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Large Aperture Systems
Apertures play a key role in remote sensing techniques as well as ensuring the accuracy and sensitivity of telescope mirrors and radar scanning. Researchers at JPL are leading several efforts to advance lightweight structures, adaptive optics, and wavefront sensing. By using high precision modeling and analysis, researchers have developed several technologies for use on current and future missions.
Advances in large-aperture systems make it possible to increase our knowledge of the universe and our planet. Large-aperture systems enable the Jet Propulsion Laboratory (JPL) to accomplish two important goals: (1) to further our understanding of the origins and evolution of the universe and the laws that govern it, and (2) to make critical measurements that improve our understanding of our home planet and help us protect our environment.
The next generation of observatories could benefit from additional advances in large-aperture-systems technology. These observatories could collect signals from across the electromagnetic spectrum to help answer key scientific questions: Is there life on other planets? What are the origin and formative processes of the Universe? What are the fundamental dynamical environmental processes of Earth?
Visible observatories currently being considered for development would require optical apertures of > 8 m, while maintaining a wavefront error of < 10 nm. Infrared observatories could investigate the infrared portion of the spectrum with a sub-10-K cryogenic telescope of similar diameter. Follow-ons to the Microwave Limb Sounder (MLS) could provide more precise measurement of the vertical profiles of atmospheric gases, temperature, pressure, and cloud properties, thereby improving our understanding of Earth’s atmosphere and global change from rising temperatures and other anthropogenic as well as natural causes. Additional Earth science missions could use radio frequency and lidar measurements to help predict earthquakes, measure global-change impact, and monitor groundwater.
Depiction of wavelengths of use to large aperture systems
Large-aperture systems would also be called upon to provide information relying on a very large portion of not only the electromagnetic but also the gravitational spectrum, as required by the distributed (formation flying) apertures supporting the Laser Interferometer Space Antenna (LISA),a joint European Space Agency (ESA)-NASA mission. Developments in large-aperture systems would also enable future smaller missions to increase their scientific return by driving down the mass and volume needed for apertures of a given size.
To enable such future missions, several key large-aperture system technologies must be developed:
Lightweight apertures - to transmit and/or collect the electromagnetic signal for measurement while maintaining a launch able mass and volume.
Lightweight, precision-controlled structures - to deploy and control the aperture elements.
Integrated, low-temperature thermal control - to establish and stabilize the aperture’s baseline temperature.
Advanced metrology - to measure the deformation of the precision structure and aperture elements.
Wavefront sensing and control - to measure the quality of the science data and correct the shape of the aperture elements and/or metering structure.
Precision-pointing systems - to acquire, point, and stabilize the line-of-sight of large apertures on desired targets while maintaining demanding pointing accuracies.
Collectively, as part of an integrated system, these technologies would facilitate the development of large-aperture systems with two key architectural capabilities: deployment and adaptive metrology and control. They would allow missions that are packaged in a compact volume, that could deploy to precise positions, and that could actively maintain their figure.
Large Aperture Technologies
Microwave reflector under test in the 25-foot solar simulator at JPL.
Large-aperture systems are fundamentally enabled by progress in reducing the weight of apertures that transmit, receive, and/or reflect electromagnetic signals for measurement.
Key areas of development focus on mass and volume. Advances are needed in three critical areas: lightweight optics, lightweight reflectors, and lightweight synthetic-aperture radar (SAR). Lightweight optics research has progressed from the Hubble Telescope’s mirror areal densities of 180 kg/m2 to the more recent Spitzer designs that have achieved areal densities of 28 and 20 kg/m2, respectively. Technology goals are to produce a diffraction-limited visible optic in a 2 m segment size with areal densities less than 10 kg/m2. Key enabling features might include actuated mirrors, fiber-reinforced materials with a zero coefficient of thermal expansion (CTE), and damping.
Lightweight reflectors for operation in the sub-millimeter region have reduced surface-figure requirements and benefit from technology developments similar to those for lightweight optics. Monolithic apertures up to 8 m in size would be required to support a follow-on mission to the Microwave Limb Sounder (MLS). These apertures need similar areal densities (5 to 10 kg/m2) but would require increased thermal stability because mission concepts dictate that they be exposed to direct sunlight. Key enabling technologies might include active materials and zero-CTE–fiber-reinforced materials.
