Predicting the Performance of Space Coronagraphs

John Krist (JPL)17 August 2016

1st 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

Dark Hole Generation Process

“Flatten” the wavefront

(phase retrieval)

Sense imageplane E-fieldλ

(DM probing)

Determine DM settings

(EFC, stroke minimization)

Evaluate new DM solution(PROPER)

Converged? YesNo

~10-6 contrast

Initial state(fabrication errors,

polarization-induced aberrations)

~10-9 contrast

~10-4 contrast

Before any WFC (10-4)

4

Flattened (10-6)

EFC Iter 1(4 x 10-8) EFC Iter 2

Add pointing & WFE jitter, finite diameter star,thermal variations, alignment tolerances, etc.

Exo-C charge 4 VVC: 495-605 nm, X polarization

No jitter0.4 mas RMS jitter

+ 1 mas star0.8 mas RMS jitter

+ 1 mas starLo

g 10(

cont

rast

)

No jitter

0.4 mas RMS jitter

0.8 mas RMS jitter

-7

-12

Circles are r = 1 & 14 λ/D

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 CoronagraphDownselect Competition Design

DM1 DM2 Shaped Pupil

Charge 4 Vortex FPM Lyot Stop

~1.5 μm stroke(optimized for 550 nm only)

r = 3.2 – 20 λ/Dimaging field

WFIRST VVC Unaberrated Contrasts

522.5 nm 550 nm 577.5 nm

r = 15 λ/D

WFIRST Coronagraphs

Lyot stop in grey superposedon AFTA obscurations

DM Patterns Focal plane mask

5.2 λc/D

3.2 λc/D

Lyot

Sto

p 1

Lyot

Sto

p 2

Lyot

Sto

p 3

FPM

HLCSPC

PIAACMC

Pupi

l mas

kFP

MLy

ot st

op

WFIRST Coronagraph Field PSF EEs

WFIRST

PSF Core Throughput

WFIRST 34.0%HLC 4.5%SPC 3.7%PIAACMC 14.0%

No jitterNo star

10

0.4 mas jitter1.0 mas star

0.8 mas jitter1.0 mas star

1.6 mas jitter1.0 mas star

HLC

SPC

PIAACMC

WFIRST Dark Holes with Pointing Jitter & Finite StarNo polarization errors

λc=550 nm10%

λc=800 nm18%

λc=550 nm10%

Log 1

0(co

ntra

st)

No polarization aberrations

PolXoptimized

PolX + PolYoptimized

Circles are r = 3 & 9 λc/D

WFIRST HLC: λc=550 nm, 15%Cross-polarization included, no jitter

WFIRST PIAACMC: λc=550 nm, 10%Cross-polarization included, no jitter

Log 1

0(co

ntra

st)

Circles are r = 1.3 & 9 λc/D

No aberrations

X polarization aberrations

No polarizationerrors

Optimizedfor X polarization

WFIRST Coronagraph Aberration Sensitivities100 picometers RMS of aberration @ 550 nm

HLC SPC

PIAACMC

61 UMa β UMa+13°

47 UMa+13°

47 UMa-13°se

ttle

sett

le

sett

le

61 UMa β UMa+13°

47 UMa+13°

47 UMa-13°se

ttle

sett

le

sett

le

OS5: Zernike Aberrations vs Time from Thermal Models

pm = picometers

Vortex Low-Order Aberration Sensitivity

Tip/Tilt

Spherical

Trefoil

Coma

Astigmatism

Focus

Tip/Tilt

Spherical

Trefoil

Coma

Astigmatism

IWAIWA

Charge 4 Vortex Charge 6 Vortex

RMS change in background contrast @ λ=550 nm with the introductionof 100 picometers RMS of the specified aberration

For very high contrast (10-10) space coronagraphs, a charge 4 vortex is notviable 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 withpolarization filtering.

Segmented Telescope Coronagraph Considerations

Effective throughputPlanet PSF morphology

Aberration sensitivitySegment-to-segment piston, global low-order, wavefront jitter

Jitter & finite stellar diameter DM patterns (ACAD)

Affect on PSF morphology, increased aberration & jitter sensitivities, stroke limitationsAlignment 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?)