Application of Precision Deformable Mirrors to Space Astronomy John Trauger, Dwight Moody Brian Gordon, Yekta Gursel (JPL) Mark Ealey, Roger Bagwell (Xinetics) Workshop on Innovative Designs for the Next Large UV/Optical Space Telescope STScI * 10 April 2003
Overview Precision deformable mirrors are an enabling technology for that active correction of large optical systems in space. Deformable mirrors developed at Xinetics specifically for application to space astronomy bring together precision control of the mirror surface, open-loop stability, high actuator count, and a compact format. The -9 characteristics of these mirrors, as measured in the laboratory at JPL, are combined with predictive optical models to illustrate their application in high-contrast space astronomy, including TPF and pre-tpf planet-survey concepts.
Surface Figure on Large Telescope Optics At right: Optical scatter from surface figure errors on primary mirrors with dimensions as large as HST is the dominant issue for high contrast and low scatter applications. HST / WFPC2 (data) 1.5-meter Kodak lightweighted mirror 2.4-meter HST primary mirror 6.5-meter Magellan primary mirror Representative Large Mirror Surface Figure PSDs HST / WFPC2 (model) At left: State of the art for surface figure errors at the critical spatial frequencies has changed little since HST despite advances in mirror construction and modern polishing technologies.
Deformable mirror technology development is ongoing At left: 21x21 mm modular deformable mirror, developed and characterized under a NASA SBIR program at Xinetics Inc, provides 441 individual actuators At right: This assembly of four actuator modules such as the one at left, with a single polished mirror facesheet and 1764 actuators, is identical to the Gen1 DM used to date in testbed experiments.
Analysis of Deformable Mirror Surface Figure At left: Measured surface figure phase map quantifies the match of the deformable mirror to a target reference mirror surface. Quilting pattern corresponds to the 1mm pitch of the underlying actuators. In color: the corresponding surface figure PSD illustrating the dark-hole control of critical spatial frequencies. At right: PSD analysis of the surface figure (PSD in Angstrom 2 cm 2 vs spatial frequency in cm -1) indicates control at spatial frequencies below Nyquist (red) and beyond Nyquist (blue), and the distinct region below Nyquist with 0.25 Angstrom rms surface figure control.
Deformable Mirror Stability Measured surface figure drift for the Xinetics DM plotted vs. actuator voltages. Open loop surface figure stability has been monitored over continuous periods as long as several months.
Deformable Mirror Surface Influence Function At left: The empirical surface deformation profile of a single actuator. Grid spacing is 0.1 mm, actuators are positioned in a square array with 1 mm pitch. At right: Mirror surface predicted by linear superposition of actuator influence profiles in an 11x11 actuator block pattern.
Diffractive wavefront propagation models At left: Predicted star background (and high contrast dark hole) at the focal plane of an actively corrected coronagraphic space telescope. At right: Propagation model includes all elements of the telescope and camera optical paths. All mirror surfaces include representative spatial low and mid-frequency figure errors. DM has been adjusted to compensate for both phase errors and phase-induced amplitude errors in the wavefront, providing the deepest contrast in the lower half of the dark hole.
Predictive models validated by the HCIT experience will be used to design future coronagraphic space telescopes and evaluate the science reach of proposed TPF and TPF-precursor missions.
