Notes on NRO telescope opportunities for exoplanets science

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Two new UofA-led activities are of potential interest for the use of the 2.4 m NRO telescope for exoplanet science. These activities indicate that high performance coronagraphy and high precision astrometry are possible with a conventional (wide field) telescope.

Coronagraphy

NASA has been funding technology development of the Phase-Induced Amplitude Apodization (PIAA) coronagraph since 2003. This coronagraphy uses lossless beam shaping as an alternative from the inefficient mask-based apodization technique. This results in coronagraph designs with nearly full throughput and angular resolution, with an inner working angle ranging from 1 to 2 lambda/D depending on the exact design details.

Current status

The coronagraph is being tested at NASA Ames and JPL. Has reached 4e-9 contrast at 2 lambda/D (JPL) and ~1e-6 contrast at 1.2 lambda/D (NASA Ames). Instrument pointing has been demonstrated to ~1e-4 lambda/D in vacuum (much better than required). All tests are in monochromatic light; polychromatic light tests will start soon.

PIAACMC and NRO telescope: Scientific return

PIAACMC for centrally obscured and segmented apertures (Guyon et al. 2012)
More details on : this webpage

A high performance version of the PIAA coronagraph compatible with centrally obscured apertures was recently developed. It can be designed to offer sub-lambda/D IWA with full throughput. This is the first coronagraph concept that shows that high performance coronagraphy is possible on a centrally obscured telescope. It is relatively new, and requires technology development for the fabrication of the focal plane mask (this has started, under NASA APRA funding, working with NASA Ames and JPL - PI: Belikov). A ground-based version will be tested in about 1 yr on sky at Subaru.
If a PIAACMC coronagraph were deployed on a 2.4 m telescope, low-resolution (R<100) spectroscopy of Earth-like planets would be possible around ~20 nearby Sun-like stars. Note that TPF-C Flight Baseline 1 with a 8 m telescope had a 4 lambda/D IWA with a ~10% coronagraph efficiency and a ~2.5 lambda/D wide PSF. This is equivalent in performance to a 2.4 m telescope with a 100% coronagraph efficiency, full angular resolution and 1.2 lambda/D IWA.

Related Activities

  • EXCEDE mission (funded by NASA for tech dev)
  • PECO study for astro2010 (funded by NASA, no longer active), a 1.4 m telescope with PIAA coronagraph
  • NASA Ames PIAA testbed (funded by NASA HQ + internal Ames funds)
  • JPL PIAA table (funded by NASA HQ / supported by ExEp program)
  • Subaru Coronagraphic Extreme-AO (SCExAO) system, funded by Japan, very similar architecture to EXCEDE
  • Proposal to NASA to demonstrate PIAACMC (PI: Cady, JPL)
  • Proposal to NASA to demonstrate cophasing of segments with PIAACMC, and its compatibility with segmented/centrally obscured pupils (PI: Guyon, UofA)

Astrometry

Concept paper (Guyon et al. 2012)
Science paper (Guyon et al. 2012)
SPIE paper with lab results (Bendek et al. 2012)
More details on : this webpage

Diffractive pupil telescope principle

Small dots (covering 1% of PM surface) are regularly placed on primary mirror. They create diffraction spikes that encode all instrumental astrometric distortions (optics, detector) and thus provide a reliable reference to measure the motion of a bright star against a background of fainter stars. This scheme is compatible with coronagraphy, and does not significantly impact wide field deep imaging.

Current status

Funded by NASA APRA grant. Tesbted at UofA demonstrates that the technique works, and a test at NASA Ames demonstrates compatibility with coronagraphy. Work will continue at NASA Ames: E. Bendek will move to NASA Ames to continue development of the technique.

Scientific return

Identification of Earth-like planets and measurement of their masses requires ~0.2 uas astrometry per visit with a moderate (<100) number of visits. According to preliminary error budgets, a 2 m telescope offers 0.2 uas astrometry if the FOV is >0.1 sq. deg.

Related activities

  • Ground-based demo (led by M. Ammons)
  • Alternate scheme by M. Shao led to NEAT mission proposed to ESA - not compatible with NRO telescope
  • Close collaboration between M. Shao group and our group

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