We summarize the current best polychromatic (∼10% to 20% bandwidth) contrast performance demonstrated in the laboratory by different starlight suppression approaches and systems designed to directly characterize exoplanets around nearby stars. We present results obtained by internal coronagraph and external starshade experimental testbeds using entrance apertures equivalent to off-axis or on-axis telescopes, either monolithic or segmented. For a given angular separation and spectral bandwidth, the performance of each starlight suppression system is characterized by the values of “raw” contrast (before image processing), off-axis (exoplanet) core throughput, and post-calibration contrast (the final 1-sigma detection limit of off-axis point sources, after image processing). Together, the first two parameters set the minimum exposure time required for observations of exoplanets at a given signal-to-noise, i.e., assuming perfect subtraction of background residuals down to the photon noise limit. In practice, residual starlight speckle fluctuations during the exposure will not be perfectly estimated nor subtracted, resulting in a finite post-calibrated contrast and exoplanet detection limit whatever the exposure time. To place the current laboratory results in the perspective of the future Habitable Worlds Observatory (HWO) mission, we simulate visible observations of a fiducial Earth/Sun twin system at 12 pc, assuming a 6 m (inscribed diameter) collecting aperture and a realistic end-to-end optical throughput. The exposure times required for broadband exo-Earth detection (20% bandwidth around λ=0.55 μm) and visible spectroscopic observations (R=70) are then computed assuming various levels of starlight suppression performance, including the values currently demonstrated in the laboratory. Using spectroscopic exposure time as a simple metric, our results point to key starlight suppression system design performance improvements and trades to be conducted in support of HWO’s exoplanet science capabilities. These trades may be explored via numerical studies, lab experiments, and high-contrast space-based observations and demonstrations.
NASA’s Habitable Worlds Observatory addresses the challenging goal of characterizing numerous Earth-like exoplanets orbiting nearby stars. While the baseline approach is to carry out the observations with a coronagraph, current planning calls for the observatory to be “starshade ready” so that it can take advantage of the superior throughput, working angle, contrast, and bandwidth when the telescope is paired with a starshade. We describe two starshade designs that together enable imaging in the UV, visible, and NIR bands, as well as a multi-starshade configuration that could efficiently discover and characterize exoplanets. Additionally, we estimate the stellar light leakage and solar light scatter from micrometeoroid impacts and show that after 10 years on orbit, the stellar leakage will have contrast below 10^-11 while solar scatter will be fainter than V=31 mag.
NASA is embarking on an ambitious program to develop the Habitable Worlds Observatory (HWO) flagship to perform transformational astrophysics, as well as directly image ∼ 25 potentially Earth-like planets and spectroscopically characterize them for signs of life. This mission was recommended by Astro2020, which additionally recommended a new approach for flagship formulation based on increasing the scope and depth of early, pre-phase A trades and technology maturation. A critical capability of the HWO mission is the suppression of starlight. To inform future architecture trades, it is necessary to survey a wide range of candidate technologies, from the relatively mature ones such as the ones described in the LUVOIR and HabEx reports to the relatively new and emerging ones, which may lead to breakthrough performance. In this paper, we present a summary of an effort, funded by NASA’s Exoplanet Exoplaration Program (ExEP), to survey potential coronagraph options for HWO. In particular, our results consist of: (1) a database of different coronagraph designs sourced from the world-wide coronagraph community that are potentially compatible with HWO; (2) evaluation criteria, such as expected mission yields and feasibility of maturing to TRL 5 before phase A; (3) a unified modeling pipeline that processes the designs from (1) and outputs values for any machine-calculable criteria from (2); (4) assessments of maturity of designs, and other criteria that are not machine-calculable; (5) a table presenting an executive summary of designs and our results. While not charged to down-select or prioritize the different coronagraph designs, the products of this survey were designed to facilitate future HWO trade studies.
