Visible nulling coronagraph testbed results

Abstract

We report on our recent laboratory results with the NASA/Goddard Space Flight Center (GSFC) Visible Nulling Coronagraph (VNC) testbed. We have experimentally achieved focal plane contrasts of 1 x 10 8 and approaching 10 9 at inner working angles of 2 * wavelength/D and 4 * wavelength/D respectively where D is the aperture diameter. The result was obtained using a broadband source with a narrowband spectral filter of width 10 nm centered on 630 nm. To date this is the deepest nulling result with a visible nulling coronagraph yet obtained. Developed also is a Null Control Breadboard (NCB) to assess and quantify MEMS based segmented deformable mirror technology and develop and assess closed-loop null sensing and control algorithm performance from both the pupil and focal planes. We have demonstrated closed-loop control at 27 Hz in the laboratory environment. Efforts are underway to first bring the contrast to > 10 9 necessary for the direct detection and characterization of jovian (Jupiter-like) and then to > 10 10 necessary for terrestrial (Earth-like) exosolar planets. Short term advancements are expected to both broaden the spectral passband from 10 nm to 100 nm and to increase both the long-term stability to > 2 hours and the extent of the null out to a ~ 10 * wavelength / D via the use of MEMS based segmented deformable mirror technology, a coherent fiber bundle, achromatic phase shifters, all in a vacuum chamber at the GSFC VNC facility. Additionally an extreme stability textbook sized compact VNC is under development.

Intelligent Optical Systems Using Adaptive Optics

“…The ring mask, shown at plane 4 is not a powered element. Although the telescope primary may be quite large, 8 meters as planned in NASA exoplanet missions [7][8][9][10][11][12] , the mitigation of Fresnel Polarization can be done at a much smaller and convenient pupil plane such as at plane 4 where the ring mask is located. The functions of the polarization compensator and ring mask can be combined, as we are describing under this effort.…”

“…Several coronagraph architectures have been proposed [7][8][9][10][11][12][13] . Each one of these will have degraded performance because of Fresnel polarization apodization.…”

“…Direct imaging of terrestrial exoplanets requires a telescope/instrument capable of controlling and suppressing unwanted star light to approximately one part in 10 12 over a wide wavelength bandpass spanning the UV to near IR spectrum [7][8][9][10][11][12] . Terrestrial exoplanets are faint requiring large aperture telescopes to record their image.…”

“…The standard expression for an a-plate with positive birefringence between crossed polarizers applies [8][9] : 2…”

Design and performance evaluation of sensors and actuators for advanced optical systems

“…The detection and characterization of planets that orbit stars other than our Sun (referred to as "exoplanets" or "extrasolar planets") will profoundly improve our understanding of the evolution of the universe. [11][12] Direct detection of exoplanets, however, poses a difficult problem because of the huge contrast between the planet and the star it orbits. The contrast between an exoplanet and its parent star is often quoted a 1 part in 10 8 for Jupiter and 1 part in 10 10 for Earth [11][12] .…”