Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Jet Propulsion Laboratory California Institute of Technology WFIRST-AFTA Coronagraph Instrument Low Order Wavefront Sensing & Control Fang Shi, R. Bartos, R. Hein, B. Kern, R. Lam, J. Moore, K. Patterson, O. Santos, E. Sidick, H. Tang, T. Truong, B. Tweddle, K. Wallace, and X. Wang Jet Propulsion Laboratory California Institute of Technology Pasadena, California 91109 Oct 8, 2014 Copyright 2014 California Institute of Technology. Government sponsorship acknowledged 1 Jet Propulsion Laboratory Outline California Institute of Technology • WFIRT-AFTA coronagraph overview • AFTA telescope wavefront drift and line-of-sight jitter • Low order wavefront sensor (LOWFS) description • LOWFS performance analysis • LOWFS testbed • Challenges of correcting low order WFE • Summary 2 Jet Propulsion Laboratory WFRIST-AFTA Coronagraph Instrument California Institute of Technology Coronagraph Instrument Exo-planet Direct imaging Telescope Diameter 2.4 m Band pass 430 – 980nm Measured sequentially in five ~10% bands Inner working angle 100 – 250 mas ~3λ/D, driven by science Outer working angle 0.75 – 1.8 arcsec Detection Limit Contrast ≤ 10-9 After post processing) Imaging Spectral Resolution Center obscuration and SM support struts By 48X48 DM Cold Jupiters, not exoearths. Deeper contrast looks unlikely due to pupil shape and extreme stability requirements 4.8" FoV 0.009" pixel scale, 1kX1K EMCCD ~70 Exo-planet Spectroscopy With IFS, R~70 across 600 – 980 nm AFTA Coronagraph Instrument will: • Characterize the spectra of over a dozen radial velocity planets. • Discover and characterize up to a dozen more ice and gas giants. • Provide crucial information on the physics of planetary atmospheres and clues to planet formation. • Respond to decadal survey to mature coronagraph technologies, leading to first images of a nearby Earth. 3 WFIRST-AFTA Coronagraph Functional Block Diagram Jet Propulsion Laboratory California Institute of Technology Star light suppression optics High contrast loop during initialization 1kX1K, EMCCD; 150K, IWA 3λ/D, OWA 12λ/D, λ(0.43-0.98um) Coronagraph FPA OTA (PM, SM) TM, relay, FSM DM #1, DM #2 Relay, Occulting Masks & Filters LOWFS Telemetry IFS LOWFS FPA Drift control loop Jitter control loop The function of LOWFS is to measure the LoS jitter and the low order WF changes Post processing IFS FPA 1kX1k, EMCCD; 150K, λ(0.6-0.98um), R~70, 17mas sampling Optics Control Detector Post processing on ground AFTA Telescope LoS Jitter and WFE Drift Jet Propulsion Laboratory California Institute of Technology • Current understanding comes from WFIRST-AFTA Cycle 3 study – Line-of-sight WFE: • Goal for LOWFS sensor: – Line-of-sight: 0.4 milli-arcsec • LoS jitter requirement may be loosened from better coulter design • Drift (<2Hz): ~14 mas • Jitter (>2Hz): ~ 4 mas – Dominant frequency around 10 Hz – Low order WFE sensing: ~10 pm at much slower rate (> minutes) – Thermal drift WFE: • Maximum ~ 50 pm over 24 hours • Typically with rate <10 pm/hour • Dominant mode: focus and coma LoS vs Wheel Speed WFE RB Thermal Drift WFE PM Thermal Drift Coronagraph RMS Contrast Sensitivity to WFE Jet Propulsion Laboratory California Institute of Technology HLC RMS Contrast vs. 0.1 nm WFE SPC RMS Contrast vs. 0.1 nm WFE • RMS contrast sensitivity curves from PROPER model – X-axis is the field distance in unit of λ/D – Y-axis is the coronagraph’s RMS contrast changes due the WFE • Contract sensitivity depends on type (Hybrid Lyot vs. Shape Pupil) and design – Shape Pupil Coronagraph (right plot) has much less sensitivity than that of Hybrid Lyot Coronagraph (left plot) • Contract sensitivity depends on WFE mode – For HLC is very sensitive to coma and spherical WFE Jet Propulsion Laboratory LOWFSC Concepts California Institute of Technology • LOWFS uses the rejected star light from the coronagraph for wavefront sensing – The rejected star light is picked from the occulter (HLC) or the field stop (SPC), hence spatially filtered to contain low order WF information only – LOWFS senses only low order wavefront changes (differential instead of absolute WFS) • Senses WF jitter at high speed needed to suppress the jitter • Senses slow varying low order WFE such as focus, astigmatism, and coma caused by the WFIRST-AFTA telescope thermal drift – Recorded WF will be used for coronagraph image post processing – Use one camera to sense both LoS jitter and low order WFE • Frame average when sense low order WFE to increase the SNR • LOWFSC uses FSM for LOS jitter