Optimization of coronagraph design for segmented aperture telescopes

Eyeglass is a very large aperture (25 - 100 meter) space telescope consisting of two distinct spacecraft, separated in space by several kilometers. A diffractive lens provides the telescope's large aperture, and a separate, much smaller, space telescope serves as its mobile eyepiece. Use of a transmissive diffractive lens solves two basic problems associated with very large aperture space telescopes; it is inherently fieldable (lightweight and flat, hence packagable and deployable) and virtually eliminates the traditional, very tight, surface shape tolerances faced by reflecting apertures. The potential drawback to use of a diffractive primary (very narrow spectral bandwidth) is eliminated by corrective optics in the telescope's eyepiece. The Eyeglass can provide diffraction-limited imaging with either single-band, multiband, or continuous spectral coverage. Broadband diffractive telescopes have been built at LLNL and have demonstrated diffraction-limited performance over a 40% spectral bandwidth (0.48 - 0.72 μm). As one approach to package a large aperture for launch, a foldable lens has been built and demonstrated. A 75 cm aperture diffractive lens was constructed from 6 panels of 1 mm thick silica; it achieved diffraction-limited performance both before and after folding. This multiple panel, folding lens, approach is currently being scaled-up at LLNL. We are building a 5 meter aperture foldable lens, involving 72 panels of 700 μm thick glass sheets, diffractively patterned to operate as coherent f/50 lens.

Context.The performance of high-contrast imaging instruments is limited by wavefront errors, in particular by non-common path aberrations (NCPAs). Focal-plane wavefront sensing (FPWFS) is appropriate to handle NCPAs because it measures the aberration where it matters the most, that is to say at the science focal plane. Phase retrieval from focal-plane images results, nonetheless, in a sign ambiguity for even modes of the pupil-plane phase.Aims.The phase diversity methods currently used to solve the sign ambiguity tend to reduce the science duty cycle, that is, the fraction of observing time dedicated to science. In this work, we explore how we can combine the phase diversity provided by a vortex coronagraph with modern deep learning techniques to perform efficient FPWFS without losing observing time.Methods.We applied the state-of-the-art convolutional neural network EfficientNet-B4 to infer phase aberrations from simulated focal-plane images. The two cases of scalar and vector vortex coronagraphs (SVC and VVC) were considered using a single post-coronagraphic point spread function (PSF) or two PSFs obtained by splitting the circular polarization states, respectively.Results.The sign ambiguity has been properly lifted in both cases even at low signal-to-noise ratios (S/Ns). Using either the SVC or the VVC, we have reached a very similar performance compared to using phase diversity with a defocused PSF, except for high levels of aberrations where the SVC slightly underperforms compared to the other approaches. The models finally show great robustness when trained on data with a wide range of wavefront errors and noise levels.Conclusions.The proposed FPWFS technique provides a 100% science duty cycle for instruments using a vortex coronagraph and does not require any additional hardware in the case of the SVC.

We study a mid-order wavefront sensor (MOWFS) to address fine cophasing errors in exoplanet imaging with future large segmented aperture space telescopes. Observing Earth analogs around Sun-like stars requires contrasts down to 10−10 in visible light. One promising solution consists of producing a high-contrast dark zone in the image of an observed star. In a space observatory, this dark region will be altered by several effects, and among them, the small misalignments of the telescope mirror segments due to fine thermo-mechanical drifts. To correct for these errors in real time, we investigate a wavefront control loop based on a MOWFS with a Zernike sensor. Such a MOWFS was installed on the high-contrast imager for complex aperture telescopes (HiCAT) testbed in Baltimore in June 2023. The bench uses a 37-segment Iris-AO deformable mirror to mimic telescope segmentation and some wavefront control strategies to produce a dark zone with such an aperture. In this contribution, we first use the MOWFS to characterize the Iris-AO segment discretization steps. For the central segment, we find a minimal step of 125 ±31 pm. This result will help us to assess the contribution of the Iris-AO DM on the contrast in HiCAT. We then determine the detection limits of the MOWFS, estimating wavefront error amplitudes of 119 and 102 pm for 10 s and 1 min exposure time with a SNR of 3. These values inform us about the measurement capabilities of our wavefront sensor on the testbed. These preliminary results will be useful to provide insights on metrology and stability for exo-Earth observations with the Habitable Worlds Observatory.

