About this ArticleJ. Kruk* (jeffrey.w.kruk[at]nasa.gov) and K. Gilbert** (kgilbert[at]stsci.edu)
Late Breaking News: On May 20, 2020, NASA announced the official name for the Wide Field Infrared Survey Telescope (WFIRST) is now the Nancy Grace Roman Space Telescope, or shortened to the Roman Space Telescope, in honor of Dr. Nancy Grace Roman, NASA’s first Chief Astronomer, who paved the way for space telescopes exploring the broader universe.
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The Wide-Field Infrared Survey Telescope (WFIRST), targeted for launch in 2025–2026, will have a field of view 100 times larger than Hubble's with comparable sensitivity and resolution. WFIRST's Wide Field Instrument (WFI) will provide survey-sized imaging and slitless spectroscopy data sets with space-based resolution, achieving survey speeds hundreds to a thousand times faster than possible with Hubble. WFIRST surveys will enable in-depth investigations in many areas of astrophysics where existing facilities have thus far provided only tantalizing glimpses. WFIRST also includes a technology demonstration Coronagraphic Instrument that will provide orders-of-magnitude improvement in contrast over existing coronagraphs, and push forward our ability to directly image and characterize exoplanets. All WFIRST data will be publicly accessible immediately upon processing, with no proprietary period. The WFIRST Project has been in Formulation since February of 2016, and recently passed its Confirmation Review.
WFIRST Mission milestones
The WFIRST Project recently completed Phase B, which is the Preliminary Design phase of the NASA Project lifecycle. Phases A and B together constitute "Formulation," in which detailed mission requirements, flight element and ground system designs, and project implementation plans are developed. The WFIRST Project at NASA's Goddard Space Flight Center worked closely throughout Phases A and B with Mission partners and stakeholders, including NASA's Jet Propulsion Laboratory (JPL), the Institute (the Science Operations Center), the Infrared Processing and Analysis Center (IPAC; the Science Support Center), and the Formulation Science Working Group.
Phase B concludes with the Mission Preliminary Design Review, and a series of management reviews that culminate in the Confirmation Review. This was passed at the end of February 2020, with direction to proceed with implementation of the Mission as presented to the Agency Program Management Council. The Project is fully funded in FY2020 and is continuing to execute the plan presented at the Confirmation Review, while Congress deliberates on the FY2021 budget.
Primary activities in Phase B included completion of detailed requirements development, preliminary design of the observatory and ground systems, definition of interfaces between all mission elements and subsystems, and development of a detailed project implementation plan. In practice, for significant portions of the observatory, the present design is in fact the final design: fabrication of the flight hardware for the instrument carrier has begun, many flight components of the spacecraft are being procured, flight NIR detectors are being delivered, and refurbishment of inherited telescope hardware is well under way. Re-figuring and polishing of the secondary mirror are complete, and the primary mirror is on schedule to be completed this summer. At that point, reassembly of the inherited portion of the telescope will begin. Engineering Units are being built for components that are new designs. Many of these are already in hand and being tested; the remainder are in various stages of fabrication. Examples of engineering units presently being tested include the WFI filters, prism and grism assemblies, and the new ASIC (application-specific integrated circuit) developed for control and readout of the NIR detectors.
Science-driven requirements for WFIRST
The wide field of view, Hubble-quality imaging, and sensitivity spanning 0.5–2.0 microns make WFIRST uniquely suited for a wide range of astrophysics investigations that cannot be addressed with any other facility. The Observatory performance requirements were defined by the needs of several key projects: studies of the expansion history and growth of structure in the universe by means of weak gravitational lensing, type Ⅰa supernovae, and galaxy spatial/redshift correlation, and by a study of exoplanet demographics by means of gravitational microlensing.
While these projects provided useful scientific guidance for defining mission performance requirements and the capabilities of the data processing system and archive, the actual WFIRST observing programs will not be specified until close to launch in order to be able to optimize the programs based on the scientific landscape at the time. The large survey programs will be designed to address as wide a range of astrophysics as possible, while enabling the objectives of the key projects. The WFIRST Project, in collaboration with mission partners, is exploring means to maximize community participation in the definition of each of these large programs. Substantial time will also be set aside for surveys proposed by members of the community to address investigations that require data not provided by the large surveys. All data will be public with no proprietary period. It is anticipated that the WFIRST archive will provide an unparalleled resource for investigations as yet unknown, hence funding will also be provided for a robust archival research program.
