Hubble Space Telescope Coronagraphs
description
Transcript of 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 = 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WFPC2WFPC2 Science Team(Unpublished)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