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Hubble Space Telescope Coronagraphs

Hubble Space Telescope Coronagraphs. John Krist Space Telescope Science Institute. Why Use HST?. High resolution with wide field of view anywhere in the sky Wavelength coverage from l = 0.2 - 2.2 m m Its stability allows significant PSF subtraction.

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Hubble Space Telescope Coronagraphs

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  1. Hubble Space Telescope Coronagraphs John Krist Space Telescope Science Institute

  2. Why Use HST? • High resolution with wide field of view anywhere in the sky • Wavelength coverage from l = 0.2 - 2.2 mm • Its stability allows significant PSF subtraction

  3. High Contrast Imaging TechniquesUsed on HST • Direct observation with PSF subtraction • Coronagraphic observation with PSF subtraction • Spatial filtering • Spectral+spatial filtering

  4. Choice of Camerasfor High Contrast Imaging Direct imagers: • WFPC2: 160” x 160”, l = 0.2-1.0 mm • STIS: 52” x 52”, l = 0.2-1.0 mm • ACS Wide Field Camera: 200” x 200”, l = 0.4-1.0 mm • ACS High Res Camera: 26” x 29”, l = 0.2-1.0 mm • NICMOS: 11” x 11” to 51” x 51”, l = 0.9–2.2 mm Coronagraphs: • ACS High Res Camera • STIS • NICMOS Camera 2: 19” x 19”

  5. Components of the HST PSF • Diffraction from obscurations • Rings, spikes • Scatter from optical surface errors • Stray light & ghosts • Diffraction from occulter (coronagraph) • Electronic & detector artifacts • CCD red scatter, detector blooming

  6. Diffraction from Obscurations HST Entrance Pupil PSF V band (no aberrations) Model

  7. Scatter from Optical Surface Errors Midfrequency Error Map Phase retrieval derived PSF 18 nm RMS wavefront error Krist & Burrows (1995) V band (ACS/HRC) Observed

  8. ACS Surface Brightness Plots ACS V band (F606W) Observed PSF Model PSF No surface errors

  9. Electronic & Detector Artifacts Electronic banding NICMOS WFPC2 Observed (I band) No Halo (model) CCD Red Halo ACS/HRC shown. Also in STIS and WFPC2 F1042M

  10. Defocused ghost Stray Light & Ghosts NICMOS (direct) F110W “Grot”

  11. PSF Subtraction Stability of HST allows diffracted and scattered light to be subtracted Reference PSF Subtraction Roll Subtraction Beta Pictoris WFPC2 WFPC2 Science Team (Unpublished) Alpha Pic Beta - Alpha Pic ACS coronagraph ACS Science Team (work in progress)

  12. Sources of PSF Mismatches • Focus changes caused by thermal variations • “Breathing” = 3-5 mm primary-secondary separation change within an orbit = 1/18-1/30 wave RMS change • Attitude changes (0 – 1/9 wave change) • Internal changes in camera • Color differences • Field position variations (WFPC2) • Star-to-occulter alignment (coronagraphs) • Lyot stop shifting (NICMOS) • Jitter

  13. Direct Observation withPSF Subtraction • Primarily used for WFPC2, but also ACS and NICMOS on occasion • PSF is subtracted using an image of another star (or roll self-subtraction) • Deep exposures saturate the detector, but bleeding is confined to columns (for CCDs) or just the saturated pixels (NICMOS)

  14. Direct Observations – WFPC2GG Tauri Circumbinary DiskScience results in Krist, Stapelfeldt, & Watson (2002) Log stretch Unsubtracted - PSFs Disk around binary T Tauri system Inner region cleared by tidal forces Integrated ring flux = 1% of stellar flux @ I band V band I band

  15. PSF is 2.5x brighter than disk here HD 141569 Direct Observations – ACS/HRC Disk around a Herbig Be star at d = 99 pc Disk flux = ~0.02% of stellar flux HD 141569 - PSF Reference PSF 7” ACS Science Team observations (unpublished)

