ASP:Absolute Spectrum Polarimeter

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ASPAbsolute Spectrum Polarimeter
CMB and Foreground Polarization With Only 4
Bolometers
Al Kogut Goddard Space Flight Center
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Absolute Spectrum Polarimeter
Team Members
Explorer-Class mission concept to measure
spectrum and polarization of sky from 30 GHz to 5
THz (1 cm to 60 ?m)
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CMB Polarization Are You Kidding?
How Do You Measure Signals at Parts-Per-Billion
Level?
Typically think in terms of kilo-pixel detector
arrays with some front-end polarization
modulator.
But there is another solution
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Photon Noise and Etendu
Photon noise density
Proportional to etendu A?
Sampled Noise
Proportional to ( A? )1/2
Sky Noise (Specific Intensity)
Proportional to ( A? )-1/2
Single-Mode Optics A? ?2 Beam size and
etendu scale with wavelength
Multi-Moded Optics A? N ?2 Beam size and
etendu constant
Number of modes N scales with wavelength
Want Multi-Moded Optics with Really Big
Detectors! Improve Sensitivity by Factor N A? /
?2
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Instrument Concept

• Polarizing Michelson Interferometer
• Interfere 2 inputs
• Optical Phase Delay
• Measure Fringe Pattern
• Big detectors A? 4 cm2 sr
• Wide Frequency Coverage Fourier Transform
  Spectrometer for spectrum 30 GHz -- 5 THz
• Multiple Nulls and Modulation
• External Calibrator Nulls Spectrum
• Spin and Phase Delay Modulate Signal
• Sun-synchronous polar orbit (COBE orbit)

Measured Fringes ? Difference Spectrum Between
Orthogonal Polarizations
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ASP Optics (unfolded)
Define 2? tophat beam on sky
Separate polarizations
Split Beams
Optical Phase Delay ?2z?/c
Re-Combine Beams
Mix Polarizations
Detect Interference Fringes
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ASP vs Polarization Challenges
4 detectors sense 44,000 modes in 40 spectral
bins for CMB
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ASP vs Polarization Challenges
4 detectors sense 44,000 modes in 40 spectral
bins for CMB
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ASP vs Polarization Challenges
4 detectors sense 44,000 modes in 40 spectral
bins for CMB
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ASP Interferograms
Forward
Reverse
Near stop to null
Far stop to null
Null to near stop
Null to far stop
Same information 4x / stroke with different
time/space symmetries Transform Real partSignal
noise, Imaginary partNoise only
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ASP Spin Modulation
Time-Ordered Data for Polarized CMB
4 RPM spin provides amplitude modulation of
entire interferogram
Noiseless Sim Sinusoidal Systematics
Powerful tool against spin-synchronous systematics
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ASP Schematic
Beam to Sky
Calibrator
2.7K ADR
Calibrator Cntwght Motor
Field Mirror
2.7 K
Collimator A
ASP Instrument
Collimator B
Polarizer Grids
Dihedral Assembly
MTM
Sunshades
Collimator C
Detectors
Collimator D
Primary Mirror B
Primary Mirror A
Cryocooler
Thermal Break
Cryocooler Compressor
Cryocooler Elec Box
Cryocooler Radiator Panels
Instrument PWAs
300 K
S/C Bus
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ASP Instrument and Observatory
Spin 4 RPM
Instrument Box (2.7 K)
Blackbody Calibator
Primary Mirror A
Thermal Break
Dihedral Mirrors
To Sun
4 Detectors (100 mK)
Radiator Panels
Polarizing Grids
Solar Panels
To Earth
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Sensitivity and Frequency Coverage

• 30 GHz to 5 THz
• (1 cm to 60 ?m)
• ?? 15 GHz (R 300)
• Noise 2 Jy/sr in each bin
• ( 2 x 10-26 W m-2 Hz-1 sr-1 )
• CMB Parameters
• ? lt 10-6
• y lt 10-7
• r ltlt 0.01

nK Sensitivity for CMB B-Mode Spatial Structure
Using Only 4 Bolometers!
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ASP Samples Cosmic History
Inflation and the Oldest Information in the
Universe
Chemical Potential ? Dark matter
annihilation/decay
Compton y distortion vs E-mode pol Reionization
and First Stars
Far-IR Background Matter at z3
Galactic Line Emission Energy Balance of
ISM Chemical Separation of ISM
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B-Mode Polarization
Sensitivity limit set by foregrounds, not noise
Tensor/scalar ratio r ltlt 0.01
r0.1
r0.01
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Secondary Science Reionization
Neutral IGM
Reionization is complex and not well
understood Pop III? Pop II? Feedback?
First Stars Pop III? Pop II?

• Reionization and Compton Scattering
• Polarization Optical depth ? ? n(z)
• Spectrum Compton distortion y ? nkTe
• Combine to get Te (ionizing spectra)

Ionized IGM
Observer
"Guaranteed Science" -- Signals must exist
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Secondary Science Far-IR Background
Thermal Dust Emission from z 3 Monopole Galaxy
Evolution Dipole Motion Anisotropy Matter power
spectrum
ASP noise is down here!
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Secondary Science Foregrounds

• Continuum Emission
• Synchrotron, Dust
• Line Emission
• CO, C, N, OIII,
• Diffuse ISM
• Temperature, Density
• Energy Balance
• Chemical Separation

300 Spectral Maps Stokes I, Q, U ?? 15 GHz
Extremely Rich Data Set!
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Now How Much Would You Pay?
You get your inflation, your dark matter, your
reionization, your infrared background, AND your
Galactic dust/ISM
Operators are standing by!
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ASP Blackbody Calibrator
Al Core
Absorber "skin"
Thermometer

• Power reflection R lt 10-6
• Based on Successful ARCADE Calibrator
• Null spectrum for interferometer
• Continuous calibration for detectors

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Detector Summary
"Harp" single-polarization bolometer
Silicon Frame
"G String" sets conductance to bath
Thermistor
"Harp String" Polarizing Absorber Doped silicon
for impedance match 3 ?m x 1.5 ? m x 13000 ?m 30
?m pitch
Thermalizer Bar
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ASP "Wide" Channel Sensitivity
Channel Sensitivity After Apodization (CMB
channels only)
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Spacecraft
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Cryogenic Design
2.7 K
ADR
0.1 K
ADR
2.7 K
Sun Shields
Instrument Isothermal with CMB (2.725 K)
Cryo-Cooler
5 K
Thermal Break
300 K
Spacecraft
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Spectral Distortions
ASP limits Compton Distortion y lt 10-7 Chemical
Potential ? lt 10-6
Compton Scattering
Compton ?T ? ? nkTe
Log I
Chemical Potential
Redshift z 106
Log ?
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ASP and Dark Matter
Dark matter annihilation injects energy into
early universe
Annihilation ?? ? qq or ?? injects energy ?E
m? Annihilation rate n2 z6 larger at early
times
Observable limit ? lt 10-6 yields m? gt 10 MeV
McDonald, Scherrer, Walker 2000, PRD, 63, 023001