The past decade has seen a significant growth in research targeted at space-based observatories for imaging exosolar planets. The challenge is in designing an imaging system for high contrast. Even with a perfect coronagraph that modifies the point spread function to achieve high contrast, wavefront sensing and control is needed to correct the errors in the optics and generate a "dark hole." The high-contrast imaging laboratory at Princeton University is equipped with two Boston Micromachines Kilo-DMs. We review here an algorithm designed to achieve high contrast on both sides of the image plane while minimizing the stroke necessary from each deformable mirror (DM). This algorithm uses the first DM to correct for amplitude aberrations and uses the second DM to create a flat wavefront in the pupil plane. We then show the first results obtained at Princeton with this correction algorithm, and we demonstrate a symmetric dark hole in monochromatic light.

For direct imaging of faint exoplanets, coronagraphs are widely used to suppress light and achieve a high contrast. Wavefront correction algorithms based on adaptive optics are introduced simultaneously to mitigate aberrations in the optical system. Stroke minimization is one of the primary control algorithms used for high-contrast wavefront control. This technique calculates the minimum deformation across the deformable mirrors' surface under the constraint that a targeted average contrast level in the search areas, namely the dark holes, is achieved. In this paper we present a modified linear constraint stroke minimization algorithm. Instead of using a single constraint on intensity averaged over all pixels, we constrain the electric field's real and imaginary part of each pixel in the dark holes. The new control algorithm can be written into a linear programming problem. Model reduction methods, including pixel binning and singular value decomposition (SVD), are further employed to avoid over-constraining the problem and to speed up computation. In numerical simulation, we find that the revised algorithm leads to more uniform dark holes and faster convergence.

The Planetary Imaging Concept Testbed Using a Recoverable Experiment - Coronagraph (PICTURE-C) mission will directly image debris disks and exozodiacal dust around nearby stars from a high-altitude balloon using a vector vortex coronagraph. The first flight of PICTURE-C launched from the NASA Columbia Scientific Balloon Facility in Ft. Sumner, NM on September 28, 2019 and ew for a total of 20 hours, with 16 hours at oat altitude above 110,000 ft. This flight successfully demonstrated many key technologies for exoplanetary direct imaging missions and all hardware components for the second, science-focused flight of PICTURE-C scheduled for the fall of 2021. These technologies include a vector vortex coronagraph, high and low-order deformable mirrors and a high speed low-order wavefront control system. The experiment also demonstrated a 60 cm off-axis telescope with a hexapod-actuated secondary mirror that aligned itself automatically during flight. This paper details the ight performance of PICTURE-C, focusing on the operation of the low-order wavefront control system and the influence of high-frequency structural vibrations. We present new structural modifications that have been made to reduce these vibrations and laboratory demonstrations of the ight 2 coronagraph, which uses a high-order 952 actuator MEMS deformable mirror to create a high-contrast dark zone.

High contrast imaging of exoplanets around nearby stars with future large segmented apertures requires starlight suppression systems optimized for such geometries, with the ability to control diffraction created by gaps between segments. The PIAACMC approach is well-suited for high high efficiency coronagraphic imaging of exoplanets at small angular separations, offering an inner working angle (IWA) as small as 1 lambda/D. We show that PIAACMC can be designed for segmented apertures and present a few representative designs. The design process can mitigate leaks due to stellar angular size and chromatic diffraction by segment gaps by co-optimizing a multi-zone diffractive focal plane mask and a Lyot stop. The resulting performance is ultimately limited by stellar angular size, and the IWA must be carefully traded against contrast and throughput at small angular separations. We show that PIAACMC's small IWA enables space-based near-IR imaging and spectroscopy of exoplanets around Sun-stars, and ground-based imaging and characterization of habitable planets around nearby M-type stars. We review the current status of PIAACMC laboratory development and near-term prospects for ground-based use.

We compare the broadband contrast performances of several Phase Induced Amplitude Apodization (PIAA) coronagraph configurations through modeling and simulations. The basic optical design of the PIAA coronagraph is the same as NASA's High Contrast Imaging Testbed (HCIT) setup at the Jet Propulsion Laboratory (JPL). Using a deformable mirror and a broadband wavefront sensing and control algorithm, we create a dark hole in the broadband point-spread function (PSF) with an inner working angle (IWA) of 2(f lambda/D)(sub sky). We evaluate two systems in parallel. One is a perfect system having a design PIAA output amplitude and not having any wavefront error at its exit-pupil. The other is a realistic system having a design PIAA output amplitude and the measured residual wavefront error. We also investigate the effect of Lyot stops of various sizes when a postapodizer is and is not present. Our simulations show that the best 7.5%-broadband contrast value achievable with the current PIAA coronagraph is approximately 1.5x10(exp -8).

As an active device, deformable mirror has been successfully used in adaptive optical systems to compensate phase aberrations and improve beam quality. However, it is often limited to correcting the phase aberrations for a flat reference wavefront. In this paper we present a new adaptive beam intensity profiles control technique based on the combination of a 19-element deformable mirror and a global genetic algorithm. This algorithm can adaptively adjust the voltages of the 19 actuators on the deformable mirror to reduce the difference between the target beam shape and the actual beam shape. The numerical simulations results show that within the mechanical constraints of the deformable mirror, this technique can create the given beam intensity profiles in a focal plane.

