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Lyot coronagraphs with band-limited masks

Lyot coronagraphs with band-limited masks. Brian Kern (JPL) 9-29-2006 (supported by all of TPF-C work to date). Overview. Coronagraph principles Broadband modeling Testbed data Masks Wavefront Sensing / Control. Lyot coronagraph principles. (Sivaramakrishnan et al. 2001).

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Lyot coronagraphs with band-limited masks

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  1. Lyot coronagraphs withband-limited masks Brian Kern (JPL) 9-29-2006 (supported by all of TPF-C work to date)

  2. Overview • Coronagraph principles • Broadband modeling • Testbed data • Masks • Wavefront Sensing / Control Band-limited Lyot coronagraphs (Kern)

  3. Lyot coronagraph principles (Sivaramakrishnan et al. 2001) Band-limited Lyot coronagraphs (Kern)

  4. Band-limited mask • Amplitude transmission mask hasfinite bandwidth (FT has finite support) • T=0 on-axis • For uniformly illuminated pupil, all transmitted on-axis light ends up at edges of Lyot plane • Off-axis light largely unchanged • Lyot stop removes “all” on-axis light (Kuchner & Traub 2002) Band-limited Lyot coronagraphs (Kern)

  5. Implementation • SpeckleCam design • No polarizingbeamsplitter • 7 reflectionsbeforeocculter • Includes3rd DM (sequential)forredundancy • Relativelysimple design 3rd DM (Krist, Trauger & Moody 2006) Band-limited Lyot coronagraphs (Kern)

  6. Throughput and IWA • Throughput vs angle is determined by width of occulter • Throughput linearly related to occulter transmission and Lyot area • Linear 4th-order max throughput 0.45 @ 4 l/D, 0.16 @ 2 l/D • Linear 8th-order (m=1, l=3) max0.30 @ 4 l/D, 0.03 @ 2 l/D • Diffractive efficiency (size ofPSF) determined by Lyot stop • Smaller IWA -> narrower occulter -> smaller Lyot stop -> bigger PSF • Bigger PSF is more sensitive to zodi adjustable width Linear sinc2(4th order) Band-limited Lyot coronagraphs (Kern)

  7. IWA considerations • Stellar size limits contrast for 4th-order mask • 4th-order maskloses 34 oftop 100 stars • 8th-order maskcan observe all top 100 stars (Crepp 2006) Band-limited Lyot coronagraphs (Kern)

  8. Optical bandwidth – DM correction • Optical surface requirements depend on bandwidth • For sequential DMs, amplitude-induced phase errors and 2nd-order propagation of phase and reflectivity variations set bandpass • Requirements are linear in R = Dl/l R=6.3 (Shaklan & Green 2006) Band-limited Lyot coronagraphs (Kern)

  9. Optical bandwidth - occulter • Lyot stop size is determined by occulter width and by longest wavelength in bandpass • Shortest wavelength in bandpass would have higher throughput if observed individually • Occulter transmission profile, phase profile should not change with l • Nominal design has 3 bands of ~ 100 nm each • One band is “discovery” band, set for best contrast • Each band has different Lyot stop to improve throughput • Bandwidth generally limited by wavefront correction contrast vs. surface requirements, rather than by occulter Band-limited Lyot coronagraphs (Kern)

  10. Aberration sensitivity: I • 8th-order masks have greatly reduced sensitivities compared to 4th-order masks (Shaklan & Green 2005) Band-limited Lyot coronagraphs (Kern)

  11. Aberration sensitivity: II • Polarization of FB1 is a non-issue with 8th-order mask • Allows design with no beamsplitter • With standard coatings, 4th-order FB1 with no polarization control limits contrast to ~ 10-9 • Polarization-induced aberrations are predominately low-order (Balasubramanian et al. 2005) Band-limited Lyot coronagraphs (Kern)

  12. Aberration sensitivity: III • DM cannot correctrandom occulter transmission errors over widebandpass becauseerrors are in focal plane • Requirements areconsistent with superpolished surfaces • Linear occulter can betranslated to avoid“bad spots” (Lay et al. 2005) (Duparre & Jakobs 1996) Band-limited Lyot coronagraphs (Kern)

  13. Requirements 8th order • Dynamic errorbudget dominatedby pointing error • Reminder ofrelaxation allowedby 8th-order mask (STDT 2006) (Shaklan STDT 2005) Band-limited Lyot coronagraphs (Kern)

  14. Broadband modeling • 3 independent packages for detailed modeling of full Fresnel propagation effects over finite bandwidths • PROPER (Krist) • MACOS + Matlab proprietary code (Sidick) • Python proprietary code (Moody) • Monochromatic binary mask modeling (Hoppe) • Avoids limitations of semi-analytic approximations • E.g., analysis for surface requirements uses 2nd-order Taylor expansions • Models point to specific problems, mitigations • E.g., effects of nonideal (complex) mask transmission, variations in Lyot stop size, speckle nulling algorithms Band-limited Lyot coronagraphs (Kern)

  15. Broadband modeling guidance • Requirements on systematic occulting mask errors are difficult to quantify analytically • Band-limited portion of mask errors are filtered by Lyot stop • More restrictive Lyot stops relax mask requirements • Nulling algorithms may be tested and optimized • “Full knowledge” about complex electric fields are available to models Band-limited Lyot coronagraphs (Kern)

