and 0.015"/pixel.

Options to upgrade to 6 spectrographs with R~3500 or 4 with R~3500 plus one with R~15,000 are being considered. Further upgrades up to 9 spectrographs are possible, dependent on funding. Starbug fiber positioners are being considered as a possible design option.


Fiber-WFOS Field of View

Fiber-WFOS Field of View with 3 spectrographs

Fiber-WFOS instrument concept

Fiber-WFOS instrument concept showing fully populated 9 spectrograph bays and 7-fiber fiber bundles with 1" width