Habitable Worlds Observatory (HWO) will search for biosignatures from Earth-size exoplanets in the habitable zones of nearby stars. The wavelength range for biosignatures used by the HabEx and LUVOIR mission concept studies was 200 nm to 2 microns and, as such, this is a candidate wavelength range for HWO. The visible wavelength range (500-1000 nm) provides for detection of water, oxygen, and Raleigh scattering; the near-ultraviolet is valuable for detection of ozone; and the near-infrared enables detection of carbon dioxide and methane for Earth-like atmospheres. Damiano et al. 2023 showed the significant improvement in spectral retrieval reliability when the NUV and NIR are both used with the visible. However, the challenge of the NUV, in addition to the technological and engineering challenges of starlight suppression in the NUV, is the drop in flux of host stars. In the NIR, the challenge is the geometric access to the habitable zone due to the wavelength dependency of the inner working angle limit of coronagraphs. For these reasons, exoplanet yields are lower in the NUV and NIR than in the visible and some instrument parameters are more critical for improving NUV and NIR yields than others. In this paper we present a new capability for performing a large number of end-to-end yield modeling simulations to enable large, multivariate parameter sweeps. We utilize this capability to calculate the Visible, NIR, and NUV yield sensitivities to the instrument parameters: aperture diameter, coronagraph core throughput, contrast, and inner working angle (IWA). We find that parameter interactions are important in determining yield, the most important of which is the interaction between contrast and IWA, but that the strength of that interaction is different in each of the three wavebands.
Decisions made early in a mission design, when information is sparse, define most of the downstream development cost. This becomes particularly problematic when uncertainties will not be revealed until later in the design life cycle. A resilient architecture is one that is adaptable to uncertainty, permitting cost-effective architectural changes as uncertainties reveal themselves. A framework is proposed for designing a resilient architecture for NASA’s Habitable Worlds Observatory (HWO). Uncertainties include knowledge of exo-Earth targets prior to launch, needed spectral bands to mitigate ambiguity in habitability, and performance limits of starlight suppression technologies. Precursor science and technology advancements drive architecture definition more than the converse. In essence, it is better to plan for, then react to, uncertainty.
The Astro2020 Decadal Survey recommended a “future large IR/O/UV telescope optimized for observing habitable exoplanets and general astrophysics” that would “search for biosignatures from a robust number of about 25 habitable zone [exo]planets,” now dubbed the Habitable Worlds Observatory (HWO). The search for biosignatures requires high quality spectra over a broad bandwidth and sufficient signal-to-noise. The combination of wavelength, spectral resolution, bandwidth, and signal-to-noise-ratio impacts the number of exo-Earths that can be spectrally characterized. Previous work (Morgan et al. 2022) evaluated the number of Earth-size, habitable zone exoplanets (denoted here as yield) that could be spectrally characterized over the wavelength range of 500 nm to 1000 nm for a 6-m diameter exoplanet direct imaging mission for coronagraph-only and hybrid coronagraph-starshade architectures for three prior knowledge cases: the nominal case of a blind-search survey, the upper-bound case of perfect prior knowledge, which is useful to determine if target depletion occurs, and the partial prior knowledge case of a hypothetical extreme precision radial velocity survey. In this paper, we extend previous exoplanet yields to include wavelengths out to 1.8 microns. Because the IWA for coronagraphs is proportional to wavelength, the achievable spectral coverage will be different for every planet detected. We present the spectral coverage achieved across individual target stars, as well as the ensemble target set, for a coronagraph-only architecture and a hybrid coronagraph + starshade architecture. We use the three prior knowledge cases. The coronagraph spectral characterization is simulated in the near infrared for each of the 10% sub-bands individually, as if it were the only spectral characterization performed during the mission, and then as a broadband spectral characterization performed in sequence over the sub-bands. The starshade achieves the broadband spectral equivalent simultaneously. We also examine the capabilities of the 60 m starshade point design and investigate the benefits of refueling.
NASA is currently responding to the recommendations of the astrophysics decadal survey via its Great Observatories Maturation Program and preparing for the maturation of the Habitable Worlds Observatory, which will have a key science goal of exoplanet detection and characterization. An important element of this process is the evaluation of the impact of mission design decisions on mission science outcomes. One approach to such science yield modeling is via Monte Carlo Mission Simulation (MCMS) - the generation of ensembles of simulated mission schedules from which performance metric statistics can be derived. MCMS requires the ability to automatically schedule such observing sequences based on a mission concept’s stated operating rules. However, inefficiencies in the scheduler can lead to suboptimal performance and decreases in expected science yield that are not driven by any design decisions. Here, we discuss approaches to quantifying the impacts of schedulability and scheduling inefficiencies on science yields, and present a new method for validating scheduler efficiency.