Lightweight, Precision-Controlled Structures
Strong, lightweight, dimensionally precise, and dynamically stable deployable structures that position lightweight apertures would be a fundamental enabling capability for future space exploration. Lightweight precision-controlled structures would involve multiple sub-technologies, including lightweight, deployable mounting structures and boom-/strut-supported membrane antennas for radar. These technologies would enable increased aperture sizes across the electromagnetic spectrum in the visible to infrared, sub-millimeter, or microwave regions involving apertures too large to be stowed unfolded in a launch shroud. Dimensional stability is an overriding structural design driver for these large deployable apertures; their stability is driven by constraints derived from system mass and stiffness, and thermal and dynamical loads.
Integrated, Low-Temperature Thermal Control
Large-aperture systems require thermal control to maintain stability. Thermal control technology could enable future astronomy/astrophysics missions, including cosmic microwave background measurement, galactic and stellar evolution observations and extrasolar planet detection. These most extreme requirements are driven by optical structures where visible wavelengths dictate the structural deformation allowed and hence dictate vibration and thermal environment control. Longer wavelengths, from the infrared to the sub-millimeter and millimeter regions of the electromagnetic spectrum, also require thermal control to meet error budgets at large aperture sizes. Very cold apertures would be needed for ultrasensitive observations at these wavelengths, with demanding requirements on uniformity of temperature across the aperture.
Integrated low-temperature thermal control encompasses multiple technologies, including milli kelvin coolers, cryo cooled apertures, integrated cooler and detector systems, and large deployed sunshades.
Metrology refers to the highly accurate measurement of the metering structure. The next generation of astrophysics missions would require precise control of active optics on flexible structures. Advances in metrology subsystems architectures, components, and data processing would be necessary, with particular emphasis on developing multiple laser-beam launchers for ultra-high-precision dimensional measurements in spatial interferometry, as well as on very-high precision dimensional measurements for future deployable radar phased-array antennas. Advanced metrology would be needed using two different approaches: point-to-point and imaging. Point-to-point metrology would require the development of high-precision metrology gauges and optical fiducials, integrated lightweight beam launchers, ultra stable lasers, and frequency shifters for metrology gauges. Imaging-based metrology would require the development of full-aperture metrology gauges and embedded grating technology. Both approaches would benefit from radiation hardened fiber-optic systems and components for routing metrology laser signals.
Wavefront Sensing and Control
Wavefront sensing and control is an essential component of active and adaptive optics. In the case of adaptive optics, which typically refers to systems targeting the correction of the deleterious effects of atmospheric turbulence, the fundamental advantages of larger apertures are: (1) more collecting power for greater sensitivity, and (2) higher spatial resolution. The latter advantage is compromised by atmospheric turbulence when observing the ground from space, or space from the ground, with meter-class apertures. Adaptive optics systems are designed to update readings of turbulence-induced wavefront distortions and to correct at the high temporal frequencies (up to ~ 3 kHz) encountered in atmospheric turbulence. On the other hand, active optics are also designed to correct various types of system deformations, such as structural and fabrication imperfections, systematic responses to thermal loads on the primary mirror, or support-structure characteristics and response. Active optics systems are also a major element in the development of lightweight, precision-controlled structures.
Active optics systems update at much slower rates and can use different correction elements. Wavefront sensing and control refers to both the hardware and software technologies that power active and adaptive optical systems. As its name suggests, wavefront sensing provides a means of measuring and comparing the measured wavefront in an optical system with the ideal wavefront. Wavefront sensing can also be employed to fingerprint an optical system, i.e., retrieving the system’s nominal optical prescription as well as indicating the origins of the observed aberrations. Wavefront control is the means by which inputs obtained from wavefront sensing are transformed into changes in deformable or otherwise reconfigurable elements within an optical system, bringing the measured wavefront into closer conformance with the ideal. High-speed adaptive optical systems utilize rapidly deformable mirrors in the optical system that are much smaller than the telescope primary mirror, yet are able to correct wavefront aberrations across the entire aperture area. Technology advancements needed for wavefront sensing and control include fast deformable mirrors with high spatial-frequency capability, high-sensitivity wavefront aberration sensors, and efficient algorithms for computation of phase-front and alignment corrections, and high-precision actuators.
