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 = 0.2 - 2.2 m

• Its stability allows significant PSF subtraction

High Contrast Imaging TechniquesUsed on HST

• Direct observation with PSF subtraction

• Coronagraphic observation with PSF subtraction

• Spatial filtering

• Spectral+spatial filtering

Choice of Camerasfor High Contrast Imaging

Direct imagers:• WFPC2: 160” x 160”, = 0.2-1.0 m• STIS: 52” x 52”, = 0.2-1.0 m• ACS Wide Field Camera: 200” x 200”, = 0.4-1.0 m• ACS High Res Camera: 26” x 29”, = 0.2-1.0 m• NICMOS: 11” x 11” to 51” x 51”, = 0.9–2.2 mCoronagraphs:• 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

PSFHST Entrance Pupil

Scatter from Optical Surface Errors

V band (ACS/HRC)Observed

18 nm RMS wavefront errorKrist & Burrows (1995)

Midfrequency Error MapPhase retrieval derived PSF

ACS Surface Brightness Plots

Observed PSF

Model PSFNo surface errors

ACS V band (F606W)

Electronic & Detector Artifacts

WFPC2

NICMOS

No Halo (model) Observed (I band)

Electronicbanding

CCD Red Halo

ACS/HRC shown.Also in STIS andWFPC2 F1042M

Stray Light & Ghosts

Defocusedghost

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 coronagraphACS Science Team(work in progress)

WFPC2WFPC2 Science Team(Unpublished)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 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)

Direct Observations – WFPC2GG Tauri Circumbinary Disk

Science results in Krist, Stapelfeldt, & Watson (2002)

V b

and

I ba

nd

- PSFsUnsubtractedLog stretch

• Disk around binary T Tauri system

• Inner region cleared by tidal forces

• Integrated ring flux = 1% of stellar flux @ I band

Direct Observations – ACS/HRC

HD 141569 - PSF

Reference PSF

HD 141569

7”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, = 0.9 - 2.2 m• Spot and Lyot stop always in-place• Occulting spot is r = 0.3” hole drilled in mirror

– Contains 2nd 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 PupilModels

Pupil after spotWith an Aligned

Lyot StopWith 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 StopModel

Misaligned Lyot StopModel

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 reduction200x reduction

NICMOS Image of HD 141569F110W (~J band)

Science results in Weinberger et al. (1999)HD 141569

Reference Star

Image1 – PSF1 Image1 – PSF2

Image2 – PSF1 Image2 – 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: = 0.2 - 1.0 m• Occulters are crossed wedges: r = 0.5”-2.8”

(21/D – 110/D @ V)• Lyot stop always in the beam• “Incomplete” Lyot stop

STIS Occulters

STIS Coronagraph PupilModels

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 dueto red halo, UV scatter

STIS Image of HD 141569HD 141569

Reference Star

HD 141569 - 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=50 mas @ 0.5 m• Multiple filters over = 0.2 - 1.0 m• Two occulting spots: r = 0.9” and 1.8” (38/D –

64/D @ V)• Occulting spots in the aberrated beam from HST,

before corrective optics

ACS Coronagraph1st (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 PSFV band, r = 0.9” spot, Arcturus (500 sec)

Shadows of largeocculting spot &

finger

Spot interiorfilled with

corrected light

Rings causedby spot diffraction

Scattered lightstreak from

unknown source

Scattered lightfrom surface errors

ACS PSF Mean Brightness Profiles (V)

Star outsideof spot

Coronagraph

│Coronagraph - PSF│(Roll subtraction)

7x reduction

6x reduction

1200x reduction 1500x reduction

Surface scatterdominated

ACS Coronagraph Image of HD 141569

7”

V band (F606W)

Science results in Clampin et al. (2003)

Disk is 2.4x brighter than PSF here

ACS Coronagraph Images of HD 141569

• 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 141569

Hard stretchDeprojectedDensity Map

DeprojectedDensity Map

3.3x fainterthan 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

focusSM = 0.5 m

focusSM = 3 m

Shift = 6 mas

Shift = 25 mas

Color Difference Focus DifferenceOcculting 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 40-100 cycles/diameter (lower value limited by occulting spot)

• Midfrequency wavefront varies by <5Å (conservative), <2Å (likely)

HST vs. Ground: HD 141569ACS Direct (V) STIS Coronagraph (U→I)

NICMOS Coronagraph (J)ACS Coronagraph (V)

Palomar AOCoronagraph (2.2 m)

Boccaletti et al. 2003(Their image)

HST can image disks in the visible – AO can’t

Spectral DeconvolutionSparks & Ford (2002)

Images courtesy of Bill Sparks

HD 130948 (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 pcMedium band filter, c = 0.5 m

Raw CODEX Image PSF Subtracted Image

J

SS

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