We present a design for an active telescope for space astronomy. The telescope is capable of both exoplanet work and general astronomy over wavelengths from ∼100 nm up to 5 μm. The primary mirror is 6 m in diameter, formed by 16 mirror segments that are precisely phased and supported on rigid body actuators and with segment optical surface figures fine-tuned using surface figure actuators. The active primary forms a large deformable mirror (DM) with wavefront error (WFE) correction at the entrance pupil. Thus the largest source of WFE can be removed at the source and is corrected over the entire field of view. This enables diffraction-limited performance at 400 nm and a more efficient optical system over a broader wavelength range than could be achieved by a small DM at a downstream relayed pupil. The telescope is passively cooled to below 100 K at Sun–Earth L2, enabling astronomical-background-limited observations out to 5 μm. Launched on a SpaceX Starship or alternatively National Aeronautics and Space Administration’s Space Launch System, the telescope requires minimal deployments. A 72-m-diameter starshade provides a contrast ratio better than 10 − 10 for exoplanet science. Near the visible region, with a 108% working bandwidth from 300 to 1000 nm, a working distance of 120 Mm provides a 51-mas inner working angle (IWA). This band can be moved to shorter or longer wavelengths by adjusting the starshade range from the telescope. Our first-ever thermal analysis of such a starshade shows that a temperature below 100 K can be achieved over a broad range of observing directions, permitting the possibility of working into the infrared. We model the yield in exoplanets that can be observed. A starshade and associated spectrograph offer significant advantages for exoplanet characterization. They enable a much broader instantaneous spectral bandwidth (here 108%) than current coronagraphs (∼10 % to 20% bandwidth), allow both polarizations to be observed simultaneously, and have higher throughput. The IWA is twice as small as can be achieved with a coronagraph and there is no outer working angle. These differences are particularly pronounced in the UV, where coronagraph performance would be strongly affected by throughput losses, wavefront aberrations, Fresnel polarization effects at surfaces, and thermal instability.
The Astro2020 Decadal Survey recommended as the next strategic astrophysics mission a 6 m class space telescope capable of high-contrast direct imaging of Earth-size exoplanets in about one hundred habitable zones of nearby sun-like stars. The expected number of imageable exoplanets for such a telescope depends on the architecture and the metrics used to evaluate those architectures. In this paper, we assess the yield of notional coronagraph-only, starshade-only, and hybrid starshade/coronagraph architectures for several metrics. We evaluate the exoplanet yield for a 20% bandwidth, SNR=5, R=70 water search metric; a 20% bandwidth, SNR=8.5, R=140 oxygen search metric; and a 4x20% bandwidth metric, SNR=8.5, R=7 for 450-700 nm and R=140 for 700-1000 nm, which is tailored for a coronagraph’s sequence of 20% bandwidth sub-spectra. We bound the number of expected exoplanets by considering three cases of a priori knowledge: the case of no prior knowledge that requires a photometric blind search for exoplanets; the theoretical case of perfect prior knowledge that skips the photometric blind search and performs only spectral characterizations using realistic mission scheduling constraints (this approach shows the upper bound and when target exhaustion is reached); and a case of partial prior knowledge via a hypothetical, future extreme precision radial velocity instrument with 3 cm/s sensitivity. This work is an initial study of the potential exoplanet science return for the Decadal-recommended large infrared/optical/UV Great Observatory (IROUV).
A key aspect of the search for earth-like exoplanets with direct imaging, is determining if the exoplanet is in the habitable zone. For mission design of potential future direct imaging missions, such as HabEx and LUVOIR, an efficient cadence of observations is needed. Previous work has shown that three epochs, spanning more than half a period, is the minimum to determine orbital parameters to 10%. One aspect that still needs improvement is the ability to fit multiple planets with limited prior information about which planet is which. Since data from direct observations is expected to consist of multiple objects at each epoch, looking at each epoch separately is not sufficient to decide whether 1) a detected object is part of an exosolar system and 2) which planet it corresponds to. Existing multi-planet trajectory matching libraries, such as “Orbits For The Impatient” (OFTI), currently require the user to specify which data points belong to which planet. This assumes that the user has already matched true-positive detections to planets. Additionally, this planet matching between detected objects needs to be taken into account when assessing the impact of observation scheduling on the accuracy of trajectory estimation. To address this need for fitting orbits to multiple objects with limited knowledge, we propose an approach that uses a Monte Carlo study of different observation schedules and planetary systems. For each case we automatically match observations to planets and check the accuracy of the match. By considering a large number of such cases, we provide constraints on the number of observations and their spacing necessary to “deconfuse” the detections. We present preliminary planet matching success rates for several observation schedules based on simulated planetary systems and assess the accuracy of trajectory fitting combined with OFTI.
