The Habitable Exoplanet Imaging Mission (HabEx) Study is one of four studies sponsored by NASA for consideration by the 2020 Decadal Survey Committee as a potential flagship astrophysics mission. A primary science directive of HabEx would be to image and characterize potential habitable exoplanets around nearby stars. As such, the baseline design of the HabEx observatory includes two complimentary starlight suppression systems that reveal the reflected light from the exoplanet – an internal coronagraph instrument, and an external, formation-flying starshade occulter. In addition, two general astrophysics instruments are baselined: a high-resolution ultraviolet spectrograph, and an ultraviolet, visible, and near-infrared (UV/Vis/NIR), multi-purpose, wide-field imaging camera and spectrograph. In this paper, we present the baseline architecture concept for a 4m HabEx telescope, including key requirements and a description of the mission and payload designs.
This paper provides an overview of a feasible design architecture that satisfies the strict pointing requirements for the 2020 Astrophysics Decadal Survey Habitable Exoplanet Observatory (HabEx) Architecture A mission concept. Microthruster technology has matured significantly in recent years, with high specific impulse and low-level disturbance making microthrusters the prime candidate for high-precision pointing in upcoming space telescope missions. HabEx’s Architecture A concept utilizes microthrusters as the main actuators for the attitude control system pointing mode and a fine steering piezo-electric-operated mirror is utilized in the inner finepointing loop of the attitude control system. Sensing is undertaken using a high-resolution, low-noise focal-plane camera that can support high readout speeds (> 100 Hz), in addition to a state-of-the-art low-order wavefront sensor, which is currently under technology development for NASA’s Wide Field Infrared Survey Telescope (WFIRST).
The Space Interferometry Mission, scheduled for launch in 2008, is an optical stellar interferometer with a 10 meter baseline capable of micro-arcsecond accuracy astrometry. A mission-enabling technology development program conducted at JPL, has yielded the heterodyne interferometric displacement metrology gauges required for monitoring the geometry of optical components of the stellar interferometer, and for maintaining stable starlight fringes. The gauges have <20 picometer linearity, <10 micron absolute accuracy, are stable to <200 pm over the typical SIM observation periods (~1 hour), have the ability to track the motion of mirrors over several meters. We discuss the technology that led to this level of performance: lowcross- talk, low thermal coefficient optics and electronics, active optical alignment, a dual wavelength laser source, and a continuously averaging, high data rate phase measurement technique. These technologies have wide applicability and are already being used outside of the SIM project, such as by the James Webb Space telescope (JWST) and Terrestrial Planet Finder (TPF) missions.
The proposed spaceborne NASA-ISRO SAR (NISAR) mission would use the repeat-pass interferometric Synthetic Aperture Radar (InSAR) technique to measure the changing shape of Earth’s surface at the centimeter scale in support of investigations in solid Earth and cryospheric sciences. Repeat-pass InSAR relies on multiple SAR observations acquired from nearly identical positions of the spacecraft as seen from the ground. Consequently, there are tight constraints on the repeatability of the orbit, and given the narrow field of view of the radar antenna beam, on the repeatability of the beam pointing. The quality and accuracy of the InSAR data depend on highly precise control of both orbital position and observatory pointing throughout the science observation life of the mission. This paper describes preliminary NISAR requirements and rationale for orbit repeatability and attitude control in order to meet science requirements. A preliminary error budget allocation and an implementation approach to meet these allocations are also discussed.
Deep-Space Optical Communications is a key emerging technology that is being pursued for high data-rate
communications, which may enable rates up to ten times more than current Ka-band technology. Increasing the
frequency of communication, from Ka-band to optical, allows for a higher data rate transfers. However, as the frequency
of communication increases, the beam divergence decreases. Less beam divergence requires more accurate and precise
pointing to make contact with the receiver. This would require a three-order-of-magnitude improvement from Ka-Band
(~ 1 mrad) to optical (~ 1 urad) in the required pointing. Finding an architecture that can provide the necessary pointing
capability is driven by many factors, such as allocated signal loss due to pointing, range to Earth, spacecraft disturbance
profile, spacecraft base pointing capability, isolation scheme, and detector characteristics. We have developed a suite of
tools to 1) flow down a set of pointing requirements (Error Budget Tool), 2) determine a set of architectures capable of
meeting the requirements (Pointing Architecture Tool), and 3) assess the performance of possible architecture over the
mission trajectory (Systems Engineering Tool). This paper describes the three tools and details their use through the
case study of the Asteroid Retrieval Mission. Finally, this paper details which aspects of the pointing, acquisition, and
tracking subsystem still require technology infusion, and the future steps needed to implement these pointing
The dynamic stability of white light fringes formed on the guide and science interferometers in SIM-Lite
along with the pointing stability of each arm of each interferometer affect the visibility of fringes and the length of
the fringe camera integration time for the observatory. Hence, tight fringe and pointing stability requirements are
needed to reduce science interferometer camera integration times, which in turn help increase the all important
instrument's observing efficiency. The SIM-Lite Instrument Dynamics and Controls (D&C) System Architecture
deals with such dynamic issues through a "tailored" system dynamics design complemented by a comprehensive
active control system. The SIM-Lite on-orbit System architecture is described in this paper. Key roles played by
the resulting D&C System are also established, while the system design is clearly linked to the four nominal phases
of on-orbit operations for the observatory (Tile to Tile slew & settling, guide star acquisition, science observation, &
science interferometer retargeting). Top driving requirements dictating system interferometric-baseline stability and
repeatability, instrument pointing stability, and fringe stability are discussed here together with the resulting high
level Error Budget. Key system sensitivities and currently known D&C related design challenges are also discussed.
