High contrast spectroscopy Michelson Summer School 2004 Bruce Woodgate, Goddard Space Flight Center

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High contrast spectroscopyMichelson Summer
School 2004Bruce Woodgate, Goddard Space Flight
Center

• Topics
• Spectra of known planets
• Examples of current attempts for high contrast
  spectroscopy
• How to do better
• Integral field spectrographs

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Why do spectroscopy?

• Spectroscopy is harder than pictures, so why
  bother? (The public wont get to see it! )
• - For detecting an object, if we dont introduce
  much detector noise or light losses, we can get
  all the photons from an object at once, so it can
  be faster than serial filter imaging.
• - For distinguishing an object from background,
  such as a planet from the scattered light of a
  parent star, or from background stars,
  spectroscopic features can identify it. Need not
  wait years for proper motion.
• - Characterize the objects atmosphere
  molecules and atoms, temperature, density
• - Radial velocity, orbit kinematics

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Examples of planets spectra
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Spectra of planets (from Karkoschka, Icarus,
1994, 111,174)
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Neptune spectrum(Melillo, 2000)
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Earth extinction spectrum (from Karkoschka,
Icarus, 111,174)
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Earthshine spectrum(Woolf et al, ApJ 574, 430)
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Keck spectrum of molecular hydrogen around
eclipsed star
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HST/STIS slit position for brown dwarf spectrum
GL 229b
GL 229a (star)
GL 229b (brown dwarf)
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Cool object spectra(Schultz et al, 1998, ApJL
482, L181)
GL 229b spectrum taken with HST/STIS
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Examples of current attempts for high contrast
spectroscopy
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STIS coronagraphic format
Real instrument uses mirrors
CCD detector
Focal plane wedge mask blocks star with
selectable width
Pupil plane mask covers 15 outer ring
Hubble space telescope corrector mirrors
Correction for spherical aberration only No
mid-frequency wavefront correction No masking of
secondary and spider diffraction
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Gas and Dust cloud condensing around AB Auriga -
Hubble/STIS coronagraphic image, PSF subtracted
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Visible
X-rays
? HH 409C
?
new HH knots??
HH 409A ?
STIS 1998 Coronagraphic Image of HD163296 (Grady
et al. 2000) placed next to Chandra 2003 X-Ray
Image (20x20) of the same field, courtesy of
Doug Swartz (USRA/NSSTC), adjusted to the same
plate scale. As HH 409A and 409C have moved out
of the field, we believe the x-ray image has
revealed one if not two new HH knots.
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Jets and Older Herbig Ae Stars

• The first indication that the conventional wisdom
  that near-ZAMS intermediate-mass stars dont have
  jets was wrong was provided by HD 163296.

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HD163296 STIS red and UV spectra
Lyman alpha red-shifted
RED G750L
UV G140M
H alpha
Lyman alpha blue-shifted
Si III
S II
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Spectroscopic formats onto 2-D detectors
Focal Plane Feed to Spectrograph Detector
Echelle (two spectral dimensions) Long
Slit (one spatial, one spectral
dimension) Tunable narrow band imager (two
spatial dimensions, serial tune for
spectral) ImageSlicer (two spatial, one
spectral dimension)
1 2 3 4
1 2 3 4
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STIS G140M Long Slit Observations over two
epochs The 1999 image is discussed in Devine et
al (2000). Lyman a observations manifest a
proper motion asymmetry between the jet and
counterjet, while observations in Si III
highlight an asymmetry in ionization. Looking at
Si III, it is also evident that the jet is
decelerating as it moves away from the star.
HH 409C
HH 409B
HH 409A
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HD163296 H AlphaFabry-Perot in coronagraphic
mode at Apache Point 3.5-m telescope
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HD163296 (H Alpha off-band)Fabry-Perot in
coronagraphic mode at Apache Point 3.5-m telescope
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HD100546 HST/STIS visible coronagraphic image,
combining 2 roll angles
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Mapping the Environment of HD 100546

• Long slit spectrum obtained in October 2001 along
  the system major axis.

UV Lyman alpha G140M
Visible
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Mapping the Environment of HD 100546

• System minor axis observed in June 2002.

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HD 100546 the Inner 100 AU
Minor axis
Major axis

• Ly a, H2, and reflection nebulosity is enhanced
  along the system minor axis to the NE of the
  star.
• Consistent with an origin in the envelope and not
  in the disk.