KEYWORDS: Coronagraphy, Planets, Stars, Signal to noise ratio, Target detection, Monte Carlo methods, Telescopes, Exoplanets, Device simulation, Observatories
The HabEx and LUVOIR mission concepts reported science yields for mission scenarios in which the instruments must search for potentially habitable planets, determine their orbits, and, if worthwhile, invest the integration time for a spectral characterization. We evaluate the impact of prior knowledge of planet existence and orbital parameters on yield for four mission concept architectures: HabEx 4m telescope with hybrid starshade and coronagraph, HabEx 4m telescope with starshade only, HabEx 4m telescope with coronagraph only, and LUVOIR B 8m telescope with coronagraph only. We use perfect prior knowledge to establish an upper bound on yield and use partial prior knowledge from a potential future extreme precision radial velocity (EPRV) instrument with 3 cm / s sensitivity. We detail a modeling framework that performs dynamically responsive observation scheduling with realistic mission constraints. We evaluate exo-Earth yields against three metrics of spectral characterization for the four mission architectures and three levels of prior knowledge (none, partial, and perfect). The EPRV provided prior knowledge increases yields by ∼30 % and accelerates by a factor of 3 to 6 the time to achieve half of the yield of the mission. Prior knowledge makes all the mission architectures more nimble and powerful, and most especially starshade-based architectures. With prior knowledge, a small telescope with a starshade can achieve comparable yield to a larger telescope with a coronagraph.
We present an analytical model for the desired kinematics of the starshade-telescope relative motion during exoplanet direct imaging observations. We combine this model with an existing deadbanding strategy published by the NASA JPL S5 Team to define a dynamics framework for deadbanding simulations. Global results of these simulations show that the fuel usage and the number of observation interruptions vary as a function of the target star ecliptic coordinates and time, meaning there exist optimal times to observe particular targets. We combine these results with the telescope pointing constraints due to the relative position of the Sun and other bright solar system objects. We show that optimally scheduling an observation could result in up to 30 more min of integration time and 26 fewer interruptions per observation, improvements of almost 300% in some cases. We also show how phasing the start time of the telescope on its halo orbit is paramount for ensuring optimal observations, providing up to 68 additional min and 31 fewer interruptions per observation. Choosing an optimal halo phasing can also increase, for some near-ecliptic target stars, the fraction of a year that the target is observable from a few percent to more than 30%.
We consider the scientific benefits and technical feasibility of a 6-m, non-deployed, cold space telescope mission concept, covering the ultraviolet, visible, near-infrared, and mid-infrared wavebands, for direct imaging of exoplanets and a broad range of astronomical investigations. The concept uses the largest practical aperture size that can be launched without deployment, for lower risk and cost. An innovative, rigid outer barrel and sunshield control temperature and stray light in a compact, Spitzer-like configuration that provides a 100-K telescope. Additional active and passive thermal features provide millikelvin temperature stability. The ultraviolet and visible instruments are based on the suite developed for the Habitable Exoplanet Observatory concept. The cold telescope enables the scientifically important addition of mid-infrared imaging and spectroscopy modes, providing background-limited imaging to 5 um wavelength. The telescope uses actively-controlled mirrors to compensate for cool-down aberrations, other optical uncertainties, and tolerances or errors that may occur in manufacturing, assembly, launch, and on-orbit operations. A starshade provides high-dynamic-range imaging and spectroscopy of exoplanets, potentially augmented by a coronagraph for exoplanet search and orbit measurement. Special attention has been paid to contamination control, assessing the feasibility of UV imaging with a cryogenic telescope. The paper will provide design details and assessment of scientific yield and technology readiness, while addressing real and perceived issues for a space telescope capable of covering this wide wavelength range.