Kite is a system level testbed for the External Metrology System of the Space Interferometry Mission (SIM). The External Metrology System is used to track the fiducials that are located at the centers of the interferometer's siderostats. The relative changes in their positions needs to be tracked to an accuracy of tens of picometers in order to correct for thermal deformations and attitude changes of the spacecraft. Because of the need for such high precision measurements, the Kite testbed was build to test both the metrology gauges and our ability to optically model the system at these levels. The Kite testbed is a redundant metrology truss, in which 6 lengths are measured, but only 5 are needed to define the system. The RMS error between the redundant measurements needs to be less than 140pm for the SIM Wide-Angle observing scenario and less than 8 pm for the Narrow-Angle observing scenario. With our current testbed layout, we have achieved an RMS of 85 pm in the Wide-Angle case, meeting the goal. For the Narrow-Angle case, we have reached 5.8 pm, but only for on-axis observations. We describe the testbed improvements that have been made since our initial results, and outline the future Kite changes that will add further effects that SIM faces in order to make the testbed more representative of SIM.
The Space Interferometry Mission (SIM) System Testbed-3 has been integrated in JPL's new Optical & Interferometry Development Laboratory. The testbed consists of a three baseline stellar interferometer whose optical layout is functionally equal to SIM's current flight layout. The main testbed objective is to demonstrate nanometer class stability of fringes in the dim star, or science, interferometer while using path length & angle feed-forward control, and while the instrument is integrated atop a flight-like flexible structure. This work marks the first time an astrometric 3-baseline interferometer is tested in air and on a flight-like structure rather than on rigid optical tables. This paper discusses the system architecture, dim star fringe tracking, and the testbed's latest experimental results.
The Space Interferometry mission's nano-meter class System Testbed has implemented an external metrology system to monitor changes in the length & orientation of the science interferometer baseline vector, which cannot be monitored directly. The output of the system is used in real time fringe tracking of dim stars. This paper describes the external metrology system, its mathematical representation, limitations, and method for estimating the length & orientation of the science baseline vector. Simulations and current system performance are presented and discussed.
Delay lines provide the pathlength compensation that makes the measurement of interference fringes possible. When used for nulling interferometry, the delay line must control pathlengths so that the null is stable and controlled throughout the measurement. We report on a low noise, low disturbance, high bandwidth optical delay line capable of meeting the TPF interferometer optical path length
control requirements at cryogenic temperatures.
SIM System Testbed 3 (STB3) features three optical interferometers sharing a common baseline, as a dynamic representation of the SIM instrument. An artificial star feeding the interferometers is installed on a separate optics bench. All three interferometers use photons captured by avalanche photo diodes (APDs) to measure the position and quality of fringes, and additional pointing precision is achieved by fast steering mirrors (FSMs) that keep the star images centered on the beam combining optics using a CCD camera. Each interferometer uses internal metrology to measure changes in its optical pathlength. External metrology beams measure changes in the baseline vector. This system acquires and tracks white light fringes with one interferometer, while the other two acquire and track laser light fringes representing the bright guide stars that will be used by SIM. The white light source represents a dim star that cannot supply enough photons for the Science interferometer to lock onto fringes in closed-loop mode; instead it operates open-loop, using pathlength corrections fed to it from the two guide interferometers and the external metrology subsystem to reject disturbances and maintain the fringes. This tracking mode is known as Pathlength Feed Forward (PFF). The precise real-time behavior required to achieve this result is implemented by a complex set of interacting software control loops. This paper describes how these loops take advantage of the benefits of the RTC Core architecture, and how they work together to accomplish STB3's objectives.