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The HD 104237 Bipolar Jet

• G140M long slit spectroscopy of HD 104237 reveals
  a microjet in Lyman a.
• The approaching jet is along PA342, with the
  counterjet along PA162.
• The approaching jet extends 1.05 from the star,
  while the counterjet can be traced 2.6 from the
  star.

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Planet transiting star HD 209458 HST/STIS
photometry (from Brown et al, 2001, ApJ, 552, 699)
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Sodium in occulting planets atmosphere HD
209458bHST/STIS G750M spectrafrom Charbonneau
et al, 2002, ApJ 568, 377
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HST/STIS echelle spectrum- format used for long
high resolution spectra- one long spectrum is
chopped into many short spectral orders
?1750A
?1150A
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Wide slit spectra (2 arcsec), to show 2 spatial
dimensions and one spectral. One spatial
dimension is convolved with the spectral dimension
Supernova 1987A red spectra
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How to go deeper

• Observe all the wavelengths, and all the relevant
  spatial points simultaneously
• Maintain high efficiency and low background

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Factors controlling signal to noise (1)
The signal to noise per resolution element (S/N)
in a measurement is (S/N) Nsig / (Nsig
Nb)1/2 where Nsig number of counts from
source Nb number of counts from
background Nscatt Nsky Ndarks
Nread Nscatt residual counts from star
(after any suppression) Nsky counts from
sky, airglow, our zodi, circumstellar,
interstellar, etc Ndarks detector dark counts
Cdarksnpt where Cdarks detector dark
rate np number of pixels per resolution
element t total exposure time Nread
equivalent counts from detector read noise
nrnp(nrms)2 where For multiple reads,
nr number of reads t/t0 where t0 max
exposure time to remove cosmic rays (nrms)2
equivalent detector read noise counts per pixel
per read
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Factors controlling signal to noise (2)
Converting to rates, N Ct, (S/N) Csigt /
(Csigt Cscattt Cskyt Cdarksnpt
(t/t0)np(nrms)2 )1/2 (S/N) Csigt1/2 / (Csig
Cscatt Csky Cdarksnp (np/t0)(nrms)2 )1/2
Renaming, (S/N) Csigt1/2 / Snoiserate1/2 Total
exposure time, t (S/N)2 Snoiserate /
Csig For an excellent coronagraphic
spectrograph (TPF-C) observing a faint planet,
using a cold detector, Csig, Cscatt, Cdarks, are
small. Then Csky or Cread (np/t0)(nrms)2 could
be the dominant noise source contributing to
Snoiserate. Read noise is dominant with the
best regular CCD (with nrms 2 electrons), if
(np/t0)(nrms)2 gt FskyAeffOd?. Csky
FskyAeffOd?, Fsky (zodi)2.5x108 ph/(cm2s.sr.µ),
Aeff4.3x104cm2, O1.5x10-14 sr, np9,
t01000s For TPF-C parameters, read noise is
dominant for d? lt 0.23µ (R10). So for spectral
resolution elements narrower than broad band
filters, zero read noise photon counters are
needed. See also Lindler and Heap simulations
later.
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Spectroscopic strategies

• High contrast spectroscopy
• Reduce scattering source
• - coronagraphy, nulling interferometry
• Reduce other backgrounds (detector, sky)
• - photon counting detector, high angular
  resolution
• 3) Observe source and background simultaneously
• - subtract background from source under same
  conditions, eg seeing, thermal, pointing,
  deformable mirror status
• 4) Select wavelength, resolution, polarization to
  improve signal to background
• - eg UV, narrow spectral band for gas line
  emission, polarization for scattered light, IR
  for thermal emission
• 5) Include as many source photons as possible
• - integral field spectroscopy
• 6) Observe background surroundings broadly to
  estimate background at source position
• - integral field spectroscopy, separate source
  and background with spectral template
• 7) Observe reference point source under similar
  conditions
• - PSF subtract

