The correction of quasi-static wavefront errors within a coronagraphic optical system will be a key challenge to overcome in order to directly image exoplanets in reflected light. These quasi-static errors are caused by mid to high-order surface errors on the optical elements as a result of manufacturing processes. Using high-order wavefront sensing and control (HOWFSC) techniques that do not introduce non-common path aberrations, the quasi-static errors can be corrected within the desired region of interest designated as the dark hole. For the future Habitable Worlds Observatory (HWO), HOWFSC algorithms will be key to attaining the desired contrasts. To simulate the performance of HOWFSC with space rated processors, optical models for a 6 m class space-borne observatory and a coronagraph have been developed. Phenomena such as the Talbot effect and beamwalk are included in the simulations using combinations of ray-based modeling and end-to-end propagation techniques. After integrating the optical models with the embedded processors, simulations with realistic computation times can be performed to understand the computational hardware performance that will be needed to maintain the desired contrasts. Here, the details of the optical models are presented along with the HOWFSC methods utilized. Initial results of the HOWFSC methods are also included as a demonstration of how system drifts will degrade the contrast and require dark hole maintenance.
High contrast imaging of extrasolar planets and circumstellar disks requires extreme wavefront stability. Such stability can be achieved with active wavefront control (WFC). The next generation of ground- and space-based telescopes will require a robust form of WFC in order to image planets at small inner working angles and extreme flux ratios with respect to the host star. WFC algorithms such as implicit Electric Field Conjugation (iEFC) reduce stellar leakage by minimizing the electric field within a given region of an image, creating a dark hole. iEFC utilizes an empirical approach to sense and remove speckles in the focal plane. While iEFC is empirically calibrated and can handle optical model errors, there are still model assumptions made during the calibration. The performance of iEFC will degrade if the system changes due to slow, optomechanical drifts. In this work, we assess the iEFC performance impacts of pupil misalignments on the deformable mirror (DM) and focal plane misalignments on the detector. We base our analysis on the MagAO-X instrument, an extreme AO system installed on the Magellan-Clay telescope, to develop iEFC misalignment tolerancing requirements for both ground- and space-based missions. We present end-to-end physical optics simulations of the MagAO-X instrument, demonstrating iEFC’s alignment tolerance.
Polarimetric differential imaging observations provide the highest contrast images of circumstellar disks in addition to providing information on dust grain scattering properties. The upcoming Nancy Grace Roman Space Telescope Coronagraph is expected to measure the linear polarization fraction of disks greater than 0.3 with an uncertainty of 0.03. One of the critical problems with polarimetric observations is the polarization aberrations generated by the telescope and polarimetric optics, which introduce errors when measuring lower SNR polarized signals. A modeling pipeline was previously developed to simulate the polarization observations of higher SNR debris disks similar without accounting for polarization aberrations. Here, we present the simulated polarimetric disk images of fainter debris disks (∼0.1mJy/arcsec2) through the Roman telescope and the HLC and SPC coronagraphs, incorporating polarization aberrations, jitter, detector, and speckle noise. The Point Response Functions are generated using PROPER for each orthogonal polarization state to account for the polarization aberrations. Finally, we compare the recovered polarization fraction of the debris disk with the input to demonstrate the polarimetric capability of the Roman Coronagraph.
Implementing high-order wavefront sensing and control (HOWFSC) algorithms on future space telescopes will require significant computing power. To enable the mission of Habitable Worlds Observatory to directly image exoplanets, we need to improve our understanding of the available performance of radiation-hardened processors. In this work, we describe the testing setup we use to evaluate HOWFSC algorithms, including Electric Field Conjugation and optical modeling on embedded processors. This testing setup enables accurate performance characterization of spaceflight-relevant CPUs and FPGAs in support of HOWFSC algorithms. We interface the embedded processors with a software model of a telescope and coronagraph to perform processor-in-the-loop testing. With this setup, we can test a range of telescope and HOWFSC algorithm configurations that are relevant to the design of future space missions, illuminating the feasibility of in-space HOWFSC algorithm execution.
