John Steeves

Design, fabrication and testing of active carbon shell mirrors for space telescope applications

A novel active mirror concept based on carbon fiber reinforced polymer (CFRP) materials is presented. A nanolaminate facesheet, active piezoelectric layer and printed electronics are implemented in order to provide the reflective surface, actuation capabilities and electrical wiring for the mirror. Mirrors of this design are extremely thin (500-850 µm), lightweight (~ 2 kg/m2) and have large actuation capabilities (~ 100 µm peak- to-valley deformation per channel). Replication techniques along with simple bonding/transferring processes are implemented eliminating the need for grinding and polishing steps. An outline of the overall design, component materials and fabrication processes is presented. A method to size the active layer for a given mirror design, along with simulation predictions on the correction capabilities of the mirror are also outlined. A custom metrology system used to capture the highly deformable nature of the mirrors is demonstrated along with preliminary prototype measurements.

Ultra-Thin Highly Deformable Composite Mirrors

Optical quality mirrors are heavy, expensive and difficult to manufacture. This paper presents a novel mirror concept based on an active laminate consisting of an ultra-thin carbon-fiber shell bonded to a piezo-ceramic active layer coated with patterned electrodes. Mirrors based on this concept are less than 1 mm thick and hence are very lightweight and flexible. They also have a large dynamic range of actuation that allows them to take up a wide range of deformed configurations. This concept is compatible with relatively high-volume manufacturing processes and can potentially achieve a significant reduction in cost in comparison to currently available active mirrors. It is also suitable for applications ranging from concentrators for solar power generation to primary mirrors for optical telescopes. The paper presents an overview of the mirror components as well as a simple design relationship for sizing the active layer. The expected performance of a preliminary design for a 1 m diameter mirror with a radius of curvature of 15 m is computed numerically, showing that a set of 96 actuators can remove an edge-to-edge manufacturing-induced cylindrical curvature of 5 mm to an RMS accuracy of 50 μm. The prescription of the mirror can also be adjusted to a radius of curvature of 11 m with an accuracy of 160 μm. The development and characterization of a proof-of-concept prototype mirror is also presented.

Precision optical edges for a starshade external occulter

The use of a starshade is one technique to perform high contrast imaging with space-based telescopes. The primary function of a starshade is to suppress light from a target star in order to image its orbiting planets. In order to provide the proper apodization function the edges of the starshade must follow a precise in-plane profile. However of equal importance is the issue of light from our own sun scattering off of the edges and entering the telescope. A method to alleviate this problem is to make the edges extremely sharp (< 1 μm terminal radius) such that the area available for scattering is minimized. The combination of these two requirements, along with the need to integrate the edges into a 30-40 m dia. deployable structure, present a number of significant engineering challenges. Substrate etching techniques are used to obtain both the intended profile as well as the edge sharpness. Current efforts implement an isotropic etching process on thin metal substrates. This paper discusses the progress towards producing a sharp optical edge at the coupon level. Samples have been characterized using scanning electron microscopy as well as a custom testbed to assess their scattered-light performance.

Starshade mechanical design for the Habitable Exoplanet imaging mission concept (HabEx)

An external occulter for starlight suppression – a starshade – flying in formation with the Habitable Exoplanet Imaging Mission Concept (HabEx) space telescope could enable the direct imaging and spectrographic characterization of Earthlike exoplanets in the habitable zone. This starshade is flown between the telescope and the star, and suppresses stellar light sufficiently to allow the imaging of the faint reflected light of the planet. This paper presents a mechanical architecture for this occulter, which must stow in a 5 m-diameter launch fairing and then deploy to about a 80 m-diameter structure. The optical performance of the starshade requires that the edge profile is accurate and stable. The stowage and deployment of the starshade to meet these requirements present unique challenges that are addressed in this proposed architecture. The mechanical architecture consists of a number of petals attached to a deployable perimeter truss, which is connected to central hub using tensioned spokes. The petals are furled around the stowed perimeter truss for launch. Herein is described a mechanical design solution that supports an 80 m-class starshade for flight as part of HabEx.

Energy-Efficient Active Reflectors with Improved Mechanical Stability and Improved Thermal Performance

Controlling surface wavefront of apertures using a distributed array of actuators to mechanically correct the surface has been widely studied. Traditional active reflector systems require a sustained voltage profile which holds each actuator at a specific strain state to control the surface of the reflector. Each actuator, typically piezoelectric, draws a small amount of power under nominal operation. This power draw is small, but can complicate mission designs that depend on a cryogenic primary reflector surface. In this study we have extended the results of our previous work to include nonlinear piezoelectric actuation for active reflector systems. By deliberately operating in the nonlinear regime, it is possible to deform the actuators in such a way that the reflector surface maintains its corrected shape without sustained power. Demonstration of unpowered primary mirror wavefront control has positioned the technology as suitable for cryogenic/infrared systems. This report describes a nonlinear piezoelectric characterization campaign, and the associated nonlinear energy-efficient active reflector demonstration.