Precision pointing would be an integral part of the next generation of large-aperture observatories, including telescopes, interferometers, coronagraphs, and Earth-observing systems. Increasingly stringent pointing accuracy and stability levels would be required for future missions to capture images of distant worlds, measure distance between dim stars, enable long science exposures to image Earth-like planets around nearby solar systems, or to investigate from space small-scale features of our planet with instruments characterized by a narrow field of view (FOV) or pencil beams.
Precision pointing requires rejection of disturbances across a wide spectrum of frequencies. Low-frequency disturbances are generally introduced by sources external to the spacecraft, such as solar pressure or atmospheric drag. High-frequency disturbances are introduced by internal sources such as reaction wheels, thrusters, or payload-cooling systems. Different disturbance sources excite different spacecraft structural modes, and pointing stability is achieved through stabilization of these vibration modes to meet performance requirements. Large-aperture, lightweight, flexible structures are particularly challenging due to control-structure interactions, requiring a control-oriented design framework. Segmented apertures bring additional challenges associated with controlling multiple segments using a large number of actuators and sensors.
The Guide 2 telescope (G2T) test bed, shown here in the vacuum chamber, has demonstrated star-tracking capability at an unprecedented 30 μas level.
Large-aperture pointing systems typically require a hierarchical control approach where the problem is decomposed into multiple coordinated control loops, defined by separating time scales and spatial degrees of freedom. Examples include alignment and calibration strategies, low-frequency active control (such as the spacecraft attitude control system), mid-frequency passive vibration isolators (such as reaction-wheel isolators, spacecraft isolators, solar array dampers), and high-frequency stabilization (such as fast-steering mirrors and hexapods). Pointing design can also be driven by special requirements such as object tracking and coordinated mirror scanning. Achieving precision pointing performance in large-aperture, lightweight, flexible structures would require advancements in such areas as hierarchical pointing systems, fine-pointing sensors and actuators, optimal fusion of multiple sensor types (fine-guidance sensors, inertial reference units, star trackers, Shack-Hartmann wavefront sensors, edge-sensors, phase-retrieval cameras, temperature sensors, etc.), knowledge-transfer systems, minimization of control-structure interactions, and structural design for maximum control authority. Multidisciplinary modeling efforts and experimental validation are essential elements in developing an advanced pointing system design. Modeling efforts involve developing high-fidelity integrated models of optical, structural, and thermal effects, actuators and sensors (including nonlinearities and hysteresis), and environmental disturbances. Experimental validation of subarcsecond pointing performance would require an investment in high-fidelity ground-based test beds.
Selected Research Topics
Large Space Structures: Self-Assembling Large Telescope
The overall goal of this three-year technical development program started in December 2009 is to create and demonstrate the technology fundamental to the development of a 100 m class coherent aperture that is both segmented and sparse.
An alternative approach already extensively investigated at JPL and elsewhere is a formation of separate satellites, each carrying a mirror segment, maintaining precision alignment while the telescope is in observation mode. The approach is based on the concept of in-space self-assembly of a mosaic mirror from separate segments. Each segment is attached to a low-cost small satellite able to execute autonomous rendezvous and docking maneuvers. Four research themes that are central to this concept: (1) architecture of a telescope with a primary aperture formed by separate mirror segments, and shape control of the mirror segments; (2) lightweight, thermally stable, active mirrors that could be made at low cost and would be able to maintain figure accuracy without thermal protection; (3) optimal guidance, navigation, and control algorithms to enable individual satellites to autonomously reach their operational configuration and dock to a central cluster; (4) validation of the performance of a distributed telescope using ground measurements on mirror segments combined with computer simulation. Each theme is being studied by a JPL/Caltech research team.
Exoplanet Exploration Coronagraph and Occulter Technology Infrastructure at JPL
A new test facility has been created at JPL to aid in the search for exoplanets. This facility contains test equipment dedicated to LYOT Coronagraph configuration, band-limited occulting masks, shaped-pupil masks, vector-vortex masks, Phase-Induced-Amplitude-Apodization (PIAA) coronagraphm, narrow or broad band coronagraph system demos, investigation of novel system configurations (e.g., DM placement), coronagraph model validation & error budget sensitivities.