The WFIRST Design Reference Mission (DRM) uses existing 2.4-meter telescope hardware, along with heritage instrument, spacecraft, and ground system architectures and hardware to meet the WFIRST science requirements. The current WFIRST observatory will be flying at the second Sun-Earth Lagrange point (SEL2). The orbit isn't a Halo, as it doesn't pass through the exact same points every 6 months. It is evolving and opening in a quasi-periodic fashion, and hence is called a Quasi-Halo.
The payload features a telescope with a 2.4-meter aperture and on-axis secondary mirror, which feeds two different instrument volumes containing the wide field and coronagraph instruments. The telescope hardware was built by Harris Corporation under contract to another agency and is being made available to NASA. The telescope is a space flight qualified 2.4-meter, obscured two-mirror system. Repurposing modifications will include conversion to a three-mirror anastigmat (TMA) optical configuration to enable a wide field-of-view instrument and replacements for hardware that was not provided to NASA. This existing hardware significantly reduces the development risk of the WFIRST payload.
The wide field instrument provides the Wide Field imaging and slitless spectroscopic capabilities required to perform the Dark Energy, Exoplanet Microlensing, and NIR surveys while the coronagraph instrument supports the Exoplanet high contrast imaging and spectroscopicy science. The wide field instrument includes two channels, a wide field channel and an integral field unit (IFU) spectrograph channel. The wide field channel includes three mirrors (two folds and a tertiary) and a filter/grism wheel to provide an imaging mode covering 0.76 — 2.0 μm and a spectroscopy mode covering 1.35 — 1.95 μm. The wide field focal plane uses 4k x 4k HgCdTe detectors with 10 μm pixels. The HgCdTe detectors are arranged in a 6x3 array, providing an active area of 0.281 deg2 . The IFU channel uses an image slicer and spectrograph to provide individual spectra of each 0.15"wide slice covering the 0.6 — 2.0 μm spectral range over a 3.00 x 3.15 arcsec field. The instrument provides a sharp point spread function, precision photometry, and stable observations for implementing the Dark Energy, Exoplanet Microlensing, and NIR surveys.
The coronagraph instrument includes an imaging mode and a spectroscopic mode to perform exoplanet direct imaging and spectroscopic characterization of planets and debris disks around nearby stars. The coronagraph imager covers a spectral range of 0.43 — 0.98 μm, providing a contrast of 10-9 with an inner working angle of 3λ/D at 430 nm. The optical train contains two deformable mirrors that form a sequential wavefront control system (WFCS) that compensates for both phase and amplitude errors in the telescope and coronagraph optics. The direct imager is a 1k x 1k format detector fed by a flip-in mirror. This branch also aids the WFCS with a pupil imaging lens for focus diversity estimation. The spectroscopy mode uses an Integral Field Spectrograph (IFS) with a nominally constant dispersion across the 0.60 — 0.98 μm spectral range with a resolving power R~70.
The coronagraph starlight suppression uses two different occulting mask architectures, a set of Hybrid Lyot and Shaped Pupil occulting masks, in a single optical design. The Shaped Pupil design is relatively insensitive to observatory jitter and provides a low risk path to demonstrating coronagraph performance on-orbit. The Hybrid Lyot can detect and characterize more planets than the Shaped Pupil design, with the help of the low-order wavefront sensing and control system to reject observatory jitter and telescope wavefront drift. Filter wheels are used to rotate the various masks into the optical path to change between the two designs. A backup architecture, a Phase Induced Amplitude Apodization Complex Mask Coronagraph, is also being studied for WFIRST. It provides greater potential science return than either the Hybrid Lyot or Shaped Pupil but requires additional technology development beyond the baseline architecture.