The Space Coronagraph Optical Bench (SCoOB) is a high-contrast imaging testbed built to demonstrate starlight suppression techniques at visible wavelengths in a space-like vacuum environment. The testbed is designed to achieve <10−8 contrast from 3 − 10λ/D in a one-sided dark hole using a liquid crystal vector vortex waveplate and a 952-actuator Kilo-C deformable mirror (DM) from Boston Micromachines (BMC). We have recently expanded the testbed to include a field stop for mitigation of stray/scattered light, a precision-fabricated pinhole in the source simulator, a Minus K passive vibration isolation table for jitter reduction, and a low-noise vacuum-compatible CMOS sensor. We report the latest contrast performance achieved using implicit electric field conjugation (iEFC) at a vacuum of ∼10−6 Torr and over a range of bandpasses with central wavelengths from 500 to 650nm and bandwidths (BW) from ≪ 1% to 15%. Our jitter in vacuum is < 3 × 10−3λ/D, and the best contrast performance to-date in a half-sided D-shaped dark hole is 2.2 × 10−9 in a ≪ 1% BW, 4 × 10−9 in a 2% BW, and 2.5 × 10−8 in a 15% BW.
High-order wavefront sensing and control (HOWFSC) is key to creating a dark hole region within the coronagraphic image plane where high contrasts are achieved. The Roman Coronagraph is expected to perform its HOWFSC with a ground-in-the-loop scheme due to the computational complexity of the electric field conjugation (EFC) algorithm. This scheme provides the flexibility to alter the HOWFSC algorithm for given science objectives. The baseline HOWFSC scheme involves running EFC while observing a bright star such as ζ Puppis to create the initial dark hole followed by a slew to the science target. The new implicit EFC (iEFC) algorithm removes the optical diffraction model from the controller, making the final contrast independent of model accuracy. While previously demonstrated with a single deformable mirror, iEFC is extended to two deformable mirror systems to create annular dark holes. First, an overview of both EFC and iEFC is presented. The algorithm is then applied to the wide-field-of-view shaped pupil coronagraph (SPC-WFOV) mode designed for the Roman Space Telescope using end-to-end physical optics models. Initial noiseless monochromatic simulations demonstrate the efficacy of iEFC as well as the optimal choice of modes for the SPC-WFOV instrument. Further simulations with a 3.6% wavefront control bandpass and a broader 10% bandpass then demonstrate that iEFC can be used in broadband scenarios to achieve contrasts below 10−8 with Roman. Finally, an electron multiplying charge-coupled device (EMCCD) model is implemented to estimate calibration times and predict the controller’s performance. Here, 10−8 contrasts are achieved with a calibration time of ∼6.8 h assuming the reference star is ζ Puppis. The results here indicate that iEFC can be a valid HOWFSC method that can mitigate the risk of model errors associated with space-borne coronagraphs, but to maximize iEFC performance, lengthy calibration times will be required to mitigate the noise accumulated during calibration.
A major goal of proposed future space observatories, such as the Habitable World Observatory, is to directly image and characterize Earth-like planets around Sun-like stars to search for habitability signatures requiring the starlight suppression (contrast) of 10−10. One of the significant aspects affecting this contrast is the polarization aberrations generated from the reflection from mirror surfaces. The polarization aberrations are the phase-dependent amplitude and phase patterns originating from the Fresnel reflections of the mirror surfaces. These aberrations depend on the angle of incidence and coating parameters of the surface. This paper simulates the polarization aberrations for an on-axis and off-axis TMA telescope of a 6.5 m monolithic primary mirror. We analyze the polarization aberrations and their effect on the coronagraphic performance for eight different recipes of mirror coatings for Astronomical filter bands g-I: three single-layer metal coatings and five recipes of protective coatings. First, the Jones pupils are estimated for each coating and filter band using the polarization ray tracing in Zemax. Then, we propagate these Jones pupils through a Vector Vortex Coronagraph and Perfect Coronagraphs using hcipy, a physical optics-based simulation framework. The analysis shows that the two main polarization aberrations generated from the four mirrors are the retardance-defocus and retardance-tilt. The simulations also show that the coating plays a significant role in determining the strength of the aberrations. The bare/oxi-aluminum and Al+18nm LiF coating outperforms all the other coatings by one order of magnitude.