The National Academies’ Decadal Survey telescope studies have produced mission design concepts that plot pathways into the future to follow on from Hubble, Spitzer, JWST and NGRST. Considering the results of the LUVOIR and HabEx studies in particular, it is clear that segmented mirrors will eventually be needed to provide very large apertures in space and that this architecture presents both a scientific opportunity and an engineering challenge. Furthermore, while HabEx and LUVOIR cover a great deal of spectrum, both fall short of the mid-IR region where general astronomy and astrophysics can be undertaken that would be impossible from terrestrial observatories and where there also exist spectral features of interest in the search for life. A telescope with similar capabilities to Habex/LUVOIR but also capable of exoplanet work in spectral regions up to 5 μm would largely bridge the gap between those proposals and TPF-I (which would have operated from about 7 μm upwards), and is therefore worthy of study. The Active Telescope for Space Astronomy (ATSA) design study presents a possible architecture and is moderately sized (6 m) to enable the use of both starshade and coronagraph technologies. While the segment gaps of a segmented primary mirror present a challenge for coronagraphy, the architecture does allow direct wavefront control at each segment of that mirror, enabling a great degree of control at the primary source of contrast degradation. While active systems (for example, deformable mirrors on WFIRST CGI) are being incorporated into telescope designs today, a fully active mirror system needs further development for a future mission. With this concept in mind, and intending to build on the LUVOIR and HabEx studies, we discuss the elements of a cooled telescope design enabling both general astrophysics and exoplanet studies from the near UV through to the near-IR.
Accurate characterization of exoplanet orbits requires multiple observations made over one or more orbital periods. We use a rejection sampling algorithm to study how uncertainty in the observation data manifests in the computed orbital parameters and how this uncertainty depends on the number and spacing of observations. We find that 3 observations equally spaced over at least one half of an orbital period robustly reduces the uncertainty in the semi-major axis and eccentricity to below 10%. We also study modifications of this revisit strategy in the presence of a central obscuration and show how null observations may still be leveraged to constrain a detected exoplanets orbital parameters. Finally, we suggest a method for incorporating photometric data into the rejection sampling framework to break degeneracy in the orbit fitting procedure and further constrain the orbital parameters.
The HabEx and LUVOIR mission concepts aim to directly image and spectrally characterize potentially habitable exoplanets. We use EXOSIMS to simulate design reference missions with observation scheduling to determine yield of exoplanets detected, spectrally characterized, and orbits determined. EXOSIMS performs dynamically responsive scheduling with realistic mission observing constraints on Monte Carlo universes of synthetic planets around known nearby stars. We use identical astrophysical inputs and the individual observing scenarios of each concept to evaluate a common comparison of the detection and spectral characterization yields of HabEx and LUVOIR. HabEx is evaluated for the 4m hybrid starshade and coronagraph architecture, the 4m coronagraph only architecture, and the 3.2 m starshade only architecture. LUVOIR is evaluated for the 15 m architecture presented in their interim report and the 9 m architecture of their final report. Yield analysis shows that both concepts can directly image and spectrally characterize earth-like planets in the habitable zone and that each concept has complementary strengths.
Since the 2010 Decadal Survey, the technologies needed for direct imaging of exoplanets advanced significantly. NASA investment in these technologies, prioritized in the 2010 Decadal Survey, have ripened to a maturity to enable direct imaging of earthlike exoplanets. For the first time since the discovery of exoplanets, a direct imaging mission can be conceived to start in less than ten years, possibly as soon as five years.
The HabEx Observatory Concept design utilizes technologies that are state of the art or near to state of the art with clear paths of development. The philosophy of the design favors as high a Technology Readiness Level (TRL) as possible to minimize risk. We discuss the HabEx technology challenges and assess the TRL expected by the submission of the Final Report in 2019. Many of the enabling technologies are at, or expected to be at, TRL 5 by 2019, and the remaining technologies are at TRL 4. We update the technology maturity roadmap with technology advances in the past year and expand it to include an Architecture option which is a 3.2 m diameter on-axis segmented aperture with a starshade only. The starshade suppresses starlight before it enters the telescope, allowing the telescope optical performance and stability to be significantly looser than for a coronagraph, thus enabling a segmented primary mirror design that can meet stability requirements with minimal advancement from the state of the art. We assess the exoplanet-driven technologies of HabEx, including starshades, coronagraphs, deformable mirrors, wavefront control, 4 m aperture mirrors, jitter mitigation, segmented mirror stability, and low-noise detectors.
