Visible Nulling Coronagraph

Details Visible Nulling Coronagraph Visible Nulling Coronagraph

Jun 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007lyot.confr..22s&link_type=abstract

Proceedings of the conference In the Spirit of Bernard Lyot: The Direct Detection of Planets and Circumstellar Disks in the 21s

Computer Science

Sound

Scientific paper

To date, nulling coronography has achieved the deepest level of starlight suppression than any other coronagraphic technique. The terms starlight suppression and contrast are often used interchangeably and incorrectly. At 3.5 lambda/D the airy function sidelobes has dropped to 0.001 of the stellar flux. Starlight suppression of 1e-7 will then produce a speckle pattern with a contrast of 1e-10. This paper describes three topics in nulling coronagraphs. The first is starlight suppression of 1.1e-7 in laser light, the level of star light suppression needed for 1e-10 contrast. A similar experiment in broadband (16%) white light has demonstrated ~1.2e-6 suppression. Both experiments were starlight suppression using a single mode optical fiber. Starlight suppression to 1e-6~1e-7 requires wavefront measurements with sub-angstrom level accuracy. This is most easily done with a wavefront sensor that works with light after the coronagraph rather than the bright starlight in front of the coronagraph. Our nulling coronagraph uses a post coronagraphic interferometer to perform two functions, 1) measure the wavefront and provide an correction/error signal for the deformable mirror, and 2) post coronagraph PSF subtraction. We show that the post coronagraph interferometer has very significant SNR advantages over other post coronagraph approaches such as speckle nulling. A secondary function of the post coronagraphic wavefront sensor is that since it measures the amplitude and phase of the wavefront of the starlight after exiting the coronagraph, that amp*exp(i*phi) can be used to estimate the residual speckle pattern for subsequent PSF subtraction. By measuring the PSF at the same time as we're measuring the science image, the stability requirements can be relaxed by many orders of magnitude. (over rotating the telescope around the line of sight for example). The post coronagraph interferometer is being used in three projects, the PICTURE project (coronagraph behind a 60 cm telescope on a sounding rocket), the Gemini Planet Imager and a study of an extreme AO coronagraph for the TMT telescope. The nulling coronagraph is being used in four projects/studies, the PICTURE project, a proposed Discovery mission EPIC, a study for an extreme AO coronagraph for the TMT and a study for an alternative coronagraphic instrument for TPF-C.

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