Future space telescopes such as the Habitable Worlds Observatory (HWO) will use coronagraphs and wavefront control to achieve the approximate 1010 starlight suppression necessary to directly image Earth-like exoplanets. Wavefront control algorithms such as Electric Field Conjugation (EFC) will control thousands of actuators at cadences of seconds or minutes. EFC uses a Jacobian matrix which maps Deformable Mirror (DM) voltages to the change in electric field at the image plane. The Jacobian matrix grows in size with the number of pixels, DM actuators, and spectral channels. EFC on proposed future telescopes like HabEx and LUVOIR will require as much as 25 GFLOPS (floating point operations per second). This level of compute density has never been achieved on radiation-hardened processors that are used on NASA Class-A missions such as the Roman Space Telescope. Previous work has focused on estimating the Compute Density (CD) of processors using assumptions about memory access characteristics and the parallelizability of algorithm implementation. Such analysis produces large uncertainty due to the assumptions necessary to compute CD. To refine the estimates of EFC compute capability of current generation processors, we determine the FLOPS performance of processors using benchmark tests which represent the operations mix and memory access patterns of EFC. The expected EFC iteration computation period on future space telescopes based on application benchmarks is reported. We have created a ray tracing optical model for the telescope assembly as well as a physical optics model for the telescope and coronagraph for the purposes of testing HOWFC algorithms. This testing can be applied to CPUs and FPGAs, representing a range of potential compute architectures.
The Roman Coronagraph is expected to perform its High-Order Wavefront Sensing and Control (HOWFSC) with a ground-in-the-loop scheme due to the computational complexity of the Electric-Field-Conjugation (EFC) algorithm. This scheme provides the flexibility to alter the HOWFSC algorithm for given science objectives. A new alternative implicit-EFC algorithm is of particular interest as it requires no optical model to create a dark-hole, making the final contrast independent of the model accuracy. The intended HOWFSC scheme involves running EFC while observing a bright star such as ζ Puppis to create the initial dark-hole, then slew to the science target while maintaining the contrast with low-order WFSC over the given observation. Given a similar scheme, the efficacy of iEFC is simulated for two coronagraph modes, namely the Hybrid Lyot Coronagraph (HLC) and the Wide-Field-of-View Shaped-Pupil-Coronagraph (SPC-WFOV). End-to-end physical optics models for each mode serve as the tool for the simulations. Initial monochromatic simulations are presented and compared with monochromatic EFC results obtained with the FALCO software. Various sets of calibration modes are tested to understand the optimal modes to use when generating an iEFC response matrix. Further iEFC simulations are performed using broadband images with the assumption that ζ Puppis is the stellar object being observed. Shot noise, read noise, and dark current are included in the broadband simulations to determine if iEFC could be a suitable alternative to EFC for the Roman Coronagraph.
Extreme wavefront correction is required for coronagraphs on future space telescopes to reach 10-8 or better starlight suppression for the direct imaging and characterization of exoplanets in reflected light. Thus, a suite of wavefront sensors working in tandem with active and adaptive optics are used to achieve stable, nanometerlevel wavefront control over long observations. In order to verify wavefront control systems, comprehensive and accurate integrated models are needed. These should account for any sources of on-orbit error that may degrade performance past the limit imposed by photon noise. An integrated model of wavefront sensing and control for a space-based coronagraph was created using geometrical raytracing and physical optics propagation methods. Our model concept consists of an active telescope front end in addition to a charge-6 vector vortex coronagraph instrument. The telescope uses phase retrieval to guide primary mirror bending modes and secondary mirror position to control the wavefront error within tens of nanometers. The telescope model is dependent on raytracing to simulate these active optics corrections for compensating the wavefront errors caused by misalignments and thermal gradients in optical components. Entering the coronagraph, a self-coherent camera is used for focal plane wavefront sensing and digging the dark hole. We utilize physical optics propagation to model the coronagraphy’s sensitivity to mid and high-order wavefront errors caused by optical surface errors and pointing jitter. We use our integrated models to quantify expected starlight suppression versus wavefront sensor signal-to-noise ratio.
Integrated optical models allow for accurate prediction of the as-built performance of an optical instrument. Optical models are typically composed of a separate ray trace and diffraction model to capture both the geometrical and physical regimes of light. These models are typically separated across both open-source and commercial software that don’t interface with each other directly. To bridge the gap between ray trace models and diffraction models, we have built an open-source optical analysis platform in Python called Poke that uses commercial ray tracing APIs and open-source physical optics engines to simultaneously model scalar wavefront error, diffraction, and polarization. Poke operates by storing ray data from a commercial ray tracing engine into a Python object, from which physical optics calculations can be made. We present an introduction to using Poke, and highlight the capabilities of two new propagation modules that add to the utility of existing scalar diffraction models. Gaussian Beamlet Decomposition is a ray-based approach to diffraction modeling that allows us to integrate physical optics models with ray trace models to directly capture the influence of ray aberrations in diffraction simulations. Polarization Ray Tracing is a ray-based method of vector field propagation that can diagnose the polarization aberrations in optical systems. Poke has been recently used to study the next generation of astronomical observatories, including the ground-based Extremely Large Telescopes (TMT, GMT, ELT) and a 6 meter space telescope (6MST) early concept for NASA’s Habitable Worlds Observatory.