  16. Model validation testbed • High Contrast Imaging Testbed (HCIT) provides experimental validation and guidance to models Lyot DM occulter Band-limited Lyot coronagraphs (Kern)

  17. Testbed layout • Testbed is classical Lyotarrangement • 32x32 DM • Optics ready for 64x64 DM • Re-imaging back end foradequate sampling on CCD • Monochromatic and broadband light sources • Broadband light generatedwith supercontinuum laser • Select bandpass usingfilters • 2%, 10% Band-limited Lyot coronagraphs (Kern)

  18. Testbed results • Monochromatic contrast to < 10-9 • Explore variations in contrast with bandwidth • Null at 785 nm with 2% bandwidth • Measure contrast at 10%bandwidth without changing DM • Agreement with model ~ 20% • Modeling shows pathfor improvement • Performance limited by systematic mask errors (dispersion) • Optimal Lyot stop improves by ~ 2x Band-limited Lyot coronagraphs (Kern)

  19. Occulting mask technology • Ideal occulting mask transmission is real-valued and independent of wavelength • No spatial variations in transmitted phase • No spatial variations in dispersion • No spatial variations in absorption spectrum • Occulting profile smooth on fl/D scale • Profiles can be grayscale or binary on smaller scales (Balasubramanian et al. 2005) Band-limited Lyot coronagraphs (Kern)

  20. Occulting mask materials • Metal evaporation for binary occulters • Si substrate, etched to leave “windows” for transmission • Variable duty-cycle approach leads to “waveguiding” polarization effects • High Energy Beam Sensitive (HEBS) glass • Glass becomes absorbing with e--beam dose • Excellent grayscale control, good absorption (108), good spatial resolution • Different exposure levels show different absorptive spectra, exposure changes index of refraction (dispersion also changes) Band-limited Lyot coronagraphs (Kern)

  21. Occulting mask experiments - I • Binary mask polarization models validated on HCIT • Prediction was that orthogonal linear polarizations seedifferent occultingmask phase • Resulting contrastshould be differentin two polarizations Data Models Nulledpolarization < 10-8 Orthogonalpolarization > 10-7 (Hoppe 2006) Band-limited Lyot coronagraphs (Kern)

  22. Occulting mask experiments - II • HEBS transmission is non-ideal in phase and modulus • Spectrometer measures intensity transmission in lab • Interferometer measures transmitted phase in lab • Phase dispersion in particular limits broadband performance • Anomalous dispersion, consistent with resonant absorber model • All HEBS formulations to date show similar dispersion • Model of broadband contrast limits matches testbed data (Halverson et al. 2005) Band-limited Lyot coronagraphs (Kern)

  23. Occulting mask development • Current HEBS and binary masks don’t reach 10-10 contrast over 10% bands (in both polarizations) • Modeling has begun on combined metallic – dielectric occulters • Occulter transmission modulus of candidate formulations maintain systematic errors for contrast < 10-10 • Control of transmitted phase should be feasible with multi-layer dielectrics Band-limited Lyot coronagraphs (Kern)

  24. Nulling algorithms • Wavefront sensing performed on same light that heads to science camera • Avoid non-common path sensing • Image-plane vs. Lyot-plane nulling • Aberrations in pupil plane cause speckles from on-axis source to scale radially with wavelength when viewed in image plane • Speckle in image plane (smeared radially) can be nulled by sinusoidal phase in pupil plane • Uses all actuators on pupil-plane DM • Speckle in Lyot plane (not smeared) can be nulled using a small number of neighboring actuators in pupil plane • Uses actuators in “band-limited” neighborhood of speckle, projected onto pupil-plane DM Band-limited Lyot coronagraphs (Kern)

  25. Sensing for null - I • Must determine complex correction from intensity images • Image-plane “Speckle nulling” applies discrete spatial frequencies to DM and minimizes image-plane intensities • Sparse nature of image-plane observations is tolerant of noise • Must iterate to correct continuous distribution of errors • Lyot-plane speckle nulling actuates noncontiguous actuators at DM and minimizes Lyot-plane intensities • Simplest technique ignores structure of occulter transform • Does not discriminate speckle image-plane position DM image occulter FT Lyot DM Band-limited Lyot coronagraphs (Kern)

  26. Sensing for null - II • Bordé-Traub algorithm senses entire correction at pupil using complicated “test pattern” • Uses linear-phase approximation (eif = 1 + if) • Applies to image plane or Lyot plane • Complexity of “test pattern” (number of degrees of freedom) determines sensitivity to noise • Detailed behavior in presence of noise not yet known • Spectral smearing in image plane sensing may require more iterations • Lyot plane nulling may lead to undesirable distribution of speckles in the dark hole • Could consider information from both Lyot and image plane Band-limited Lyot coronagraphs (Kern)

  27. Model of nulling algorithm • MonochromaticBordé-Traub nulling algorithm • Start from “blank”DM setting • 20 iterations to10-9, 30 to 6x10-10 • Includes opticaleffects not presentin linear-phaseanalysis (Krist & Bordé / PROPER) Band-limited Lyot coronagraphs (Kern)

  28. Summary • Con: Band-limited masks have lower throughput, larger PSF than some coronagraph designs • Pro: 8th-order mask offers excellent rejection of low-order aberrations • Allows relaxed requirements / no polarization control • Pro: Low optical complexity • Pro: Lower risk (well validated models) Band-limited Lyot coronagraphs (Kern)

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