The Galaxy Evolution Probe: a concept for a mid and far-infrared space observatory

The Galaxy Evolution Probe (GEP) is a concept for a mid and far-infrared space observatory designed to survey sky for star-forming galaxies from redshifts of z = 0 to beyond z = 4. Furthering our knowledge of galaxy formation requires uniform surveys of star-forming galaxies over a large range of redshifts and environments to accurately describe star formation, supermassive black hole growth, and interactions between these processes in galaxies. The GEP design includes a 2 m diameter SiC telescope actively cooled to 4 K and two instruments: (1) An imager to detect star-forming galaxies and measure their redshifts photometrically using emission features of polycyclic aromatic hydrocarbons. It will cover wavelengths from 10 to 400 μm, with 23 spectral resolution R = 8 filter-defined bands from 10 to 95 μm and five R = 3.5 bands from 95 to 400 μm. (2) A 24 – 193 μm, R = 200 dispersive spectrometer for redshift confirmation, identification of active galactic nuclei, and interstellar astrophysics using atomic fine-structure lines. The GEP will observe from a Sun-Earth L2 orbit, with a design lifetime of four years, devoted first to galaxy surveys with the imager and second to follow-up spectroscopy. The focal planes of the imager and the spectrometer will utilize KIDs, with the spectrometer comprised of four slit-coupled diffraction gratings feeding the KIDs. Cooling for the telescope, optics, and KID amplifiers will be provided by solar-powered cryocoolers, with a multi-stage adiabatic demagnetization refrigerator providing 100 mK cooling for the KIDs.

Active mirrors for future space telescopes

The demanding science goals of future astrophysics missions currently under study for the 2020 Decadal Survey impose significant technological requirements on their associated telescopes. These concepts currently call for apertures as large as 15 m (LUVOIR), and operational temperatures as low as 4 Kelvin (OST). Advanced mirror technologies, such as those implementing a high degree of actuation at the primary, can help to overcome the challenges associated with these missions by providing in-situ wavefront correction capabilities. Active mirrors can also greatly reduce the cost/complexity associated with mirror fabrication as well as system I and T as on-orbit performance specifications can be achieved under a variety of test conditions (i.e. room/cryogenic temperatures, 0g/1g). JPL has significant experience in this area for visible/near-infrared applications, however future mission requirements create a new set of challenges for this technology. This paper presents design, analysis, and test results for lightweight silicon-carbide mirrors with integrated actuation capabilities. In particular, studies have been performed to test the performance of these mirrors at cryogenic temperatures.

Development of low-scatter optical edges for starshades

Starshades, combined with future space telescopes, provide the ability to detect Earth-like exoplanets in the habitable zone by producing high contrast ratios at small inner working angles. The primary function of a starshade is to suppress light from a target star such that its orbiting planets are revealed. In order to do so, the optical edges of the starshade must maintain their precise in-plane profile to produce the necessary apodization function. However, an equally important consideration is the interaction of these edges with light emanating from our own Sun as scattered and/or diffracted sunlight can significantly degrade the achievable contrast. This paper describes the technical efforts performed to obtain precision, low-scatter optical edges for future starshades. Trades between edge radius (i.e. sharpness) and surface reflectivity have been made and small-scale coupons have been produced using scalable manufacturing processes. A custom scattered light testbed has been developed to quantify the magnitude of scattered light over all sun angles. Models have also been developed to make predictions on the level of reflected and/or diffracted light for various edge architectures. The results of these studies have established a current baseline approach which implements photochemical etching techniques on thin metal foils.

Advances in starshade technology readiness for an exoplanet characterizing science mission in the 2020's

The discovery of thousands of exoplanets is generating increasing interest in the direct imaging and characterization of these planets. Starshade, an external occulter, could fly in formation between a telescope and distant star, blocking out the light from the star, and enabling us to focus on the light of any orbiting planets. Recent technology developments in coordination with system level design, has added much needed detail to define the technology requirements for a science mission that could launch in the 2020’s. This paper addresses the mechanical architecture, the successful efforts to date, the current state of design for the mechanical system, and upcoming technology efforts.

Multilayer active shell mirrors for space telescopes

A novel active mirror technology based on carbon fiber reinforced polymer (CFRP) substrates and replication techniques has been developed. Multiple additional layers are implemented into the design serving various functions. Nanolaminate metal films are used to provide a high quality reflective front surface. A backing layer of thin active material is implemented to provide the surface-parallel actuation scheme. Printed electronics are used to create a custom electrode pattern and flexible routing layer. Mirrors of this design are thin (< 1.0 mm), lightweight (2.7 kg/m2), and have large actuation capabilities. These capabilities, along with the associated manufacturing processes, represent a significant change in design compared to traditional optics. Such mirrors could be used as lightweight primaries for small CubeSat-based telescopes or as meter-class segments for future large aperture observatories. Multiple mirrors can be produced under identical conditions enabling a substantial reduction in manufacturing cost and complexity. An overview of the mirror design and manufacturing processes is presented. Predictions on the actuation performance have been made through finite element simulations demonstrating correctabilities on the order of 250-300× for astigmatic modes with only 41 independent actuators. A description of the custom metrology system used to characterize the active mirrors is also presented. The system is based on a Reverse Hartmann test and can accommodate extremely large deviations in mirror figure (> 100 μm PV) down to sub-micron precision. The system has been validated against several traditional techniques including photogrammetry and interferometry. The mirror performance has been characterized using this system, as well as closed-loop figure correction experiments on 150 mm dia. prototypes. The mirrors have demonstrated post-correction figure accuracies of 200 nm RMS (two dead actuators limiting performance).