WFIRST-AFTA Coronagraphic Operations: Lessons Learned from the Hubble Space Telescope and the James Webb Space Telescope
ABSTRACT
The coronagraphic instrument currently proposed for the WFIRST-AFTA mission will be the first example of a space-based coronagraph optimized for extremely high contrasts that are required for the direct imaging of exoplanets reflecting the light of their host star. While the design of this instrument is still in progress, this early stage of development is a particularly beneficial time to consider the operation of such an instrument. In this paper, we review current or planned operations on the Hubble Space Telescope (HST) and the James Webb Space Telescope (JWST) with a focus on which operational aspects will have relevance to the planned WFIRST-AFTA coronagraphic instrument. We identify five key aspects of operations that will require attention: 1) detector health and evolution, 2) wavefront control, 3) observing strategies/post-processing, 4) astrometric precision/target acquisition, and 5) polarimetry. We make suggestions on a path forward for each of these items.
1. INTRODUCTION
The WFIRST-AFTA coronagraphic instrument (CGI) enables ground-breaking exoplanet discoveries using optimized space-based coronagraphic high contrast imaging. Its goal of 10−9 contrast at an inner working angle of 0.1′′ relative to an astrophysical point source will result in the direct detection of exoplanet candidates around nearby stars that are directly reflecting light from their hosts and push to within an order of magnitude of the requirements for a large aperture space telescope to directly image nearby terrestrial planets. The CGI design currently consists of a Hybrid Lyot Coronagraph (HLC) mode that will be primarily used for planet detection. There will also be a Shaped Pupil Coronagraph (SPC) mode, which will have more moderate contrast and a smaller field of view. The SPC coupled with an integral field spectrograph (IFS) mode will provide spectral characterization of exoplanets in addition to direct imaging. Both imaging and spectroscopic modes will use Electron Multiplying Charge Coupled Devices (EMCCDs),1 which provide high sensitivity for faint sources in the optical bandpass.
Achieving the goals of the CGI requires not only a sound design for launch, but a comprehensive and complete plan for operating the instrument and adapting to its changes throughout the duration of the mission. The present manuscript is aimed at highlighting the synergies between HST, JWST, and WFIRST-AFTA in coronagraphic operations. It relies on information regarding the instrument presented in the 2015 SDT report.2 Aspects discussed here are lessons learned from 15+ years of coronagraphy with HST and plans for JWST. HST has had optical and Near-IR coronagraphic capabilities in space for most of its lifetime, while JWST will represent the first NASA flagship observatory with a diverse suite of various high contrast imaging approaches. The operation of the coronagraphs for each mission have direct ties to the WFIRST-AFTA CGI.
2 Hubble Space Telescope Operations
Over its 25-year lifetime, HST has had four separate instruments with high contrast coronagraphic imaging capability, though none of which were specifically optimized for high contrast imaging at extreme inner working angles. As such, HST’s high contrast instruments have achieved high contrast (such as the 10−9 contrast detection of Fomalhaut b at r=9′′) or small inner working angles (0.25′′ for STIS), but not both simultaneously. This is primarily because there is no instrumentation that is capable of significantly suppressing diffracted starlight from the wings of the telescope’s point spread function, which are more severe from mid-frequency wavefront errors on the primary mirror. One of the four original instruments on Hubble, the Faint Object Camera, had occulting fingers that were not highly used.3 With the second servicing mission of HST the Space Telescope Imaging Spectrograph (STIS)4 and Near-infrared Imager and Multi-Object Spectrometer (NICMOS)5 were installed, which in concert with Reference Difference Imaging (RDI) and Angular Differential Imaging (ADI) allowed for high quality, high contrast images of faint companions, circumstellar debris disks, and protoplanetary disks at the level of 10−4 contrast at inner working angles of 0.3′′ . 6–8 High contrast images of other astrophysical objects, such as the host galaxies of quasars were also imaged.9 While NICMOS is no longer operational, archival coronagraphic data is still being utilized in concert with modern post-processing techniques, which we discuss further in Section 3. STIS possesses two focal plane wedges and two occulting bars with a slightly undersized Lyot stop in the pupil plane. NICMOS possessed a hole bored into a mirror that directed light to its second camera which served as an occulting spot as well as a cold mask that acted as a crude Lyot stop. The High Resolution Channel (HRC) on the Advanced Camera for Surveys (ACS),10 launched during servicing mission 3B, possessed two coronagraphic spots with inner working angles of 0.9′′ and 1.8′′, which provided high contrast, deep observations of many disks across broadband optical filters.
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