Diffractive analysis of limits of an occulter experiment

An external occulter is a specially-shaped spacecraft own along the line-of-sight of a space telescope to block starlight before reaching its entrance pupil. Using optimization methods, occulter shapes can be designed to most effectively block starlight. A full-scale occulter cannot be tested on the ground and its performance must be predicted; therefore the fidelity of the optical propagation models used for design and performance prediction must be verified under scaled conditions. In this paper we present both contrast and suppression laboratory measurements for a scaled occulter, and perform a diffractive analysis to determine the factors limiting performance of the laboratory occulter

Progress on optical verification for occulter-based high contrast imaging

An external occulter is a specially-shaped spacecraft flown in formation with a telescope. It enables high-contrast imaging of the dim planetary companions of the neighboring solar system by blocking starlight before it reaches the entrance pupil. Occulters have to be designed via optimization methods that account for diffraction to most effectively block starlight. To predict occulter performance, we must verify the fidelity of the optical propagation models under scaled conditions. In this paper, we measure the contrast of a scaled occulter. The validity of the contrast calibration is determined using a baseline circular occulter. We verify contrast better than 10-10, however the measurements do not perform as well as the prediction from theoretical modelling. We attribute this difference to glint scattering off mask edges. © 2013 SPIE

Shaped pupil coronagraphy with WFIRST-AFTA

The recently completed study of using one of the AFTA telescopes for a potential WFIRST mission included a coronagraph instrument for exoplanet imaging. The challenge is to design a coronagraph that achieves the desired high contrast in the presence of the complicated on-axis optical architecture of the AFTA. This is especially difficult if contrast levels as small as 10 -9 must be achieved at only 3λ/D from the star. In this paper we present shaped pupil designs using our new two-dimensional formulation. These designs also include constraints given by the wavefront control system, a necessary element of a complete high-contrast system in space. We have computed various shaped pupils for different contrast floors, inner working angles, and high-contrast region shapes. Two main types of masks are presented: discovery masks that offer wide discovery space with moderate inner working angles, and characterization masks which are designed for narrower discovery space and smaller inner working angles. Discovery and characterization masks would be used to image planets at different distances from the star at the same wavelengths, or to image the same planets at different wavelengths. © 2013 SPIE

Hybrid coronagraphic design: Optimization of complex apodizers

To spectrally characterize Earth-like planets around nearby stars with a coronagraph, an extreme adaptive optics (ExAO) system is mandatory. The correction of the amplitude and phase aberrations in the wavefront on both sides of the image plane and in sufficiently large bandwidths can be done with two deformable mirrors (DM) in a pupil mapping configuration. While this system is primarily intended to correct for aberrations, it can potentially be used to improve the contrast beyond the nominal value set by the coronagraph; the two DMs can be seen as a complex apodizer. We present solutions to two types of numerical optimization problems. Our first approach consists in maximizing the sum of the real and the imaginary parts of the electric field in the pupil plane, while constraining the intensity of the electric field in chosen regions of the the subsequent image plane to be less than a chosen extremum. The solutions can be translated in term of modulus and phase. The optimal modulus is very close to 1, and the high-contrast is induced by a binary phase shift, which cannot be induced with current deformable mirrors. Our second approach consists in directly optimizing the stroke commands sent to a deformable mirror. Solutions are computed by either solving successive linear optimizations or non-linear optimizations. For a telescope with a 30% central obscuration, a 3λ/D inner working angle and a 10λ/D outer working angle, a 10-6-10-7 is reached after a dozen iterations, and the coronagraph has a 60-80% throughput. Shaped pupils are then computed to lower that contrast down to 10-9-10-10. © 2013 SPIE

Space-based planet detection using two MEMS DMs and a shaped pupil

NASA and the astronomy community hope to soon launch a new space-based telescope to detect and characterize extrasolar planets. Detecting extrasolar planets with angular separations and contrast levels similar to Earth requires not only a large space-based observatory but also advanced starlight suppression techniques. One promising approach is coronagraphy via shaped pupils. Shaped pupil coronagraphs are binary pupil functions that modify the point spread function of a telescope to produce regions of high contrast. Unfortunately, the contrast performance of coronagraphs is highly sensitive to optical errors, thus necessitating wavefront control to retrieve the necessary contrast levels. Using two MEMS deformable mirrors in series with the coronagraph allows us to control both the phase and amplitude aberrations over a finite wavelength range. Given an estimate of the wavefront we have developed an optimal controller that minimizes actuator strokes on the deformable mirrors subject to a constraint that it achieve a targeted contrast level in a defined region of the image. To provide an estimate for the controller that is accurate enough to converge to a solution that achieves the required ten orders of magnitude, the electric field must be estimated using the science camera to avoid any non-common path errors. The estimate is found by either using a batch process or Kalman filter technique which uses multiple image pairs with conjugated deformable mirror settings to estimate the field prior to evaluating the control shape. This paper outlines the algorithms used and presents our laboratory results. © 2012 SPIE

