Exoplanet telescope diffracted light minimized: the pinwheel-pupil solution

Abstract

Terrestrial exoplanets shine in light reflected from a parent star. Optical spectra are required to provide evidence of a life-supporting environment. Exoplanets are very faint and their optical spectra are contaminated by the spectrum of the parent star. High angular resolution provided by large apertures is needed to distinguish between the spectrum of the exoplanet and its star. Today, large aperture telescopes use segmented primary mirrors that employ close-packed hexagonal segments. The telescope primary mirror is periodically discontinuous with straight lines. These discontinuities scatter unwanted radiation from the much brighter parent star across the field of view to obscure the light from the very faint terrestrial exoplanet. These discontinuities, which mimic a diffraction grating, result in a non-uniform distribution of background light across the image plane. This non-uniformity masks or hides exoplanets from view, to reduce the number of exoplanets that can be observed with a large aperture telescope or to reduce the quality of spectra and thus lead to misinterpretation of data. Here we introduce the concept of the pinwheel pupil whose unique diffraction pattern significantly reduces the non-uniform distribution of background radiation. Diffraction patterns from pinwheel pupils are compared to the monolithic filled aperture, the classical Cassegrain, the 60-degree symmetry of the hexagonal segments (JWST, E-ELT, etc.). Diffraction "spikes" are reduced by at least 10 5. We discuss the "pinwheel pupil" advantages to spectroscopy, image processing, and observatory operations. We show that, segment fabrication of curved-sided mirrors is not more difficult than fabrication of hexagonal mirror segments.. This is the report of quantitative study of Fraunhofer (far field) diffraction patterns produced by three different topologies or architectures of mirror segmentation, when illuminated by a plane wave of monochromatic white-light. A plot, in angular units of the intensity as a function of azimuth, Phi f , within annular rings at different FOVs, centered on the system axis of the diffraction pattern will be presented. The advantages of the segmented pinwheel pupil is discussed.

Topological pupil segmentation and point spread function analysis for large aperture imaging systems

“…[16][17][18][19][20] Many creative ways of segmentation have been suggested. 19,[21][22][23] After several years of study, three topologies for large aperture primary mirrors have evolved. These are shown in Fig 2. They are: circular monolithic, circular segments, and hexagonal mirrors mosaiced with hexagonal segments.…”

“…2) which 1) minimized diffraction confusion, 2) increases transmittance, and 3) lowers manufacturing complexity and corresponding costs. 22,23 Descriptive names for this innovative topology are the pinwheel aperture and pinwheel pupil segmentation.…”

“…In the same year, Breckinridge and Harvey further illustrate the high possibility of manufacturing a curvesided segment, using mirror substrates, processing steps, and polishing methods that are all available with currently existing technology. 22 In 2019, Breckinridge indicated a hex segmented aperture may produce Point Spread Function (PSF) that has many false-positive identifications of exoplanets, which may mask an actual exoplanet. 23…”

“…The arc geometry of the combination of every spoke in a "Pinwheel" pupil is controlled by Total Spokes Angle, which needs to be the integer multiple of 180° in order to have a bow-tie effect on PSFs. 22,31 The Total Spokes Angle is defined as…”

“…For 3-Ring Hex and Circ, because of the different grating-like period from the repeating gaps in different directions of the pupil plane, different flares in different directions create many fake stars/exoplanets images spreading in image plane. 22…”