The HabEx mission concept is intended to directly image planetary systems around nearby stars, and to perform a wide range of general astrophysics and solar system observations. The baseline HabEx design would use both a coronagraph and a starshade for exoplanet discovery and characterization. We describe a lower-cost alternative HabEx mission design, which would only use a starshade for exoplanet science. The starshade would provide excellent exoplanet science performance, but for a smaller number of detected exoplanets of all types, including exoEarth candidates, and a smaller fraction of exoplanets with measured orbits. The full suite of HabEx general astrophysics and solar-system science would be supported.
The Habitable Exoplanet Observatory (HabEx) is one of four mission concepts under study for the 2020 Astrophysics Decadal Survey. Its goal is to directly image and spectroscopically characterize planetary systems in the habitable zone around nearby sun-like stars. Additionally, HabEx will perform a broad range of general astrophysics science enabled by 115 to 2500 nm spectral range and 3 x 3 arc-minute FOV. Critical to achieving the HabEx science goals is a large, ultrastable UV/Optical/Near-IR (UVOIR) telescope. The baseline HabEx telescope is 4-meter off-axis unobscured, diffraction limited at 400 nm with wavefront stability on the order of a few 10s of picometers. The technology readiness level (TRL) to manufacture and test the HabEx baseline primary mirror is assessed to be at TRL-6 for all but two TRL-4 technologies: 1) non-destructive process to quantify CTE homogeneity of a 4-m mirror substrate with a spatial sampling of at least 100 x 100 to better than +/- 1 ppb/K; and, 2) process to quantify self-weight gravity deflection to better than 4-nm rms over a 100 x 100 spatial sampling. This paper reviews the technology needs to manufacture the HabEx primary mirror, assesses their TRL and proposes a roadmap to mature the two remaining technologies to TRL-6.
HabEx Architecture A is a 4m unobscured telescope mission concept optimized for direct imaging and spectroscopy of potentially habitable exoplanets, and also enables a wide range of general astrophysics science. The exoplanet detection and characterization drives the enabling core technologies. A hybrid starlight suppression approach of a starshade and coronagraph diversifies technology maturation risk. In this paper we assess these exoplanet-driven technologies, including elements of coronagraphs, starshades, mirrors, jitter mitigation, wavefront control, and detectors. By utilizing high technology readiness solutions where feasible, and identifying required technology development that can begin early, HabEx will be well positioned for assessment by the community in 2020 Astrophysics Decadal Survey.
KEYWORDS: Systems modeling, Space telescopes, Systems engineering, Exoplanets, Monte Carlo methods, Diffraction, Telescopes, Optical instrument design, Space observatories
We present yield modeling results for the HabEx concept study using EXOSIMS. EXOSIMS (Exoplanet Open-Source Imaging Mission Simulator) provides a parametric estimate of science yield of mission concepts using contrast curves from physics-based diffraction model codes and Monte Carlo simulations of design reference missions using realistic observing constraints.
Two baseline architecture configurations and two extended configurations are compared. We compare a configuration with a coronagraph to a configuration with a starshade for both detection and spectral characterization. The input parameters, including astrophysical assumptions, are detailed. We show sensitivity to key design parameters around design space local to the point designs. The yield results provide an analysis of the relative performance of telescope and instrument design that enable system engineering decisions.
EXOSIMS is an open-source simulation tool for parametric modeling of the detection yield and characterization of exoplanets. EXOSIMS has been adopted by the Exoplanet Exploration Programs Standards Definition and Evaluation Team (ExSDET) as a common mechanism for comparison of exoplanet mission concept studies. To ensure trustworthiness of the tool, we developed a validation test plan that leverages the Python-language unit-test framework, utilizes integration tests for selected module interactions, and performs end-to-end crossvalidation with other yield tools. This paper presents the test methods and results, with the physics-based tests such as photometry and integration time calculation treated in detail and the functional tests treated summarily. The test case utilized a 4m unobscured telescope with an idealized coronagraph and an exoplanet population from the IPAC radial velocity (RV) exoplanet catalog. The known RV planets were set at quadrature to allow deterministic validation of the calculation of physical parameters, such as working angle, photon counts and integration time. The observing keepout region was tested by generating plots and movies of the targets and the keepout zone over a year. Although the keepout integration test required the interpretation of a user, the test revealed problems in the L2 halo orbit and the parameterization of keepout applied to some solar system bodies, which the development team was able to address. The validation testing of EXOSIMS was performed iteratively with the developers of EXOSIMS and resulted in a more robust, stable, and trustworthy tool that the exoplanet community can use to simulate exoplanet direct-detection missions from probe class, to WFIRST, up to large mission concepts such as HabEx and LUVOIR.
