Segmented Space Telescopes Research Articles

Fiber-Connected Wavefront-Sensing System for Large Segmented Space Telescopes

Fiber-Connected Wavefront-Sensing System for Large Segmented Space Telescopes

Low-order wavefront control using a Zernike sensor through Lyot coronagraphs for exoplanet imaging

Context. Wavefront sensing and control (WFSC) will play a key role in improving the stability of future large segmented space telescopes while relaxing the thermo-mechanical constraints on the observatory structure. Coupled with a coronagraph to reject the light of an observed bright star, WFSC enables the generation and stabilisation of a dark hole (DH) in the star image to perform planet observations. Aims. While WFSC traditionally relies on a single wavefront sensor (WFS) input to measure wavefront errors, the next generation of instruments will require several WFSs to address aberrations with different sets of spatial and temporal frequency contents. The multiple measurements produced in such a way will then have to be combined and converted to commands for deformable mirrors to modify the wavefront subsequently. Methods. We asynchronously operate a loop controlling the high-order modes digging a DH and a control loop that uses the rejected light by a Lyot coronagraph with a Zernike wavefront sensor to stabilize the low-order aberrations. Using the HiCAT testbed with a segmented telescope aperture, we implement concurrent operations and quantify the expected cross-talk between the two controllers. We then present experiments that alternate high-order and low-order control loops to identify and estimate their respective contributions. Results. We show an efficient combination of the high-order and low-order control loops, keeping a DH contrast better than 5 × 10−8 over a 30 min experiment and stability improvement by a factor of 1.5. In particular, we show a contrast gain of 1.5 at separations close to the DH inner working angle, thanks to the low-order controller contribution. Conclusions. Concurrently digging a DH and using the light rejected by a Lyot coronagraph to stabilize the wavefront is a promising path towards exoplanet imaging and spectroscopy with future large space observatories.

Adaptive optics performance of a simulated coronagraph instrument on a large, segmented space telescope in steady state

Directly imaging Earth-like exoplanets (``exoEarths'') with a coronagraph instrument on a space telescope requires a stable wavefront with optical path differences limited to tens of picometers RMS during exposure times of a few hours. While the structural dynamics of a segmented mirror can be directly stabilized with telescope metrology, another possibility is to use a closed-loop wavefront sensing and control system in the coronagraph instrument that operates during the science exposures to actively correct the wavefront and relax the constraints on the stability of the telescope. In this paper, we present simulations of the temporal filtering provided using the example of LUVOIR-A, a 15~m segmented telescope concept. Assuming steady-state aberrations based on a finite element model of the telescope structure, we (1)~optimize the system to minimize the wavefront residuals, (2)~ use an end-to-end numerical propagation model to estimate the residual starlight intensity at the science detector, and (3)~predict the number of exoEarth candidates detected during the mission. We show that telescope dynamic errors of 100~pm~RMS can be reduced down to 30~pm~RMS with a magnitude 0 star, improving the contrast performance by a factor of 15. In scenarios where vibration frequencies are too fast for a system that uses natural guide stars, laser sources can increase the flux at the wavefront sensor to increase the servo-loop frequency and mitigate the high temporal frequency wavefront errors. For example, an external laser with an effective magnitude of -4 allows the wavefront from a telescope with 100~pm~RMS dynamic errors and strong vibrations as fast as 16~Hz to be stabilized with residual errors of 10~pm~RMS thereby increasing the number of detected planets by at least a factor of 4.

Low-order wavefront control using a Zernike sensor through Lyot coronagraphs for exoplanet imaging

Context.The combination of large segmented space telescopes, coronagraphy, and wavefront control methods is a promising solution for producing a dark hole (DH) region in the coronagraphic image of an observed star in order to study planetary companions. The thermal and mechanical evolution of such a high-contrast instrumental setup leads to wavefront drifts that degrade the DH contrast during the observing time, thus limiting the ability to retrieve planetary signals.Aims.Lyot-style coronagraphs are starlight-suppression systems that remove the central part of the image for an unresolved observed star, that is, the point spread function, with an opaque focal plane mask (FPM). When implemented with a flat mirror containing an etched pinhole, the mask rejects part of the starlight through the pinhole which can be used to retrieve information about low-order aberrations.Methods.We propose an active control scheme using a Zernike wavefront sensor (ZWFS) to analyze the light rejected by the FPM, control low-order aberrations, and stabilize the DH contrast. We first present the concept formalism and then describe how we characterized the sensor behavior in simulations and in the laboratory. We performed experimental tests to validate a wavefront control loop using a ZWFS on the HiCAT testbed.Results.By controlling the first 11 Zernike modes, we show a decrease in the standard deviation of the wavefront error by a factor of up to 9 between open- and closed-loop operations using the ZWFS. In the presence of wavefront perturbations, we show the ability of this control loop to stabilize a DH contrast around 7 × 10−8with a standard deviation of 7 × 10−9.Conclusions.Active control with a ZWFS proves to be a promising solution in Lyot coronagraphs with an FPM-filtered beam for controlling and stabilizing low-order wavefront aberrations and DH contrast for exoplanet imaging with future space missions.

