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  =  m Its stability allows significant PSF subtraction

High Contrast Imaging Techniques Used on HST Direct observation with PSF subtraction Coronagraphic observation with PSF subtraction Spatial filtering Spectral+spatial filtering

Choice of Cameras for High Contrast Imaging Direct imagers: WFPC2: 160” x 160”, =  m STIS: 52” x 52”, =  m ACS Wide Field Camera: 200” x 200”, =  m ACS High Res Camera: 26” x 29”, =  m NICMOS: 11” x 11” to 51” x 51”, = 0.9–2.2  m Coronagraphs: ACS High Res Camera STIS NICMOS Camera 2: 19” x 19”

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

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

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

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

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

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

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Beta Pictoris Alpha Pic Beta - Alpha Pic ACS coronagraph ACS Science Team (work in progress) WFPC2 WFPC2 Science Team (Unpublished) Reference PSF Subtraction Roll Subtraction

Sources of PSF Mismatches Focus changes caused by thermal variations –“Breathing” = 3-5  m 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

Direct Observation with PSF 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)

Direct Observations – WFPC2 GG Tauri Circumbinary Disk Science results in Krist, Stapelfeldt, & Watson (2002) V band I band - PSFsUnsubtractedLog stretch Disk around binary T Tauri system Inner region cleared by tidal forces Integrated ring flux = 1% of stellar I band

Direct Observations – ACS/HRC HD PSF Reference PSF HD ” ACS Science Team observations (unpublished) PSF is 2.5x brighter than disk here Disk around a Herbig Be star at d = 99 pc Disk flux = ~0.02% of stellar flux

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

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

NICMOS Coronagraph Pupil Models Pupil after spot With an Aligned Lyot Stop 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

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

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

NICMOS Image of HD F110W (~J band) Science results in Weinberger et al. (1999) HD Reference Star Image1 – PSF1Image1 – PSF2 Image2 – PSF1Image2 – PSF2

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

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”)

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

STIS Occulters

STIS Coronagraph Pupil Models After Occulter, Before Lyot Stop After Lyot Stop

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

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

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

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

ACS/HRC Coronagraph Selectable mode in the HRC: the occulting spots and Lyot stop flip in on command CCD, 25 mas pixels, PSF FWHM=  m Multiple filters over  =  m Two occulting spots: r = 0.9” and 1.8” (38 /D – 64 V) Occulting spots in the aberrated beam from HST, before corrective optics

ACS Coronagraph 1 st (Aberrated) Image Plane Model r =1.8” (96%) r = 0.9” (86%)

ACS Coronagraph Pupil Models Pupil After Spot Pupil After Lyot Stop

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

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

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

ACS Coronagraph Images of HD 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 B V I

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

ACS Coronagraph Point Source Detection Limits

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

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)

Sources of PSF Mismatches Focus changes caused by thermal variations –“Breathing” = 3-5  m 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

Sensitivity to PSF Mismatches: ACS Coronagraph+Disk at V (Models) A0V-A5V K7V-K4V  focus SM = 0.5  m  focus SM = 3  m Shift = 6 mas Shift = 25 mas Color DifferenceFocus Difference Occulting Spot Shift

ACS Coronagraph Sensitivity to Breathing (  Z4 = 1/36 wave) (  Z4 = 1/120 wave)

ACS Coronagraph Sensitivity to Color

ACS Coronagraph Sensitivity to Decentering

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 cycles/diameter (lower value limited by occulting spot) Midfrequency wavefront varies by <5Å (conservative), <2Å (likely)

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

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

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)

CODEX: Our Solar System at 4 pc Medium band filter, c = 0.5  m Raw CODEX ImagePSF Subtracted Image J S S J 5”

CODEX Azimuthal profile plot

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