correction and a suitable corrector for low order WFE correction – FSM loop bandwidth is driven by AFTA LOS jitter – Low order WFE corrector should avoid to corrupt coronagraph’s high order WF control • LOWFS is closed-loop only for low order wavefront error. It lacks the sensitivity to high spatial order WFE • DM calibration is very important • Combine the LOWFS mask (a phase dimple of π/2 in phase and diameter of ~1.2 λ/D) on the coronagraph star light rejection mask reduces the non-common path error Jet Propulsion Laboratory Zernike Wavefront Sensor California Institute of Technology ZWFS Fourier Modes • Zernike WFS (ZWFS) measures WFE from interference between the abberated WF and the reference WF generated by a phase Input plate • Phase plate is at the center of occulting mask • ZWFS sensed pupil is imaged to CCD at 8x8 pixels • Phase shift of π/2 in ref portion to accentuate the WFE – CCD brightness variation proportional to the WFE: ∆Ι ~ ±2φ – ZWFS uses differential image to sense the delta WFE Phase Image Plane Pupil • AFTA-C LOWFS mask will be reflective instead of transmissive Plane Image shown in this conceptual illustration • Working with 10% band filters, higher bandwidth (20%) is ok E (η ,υ ) = F [P(u , v )] ∗ F [A(1 + ε (u , v ) + i ϕ (u , v ))] E (η ,υ ) = A PSF (η ,υ )ei θ + A PSF (η ,υ ) ∗ F [ε (u , v ) + i ϕ (u , v )] E (u, v ) = P (u, v ) ⋅ A(1 + ε (u, v ))ei ϕ (u ,v ) ( I = E ⋅ E * = A2 1 + 2ϕ + ε 2 + ϕ 2 with θ = π/2 ) Jet Propulsion Laboratory ZWFS Analysis Approach California Institute of Technology Diffraction model using “SemiAnalytical Method (SAM) or FFT: Example on ZWFS Signal (FFT Model) 1. Generate ZWFS intensity output (reference) with zero input phase and down sampled to 8x8 2. Adding photon noise and detector read/dark noise and down sampled to 8x8 pixels 3. Generate ZWFS intensity output with aberration on the input phase Φ 4. Adding photon noise and detector read/dark noise and down sampled to 8x8 pixels 5. Subtract out reference frame and reconstruct the sensed phase using signal-to-Zernike coeff matrix which is precalculated at noise-free case 6. WFE is computed with pseudo inverse of signal-to-Zernike coeff matrix Input Pupil & WFE HLC configuration LOWFS is illustrated here. SPC LOWFS is similar except the pupil support is that of SPC mask. Higher spatial frequency contents (>6 λ/D) are not being collected by LOWFS High Resolution ZWFS Signal Pixelated ZWFS Signal *SAM reference: R. Soummer, etal, "Fast computation of Lyot-style coronagraph propagation", Optics Express, Vol. 15, No. 24, 26 Nov. 2007 Zernike WFS Pixelated Signals for each WFE Modes Jet Propulsion Laboratory California Institute of Technology Pixelated ZWFS signals of Z2-Z11 WFE modes • Used for WF sensing • Noise will be applied on these signals for performance evaluation ZWFS Noise Performance with AFTA-C HLC Jet Propulsion Laboratory California Institute of Technology Noise Equivalent Angle (tilt) Noise Equivalent Low Order WFE • K0V star spectral profile • ZWFS wavelength band shown in the plot: 561 nm ± 64 nm • WFS camera exposure of 1 msec (read out at 1 KHz frame rate) – Low order WFE from thermal drift veries much slower which allows senso’s images to be averaged to impove S/N • Include camera (CCD39) QE, readout noise, and dark current specifications • Each result data point is a mean of 100 Monte-Carlo realizations • Noise performance is very similar with & without WFE presence Preliminary LoS Jitter Suppression Estimate Jet Propulsion Laboratory California Institute of Technology • Simulink model for LoS loop – Integrator for LOWFS over sampling period – Normally distributed sensor noise based on star magnitude ZWFS sensor study – Karman filter to reduce servo’s response to ZWFS sensor noise – Fitted model for FSM dynamics from measurement – PID controller • Primary limitation is photon noise/sensing speed • Very quiet between 15 - 40 rev/s • Strong resonances greater than ~130 Hz tough to kill • Faint target stars may make it tough to meet jitter requirements if wheel speed is not properly “selected” before observation – Data editing / smart operation to avoid high wheel seed Jitter Amplifier & FSM Controller LOWFS OTA Simulator and LOWFSC Testbed Pinhole PM OAP1 OAP4 Jitter Mirr OAP2 OAP3 FSM (pupil) Fold Mirr Focusing Fold Mirror LOWFS OAP Occulter & ZWFS Mask L2 L1 L3 4’ x 3’ LOWFS Bench 1.5’ x 2.5’ OTA Bench SM (pupil) CCD Coro Sci • OTA simulator consists of miniature WFIRST-AFTA telescope, relay optics, and a small jitter mirror conjugated to the pupil (PM) • OTA