Hubble Space Telescope Coronagraphs

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Hubble Space Telescope Coronagraphs

• John Krist
• Space Telescope Science Institute

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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

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High Contrast Imaging TechniquesUsed on HST

• Direct observation with PSF subtraction
• Coronagraphic observation with PSF subtraction
• Spatial filtering
• Spectralspatial filtering

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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 0.92.2 mm
• Coronagraphs
• ACS High Res Camera
• STIS
• NICMOS Camera 2 19 x 19

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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

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Diffraction from Obscurations
HST Entrance Pupil
PSF
V band (no aberrations) Model
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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
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ACS Surface Brightness Plots
ACS V band (F606W)
Observed PSF
Model PSF No surface errors
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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
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Stray Light Ghosts
NICMOS (direct) F110W
Grot
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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WFPC2 WFPC2 Science Team (Unpublished)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)
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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

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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)

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Direct Observations WFPC2GG Tauri Circumbinary
DiskScience results in Krist, Stapelfeldt,
Watson (2002)
- PSFs
Unsubtracted
Log stretch

• Disk around binary T Tauri system
• Inner region cleared by tidal forces
• Integrated ring flux 1 of stellar flux _at_ I
  band

V band
I band
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Direct Observations ACS/HRC
HD 141569
Disk around a Herbig Be star at d 99 pc Disk
flux 0.02 of stellar flux
HD 141569 - PSF
Reference PSF
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ACS Science Team observations (unpublished)
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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

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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 l1.6 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
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NICMOS Coronagraph PupilModels
With an Aligned Lyot Stop
Pupil after spot
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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
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NICMOS PSF Mean Brightness Profiles (F110W)
Normal PSF
Coronagraph
Coronagraph - PSF (Roll subtraction)
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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
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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

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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)

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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 _at_ V)
• Lyot stop always in the beam
• Incomplete Lyot stop

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STIS Occulters
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STIS Coronagraph PupilModels
After Occulter, Before Lyot Stop
After Lyot Stop
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STIS PSF Mean Brightness Profiles
Wings high due to red halo, UV scatter
Direct
Coronagraph
Coronagraph - PSF (Roll subtraction)
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STIS Image of HD 141569
HD 141569
HD 141569 - Reference Star
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Reference Star
Science results in Mouillet et al. (2001)
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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

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STIS Coronagraph Problems

• Incomplete Lyot stop results in low diffracted
  light supression
• Unfiltered imaging
• Wedge position not constant

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ACS/HRC Coronagraph

• Selectable mode in the HRC the occulting spots
  and Lyot stop flip in on command
• CCD, 25 mas pixels, PSF FWHM50 mas _at_ 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 _at_ V)
• Occulting spots in the aberrated beam from HST,
  before corrective optics

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ACS Coronagraph1st (Aberrated) Image PlaneModel
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ACS Coronagraph Pupil Models
Pupil After Lyot Stop
Pupil After Spot
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ACS Coronagraph PSFV band, r 0.9 spot,
Arcturus (500 sec)
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ACS PSF Mean Brightness Profiles (V)
Star outside of spot
Coronagraph
Coronagraph - PSF (Roll subtraction)
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ACS Coronagraph Image of HD 141569
V band (F606W)
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Science results in Clampin et al. (2003)
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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
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ACS Coronagraph Image of HD 141569
Deprojected Density Map
Deprojected Density Map
Hard stretch
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ACS Coronagraph Point Source Detection Limits
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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

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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)

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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

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Sensitivity to PSF MismatchesACS
CoronagraphDisk at V (Models)
Occulting Spot Shift
Color Difference
Focus Difference
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ACS Coronagraph Sensitivity to Breathing
(dZ4 1/36 wave)
(dZ4 1/120 wave)
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ACS Coronagraph Sensitivity to Color
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ACS Coronagraph Sensitivity to Decentering
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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 lt5Å
  (conservative), lt2Å (likely)

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HST vs. Ground HD 141569
ACS Direct (V)
STIS Coronagraph (U?I)
Palomar AO Coronagraph (2.2 mm) Boccaletti et al.
2003 (Their image)
NICMOS Coronagraph (J)
ACS Coronagraph (V)
HST can image disks in the visible AO cant
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Spectral DeconvolutionSparks Ford
(2002)Images courtesy of Bill Sparks
HD 130948 (ACS Coronagraph)
After Spectral Deconvolution
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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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CODEX Our Solar System at 4 pc
Medium band filter, lc 0.5 mm
Raw CODEX Image
PSF Subtracted Image
S
S
J
J
5
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CODEX Azimuthal profile plot
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The Future of HST High Contrast Imaging

• WFC3(?) UV-Vis near-IR cameras
• No coronagraphs or occulters
• WFPC2 Cumulative radiation damage taking its
  toll (WFPC2 would be replaced by WFC3)
• STIS ACS Can continue for years
• NICMOS Can continue, but may need to be turned
  off if power system (battery) begins to
  deteriorate
• Gyroscope failure
• Would result in increased jitter (3 mas now,
  perhaps up to 30 mas on 2 gyros)
• NICMOS small-diameter STIS coronagraphic
  observations probably discontinued
• ACS coronagraph might possibly continue, but
  depends on jitter repeatability