The Roman coronagraph instrument will demonstrate high-contrast imaging technology, enabling the imaging of faint debris disks, the discovery of inner dust belts, and planets. Polarization studies of debris disks provide additional information on dust grains’ size, distribution, and shape. The Roman coronagraph uses a polarization module comprising two Wollaston prism assemblies to produce four orthogonally polarized images (I0, I90, I45, and I135), each measuring 3.2 arcsecs in diameter and separated by 7.5 arcsecs in the sky. The expected RMS error in the linear polarization fraction measurement is 1.66% per resolution element of 3 by 3 pixels. We present a mathematical model to simulate the polarized intensity images through the Roman CGI, including the instrumental polarization and other uncertainties. We use disk modeling software, MCFOST, to model q, u, and polarization intensity of the debris disk, Epsilon-Eridani. The polarization intensities are convolved with the coronagraph throughput incorporating the PSF morphology. We include model uncertainties, detector noise, speckle noise, and jitter. The final polarization fraction of 0.4±0.0251 is obtained after post-processing and speckle noise removal.
Exceptional wavefront correction is required for coronagraphs on future space observatories to reach 10-10 contrasts for direct imaging of rocky exoplanets around Sun-like stars. This picometer level wavefront correction must be stable over long periods of time and should be limited only by photon noise and wavefront sensing architecture. Thus, wavefront errors that arise from optical surface errors, thermal gradients, pointing induced beamwalk, and polarization aberration must be tightly controlled. A self-coherent camera (SCC) allows for image plane correction of mid-spatial frequency errors and a continuous means of dark-hole maintenance. By introducing a reference pinhole at the Lyot stop of a coronagraph, coherent starlight can be interfered with image plane speckles while leaving incoherent planet light untouched. A coronagraph model was created using High Contrast Imaging in Python (HCIPy) to simulate the SCC. Using these tools, realistic input disturbances can be introduced to analyze wavefront sensor performance. Using our model, we first demonstrate the necessity of a complimentary low-order wavefront sensor (LOWFS) to be paired with the SCC. Next, we discuss considerations when creating the modified Lyot stop of an SCC. Finally, a tolerance analysis of the SCC in the presence of optical surface errors, beamwalk due to pointing errors, photon noise, and detector read noise is presented.
The development of spaceborne coronagraphic technology is of paramount importance to the detection of habitable exoplanets in visible light. In space, coronagraphs are able to bypass the limitations imposed by the atmosphere to reach deeper contrasts and detect faint companions close to their host star. To effectively test this technology in a flight-like environment, a high-contrast imaging testbed must be designed for operation in a thermal vacuum (TVAC) chamber. A TVAC-compatible high-contrast imaging testbed is undergoing development at the University of Arizona inspired by a previous mission concept: The Coronagraphic Debris and Exoplanet Exploring Payload (CDEEP). The testbed currently operates at visible wavelengths and features a Boston Micromachines Kilo-C DM for wavefront control. Both a vector vortex coronagraph and a knife-edge Lyot coronagraph operating mode are under test. The optics will be mounted to a 1 × 2 meter pneumatically isolated optical bench designed to operate at 10−8 torr and achieve raw contrasts of 10−8 or better. The validation of our optical surface quality, alignment procedure, and first light results are presented. We also report on the status of the testbed’s integration in the vaccum chamber.
The 2020 Decadal Survey on Astronomy and Astrophysics endorsed space-based high contrast imaging for the detection and characterization of habitable exoplanets as a key priority for the upcoming decade. To advance the maturity of starlight suppression techniques in a space-like environment, we are developing the Space Coronagraph Optical Bench (SCoOB) at the University of Arizona, a new thermal vacuum (TVAC) testbed based on the Coronagraphic Debris Exoplanet Exploring Payload (CDEEP), a SmallSat mission concept for high contrast imaging of circumstellar disks in scattered light. When completed, the testbed will combine a vector vortex coronagraph (VVC) with a Kilo-C microelectromechanical systems (MEMS) deformable mirror from Boston Micromachines Corp (BMC) and a self-coherent camera (SCC) with a goal of raw contrast surpassing 10−8 at visible wavelengths. In this proceedings, we report on our wavefront sensing and control efforts on this testbed in air, including the as-built performance of the optical system and the implementation of algorithms for focalplane wavefront control and digging dark holes (regions of high contrast in the focal plane) using electric field conjugation (EFC) and related algorithms.