Evolution of WFIRST's design during Formulation
While many design details of the WFIRST observatory have evolved since the Mission Concept Review in December 2015, most of the characteristics of interest to users have not changed. The observatory will be placed in a halo-like orbit about the second Sun-Earth Lagrange point, with a field of regard defined by lines of sight with angles relative to the Sun of 54 degrees to 126 degrees. This means that a little more than half the sky is accessible at any given time, and regions within 36 degrees of the ecliptic poles are accessible continuously. The average daily data volume that can be transmitted to the ground is 11 terabits, up to a maximum of 16 terabits. As the mission is required to survey large areas of the sky, the observatory design is optimized for rapid slewing and settling. A slew of 0.4 degrees, the size of the narrow dimension of the wide-field channel field of view (Figure 1), will take just over 60 seconds including guide star acquisition at the new field. Examples of net survey speed for a variety of observing programs are provided in Akeson et al. 2019 (arXiv:1902.05569). The prime mission duration is five years after commissioning, with propellant sized for ten years of operations. The observatory is designed to be robotically refuellable, so extension of the mission life beyond ten years is possible.
The WFI focal plane consists of 18 4K x 4K near-infrared HgCdTe detectors with a pixel scale of 0.11 arcseconds per pixel; the total active area is 0.281 square degrees—roughly the size of the full Moon. There are seven filters with a total bandpass of 0.48–2.0 microns. The WFI also contains a grism and prism for wide-field slitless spectroscopy. The grism bandpass is 1.0–1.93 microns, which for Halpha-emitting galaxies corresponds to redshifts 0.55–1.9, and the dispersion is designed to provide 0.1% redshift precision. The multiplexing enabled by this wide field-of-view grism is critical for the large surveys envisioned for WFIRST: for a representative wide-area galaxy redshift survey program, there will be roughly 2000 redshifts measured in this range per WFIRST pointing. The prism bandpass is 0.75–1.8 microns, with a resolving power of 80–100 for most of the bandpass, and increasing sharply below 1.0 microns to ~180 at the blue cutoff. The prism parameters were optimized for measurements of type Ⅰa supernovae, but the low dispersion and high throughput of the prism provide a high-sensitivity alternative to the grism for studies that don't require the higher spectral resolution of the grism.
The second instrument on WFIRST is a coronagraph, which will be the first coronagraph in space with active wavefront control. It includes both shaped-pupil and hybrid-Lyot architectures, autonomous high-order and low-order wavefront control, and ultra-low-noise detectors. Downstream from the coronagraph are filters, a selection of slits and prisms for spectroscopy, and polarizers, followed by an electron-multiplying CCD camera operating at visible wavelengths. The coronagraph is a technology demonstration instrument, so there are no formal scientific performance requirements. However, projected performance is that flux ratios between host star and potential planets of 10–8 to 10–9 are achievable, which would enable characterization of giant exoplanet atmospheres and exozodiacal dust and debris disks in reflected light. The present plan is to make the coronagraph available to the community through a Participating Scientist Program. This approach is common for planetary science instruments, but the details of how best to define this for the WFIRST coronagraph are still under discussion.
Find a summary of the imaging and spectroscopic capabilities of the WFI and coronagraph, as well as sensitivity estimates. Optical reference information files for the Phase B design are also available.
Future WFIRST development
The Formulation Science Working Group (FSWG) and the Science Investigation Teams have been an integral part of the requirements development and validation efforts throughout Formulation. In recent months, and continuing into Phase C, the Project and the science teams are working closely with the Institute and IPAC on beginning detailed planning for data processing and analysis. The WFIRST Project and its mission partners plan to engage the broader community in these efforts in the coming years in order to maximize the scientific potential of the WFIRST mission. One step in that process is the formation of a WFIRST Science Input Group (WSIG) that will provide community input to the Project and NASA Headquarters regarding high-level policy questions, and technical input to the Project regarding engineering trade study questions that arise after the present FSWG concludes in 2021. Self-nominations for participation in the WSIG are encouraged; the Project is looking forward to this new opportunity for engaging the broader scientific community. The STScI Director has also convened a WFIRST Advisory Committee (WSTAC), made up of members representing the U.S. and international astronomical communities, to advise him on optimum strategies and priorities for the science program and science operations of WFIRST to maximize the observatory's scientific productivity. Finally, the WFIRST Project anticipates a number of proposal opportunities beginning in 2021 that solicit a range of WFIRST preparatory science programs.
More information on the WFIRST Mission can be found at the sites of NASA and Mission Partners:
*Dr. Jeffrey Kruk at NASA Goddard Space Flight Center is the WFIRST Project Scientist.
**Dr. Karoline Gilbert at STScI is the WFIRST Mission Scientist.