In this paper, we present some ideas regarding the optics and imaging aspects of <i>granular spacecraft</i>. Granular spacecraft are complex systems composed of a spatially disordered distribution of a large number of elements, for instance a cloud of grains in orbit. An example of this application is a spaceborne observatory for exoplanet imaging, where the primary collecting aperture is a cloud of small particles instead of a monolithic aperture.
The optical vortex coronagraph is a promising scheme for achieving high contrast low loss imaging
of exoplanets as close as 2λ/<i>D</i> from the parent star. We describe results using a high precision
vortex lens that was fabricated using electron-beam lithography. We also report demonstrations of
the coronagraph on a telescope employing a tip-tilt corrector.
The goal of the Terrestrial Planet Finder Mission is to detect and characterize Earth-like planets. Detection of these faint objects, which appear very close to their parent stars, requires a coronagraph capable of achieving better than 10<sup>-10</sup> starlight suppression within a few Airy rings of the stellar image. The coronagraph is also required to maintain this high stellar extinction over a 100nm spectral bandwidth. To ease requirements on the telescope, a high planet light throughput and low sensitivity to wave front aberrations are also desirable features. An optical vortex coronagraph is a promising candidate architecture, which makes use of a spiral phase plate placed in an intermediate image plane to null out the stellar signal. This architecture has the advantage of high stellar extinction, high planet light throughput, and low sensitivity to wave front aberrations. Here we report the high contrast performance of an optical vortex coronagraph limited by the manufacturability of the spiral phase plate.
We derive the requirements on the surface height uniformity and reflectivity uniformity of the Terrestrial Planet Finder Coronagraph telescope and instrument optics for spatial frequencies within and beyond the spatial control bandwidth of the wave front control system. Three different wave front control systems are considered: a zero-path difference Michelson interferometer with two deformable mirrors at a pupil image; a sequential pair of deformable mirrors with one placed at a pupil image; and the Visible Nuller spatially-filtered controller. We show that the optical bandwidth limits the useful outer working angle.
An optical vortex may be characterized as a dark core of destructive interference in a beam of spatially coherent light. This dark core may be used as a filter to attenuate a coherent beam of light so an incoherent background signal may be detected. Applications of such a filter include: eye and sensor protection, forward-scattered light measurement, and the detection of extra-solar planets. Optical vortices may be created by passing a beam of light through a vortex diffractive optical element, which is a plate of glass etched with a spiral pattern, such that the thickness of the glass increases in the azimuthal direction. An optical vortex coronagraph may be constructed by placing a vortex diffractive optical element near the image plane of a telescope. An optical vortex coronagraph opens a dark window in the glare of a distant star so nearby terrestrial sized planets and exo-zodiacal dust may be detected. An optical vortex coronagraph may hold several advantages over other techniques presently being developed for high contrast imaging, such as lower aberration sensitivity and multi-wavelength operation. In this manuscript, I will discuss the aberration sensitivity of an optical vortex coronagraph and the key advantages it may hold over other coronagraph architectures. I will also provide numerical simulations demonstrating high contrast imaging in the presence of low-order static aberrations.