Advances in optical engineering for future telescopes

Precision wavefront control on future segmented-aperture space telescopes presents significant challenges, particularly in the context of high-contrast exoplanet direct imaging. We present a new wavefront control architecture that translates the ground-based artificial guide star concept to space with a laser source on board a second spacecraft, formation flying within the telescope’s field of view. We describe the motivating problem of mirror segment motion and develop wavefront sensing requirements as a function of guide star magnitude and segment motion power spectrum. Several sample cases with different values for transmitter power, pointing jitter, and wavelength are presented to illustrate the advantages and challenges of having a non-stellar-magnitude noise limited wavefront sensor for space telescopes. These notional designs allow increased control authority, potentially relaxing spacecraft stability requirements by two orders of magnitude and increasing terrestrial exoplanet discovery space by allowing high-contrast observations of stars of arbitrary brightness.

/pdf/laser-guide-star-for-large-segmented-aperture-space-2wglh8m3t0.pdf

Laser Guide Star for Large Segmented-aperture Space Telescopes. I. Implications for Terrestrial Exoplanet Detection and Observatory Stability

The ultimate astronomical observatory would be a formation flying interferometer in space, immune to atmospheric turbulence and absorption, free from atmospheric and telescope thermal emission, and reconfigurable to adjust baselines according to the required angular resolution. Imagine the near/mid-infrared sensitivity of the JWST and the far-IR sensitivity of Herschel but with ALMA-level angular resolution, or imagine having the precision control to null host star light across 250m baselines and to detect molecules from the atmospheres of nearby exo-Earths. With no practical engineering limit to the formation's size or number of telescopes in the array, formation flying interferometry will revolutionize astronomy and this White Paper makes the case that it is now time to accelerate investments in this technological area. Here we provide a brief overview of the required technologies needed to allow light to be collected and interfered using separate spacecrafts. We emphasize the emerging role of inexpensive smallSat projects and the excitement for the LISA Gravitational Wave Interferometer to push development of the required engineering building-blocks. We urge the Astro2020 Decadal Survey Committee to highlight the need for a small-scale formation flying space interferometer project to demonstrate end-to-end competency with a timeline for first stellar fringes by the end of the decade.

pdf/a-realistic-roadmap-to-formation-flying-space-interferometry-18500i9rdm.pdf

A Realistic Roadmap to Formation Flying Space Interferometry

Microelectromechanical systems (MEMS) deformable mirrors (DMs) can provide high-precision wavefront control with a small form-factor, low power device. This makes them a key technology option for future space telescopes requiring adaptive optics for high-contrast imaging of exoplanets with a coronagraph instrument. The Deformable Mirror Demonstration Mission (DeMi) CubeSat payload is a miniature space telescope designed to demonstrate MEMS DM technology in space for the first time. The DeMi payload contains a 50-mm primary mirror, an internal calibration laser source, a 140-actuator MEMS DM from Boston Micromachines Corporation, an image plane wavefront sensor, and a Shack–Hartmann wavefront sensor (SHWFS). The key DeMi payload requirements are to measure individual actuator wavefront displacement contributions to a precision of 12 nm and correct both static and dynamic wavefront errors in space to less than 100-nm RMS error. The DeMi mission will raise the technology readiness level of MEMS DM technology from a five to at least a seven for future space telescope applications. We summarize the DeMi optical payload design, calibration, optical diffraction model, alignment, integration, environmental testing, and preliminary data from in-space operations. Ground testing data show that the DeMi SHWFS can measure individual actuator deflections on the MEMS DM to within 10 nm of interferometric calibration measurements and can meet the 12-nm precision mission requirement for actuator deflection voltages between 0 and 120 V. Payload data from throughout environmental testing show that the MEMS DM and DeMi payload survived environmental testing and provides a valuable baseline to compare with space data. Initial data from space operations show the MEMS DM actuating in space with a median agreement between individual actuator measurements from space and equivalent ground testing data of 12 nm.

/pdf/optical-calibration-and-first-light-for-the-deformable-53u34ngtxf.pdf

Optical calibration and first light for the deformable mirror demonstration mission CubeSat (DeMi)

The CubeSat Laser Infrared CrosslinK (CLICK) mission is a technology demonstration of a 1.5U laser communications terminal for an intersatellite link. The terminal is deployed on a pair of 3U CubeSats in Low Earth Orbit (LEO). The pointing, acquisition, and tracking (PAT) approach includes both coarse and fine systems. The coarse tracking system uses a beacon laser transmitter and receiver camera. The fine tracking system uses a fast steering mirror and quadrant photodiode. The communications transmit and receive paths include a refractive telescope, transmit laser collimator, and avalanche photodetector (APD) receiver. The communications laser full-width, half maximum (FWHM) beam divergence angle is 14.6 arcseconds, and the beacon laser FWHM divergence is 0.75◦ (2700 arcseconds). The opto-mechanical design process includes prediction & verification of assembly alignment & calibration, thermoelastic effects, structural modes & static loading, and fastener analysis. The opto-mechanical assembly has the sensors and laser transmitters kinematically mounted to enable on-ground calibration to less than 25.4 μm decenter, or 0.1◦ tip/tilt. The thermoelastic alignment error between the payload and bus star tracker is estimated via finite element analysis to be less than 9 arcseconds. The payload optical bench is designed with custom thermal isolation and control to maintain 20 ± 10 ◦C. The thermal modeling of the payload is described in detail. Structural static loading and fastener analyses of the CLICK payload under launch loads of 30 G verify margins of safety are greater than 10 and above the recommended values. Modal analyses predict the first resonant frequency to be 888 Hz, above typical vehicle structural vibration ranges with a factor of safety greater than 3.5.

/pdf/optomechanical-design-and-analysis-for-nanosatellite-laser-3rpjxd9i7r.pdf

Optomechanical Design and Analysis for Nanosatellite Laser Communications

Coronagraphs on future space telescopes will require precise wavefront correction to detect Earth-like exoplanets near their host stars. High-actuator count microelectromechanical system (MEMS) deformable mirrors provide wavefront control with low size, weight, and power. The Deformable Mirror Demonstration Mission (DeMi) payload will demonstrate a 140 actuator MEMS Deformable Mirror (DM) with 5:5 μm maximum stroke. We present the flight optomechanical design, lab tests of the flight wavefront sensor and wavefront reconstructor, and simulations of closed-loop control of wavefront aberrations. We also present the compact flight DM controller, capable of driving up to 192 actuator channels at 0-250V with 14-bit resolution. Two embedded Raspberry Pi 3 compute modules are used for task management and wavefront reconstruction. The spacecraft is a 6U CubeSat (30 cm x 20 cm x 10 cm) and launch is planned for 2019.

The deformable mirror demonstration mission (DeMi) CubeSat: optomechanical design validation and laboratory calibration

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The deformable mirror demonstration mission (DeMi) CubeSat: optomechanical design validation and laboratory calibration