  16. Using a Coronagraph • Suppresses the perfect diffraction structure • Does not suppress scatter from surface errors prior to occulter • Reduces sensitivity to PSF mismatches caused by focus changes & color differences • Occulting spot prevents detector saturation, ghosts, and scattering by subsequent surfaces • Deeper exposures possible

  17. NICMOS Coronagraph • 0.076” pixels, l = 0.9 - 2.2 mm • Spot and Lyot stop always in-place • Occulting spot is r = 0.3” hole drilled in mirror • Contains 2nd dark Airy ring at l=1.6 mm (spot diameter = 4.3l/D, 83% of light) • Rough edge scatters some light (“glint”) • Useful inner radius ~0.5” • Spot in corner of field 0.6”

  18. With a Misaligned Lyot Stop Stop does not block spiders, secondary, edge Stop “wiggles” causing PSF variations Too-small spot causes “leakage” of light into pupil NICMOS Coronagraph PupilModels With an Aligned Lyot Stop Pupil after spot

  19. Effects of NICMOS Lyot Stop Misalignment F110W (~J band) Aligned Lyot Stop Model Misaligned Lyot Stop Model Observed Misalignment results in 2x more light in the wings + spikes

  20. 3x reduction 200x reduction 500x reduction NICMOS PSF Mean Brightness Profiles (F110W) Normal PSF Coronagraph │Coronagraph - PSF│ (Roll subtraction)

  21. NICMOS Image of HD 141569F110W (~J band)Science results in Weinberger et al. (1999) HD 141569 Image1 – PSF1 Image1 – PSF2 Reference Star Image2 – PSF1 Image2 – PSF2

  22. NICMOS Coronagraph Advantages • Only HST camera to cover near-IR • Small spot allows imaging fairly close to star • Lower background compared to ground-based telescopes

  23. NICMOS Coronagraph Problems • Poorly matched spot/Lyot stop sizes result in low diffracted light suppression • Small spot results in sensitivity to offsets & focus changes • Lyot stop position “wiggles” over time • Numerous electronic artifacts and blocked pixels (“grot”)

  24. STIS Coronagraph • Primarily a spectrograph • CCD, 0.05” pixels, PSF FWHM = 50 mas, 52” x 52” field • Unfiltered imaging: l = 0.2 - 1.0 mm • Occulters are crossed wedges: r = 0.5”-2.8” (21l/D – 110l/D @ V) • Lyot stop always in the beam • “Incomplete” Lyot stop

  25. STIS Occulters

  26. STIS Coronagraph PupilModels After Occulter, Before Lyot Stop After Lyot Stop

  27. 2x reduction 6x reduction 1200x reduction 5000x reduction STIS PSF Mean Brightness Profiles Wings high due to red halo, UV scatter Direct Coronagraph │Coronagraph - PSF│ (Roll subtraction)

  28. HD 141569 STIS Image of HD 141569 HD 141569 - Reference Star 7” Reference Star Science results in Mouillet et al. (2001)

  29. STIS Coronagraph Advantages • Smallest wedge widths allow imaging to within ~0.5” of central source • Occulter largely eliminates CCD red halo and ghosts seen in direct STIS images

  30. STIS Coronagraph Problems • Incomplete Lyot stop results in low diffracted light supression • Unfiltered imaging • Wedge position not constant

  31. ACS/HRC Coronagraph • Selectable mode in the HRC: the occulting spots and Lyot stop flip in on command • CCD, 25 mas pixels, PSF FWHM=50 mas @ 0.5 mm • Multiple filters over l = 0.2 - 1.0 mm • Two occulting spots: r = 0.9” and 1.8” (38l/D – 64l/D @ V) • Occulting spots in the aberrated beam from HST, before corrective optics