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TPF Spectroscopic Requirements
Wavelength range 0.5 1.0 microns Resolving
Power 70 (Nyquist sampled if read noise
zero) Spatial sampling (i) Nyquist sample 6.0
meter diffraction limit at 0.5 microns (0.018
arcsec ) (ii) Elliptical Nyquist sample 6.0 x
3.5 m diffraction limit at 0.5 microns (0.018 x
0.031 arcsec ) Spatial coverage Cover
coronagraphic dark hole ( 1.8 arcsec square if
have 96 x 96 DM actuators)
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TPF SPECTROSCOPY - TRADE BETWEEN SLIT AND
INTEGRAL FIELD SPECTROGRAPH
Property Slit (towards star) IFU
Transmission (using prism) 0.8 0.8 Roll
alignment for point source Needed Not
Needed Alignment (slit to star) Difficult Easy
Multiple planets? No Yes Disk spectra Less
sensitive More sensitive Help find
planets? No Yes, if buried in speckles
Dark hole edge
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Traditional Integral Field Techniques
Focal Plane Feed to Spectrograph Detector
Lenslet Array Fiber Bundle Image Slicer
1 2 3 4
1 2 3 4
Based on a figure from Content, 1998.
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INTEGRAL FIELD UNIT OPTIONS
Lenslet array concentrates and separates images
to allow room to interleave spectra - examples
CFHT/Tiger, Oasis, SAURON, SNIFS, OSIRIS,
MEIFU - smallest and lightest - high
throughput - large spatial format - limited
spectral elements, use for low spectral
resolution or small spectral range 2 Mirror
image slicer array rectangle to line
reformat - examples MEIFU, MUSE, JWST/NIRSpec,
SNAP, GNIRS, KMOS, SPIFFI - intermediate weight
and size - high throughput - small spatial
format - large spectral format Fiber/lenslet
array rectangle to line reformat - examples
SMIRFS-IFU, GMOS, SILFID, INTEGRAL - large and
heavy - lowest throughput (fiber light
losses) - OK for very wide field ground-based MOS
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OSIRIS James Larkin (PI), Alfred Krabbe
(Co-PI), Andreas Quirrenbach(PS), Sean Adkins,
Ted Aliado, Paola Amico, Matthew Barczys, George
Brims, John Canfield, Thomas Gasaway, Christof
Iserlohe, Evan Kress, Ken Magnone, Nick Magnone,
Michael McElwain, Juleen Moon, Gunnar Skulason,
Inseok Song, Michael Spencer, and Jason Weiss

• Lenslet Array Integral Field Spectrograph
• Dissects arcsecond sized regions of the sky in 2
  dimensions

• Spectral resolution sufficient to take advantage
  of low background between OH sky lines (R3900)
• Full z, J, H, or K spectra with single exposure
  (1700 pixels)
• Very sensitive due to the suppression of
  atmospheric emission lines, the lack of slit
  losses and the low noise detector.
• Size 1.5 tons gt
• Vacuum chamber is 1 m3
• About 200 kg is taken to 70 K

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Design Summary(from Larkin)
Spectrograph
Collimator Optics
Lenslet Array
Cold Pupil
AO Focus
Grating
Filters
Pupil Plane
R.I. Camera Singlet
R. I. Collimating Singlet
Camera Optics
Focal Plane
Lenslet
Detector
Reimaging Optics
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LensletArray(from Larkin)

• MEMs Opticals design is fused silica, biconvex
  elements. Thickness is 1.0 mm with EFL of 0.8 mm.
  Pitch is 250 microns.
• 72x72 lenslet square area centered in 1.5
  diameter circular substrate.
• 2-3 microns of rounding between elements (98
  fill factor)
• 2 micron alignment front to back
• Sub-micron accuracy of pitch
• 1 variation in EFL across array.

