Starshade formation flying I: optical sensing

Abstract

A key challenge for starshades is formation flying. To successfully image exoplanets, the telescope boresight and starshade must be aligned to approximately one meter at separations of tens of thousands of kilometers. This challenge has two parts: first, the relative position of the starshade with respect to the telescope must be sensed; second, sensor measurements must be combined with a control law to keep the two spacecraft aligned in the presence of gravitational and other disturbances. In this work, we present an optical sensing approach using a pupil imaging camera in a 2.4-meter telescope that can measure the relative spacecraft bearing to a few centimeters in one second, much faster than any relevant dynamical disturbances. A companion paper will describe how this sensor can be combined with a control law to keep the two spacecraft aligned with minimal interruptions to science observations.

Analytical model for starshade formation flying with applications to exoplanet direct imaging observation scheduling

“…Adherence to the axial distance constraints along the LOS enables observations in specific science bands based on the telescope and starshade size. 7 In reality, the actual starshade position S can vary by up to 250 km axially but only 1 m laterally from the LOS. 4 We assume, for the desired trajectory D, a constant separation distance s and measure deviations from the desired trajectory using Eq.…”

“…[3][4][5] This level of precision control, fortunately, has been demonstrated both analytically and in experimental simulations. [6][7][8] The starshade, in many designs, carries out this precision control using a bipropellant engine: it is allowed to drift from its desired positioning and fires its thruster only when it reaches the lateral tolerance of 1 m from the LOS. 4,9 However, every burst of propellant used to maneuver the starshade back toward its desired trajectory creates a transient plume that reflects light and prevents a successful observation.…”

Solar glint from uncoated starshade optical edges

“…18 The starshade technology program has defined a set of eight milestones 10 to satisfy Technology Readiness level 5, in which a technology is tested at the component or breadboard level in a relevant environment. 19 The set of milestones includes high contrast optical diffraction and perturbation sensitivity tests (Milestones 1 and 2 [20][21][22] ), optical edge glint (Milestone 3, this work), formation flying (Milestone 4 23,24 ), petal deployment, thermal cycling, and thermal stability (Milestones 5 and 6, under review), and central disk deployment, thermal cycling, and thermal stability (Milestones 7 and 8 18 ). The plan is to continue this work through TRL 6 in preparation for mission opportunities such as SRM and HabEx.…”

Optical verification experiments of sub-scale starshades

“…In pupil imaging mode, we observe the out-of-band diffraction pattern incident on the entrance pupil and use the bright spot of Arago to align the camera with the starshade, precisely what is done in the formation flying scheme to maintain starshade alignment. 30,31 To perform calibration measurements, a neutral density filter (optical density >10 −7 ) is toggled into the optical train by a motorized stage. A linear polarizer on a motorized rotation stage serves as a polarization analyzer.…”