  32. r =1.8” (96%) r = 0.9” (86%) ACS Coronagraph1st (Aberrated) Image PlaneModel

  33. ACS Coronagraph Pupil Models Pupil After Lyot Stop Pupil After Spot

  34. Scattered light from surface errors Shadows of large occulting spot & finger Spot interior filled with corrected light Rings caused by spot diffraction Scattered light streak from unknown source ACS Coronagraph PSFV band, r = 0.9” spot, Arcturus (500 sec) 29”

  35. Surface scatter dominated 7x reduction 6x reduction 1500x reduction 1200x reduction ACS PSF Mean Brightness Profiles (V) Star outside of spot Coronagraph │Coronagraph - PSF│ (Roll subtraction)

  36. Disk is 2.4x brighter than PSF here ACS Coronagraph Image of HD 141569 V band (F606W) 7” Science results in Clampin et al. (2003)

  37. ACS Coronagraph Images of HD 141569 B • Disk is redder than the star • No internal color variations • Moderate forward scattering • g = 0.25 – 0.35 • Integrated disk flux is ~0.02% of stellar flux V I

  38. 3.3x fainter than PSF here ACS Coronagraph Image of HD 141569 Deprojected Density Map Deprojected Density Map Hard stretch

  39. ACS Coronagraph Point Source Detection Limits

  40. ACS Coronagraph Advantages • Greatest supression of diffracted light • Only coronagraph in which residual PSF is dominated by surface error scatter • Highest resolution & sampling • Variety of filters

  41. ACS Coronagraph Problems • Large spots (inner working radius ~1.2”) • Spots move over time • Occulting spot interior begins to saturate in short time on bright targets (~2 sec for Vega)

  42. Sources of PSF Mismatches • Focus changes caused by thermal variations • “Breathing” = 3-5 mm primary-secondary separation change within an orbit = 1/18-1/30 wave RMS change • Attitude changes (0 – 1/9 wave change) • Internal changes in camera • Color differences • Field position variations (WFPC2) • Star-to-occulter alignment (coronagraphs) • Lyot stop shifting (NICMOS) • Jitter

  43. DfocusSM = 0.5 mm A0V-A5V Shift = 6 mas K7V-K4V DfocusSM = 3 mm Shift = 25 mas Sensitivity to PSF Mismatches:ACS Coronagraph+Disk at V (Models) Occulting Spot Shift Color Difference Focus Difference

  44. ACS Coronagraph Sensitivity to Breathing (dZ4 = 1/36 wave) (dZ4 = 1/120 wave)

  45. ACS Coronagraph Sensitivity to Color

  46. ACS Coronagraph Sensitivity to Decentering

  47. HST Midfrequency Wavefront Stability • Stability derived from subtraction of ACS coronagraph B-band images of Arcturus separated by 24 hrs • Modeling used to estimate residual errors due to focus and star-to-spot alignment differences • Measured 40-100 cycles/diameter (lower value limited by occulting spot) • Midfrequency wavefront varies by <5Å (conservative), <2Å (likely)

  48. HST vs. Ground: HD 141569 ACS Direct (V) STIS Coronagraph (U→I) Palomar AO Coronagraph (2.2 mm) Boccaletti et al. 2003 (Their image) ACS Coronagraph (V) NICMOS Coronagraph (J) HST can image disks in the visible – AO can’t

  49. Spectral DeconvolutionSparks & Ford (2002)Images courtesy of Bill Sparks HD 130948 (ACS Coronagraph) After Spectral Deconvolution

  50. What Might Have Been: CODEX • Proposed optimized HST coronagraph with • High density deformable mirror (140 actuators/D) • Active focus and tip/tilt sensing and control • Selection of Lyot stops & Gaussian occulting spots • DM optimization algorithm corrects wavefront & amplitude errors over ½ of r = 5” field at a given wavelength • Was one of two proposed instruments considered selectable, but COS spectrograph chosen • Would have easily detected nearby Jovian planets • PI = Bob Brown (STScI)

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