High-contrast Demonstration of an Apodized Vortex Coronagraph

Abstract High-contrast imaging is the primary path to the direct detection and characterization of Earth-like planets around solar-type stars; a cleverly designed internal coronagraph suppresses the light from the star, revealing the elusive circumstellar companions. However, future large-aperture telescopes (>4 m in diameter) will likely have segmented primary mirrors, which cause additional diffraction of unwanted stellar light. Here we present the first high-contrast laboratory demonstration of an apodized vortex coronagraph, in which an apodizer is placed upstream of a vortex focal plane mask to improve its performance with a segmented aperture. The gray-scale apodization is numerically optimized to yield a better sensitivity to faint companions assuming an aperture shape similar to the LUVOIR-B concept. Using wavefront sensing and control over a one-sided dark hole, we achieve a raw contrast of 2 × 10−8 in monochromatic light at 775 nm, and a raw contrast of 4 × 10−8 in a 10% bandwidth. These results open the path to a new family of coronagraph designs, optimally suited for next-generation segmented space telescopes.

Modular assembled space telescope

We present a new approach to building a modular segmented space telescope that greatly leverages the heritage of the Hubble Space Telescope and the James Webb Space Telescope. The modular design in which mirror segments are assembled into identical panels allows for economies of scale and for efficient space assembly that make a 20-m aperture approach cost effective. This assembly approach can leverage NASA’s future capabilities and has the power to excite the public’s imagination. We discuss the science drivers, basic architecture, technology, and leveraged NASA infrastructure, concluding with a proposed plan for going forward.

Minimizing the wavefront error degradation for primary mirror segments with failed hexapod actuators

The performance of a segmented space telescope depends in part upon the ability to maintain the alignment and phasing of its primary mirror segments. Failures of segment control actuators pose a threat to mission success, but their effects can be mitigated by using the remaining segment actuators to optimize the pose of each affected segment. This paper considers the effect of actuator failures on the final wavefront error of a segmented space telescope whose primary mirror consists of 18 hexagonal segments, each controlled by a 3-6 hexapod. Optimization algorithms that minimize the wavefront error for single- and multiple-failure cases are developed, and simulation results are presented. When one actuator fails, the affected segment can still attain a pose with zero wavefront error by exploiting the rotational symmetry of the primary. When two actuators fail, the resulting wavefront error depends upon which hexapod legs fail and at what lengths; cases where both legs of a bipod fail are an order of magnitude worse than other cases. Finally, Monte Carlo simulations of many failures randomly distributed across an initially well-phased segmented primary show that more than 10% of the actuators must fail before the root-mean-square wavefront error degrades significantly.

Study on the design and Zernike aberrations of a segmented mirror telescope

The segmented mirror telescope is widely used. The aberrations of segmented mirror systems are different from single mirror systems. This paper uses the Fourier optics theory to analyse the Zernike aberrations of segmented mirror systems. It concludes that the Zernike aberrations of segmented mirror systems obey the linearity theorem. The design of a segmented space telescope and segmented schemes are discussed, and its optical model is constructed. The computer simulation experiment is performed with this optical model to verify the suppositions. The experimental results confirm the correctness of the model.

分块式空间望远镜波前传感及控制方法的仿真研究

分块式空间望远镜波前传感及控制方法的仿真研究

空间拼接主镜望远镜共相位检测方法

空间拼接主镜望远镜共相位检测方法

Evaluation of least-squares phase-diversity technique for space telescope wave-front sensing

Because of mechanical aspects of fabrication, launch, and operational environment, space telescope optics can suffer from unforeseen aberrations, detracting from their intended diffraction-limited performance goals. We give the results of simulation studies designed to explore how wave-front aberration information for such near-diffraction-limited telescopes can be estimated through a regularized, low-pass filtered version of the Gonsalves (least-squares) phase-diversity technique. We numerically simulate models of both monolithic and segmented space telescope mirrors; the segmented case is a simplified model of the proposed next generation space telescope. The simulation results quantify the accuracy of phase diversity as a wave-front sensing (WFS) technique in estimating the pupil phase map. The pupil phase is estimated from pairs of conventional and out-of-focus photon-limited point-source images. Image photon statistics are simulated for three different average light levels. Simulation results give an indication of the minimum light level required for reliable estimation of a large number of aberration parameters under the least-squares paradigm. For weak aberrations that average a 0.10lambda pupil rms, the average WFS estimation errors obtained here range from a worst case of 0.057lambda pupil rms to a best case of only 0.002lambda pupil rms, depending on the light level as well as on the types and degrees of freedom of the aberrations present.