The WFIRST-AFTA coronagraphic imager : the first high-contrast, small inner working angle instrument in space

Séminaire IPAG de Alexis Carlotti (IPAG), jeudi 15 mai 2014 à 11h00, IPAG seminar room

NASA recently acquired two "Hubble class" 2.4m mirror telescopes from the national reconnaissance office One - WFIRST-AFTA - was swiftly chosen to replace the previously cancelled 0.7m WFIRST telescope, with a new launch scheduled for 2021. A wide-field imaging camera with a grism and an IFU spectrograph will be used to address the main scientific objectives of WFIRST : searching for dark energy, and surveying exoplanets through microlensing. The much larger diameter of WFIRST-AFTA has also made the case for a highly-anticipated technology demonstrator for a high-contrast, small inner working angle coronagraph. This instrument is seen as a first step towards a future large mission capable of detecting and characterizing Earth-size rocky planets in the habitable zone of nearby stars. The most ambitious objective of AFTA’s coronagraphic imager will be the characterization of 1e-9 contrast planets at 0.1-2 arcsec from the star in a 430-980nm band. The prime configuration of the AFTA coronagraphic imager will be an occulting mask coronagraph using pupil and image plane masks. A backup configuration based on a phase induced amplitude apodized coronagraph with complex focal plane masks is also investigated. No matter which coronagraph is used, a wavefront control system using two deformable mirrors in series will be mandatory to cancel the phase and amplitude aberrations that would otherwise limit the performance of the instrument to a 1e-6 contrast. I will present the design of the WFIRST-AFTA coronagraphic imager, which takes into account the large central obscuration (30-35%) and the thick spiders of the telescope. I will detail the tradeoffs of the coronagraphic design, and the interaction between the coronagraphs and the wavefront control system. In particular I will discuss the possibility of using the two deformable mirrors as a phase induced amplitude apodizer, to relax the specifications on the coronagraph. Finally I will talk about the benefits of the ongoing research effort associated with AFTA for the instruments of the E-ELT and for those of a future large space telescope.