We report on our ongoing development of EXOSIMS and mission simulation results for WFIRST. We present the interface control and the modular structure of the software, along with corresponding prototypes and class definitions for some of the software modules. More specifically, we focus on describing the main steps of our high-fidelity mission simulator EXOSIMS, i.e., the completeness, optical system and zodiacal light modules definition, the target list module filtering, and the creation of a planet population within our simulated universe module. For the latter, we introduce the integration of a recent mass-radius model from the FORECASTER software. We also provide custom modules dedicated to WFIRST using both the Hybrid Lyot Coronagraph (HLC) and the Shaped Pupil Coronagraph (SPC) for detection and characterization, respectively. In that context, we show and discuss the results of some preliminary WFIRST simulations, focusing on comparing different methods of integration time calculation, through ensembles (large numbers) of survey simulations.
NASA's Astrophysics Division plans to initiate mission concept studies in 2016 of large candidate astrophysics missions for consideration by the 2020 Decadal Survey. The studies are expected to include two mission concepts capable of directly imaging exo-earths (HabEx and LUVOIR). Direct imaging of an exo-earth begins with starlight suppression, which is required at a depth of 10-10 in the visible for an earth-sun twin. The current results of laboratory coronagraphs are approaching the levels needed for the direct detection and characterization of an exo-earth. Other critical technologies are needed, such as ultra-low noise detectors, large format deformable mirrors, a large aperture space telescope, and sophisticated post-processing algorithms. While technologically challenging, the goal is not impossible; many of the required technologies are already at TRL 3 and beyond. After the successful on-orbit operation of WFIRST-AFTA in the next decade, some of the technologies will be at TRL 9. This paper summarizes the needed technologies that NASA's Exoplanet Exploration Program is prioritizing for maturation.
Lead magnesium niobate (PMN) actuators are electrostrictive actuators with high dynamic range used in deformable
mirrors. Actuator fault detection in deformable mirrors typically occurs through optical testing. We developed a nonoptical
method for detecting actuator faults via low electric field resonance testing. The low electric field resonance
method is standard practice for characterizing piezoelectric materials. The piezoelectric/electrostrictive coefficient
couples the electrical and mechanical impedance of the actuator; a change in the mechanical boundaries (force) on the
actuator results in a shift of the impedance resonances. We demonstrate experimentally that a PMN actuator can fracture
but retain functionality under compression and that the fracture can be detected by measuring the impedance resonances
at various bias voltages (various values of tension and compression). A concurrent optical test using a displacement
interferometer was used to corroborate the results. We propose the impedance resonance approach as a non-optical fault
detection test for in-situ actuators.
We have developed a new, adaptive cross-correlation (ACC) algorithm to estimate with high accuracy the shift as large
as several pixels in two extended-scene images captured by a Shack-Hartmann wavefront sensor (SH-WFS). It
determines the positions of all extended-scene image cells relative to a reference cell using an FFT-based iterative
image-shifting algorithm. It works with both point-source spot images as well as extended scene images. We have also
set up a testbed for extended-scene SH-WFS, and tested the ACC algorithm with the measured data of both point-source
and extended-scene images. In this paper we describe our algorithm and present our experimental results.
We have implemented a testbed to demonstrate wavefront sensing and control on an extended scene using Shack-Hartmann and MGS phase retrieval simultaneously. This dual approach allows for both high sensitivity and high dynamic range wavefront sensing. Aberrations are introduced by a silicon-membrane deformable mirror. The detailed characterization of this mirror and its sensitivity matrix are presented. The various Shack-Hartmann algorithms, including a maximum likelihood approach are discussed and compared to phase retrieval results using a point source. The next phase of the testbed will include results with extended scenes.
The Dual Anamorphic Reflector Telescope (DART) is an architecture for large aperture space telescopes that enables the use of membranes. A membrance can be readily shaped in one direction of curvature using a combination of boundary control and tensioning, yielding a cylindrical reflector. Two cylindrical reflectors (orthogonal and confocal) comprise the 'primary mirror' of the telescope system. The aperture is completely unobstructed and ideal for infrared and high contrast observations. The DART high precision testbed researches fabrication, assembly, adjustment and characterization of 1 meter cylindrical membrane reflectors made of copper foil or kapton. We have implemented two metrology instruments: a non-contacting, scanning profilometer and an infrared interferometer. The profilometer is a laser confocal displacement measuring unit on an XYZ scanning stage. The infrared interferometer used a cylindrical null lens that tests a subaperture of the membrane at center of curvature. Current surface figure achieved is 25 μm rms over a 50 cm diameter aperture.
