Demonstration of Pointing Stability to Enable Astrophysics with Rotating Synthetic Aperture Telescopes

Novel rotating synthetic aperture telescope designs have been proposed for space to maximize telescope resolution and collecting area given the available volume on launch vehicles. These large aperture diameter, low mass systems have applications in exoplanet detection as well as planet surveying and have been proposed as a way to maximize the aperture of missions from CubeSats to future great observatories. A deployable strip-like telescope has the potential to increase the light gathering power and resolution of a 3U CubeSat to hundreds of square centimeters, increasing the sensitivity to events such as exoplanet transits, astroseismology, and active galactic nuclei. Additionally, a deployable telescope allows resolutions exceeding ground-based observatories at a fraction of the cost of comparable ground systems. The proposed effort will mature the pointing control needed for a spinning strip aperture telescope through laboratory demonstration. The state-of-the art for current nanosatellite body pointing is approximately 1 arcsecond for conventional aperture. We will design a system to meet or exceed this pointing stability with a larger aperture and demonstrate pointing performance both in simulation and with hardware. This work builds on an architecture study of a strip aperture telescope developed by an MIT student team and will leverage the strong student hardware program at the MIT Space Systems Laboratory. The pointing and control demonstration will complement collaborative efforts led by University of Maryland focusing on high-contrast imaging and image reconstruction and Raytheon focusing on performance scaling laws to enable architecture trades. A primary objective of this work is to mature an existing strip aperture dynamics and controls testbed (DCT) designed and built by the student team. The DCT is designed to test the controllablity of the spinning strip aperture satellite, demonstrate the necessary pointing capability and explore the dynamic effects of spinning on the optical geometry. The testbed sits on an air bearing inside a Helmholtz cage to allow 3-degree of freedom angular motion and simulate the effects of the Earth's magnetic field. The testbed is controlled by four reaction wheels and includes an inertial measurement unit (IMU) and star tracker to close the loops around the satellite pointing. Mirror surrogates and mass tuning stages are used to simulate the inertia and mass properties of a spinning strip aperture telescope. A second objective is to demonstrate sensing and control in a laboratory environment, given a deployed strip aperture. Dynamical problems of large asymmetric structures include control of bending modes as well as dynamic loading due to photon pressure and solar wind and reaction wheel driven harmonics. We propose to develop an analytic control solution encompassing these problems, starting with a simple star tracker coupled to the dynamical model. We will translate this control model to an optical testbed using a CubeSat scale model, injecting jitter and higher order perturbations from realistic attitude determination and control systems to validate control system performance using a star tracker as the control signal. The final objective is a simulation that combines data from the DCT at MIT with that from optical testbeds at University of Arizona to validate end-to-end system pointing performance. This work will include design of an optical sensor which uses the strip aperture both for science observations and to sense the system state, measuring deformation and orientation relative to a target star or astrophysical scene and providing appropriate control signals to maximize the image quality. The end product of the proposed study is a validated dynamical model of a deployable strip aperture CubeSat telescope that can serve as the starting point for future astrophysics CubeSat missions and feasibility studies of larger strip aperture mission concepts.

The Astrophysics Research and Analysis program (APRA) supports suborbital and suborbital-class investigations, development of detectors and supporting technology, laboratory astrophysics, and limited ground based observing. Basic research proposals in these areas are solicited for investigations that are relevant to NASA's programs in astronomy and astrophysics, including the entire range of photons, gravitational waves, and particle astrophysics. The emphasis of this solicitation is on technologies and investigations that advance NASA astrophysics missions and goals.

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