Mathematical and computational modeling of a ferrofluid deformable mirror for high-contrast imaging

Deformable mirrors (DMs) are an enabling and mission-critical technology in any coronagraphic instrument designed to directly image exoplanets. A new ferrofluid deformable mirror technology for high-contrast imaging is currently under development at Princeton, featuring a flexible optical surface manipulated by the local electromagnetic and global hydraulic actuation of a reservoir of ferrofluid. The ferrofluid DM is designed to prioritize high optical surface quality, high-precision/low-stroke actuation, and excellent low-spatial-frequency performance|capabilities that meet the unique demands of high-contrast coronagraphy in a space-based platform. To this end, the ferrofluid medium continuously supports the DM facesheet, a configuration that eliminates actuator print-through (or, quilting) by decoupling the nominal surface figure from the geometry of the actuator array. The global pressure control allows independent focus actuation. In this paper we describe an analytical model for the quasi-static deformation response of the DM facesheet to both magnetic and pressure actuation. These modeling efforts serve to identify the key design parameters and quantify their contributions to the DM response, model the relationship between actuation commands and DM surface-profile response, and predict performance metrics such as achievable spatial resolution and stroke precision for specific actuator configurations. Our theoretical approach addresses the complexity of the boundary conditions associated with mechanical mounting of the facesheet, and makes use of asymptotic approximations by leveraging the three distinct length scales in the problem|namely, the low-stroke (~nm) actuation, facesheet thickness (~mm), and mirror diameter (~cm). In addition to describing the theoretical treatment, we report the progress of computational multiphysics simulations which will be useful in improving the model fidelity and in drawing conclusions to improve the design

Mathematical and computational modeling of a ferrofluid deformable mirror for high-contrast imaging

Deformable mirrors (DMs) are an enabling and mission-critical technology in any coronagraphic instrument designed to directly image exoplanets. A new ferrofluid deformable mirror technology for high-contrast imaging is currently under development at Princeton, featuring a flexible optical surface manipulated by the local electromagnetic and global hydraulic actuation of a reservoir of ferrofluid. The ferrofluid DM is designed to prioritize high optical surface quality, high-precision/low-stroke actuation, and excellent low-spatial-frequency performance|capabilities that meet the unique demands of high-contrast coronagraphy in a space-based platform. To this end, the ferrofluid medium continuously supports the DM facesheet, a configuration that eliminates actuator print-through (or, quilting) by decoupling the nominal surface figure from the geometry of the actuator array. The global pressure control allows independent focus actuation. In this paper we describe an analytical model for the quasi-static deformation response of the DM facesheet to both magnetic and pressure actuation. These modeling efforts serve to identify the key design parameters and quantify their contributions to the DM response, model the relationship between actuation commands and DM surface-profile response, and predict performance metrics such as achievable spatial resolution and stroke precision for specific actuator configurations. Our theoretical approach addresses the complexity of the boundary conditions associated with mechanical mounting of the facesheet, and makes use of asymptotic approximations by leveraging the three distinct length scales in the problem|namely, the low-stroke (~nm) actuation, facesheet thickness (~mm), and mirror diameter (~cm). In addition to describing the theoretical treatment, we report the progress of computational multiphysics simulations which will be useful in improving the model fidelity and in drawing conclusions to improve the design

Shaped pupil design for future space telescopes

© 2014 SPIE.Several years ago at Princeton we invented a technique to optimize shaped pupil (SP) coronagraphs for any telescope aperture. In the last year, our colleagues at the Jet Propulsion Laboratory (JPL) invented a method to produce these non-freestanding mask designs on a substrate. These two advances allowed us to design SPs for two possible space telescopes for the direct imaging of exoplanets and disks, WFIRST-AFTA and Exo-C. In December 2013, the SP was selected along with the hybrid Lyot coronagraph for placement in the AFTA coronagraph instrument. Here we describe our designs and analysis of the SPs being manufactured and tested in the High Contrast Imaging Testbed at JPL.We also explore hybrid SP coronagraph designs for AFTA that would improve performance with minimal or no changes to the optical layout. These possibilities include utilizing a Lyot stop after the focal plane mask or applying large, static deformations to the deformable mirrors (nominally for wavefront correction) already in the system