simulator uses the precision motion of the powered optics to simulate the telescope jitter and drifts – Line-of-sight jitter is simulated by a small mirror conjugated to pupil on a PZT stage – Small (~nm) low order (focus, stigmatism, coma, and spherical) wavefront error can be generated by moving powered optics on OTA simulator (PM, SM, and OAP2) using PZT actuators – All PZT actuators have strain gauge to ensure the precision of their motion Input WF Errors and Resulted Pre-Control 10% Mean Contrast Jet Propulsion Laboratory California Institute of Technology • Simulation of LOWFSC correction to a hypothetical thermal drift using a DM which has calibration error and fitting error (preliminary results) – Three commonly occurred wavefront drift (focus, astigmatism, and coma) are used as the WFE input to examine the WFC with DM • Based on the analysis of WFIRST-AFTA telescope thermal drift • 0.1 nm WFE in 24 hour thermal cycle (lower left plot) – The coronagraph (HLC) contrast corresponding to the three WFE modes(lower right plot) • Coronagraph has larger sensitivity to Coma. Jet Propulsion Laboratory DM Gain Calibration Errors Model California Institute of Technology DM w/ Act. Err. Example OPDnom = Nominal OPD without thermal errors (post EFC) OPDpre = Thermal + Previous Residual + OPDnom OPDsense = Z4-Z11 Fit to (Thermal + Previous Residual) OPDwfc = G x ∆u’ OPDpost = OPD after Z4-Z11 control = OPDpre - OPDwfc ∆OPDres(i) = OPDpost - OPDnom = Residual OPD after ith time-series Z4-Z11 control ∆OPDres(i) is introduced into DM1 before (i+1)th time-series Z4Z11 control Incremental Commands: Modified Static Gain Errors ∆u' = ∆u × (1 + δ ) × (1 + γ ) True (OPDsense) Time-dependent gain errors (Different at different time cases) RMS Residual Wavefront Errors After LOWFSC Correction Using DM with Calibration Error Jet Propulsion Laboratory California Institute of Technology • Post low order wavefront correction using DM – No WFE sensing error in this simulation. WFSC is quasi-static: the thermal drift is very slow compared to WFSC loop bandwidth – WFS senses only low order wavefront (Z2 - Z11) but WFE is calculated to include DM calibration error and fitting error induced high order WFE – DM gain calibration error varies from 5% to 30% • The post-WFSC residual WFE shows the DM calibration gain error greatly impact the system WF after the chasing the thermal disturbance Broadband (10%) Contrast: After LOWFSC Control Using DM with Calibration Error Jet Propulsion Laboratory California Institute of Technology • HLC broadband (10%) contrast after the low order wavefront correction using DM • The DM calibration gain error greatly impact the coronagraph contrast after the chasing the thermal disturbance • This type of DM may not be ideal for low order WF correction. Options for LOWFSC: – Better DM calibration and smarter control. Can we do smatter things about DM act influence function? – Using powered optics to correct low order wavefront correction. This will make the system more complicated – Low order wavefront sensing only and use the recorded WF information to do the post-PSF processing Jet Propulsion Laboratory Summary California Institute of Technology • Wavefront error from the WIRST-AFTA telescope jitter and thermal drift creates many challenges for the coronagraph’s performance • Zernike WFS is used to sense the low order wavefront drift and line-ofsight jitter using the rejected star light from the coronagraph • Working in differential mode the ZWFS can provide the sensitivity needed for the coronagraph • A testbed is being built to simulate WFIRS-AFTA telescope jitter & drift and to test the LOWFSC sub-system • Tight requirement for deformable mirror actuator calibration is needed if the DM is to be used for low order wavefront correction • This work is funded by NASA Exoplanet program. 18 Backups 19 ZWFS Phase Mask Manufacturing Error Tolerance Jet Propulsion Laboratory California Institute of Technology Noise Equivalent Angle (tilt) • • • • Noise Equivalent Low Order WFE SPC characterization mask is used for this analysis (similar result for other configurations) When phase-shift mask depth is off from its design specification, without resconstructor re-calibration, phase-shift mask depth error translates directly to sensing slope error and noise performance is degraded as expected However, with reconstructor recalibrated using the measured mask depth, noise performance is improved dramatically 5% manufacturing error on mask depth does not change much the noise performance