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WFIRST Mission Update

Jeffrey Kruk, jeffrey.w.kruk [at] nasa.gov

Abstract

The Wide-Field InfraRed Survey Telescope (WFIRST) is designed to have a field of view 100 times larger than Hubble's with comparable sensitivity and resolution, providing survey-sized data sets with space-based resolution.  WFIRST's surveys will explore the nature of dark energy and discover and characterize exoplanets, and will revolutionize a large number of topics in general astrophysics. The WFIRST Project has been in Formulation since February of 2016, and recently completed a major mission review. 

WFIRST field of view
Figure 1: The field of view of WFIRST's Wide-Field Instrument (WFI), overlaid on a mosaic of the Andromeda galaxy.  The mosiac was generated from over 400 separate pointings of the Hubble Space Telescope, obtained by the Panchromatic Hubble Andromeda Treasury (PHAT), a MultiCycle Treasury program (PI Dalcanton).  The 18 4k x 4k near-IR detectors of the WFI will be able to image the same area in only 2 pointings.

 

WFIRST Mission milestones

The Wide-Field InfraRed Survey Telescope (WFIRST) Project is nearing completion of Phase A. Phase A is the first half of the Formulation phase of the NASA Project lifecycle, in which mission requirements, conceptual designs, and a preliminary project implementation plan are developed, and new technologies are brought to a level of maturity suitable for flight.

WFIRST formally entered Phase A in February 2016 after a successful Mission Concept Review in December 2015.  The WFIRST Project at NASA's Goddard Space Flight Center worked closely throughout Phase A with Mission partners and stakeholders, including NASA's Jet Propulsion Laboratory (JPL), the Institute, the Infrared Processing and Analysis Center (IPAC), and the Formulation Science Working Group.

Phase A culminates in a combined System Requirements Review (SRR) and Mission Definition Review (MDR), which assess whether or not the mission requirements are necessary and sufficient to meet the Objectives, and that the mission design implements the requirements within the available cost and schedule resources. This review was successfully completed in February 2018, and the final decision on initiation of Phase B is expected in mid April. The WFIRST Project is continuing to execute the FY18 plan while Congress deliberates on the FY19 budget.

Primary activities in Phase B include 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. Any remaining technology development must also be concluded.  In practice, considerable progress was made on the designs of the optics and supporting structure and thermal control systems, and for some of the electronics. Those designs will continue to mature in Phase B as efforts on the other aspects of the design ramp up. 

Science-driven requirements for WFIRST

The wide field of view (Figure 1), 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 the spatial-redshift correlation of galaxies, and a study of exoplanet demographics by means of gravitational microlensing. 

While these key projects provided useful scientific guidance for requirement definition, the actual WFIRST observing programs will be 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 programs will also be designed to address as wide a range of astrophysics as possible. We are exploring means to maximize community participation in 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 core surveys. All observing time will be awarded competitively, and 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

WFIRST Orbit
Figure 2. The halo-like orbit of WFIRST about the second Sun-Earth Lagrange point is shown in blue.

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 (Figure 2), 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. 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, will take just over 60 seconds including guide star acquisition at the new field. The prime mission duration is five years after commissioning, with propellant sized for ten years of operations. The observatory is designed to be serviceable, so extension of the mission life beyond ten years would be possible.

During Phase A, multiple studies were conducted exploring the impact of design choices on the observatory and the science potential of WFIRST data. One of the trade studies conducted during Phase A was optimization of the telescope temperature. The warmest components of the telescope are now designed for a temperature under 270K, as opposed to the 284K temperature at MCR, resulting in a significant decrease in thermal background at long wavelengths. For limiting magnitudes typical of the planned high-latitude galaxy survey, the reduction in exposure time is roughly a factor of two in the reddest filter, and more for fainter sources.

The Wide-Field instrument (WFI) comprises the Wide-Field Channel (WFC) and Integral-Field Channel (IFC). The WFC 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. The main change since MCR was the addition of a filter spanning 0.48–0.76 microns, so there are now seven filters with a total bandpass of 0.48–2.0 microns. The extension of the bandpass towards short wavelengths greatly enhances the capabilities of WFIRST for studies of stellar populations, provides an additional means of testing for astrophysical systematic effects in Type Ia supernovae studies, and generally increases flexibility in responding to future opportunities. The grism bandpass is presently 1.0–1.93 microns, which for Hα-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.  Further optimization of the filter bandpasses and of the grism dispersion will be assessed in Phase B, but large changes are not anticipated. 

One big change in the WFC design since MCR was cooling of the focal plane passively instead of by means of a cryocooler. This was enabled by a complete overhaul in the layout of the optics that moved the focal plane much closer to the exterior surface of the observatory. Passive cooling of the focal plane reduces development risk for the instrument, but also increases the likelihood of extending the operating lifetime well in excess of the five-year design requirement.

The Integral Field Channel employs two image slicers that feed a common spectrometer and focal plane. The image slicers provide fields of view that are 9 square arcsec and 36 square arcsec in size, with spatial sampling of 0.15" and 0.3", respectively. The bandpass is 0.42–2.0 microns, with spectral resolution varying from 75 to 150. The IFC has been optimized for spatially resolved spectroscopy of faint sources in general, and Type Ia supernovae in particular. The spatial sampling provided by the image slicer enables more accurate subtraction of the host galaxy light from that of the supernova and greatly reduces the zodiacal light background in comparison to the slitless spectra provided by the grism. The resulting spectra will be used for typing and sub-typing of supernovae, measuring their redshifts, and measuring their spectral energy distributions for determination of the reddening caused by dust in the host galaxies. The IFC is slated to be contributed by one of the international partners; details of the IFC design will be established when the partnership is finalized.

The second major 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 an imaging camera and an integral field spectrograph, both 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 10-8–10-9 will be  achievable, which would enable characterization of giant exoplanet atmospheres and exozodiacal dust and debris disks. 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. 

Future WFIRST development

The Formulation Science Working Group and the Science Investigation Teams have been an integral part of the requirements development and validation efforts throughout Phase A. As we move into Phase B, the Project and the science teams will be working closely with the Institute and IPAC as increasing emphasis is given to approaches to data processing and analysis, and towards means of optimizing survey definition and observing policies in general. We plan to engage the broader community in these efforts in order to maximize the scientific potential of the WFIRST mission. We anticipate a competition in 2021 that solicits a follow-on Science Working Group that would design the observational program.

Jeffrey Kruk is the WFIRST Project Scientist at NASA's Goddard Space Flight Center.