Browsing UA Faculty Research by Subjects
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Stealth telescopes for space and ground astronomyIn this paper we examine several contrast-degrading static signature sources present in current terrestrial exoplanet Lyot Coronagraph/Telescope optical systems. These are: Unnecessary optical surfaces, which increase cost, absorption, scatter, wavefront control and alignment issues. A suggested solution is to make every effort to investigate innovative solutions to reduce the number of optical surfaces during the early design phase. Consider free-form optics. Diffraction from secondary support systems and classical hexagon segmented apertures, which masks the low IWA terrestrial exoplanets. A suggested mitigation is to investigate curved secondary support systems and a pinwheel architecture for the deployable primary aperture. Polarization Fresnel and form birefringence aberrations, which distort the system PSF, introduce absorption, scatter and wavefront control issues. Mitigation is to reduce all ray-angles of incidence to a minimum, investigate zero-loss polarization compensation wavefront technology, and investigate metal thin film deposition processes required to minimize form birefringence in large-area high-reflectivity coatings. Small-angle specular or resolved angle scattered light, which places a narrow halo of incoherent light around the base of the PSF. There is no requirement on mirror smooth-surface scatter. Investigate the physical source of the small angle scatter & develop mirror polishing & thin film deposition processes to minimize scatter.
Terrestrial exoplanet coronagraph image quality: study of polarization aberrations in Habex and LUVOIR updateDirect imaging and spectroscopy of terrestrial exoplanets requires the control of vector electromagnetic fields to approximately one part in ten to the fifth over a few milliarc second FOV to achieve the necessary 10-10 intensity contrast levels. Observations using space telescopes are necessary to achieve these levels of diffracted and scattered light control. The highly reflecting metal mirrors and their coatings needed to image these very faint exoplanets introduce polarization into the wavefront, which, in turn affects image quality and reduces exoplanet yield unless corrected. To identify & create the technologies and the electro-optical/mechanical-spacecraft systems models that will achieve these levels, NASA is currently developing two mission concepts, each with their own hardware vision. These are: The Habex, a habitable planet explorer and the LUVOIR, a Large Ultra-Violet Optical-Infrared space telescope system. This paper reports the results of polarization ray-tracing the HabEx detailed optical prescription provided by the project to the authors in the fall of 2017. Diattenuation and retardance across both the exit pupil associated with the occulting mask and the exit pupil associated with the coronagraph image plane are given as well as the corresponding Jones pupil matrices. These are calculated assuming isotropic coatings on all mirrors. Analysis and physical measurements indicates that the specification of the primary mirror for exoplanet coronagraphs will need to include a constraint on spatially varying polarization reflectivity (anisotropic coatings). The Jones exit-pupil phase terms, phi XX and phi YY just before the occulting mask differ in shape and are displaced one from the other by about 10 milli-waves. This shows that A/O, which corrects for geometric path differences, cannot completely correct for wavefront errors introduced by polarization for this particular prescription for HabEx. We suggest that these differences may be corrected by adjusting the opto-mechanical design to change angles of incidence on mirrors and corrected by adjusting the design of the dielectric coatings on the highly-reflecting mirror surfaces. Super-posing the phase of XX onto the phase of YY and then correcting using A/O will assure maximum power transmittance through the system and best contrast. These aspects require further investigation.