The StarLight mission aimed to place the first formation flying optical interferometer into space in year 2006. Utilizing two spacecraft to form a long baseline Michelson interferometer, it would measure white light fringes on a number of partially resolved stars of magnitudes >5 in the wavelength range 600 to 1000 nm. The interferometer baseline is variable between 30 and 125 m, and also has a fixed 1.3 m mode. The spacecraft are flown in a parabolic geometry which requires an optical delay line to build up more than 14 m of delay on one arm of the interferometer. To obtain high fringe visibility, starlight wavefront, pointing and intensity must be preserved through 22 reflections from mirrors and beamsplitters. The alignment of a total of 27 optics is maintained through careful thermal design and the use of two actuated mirrors on each arm. This paper describes the optical layout, including the beam combiner design which allows star tracking, optical system alignment and fringe formation on a single CCD. The effects of diffraction of the starlight transferred from a distant spacecraft and from optical surface imperfections are modeled. Other contributors to the visibility budget and the resulting variation of fringe visibility across the focal plane are discussed.
Planet detection around a bright star is dependent on the resolution of the imaging system and the degree of light suppression of the star relative to the planet. We present a concept and a scaled precursor for a visible light Terrestrial Planet Finding (VTPF) mission. Its major feature is an imaging system for planet detection using a nulling interferometer behind a single aperture telescope. This configuration is capable of detecting earth-like planets with a 4m aperture using both imaging and spectroscopic imaging modes. We will describe the principles of the system, and show results of studies demonstrating its feasibility.
Nulling stellar interferometry may enable the discovery of earth-like planets around other stars. In nulling mode, the zero order fringe is destructive and on axis, thus cancelling light from a bright source and allowing detection of dimer off-axis features. To create a deep on-axis null, the phase must be shifted half a wave achromatically over a broad band. The phase shift is created by adding optical path thickness with dielectric plates. Plates of different materials can balance dispersion. The nulling solutions found for TPF (infrared) and SIM (visible) are promising.
This paper presents the final results of a dissertation that
developed a nulling beam combiner testbed. The deepest null
achieved over the spectral region of 600 to 800 nm was 7x10-3. The test bed revealed the extreme challenges
of this technique and provided very valuable lessons to enable
further implementations.
The testbed first achromatized the null by actively controlling
the optical thicknesses of the plates. The phase as a function of
wavelength was measured by PSI on a spectrally dispersed fringe.
The phase was fit to a model to determine the optical thicknesses.
The eigenfunctions of the model were nearly collinear and
consequently the dynamic range required of the phase data was very
high and not supported by the hardware. The testbed then searched
for the null fringe and locked on the null using a 300 Hz servo
loop and on a grey fringe. The OPD was stabilized to 6 nm peak-to-valley.
This paper describes a laboratory experiment of achromatic nulling in the visible to SIM requirements. The experiment employs phase shifting interferometry techniques on a spectrally dispersed fringe to measure the phase as a function of wavelength. This phase is then used by a control system that adjusts the tilt of the plates and the air path difference until a satisfactory level of null is achieved. A fast servo adjusts the air path difference to stabilize the phase to the nanometer level. The paper includes a discussion of the design issues, the experimental measure of phase versus wavelength, a description of the control system, and a preliminary measure of the null.
The proposed New Millennium Interferometer consists of three spacecraft separated by up to several kilometers. A heterodyne laser metrology system is proposed to measure the relative distances between the spacecraft. Because diffraction losses for a round-trip measurement are prohibitively large, a two-laser metrology system has been suggested in which each spacecraft has both a laser and a receiver. The system has been successfully demonstrated with a one meter baseline and verified by a conventional single- laser system in a laboratory experiment. The precision was limited by thermal effects in the room environment for time scales greater than one minute. The single-laser system obtained a precision of 3 nm for integration times up to 0.5 seconds. The two-laser system obtained a precision of 20 and was limited by self-interference and electronics noise. The resolution of the two-laser metrology system was (lambda) 30.
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