This apparatus offers potential advantages of performance and manufacturability over conventional coronagraphs.
A holographic vortex coronagraph (HVC) has been proposed as an improvement over conventional coronagraphs for use in high-contrast astronomical imaging for detecting planets, dust disks, and other broadband light scatterers in the vicinities of stars other than the Sun. Because such light scatterers are so faint relative to their parent stars, in order to be able to detect them, it is necessary to effect ultra-high-contrast (typically by a factor of the order of 1010) suppression of broadband light from the stars. Unfortunately, the performances of conventional coronagraphs are limited by low throughput, dispersion, and difficulty of satisfying challenging manufacturing requirements. The HVC concept offers the potential to overcome these limitations.
A key feature of any coronagraph is an occulting mask in the image plane of a telescope, centered on the optical axis of the telescope. In a conventional coronagraph, the occulting mask is an opaque amplitude mask that obstructs the central starlight when the optical axis points toward the star in question. In the HVC, the occulting mask is a holographic vortex grating, which may be created by etching or otherwise forming the interference pattern of a helical phase ramp and a plane wave beam into an optical surface. As shown in the figure, the interference pattern has a forked appearance.
Light incident perpendicular to the grating is diffracted into discrete orders at angles given by θn = sin–1(nλ / d), where n is the diffraction order, λ is the wavelength of the diffracted light, and d is the lateral distance between adjacent grooves in the grating. In addition, the grating could be blazed to concentrate the diffracted light primarily into one order. If the grating is blazed to concentrate the light into the first (n = 1) order, then almost all of the light from a star or any other on-axis source will be transformed into a beam having a helical wavefront. Total destructive interference occurs along the axis of the helix over a broad wavelength band, attenuating the light from the star or other on-axis source.
The holographic vortex grating in the HVC is placed at the focus of the telescope and is designed and fabricated so as to almost completely suppress light from an on-axis star without significantly affecting images of planets or other light scatterers near the star. The starlight removed from the exit pupil appears outside exit pupil, whereas the light from scatterers near the star appears within the exit pupil. A Lyot stop — an aperture stop to block the starlight while passing the light from nearby scatterers — is placed in the exit pupil.
On the basis of previous research, it is anticipated that in comparison with a conventional coronagraph, the HVC would be less sensitive to aberrations, would yield higher throughput of light from scatterers near stars, and would offer greater planet/star contrast. On the basis of previous achievements in the fabrication of gratings similar to holographic vortex gratings, it appears that the grating for the HVC could readily be fabricated to satisfy initial requirements for imaging of extrasolar planets.
This work was done by David Palacios of Caltech for NASA’s Jet Propulsion Laboratory. For more information, download the Technical Support Package (free white paper) at www.techbriefs.com/tsp under the Physical Sciences category. NPO-45047
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