Predicting the Performance of Space Coronagraphs John Krist (JPL) 17 August 2016 1 st International Vortex Workshop
Determine the Reality of a Coronagraph through End-to-End Modeling Use End-to-End modeling to predict performance in a realistic system Deformable mirrors & wavefront control Realistic aberrations (fabrication, coating, polarization) on all optics Surface-to-surface propagation Evaluate the tolerance to low-order aberrations (after LOWFC & fast steering correction) Pointing jitter Wavefront jitter (reaction wheel vibrations of optics) Thermal changes Stellar diameter Determine useful throughput How much light ends up in the core of the planet s PSF? Tolerance to misalignments, distortions, magnification errors
Recent Space Coronagraph Studies Exo-C Probe Study 1.4 m off-axis (clear aperture, f/2.5 primary, mild polarization impact) system optimized for coronagraphy Hybrid Lyot, Vector Vortex, classical PIAA coronagraphs evaluated WFIRST 2.4 m on-axis (obscured, f/1.2 primary, significant polarization impact) system not optimized for coronagraphy After downselect competition in 2013, Hybrid Lyot & Shaped Pupil were baselined, PIAACMC is backup Both utilize LOWFS to correct time-dependent low-order errors with the DM, fast steering mirror, & focus correction mirror
Initial state (fabrication errors, polarization-induced aberrations) Dark Hole Generation Process Before any WFC (10-4 ) Flattened (10-6 ) EFC Iter 1 (4 x 10-8 ) EFC Iter 2 Flatten the wavefront (phase retrieval) ~10-4 contrast ~10-6 contrast Sense image plane E-field λ (DM probing) Determine DM settings (EFC, stroke minimization) Evaluate new DM solution (PROPER) No Converged? Yes Add pointing & WFE jitter, finite diameter star, thermal variations, alignment tolerances, etc. ~10-9 contrast
Exo-C charge 4 VVC: 495-605 nm, X polarization No jitter 0.4 mas RMS jitter + 1 mas star 0.8 mas RMS jitter + 1 mas star -7 Log 10 (contrast) -12 Circles are r = 1 & 14 λ/d No jitter 0.8 mas RMS jitter 0.4 mas RMS jitter On a 12 m unobscured scope, the 0.8 mas jitter result shown here is equivalent to 0.09 mas jitter, or a ~0.36 mas diameter star.
WFIRST Vortex Coronagraph Downselect Competition Design DM1 DM2 Shaped Pupil ~1.5 μm stroke (optimized for 550 nm only) Charge 4 Vortex FPM Lyot Stop r = 3.2 20 λ/d imaging field
WFIRST VVC Unaberrated Contrasts 522.5 nm 550 nm 577.5 nm r = 15 λ/d
DM Patterns WFIRST Coronagraphs Focal plane mask 5.2 λ c /D HLC SPC Lyot stop in grey superposed on AFTA obscurations FPM Lyot Stop 1 FPM Lyot Stop 2 Lyot stop Pupil mask 3.2 λ c /D PIAACMC Lyot Stop 3
WFIRST Coronagraph Field PSF EEs WFIRST PSF Core Throughput WFIRST 34.0% HLC 4.5% SPC 3.7% PIAACMC 14.0%
WFIRST Dark Holes with Pointing Jitter & Finite Star No polarization errors No jitter No star 0.4 mas jitter 1.0 mas star 0.8 mas jitter 1.0 mas star 1.6 mas jitter 1.0 mas star HLC λ c =550 nm 10% SPC λ c =800 nm 18% PIAACMC λ c =550 nm 10% 10
WFIRST HLC: λ c =550 nm, 15% Cross-polarization included, no jitter No polarization aberrations Pol X optimized Pol X + Pol Y optimized Log 10 (contrast) Circles are r = 3 & 9 λ c /D
WFIRST PIAACMC: λ c =550 nm, 10% Cross-polarization included, no jitter No polarization errors Optimized for X polarization X polarization aberrations Log 10 (contrast) No aberrations Circles are r = 1.3 & 9 λ c /D
WFIRST Coronagraph Aberration Sensitivities 100 picometers RMS of aberration @ 550 nm HLC SPC PIAACMC
OS5: Zernike Aberrations vs Time from Thermal Models 61 UMa β UMa +13 settle settle 47 UMa +13 settle 47 UMa -13 61 UMa β UMa +13 settle settle 47 UMa +13 settle 47 UMa -13 pm = picometers
Vortex Low-Order Aberration Sensitivity RMS change in background contrast @ λ=550 nm with the introduction of 100 picometers RMS of the specified aberration Charge 4 Vortex Charge 6 Vortex Coma IWA IWA Spherical Astigmatism Tip/Tilt Trefoil Tip/Tilt Astigmatism Trefoil Focus Coma Spherical For very high contrast (10-10 ) space coronagraphs, a charge 4 vortex is not viable due to its high astigmatism sensitivity, but a charge 6 vortex is. A charge 4 vortex would not work on WFIRST due to polarization, even with polarization filtering.
Segmented Telescope Coronagraph Considerations Effective throughput Planet PSF morphology Aberration sensitivity Segment-to-segment piston, global low-order, wavefront jitter Jitter & finite stellar diameter DM patterns (ACAD) Affect on PSF morphology, increased aberration & jitter sensitivities, stroke limitations Alignment tolerances Mask-to-pupil registration, pupil distortion
Coronagraph Performance Metrics Diffraction suppression Residual background contrast Post-wavefront control residuals Dynamic aberrations Inner working angle Practical inner working angle Finite stellar diameter Polarization-induced aberrations Dynamic aberrations (pointing, fast aberrations) Transmission Useful throughput How much light ends up in the core of the planet s PSF? How much background is in the core (narrow or wide core?)