Estimation of TPF coronagraph image contrast Four coronagraphs have been computed. These include: 6m x 4m elliptical PM, mid-frequency surface PSD shaped as HST with surface errors reduced by half Four DM configurations: 96x64, 128x86, 160x107, and 192x128 actuators. Linear sinc-squared occulter and matching lyot stop. Upper curve and data points in the plot of relative intensity vs. angular separation at right show the PSF of the 6mx4m TPF telescope at 0.75 microns with a 20% spectral bandpass. Lower curve and data points are the corresponding PSF at the coronagraph focal plane. Contrast, as defined below and plotted in the following slides, is computed from similar data for all four cases. Coronagraph contrast performance is shown for each case as a plot of contrast parameter C vs. angular separation from the occulted central star. C is defined as the ratio of surface brightness in the scattered background (speckle pattern) in the dark field surrounding the star at any given distance from the star (integrated over the area of one Airy disk) divided by the corresponding flux from the unocculted central star (again integrated over the area of one Airy disk). Telescope and Coronagraph PSFs with the 128x86 DM
6m x 4m TPF PM, 0.5x HST surface figure Linear sinc-squared occulter (3lambda/D) 192 x 128 actuator DM, 1 A accuracy 0.750 microns, 20% spectral bandpass Plotted data at right are the computed contrast parameter C vs. angular separation from the occulted star in the coronagraph science field of view. Black dots specify individual critically sampled pixels in the focal plane. Red curve is the median contrast over all azimuths within +/- 30 degrees of the preferred axis at each radius. Blue curve indicates the profile of the focal plane occulting mask. TPF Coronagraph Sims Exo Solar System at 5 pc Venus, Earth, Mars plus Jupiter at 2.5, 5.0. 7.5. 10.0 12.5 AU
Simulated JWST optical wavefront
Estimation of NIRCam coronagraph image contrast Eight cases have been computed. These include: Two candidate PM configurations: 18 segments (1.31 m edge-to-edge) and 36 segments (0.92 m edge-to-edge). Two wavelengths of special interest: 2.0 and 4.6 microns with 10% spectral bandpass. Two leading Lyot coronagraph configurations: circular Bessel-squared and linear Sinc-squared occulters. Upper curve and data points in the plot of relative intensity vs. angular separation at right show the PSF of the 18-hex JWST telescope at 4.6 microns with a 10% spectral bandpass. Lower curve and data points are the corresponding PSF at the coronagraph focal plane. Contrast, as defined below and plotted in the following slides, is computed from similar data for all eight cases. Coronagraph contrast performance is shown for each case as a plot of contrast parameter C vs. angular separation from the occulted central star. C is defined as the ratio of surface brightness in the scattered background (speckle pattern) in the dark field surrounding the star at any given distance from the star (integrated over the area of one Airy disk) divided by the corresponding flux from the unocculted central star (again integrated over the area of one Airy disk).
JWST NIRCam Coronagraph Sims Circular Occulter (Bessel-squared) 4.6 microns, 10% bandpass 18 hexagon JWST Plotted data at right are the computed contrast parameter C vs. angular separation from the occulted star in the coronagraph science field of view. Black dots specify individual critically sampled pixels in the focal plane. Red curve is the median contrast over all azimuths at each radius. Blue curve indicates the profile of the focal plane occulting mask. Focal plane occulting mask PM phase map (cyan) with Lyot mask (superimposed in red) JWST PSF at 4.6 microns, logarithmic contrast stretch Coronagraph PSF, same contrast stretch
HCIT Provides Laboratory Validations for Key Technologies Computational diffractive optical propagation models are used in concert with testbed experimentation to evaluate the optical components, quantify alignment and stability, drive the wavefront sensing algorithms, and guide the testbed experimental program. Precision deformable mirror and two CCD cameras test a broad range of algorithms for the correction of wavefront phase and amplitude errors. Coronagraphic masks in the focal plane and lyot pupil plane provides a test for all known coronagraph configurations and apodizations. Aspheric mirrors in pupil and intermediate planes test the control of scattered light arising from surface figure imperfections in the optics. Vacuum chamber provides simulation of the thermal and optical control available in a space environment. Will be available for development of TPF high-contrast imaging concepts and technologies.
Initial coronagraph layout of the HCIT optical table Corrected star image Apodized occulting mask Wavefront phase correction Illumination at pupil plane Apodized lyot mask High contrast coronagraphic field
HCIT laboratory environment All experiments to date have taken place in the ambient laboratory environment under a clean tent. The HCIT optical table was transfered to a vibration-isolated vacuum facility in April 2003.
Conclusions Applications are envisioned for high contrast and low scatter imaging in future space telescope mision inthe UV, optical, and NIR. Deformable mirrors provide active wavefront correction to Angstrom RMS accuracy in a space-simulating vacuum-thermal environment. Optical propagation models predict the corresponding improvement in space telescope imaging and contrast. Laboratory validations of DM technology is now (April 2003) moving to the HCIT facility at JPL, where a DM can be placed in the context of a space telescope imaging system. Gen2 DMs from Xinetics will be delivered, calibrated, and integrated on the HCIT this spring. DM development is ongoing -- with the goal of robust, compact, and precise active wavefront correction and high actuator count.
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