The Space Interferometry Mission's (SIM) shared-baseline astrometric interferometer System Test Bed 3 (STB3) has been constructed at JPL. STB3's objective is to use two of its interferometers (guides) for low frequency (0 to 1 Hz) fringe stabilization in the third one (science). This approach - being proposed for the first time in the context of space based observatories - is needed given the dim nature of science stars to be observed by SIM. Fringe stability is mostly affected by the low frequency attitude motion of the test bed's instrument table, with the inevitable exception of instrument vibration, thermal drift, and atmospheric fluctuations. Relative changes in table attitude cause optical path changes in the guide interferometers, which are tracked, linearly combined and fed forward to the science interferometer's active delay line to stabilize its optical path. This technique for tracking fringes in the science interferometer is possible because the position of the guide stars relative to the science star is well known. This open loop fringe tracking technique is dubbed Path-length Feed Forward, or PFF. In STB3, current fringe stability in the science interferometer using the PFF technique is at 50 to 60 nanometers RMS (from 0 to 500 Hz). Compare this to 15 to 20 nm RMS fringe stability in the guide interferometers, which operate in closed loop mode. Vibration, thermal drift and atmospherics in the science and guide interferometers are largely eliminated with the use of an internal metrology system. By design, mechanical vibrations are above the bandwidth of the interferometer system, and are passively rejected. Nevertheless, the internal metrology system can easily reject current low-level vibrations in STB3 down to the 6-nanometer RMS level.
Fringe tracking error in the science interferometer due to atmospherics is currently about 40 nanometers RMS at frequencies below 1.0 Hz. In SIM, the error in this low frequency band must be no more than 6 nm RMS. This error arises because the optical path stabilized by the internal metrology system is not equal to taht of the starlight, so not all atmospheric fluctuations in the starlight path can be stabilized. Therefore, there is a need to reduce the strength of atmospheric fluctuations or to filter them from the PFF command. In STB3 the strength of atmospheric fluctuations is already reduced with the use of optical path enclosures, which brought these fluctuations down from ~170nm RMS to their current levels of ~66nm RMS with a spread of 20nm. Simulations show that signal to noise ratios are generally not sufficient to filter atmospheric errors on-line.
Future space-based optical interferometers such as the Space Interferometer Mission require fringe stabilization to the level of nanometers in order to produce astrometric data at the micro-arc-second level. Even the best attitude control system available to date will not be able to stabilize the attitude of a several thousand pound spacecraft to a few milli-arc-seconds. Active pathlength control is usually implemented to compensate for attitude drift of the spacecraft. This issue has been addressed in previous experiments while tracking bright stars. In the case of dim stars, as the sensor bandwidth falls below one hertz, feedback control will not provide sufficient rejection. However, stabilization of the fringes from a dim-star down to the nanometer level can be done open loop using information from additional interferometers looking at bright guide stars. The STB3 testbed developed at the Jet Propulsion Laboratory features three optical interferometers sharing a common baseline, dynamically representative to the SIM interferometer. An artificial star feeding the interferometers is installed on a separate optics bench. Voice coils are used to simulate the attitude motion of the spacecraft by moving the entire bench. Data measured on STB3 show that fringe motion of a dim star due to spacecraft attitude changes can be attenuated by 80 dB at 0.1Hz without feedback control, using only information from two guide stars. This paper describes the STB3 setup, the pathlength feed-forward architecture, implementation issues and data collected with the system.
A system for active suppression of structural vibrations has been developed. The system consists of piezoelectric ceramic actuator and sensor elements which can be either bonded on to or embedded in structural components. For active damping, these are placed at locations of high modal strain energy. For active isolation, locations of high disturbance transmissibility are chosen. Small analog and digital control electronics units have been developed which include all sensing, processing, and actuator drive electronics. The analog unit is appropriate for active damping using strategies such as positive position and integral force feedback. Damping levels in structures has been increased from 0.1% to 100% using a single analog controller. The digital system is capable of executing any algorithm having two inputs and two outputs. Active damping using feedback and active force cancellation using feedforward have been demonstrated. Block diagrams, specifications, photographs, and test results describing the elements of the modular vibration suppression system are presented.
TRW has been implementing active damping compensators on smart structures for the past five years. Since that time there have been numerous publications on the use of impedance matching techniques for structural damping augmentation. The idea of impedance matching compensators came about by considering the flow of power in a structure undergoing vibration. The goal of these compensators is to electronically dissipate as much of this flowing power as possible. This paper shows the performance of impedance matching compensators used in smart structures to be comparable to that of active damping compensators. Theoretical comparisons between active damping and impedance matching methods are made using PZT actuators and sensors. The effects of these collocated and non-collocated PZT sensors and actuators on the types of signals they sense and actuate are investigated. A method for automatically synthesizing impedance matching compensators is presented. Problems with implementing broad band active damping and impedance matching compensators on standard Digital Signal Processing (DSP) chips are discussed. Simulations and measurements that compare the performance of active damping and impedance matching techniques for a lightly damped cantilevered beam are shown.