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Example of microlens array- for Supernova
factory integral field spectrograph (SNIFS)
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A symmetric magnifier for the spectrograph
Imager mirror
Insertable convex mirror
Dark hole
Beam from coronagraph
Imager field
Outer field angles
To spectrograph
Microlens array
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Microlens element of array for IFS
Microlens (eg 250µ dia, f/4)
From magnification stage
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Microlens-based Integral Field Spectrograph layout
Precede by magnification stage
Insert for modes 2 and 3
Microlens (element of array)
Insert for mode 3
Focal plane
Insert for mode 1
Mask insert for mode 2
Change Field lens now precedes microlens
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TPF prime planet/speckle spectral IFS data
format- at microlens focus, and projected onto
detector, without disperser
FWHM of diffraction limit
Microlens
Spot at focus of microlens
200 x 200 microlens array. 20 pix separation. 5
spectra interleaved, each separated by 4 pix. 4k
x 4k detector
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TPF prime planet/speckle spectral IFS data
format- without disperser, showing detector
pixel spacing
FWHM of diffraction limit
Microlens
Spot at focus of microlens
Detector pixel row spacing
200 x 200 microlens array. 20 pix separation. 5
spectra interleaved, each separated by 4 pix. 4k
x 4k detector
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TPF prime planet/speckle spectral IFS data
format- with disperser
200 x 200 microlens array. 20 pix separation. 5
spectra interleaved, each separated by 4 pix. 4k
x 4k detector
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TPF prime planet/speckle spectral IFS data
format- with prism disperser
TPF baseline, 100 pixels for R70
200 x 200 microlens array. 20 pix separation. 5
spectra interleaved, each separated by 4 pix. 4k
x 4k detector
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TPF prime and auxiliary candidate spectrographic
capabilities
Suggested point design layouts for science
discussion and prioritization. 0.5 -1.0 micron
range. 4k x 4k detector. 6.5 m telescope.
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General astrophysics short high resolution
spectral IFS data format- with grating disperser
and filter range blocker
Mode 1, 100 pixels for R200 20,000, for
selected short spectral regions
200 x 200 microlens array. 20 pix separation. 5
spectra interleaved, each separated by 4 pix. 4k
x 4k detector
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General astrophysics long medium resolution
spectral IFS data format- with grating disperser
Mode 2, 4000 pixels for R3000, for long spectra
Spatial format
200 x 5 microlens array. 20 pix separation. 5
spectra interleaved, each separated by 4 pix. 4k
x 4k detector
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IFS microlens concentration factors
Concentration factor limitation contributors
diffraction, projection, chromatic focus change,
monochromatic aberrations.
Diffraction The FWHM at the focus of the
microlens is given by dDf?/D, . where D lens
diameter, ffocal length. Concentration factor,
CD D/d D2/f? .for CD large, lenses should
be large and fast. (For OAO 250µ f/4, CD 62,
dD 4µ) Projection Concentration factor, CP
D/dp D/ff ?/f, .where f/in f, and f/out
? (For a 10-m dia telescope (TPF), for a 250µ
dia microlens to Nyquist sample the diffraction
limit diameter of 0.020 arcsec at ? 0.5µ, need
f/ 500. Then CP 500/4 125.) Chromatic
focus change Concentration factor CF D/dc For
250µ lens, from ray trace, best focus spot
diameter 2.4µ. Then CF 100 Aberration
From ray trace of spherical lens, best focus spot
diameter 4.0µ. Then CA 62 Combined
concentration factor C v(1/SCi2) 38, for the
example above (250µ f/4 lenslets with f/500
input). Then with 4 pixel spacing between
spectra, can fit 9 spectra between image points.
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Alternative Reflective Image Slicer Integral
Field Spectrograph
Detector
Slicer focussing elements
Entrance slits
Slicer focal plane
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Slicer Difficulties

• In a slicer spectrograph, one spatial axis (along
  the slice) is not sampled until the detector, so
  all optical components including the grating
  introduce non-common path errors.
• Each slice has different non-common path errors.
• Introduced polarization is also different for
  each slice.

From Content, 98
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Example Image Slicer detector format- for 4 rows
of re-imaging mirrors
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Adjusting for the Elliptical Primary- an
asymmetric magnifier for the spectrograph
Asymmetric magnifier to sample 8 m x 3.5 m
diffraction limit at 0.5 microns
Insertable convex cylindrical mirror, vertically
diverging
Imager mirror
Fixed convex cylindrical mirror, horizontally
diverging
Dark hole
Beam from coronagraph
Imager field
Fold mirror
Outer field angles
Microlens array
Dark hole
To spectrograph
Spectrograph field
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Planet Detection Simulations(from Lindler and
Heap, GSFC, at Caltech TPF-C meeting June 2004)

• Four visits/two rolls per target
• Exposure times set to detect Earth-sized planet
  at the scaled Mars distance. (limited to a
  maximum of 14 days for all visits)
• Monte-Carlo simulation with random circular
  orbits
• he 0.1
• PSF reset every 10,000 seconds to reduce speckle
  residual
• Plots color coded by stellar luminosity
• Spectra integrated over entire wavelength range
  to compute S/N

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6 x 3.5 Imaging Mode Readnoise3,
Darkrate0.001
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6 x 3.5 IFU Rpower10 Readnoise3
Darkrate0.001
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6 x 3.5 IFU Rpower10 Readnoise1
Darkrate0.001
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Planet Detection via Imaging vs. R10
Spectroscopy

• Imaging
• Standard CCDs can be used
• Requires Telescope roll to remove speckles
• Requires a very stable PSF

• Spectroscopy
• Requires detector with readnoise lt 1e-
• No telescope dithers needed
• Planet confirmation information obtained
• Reduced PSF stability required