The current generation of ground-based coronagraphic instruments uses\ndeformable mirrors to correct for phase errors and to improve contrast levels\nat small angular separations. Improving these techniques, several space and\nground based instruments are currently developed using two deformable mirrors\nto correct for both phase and amplitude errors. However, as wavefront control\ntechniques improve, more complex telescope pupil geometries will soon be a\nlimiting factor for these next generation coronagraphic instru- ments. The\ntechnique presented in this proceeding, the Active Correction of Aperture\nDiscontinuities method, is taking advantage of the fact that most future\ncoronagraphic instruments will include two deformable mirrors, and is proposing\nto find the shapes and actuator movements to correct for the effect introduced\nby these complex pupil geometries. We here present the results of the\nparametric analysis realized on the WFIRST pupil for which we obtained high\ncontrast levels with several deformable mirror setups (size, separation between\nthem), coronagraphs (Vortex charge 2, vortex charge 4, APLC) and spectral\nbandwidths. However, because contrast levels and separation are not the only\nmetrics to maximize the scientific return of an instrument, we also included in\nthis study the influence of these deformable mirror shapes on the throughput of\nthe instrument and sensitivity to pointing jitters. Finally, we present results\nobtained on another potential space based telescope segmented aperture. The\nmain result of this proceeding is that we now obtain comparable performance\nthan the coronagraphs previously designed for WFIRST. First result from the\nparametric analysis strongly suggest that the 2 deformable mirror set up (size\nand distance between them) has a important impact on the performance in\ncontrast and throughput of the final instrument.\n

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.

The monolithic primary mirror of an 8m telescope, JNLT (Subaru), under construction will be made by fusion-bonding a number of hexagonal blank segments. These segments, made of zero expansion glass material, have very small coefficients of thermal expansion (CTE's) with averages in the range of -10 to +10ppb/°C; however, there are differences in CTE between individual segments, which result in the nonnegligible thermal deformation of the monolithic mirror. The deformation of the mirror can be evaluated from the measured CTE's of individual segments, once the arrangement of segments is specified. One can, therefore, pose a combinatorial optimization problem of finding the optimum solution for the arrangement of segments, that gives the minimum thermal deformation of the fused primary mirror.In this paper, we show that this combinatorial optimization problem can be solved by a method using the simulated annealing approach. We demonstrate that the estimated deformation of the mirror can be reduced to a negligibly small level by applying the method to finding the optimum arrangement of segments in a fused mirror blank.

The onset of a typical bacterial growth curve shows a period of very slow increase in population counts. This is a period of physiological adaptation to new environmental conditions. While in mathematical biology much progress was made in recent years to describe physiologically structured populations, these models typically have too many degrees of freedom to easily allow a model identification against experimental data. Therefore, and for all practical purposes, microbiologists have proposed simpler models of physiological adaptation in the past, usually in connection with standard growth curves. In this paper we compare the performance of four such lag-time models, each of which described by a scalar differential equation, when combined with a model of a siderophore producing bacterial population under iron limitation. In each case this yields a system of five nonlinear ordinary differential equations that we compare against experimental data, by solving the associated vector optimization problem. Our main finding is that a big step in accuracy is made already by including a simple lag-time model that only introduces one additional degree of freedom in the parameter identification problem (the initial state of health of the population), and that this can be reliably improved if a further degree of freedom, describing the dynamics of the physiological recovery process, is included. The vector optimization problem is solved by scalarizing it with a linear functional and solving the resulting scalar optimization problem. The growth parameters that are identified in this procedure are found to be robust with respect to the scalarization coefficient.

Integrated design of ORC process and working fluid using process flowsheeting software and PC-SAFT

A low cost adaptive optical system for improving solid-state laser beam quality has been set up. This system consists of a deformable mirror, a high voltage amplifier, a set of solid-state laser system, a CCD camera, a control software and corresponding optimization algorithm. This adaptive optical system doesn't employ a wave-front sensor to detect the phase information, but optimize the light intensity within a fixed aperture in the focal plane by a 19-element piezoelectricity deformable mirror. In order to find the optimum surface profile of the deformable mirror, which is applied to correct the phase aberrations in a solid-state laser system, a global genetic algorithm is introduced. The far-field light intensity signal, which is measured by a CCD camera, is used as a fitness function of the genetic algorithm. In this paper, the performance and efficiency of this wave-front sensor-less adaptive optical system based on the genetic algorithm are presented. Both the simulation results and the experimental results are given and discussed.