The Nancy Grace Roman Space Telescope Coronagraph Instrument will be the first large scale coronagraph mission with active wavefront control to be operated in space and will demonstrate technologies essential to future missions to image Earth-like planets. Consisting of multiple coronagraph modes, the coronagraph is expected to characterize and image exoplanets at 1E-8 or better contrast levels. An object-oriented physical optics modeling tool called POPPY provides flexible and efficient simulations of high-contrast point spread functions (PSFs). As such, three coronagraph modes have been modeled in POPPY. In this paper, we present the recent testing results of the models and provide quantitative comparisons between results from POPPY and existing tools such as PROPER/FALCO. These comparisons include the computation times required for PSF calculations. In addition, we discuss the future implementation of the POPPY models for the POPPY front-end package WebbPSF, a widely used simulation tool for JWST PSFs.
The Nancy Grace Roman Space Telescope Coronagraph Instrument (CGI) will be capable of characterizing exoplanets in reflected light and will demonstrate space technologies essential for future missions to take spectra of Earthlike exoplanets. As the mission and instrument move into the final stages of design, simulation tools spanning from depth of search calculators to detailed diffraction models have been created by a variety of teams. We summarize these efforts, with a particular focus on publicly available datasets and software tools. These include speckle and point-spread-function models, signal-to-noise calculators, and science product simulations (e.g. predicted observations of debris disks and exoplanet spectra). This review is intended to serve as a reference to facilitate engagement with the technical and science capabilities of the CGI instrument.
The Coronagraphic Debris Exoplanet Exploring Payload (CDEEP) is a Small-Sat mission concept for high contrast imaging of circumstellar disks. CDEEP is designed to observe disks in scattered light at visible wavelengths at a raw contrast level of 10-7 per resolution element (10-8 with post processing). This exceptional sensitivity will allow the imaging of transport dominated debris disks, quantifying the albedo, composition, and morphology of these low-surface brightness disks. CDEEP combines an off-axis telescope, microelectromechanical systems (MEMS) deformable mirror, and a vector vortex coronagraph (VVC). This system will require rigorous testing and characterization in a space environment. We report on the CDEEP mission concept, and the status of the vacuum-compatible CDEEP prototype testbed currently under development at the University of Arizona, including design development and the results of simulations to estimate performance.
End-to-end simulation of the influence of the optical train on the observed scene is important across optics and is particularly important for predicting the science yield of astronomical telescopes. As a consequence of their goal of suppressing starlight, coronagraphic instruments for high-contrast imaging have particularly complex field-dependent point-spread-functions (PSFs). The Roman Coronagraph Instrument (CGI), Hybrid Lyot Coronagraph (HLC) is one example. The purpose of the HLC is to image exoplanets and exozodiacal dust in order to understand dynamics of solar systems. This paper details how images of exoplanets and exozodiacal dust are simulated using some of the most recent PSFs generated for the CGI HLC imaging mode. First, PSFs are generated using physical optics propagation techniques. Then, the angular offset of pixels in image scenes, such as exozodiacal dust models, are used to create a library of interpolated PSFs using interpolation and rotation techniques, such that the interpolated PSFs correspond to angular offsets of the pixels. This means interpolation needs only be done once and an image can then be simulated by multiplying the vector array of the model astrophysical scene by the matrix array of the interpolated PSF data. This substantially reduces the time required to generate image simulations by reducing the process to matrix multiplication, allowing for faster scene analysis. We will detail the steps required to generate coronagraphic scenes, quantify the speed-up of our matrix approach versus other implementations, and provide example code for users who wish to simulate their own scenes using publicly available HLC PSFs.
The WFIRST Coronagraph Instrument (CGI) will image the environment close to stars at orders of magnitude higher sensitivity than current observatories. In addition to directly imaging giant exoplanets, WFIRST CGI has unprecedented sensitivity to scattered light from circumstellar dust. Most modeling has been confined to the dark-hole regime of the coronagraph (approximately 0.15 arcsec to 1 arcsec). This work uses publicly available field-dependent point spread functions to model an exozodiacal disk within the 0.15 arcsec inner working angle. For this simple Solar System-like test case, we find an approximately 25% increase in the transmitted exozodiacal flux due to light inside the inner working angle. We also describe plans to accelerate and extend this modeling to a wider range of geometries, and to quantify the impact on post-processing and source detection.
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