Transcript
Observing with Bolometer Arrays
Brian Mason (NRAO) 15jul13 7th NAIC/NRAO single dish school
What is a Bolometer?
C: “Heat Capacity” [Joules/Kelvin] G: “Thermal Conductance” [Joules/sec/Kelvin]
Destroys EM-wave phase information instantly! Measures intensity very sensitively. 2
What is a Bolometer?
Lowest possible noise for a phase-preserving amplifier:
C: “Heat Capacity” [Joules/Kelvin] Minimum Conductance” noise gets *added* to the intrinsic G: “Thermal [Joules/sec/Kelvin] Tsky noise
Destroys EM-wave phase information instantly! Measures intensity very sensitively. 3
Bolometer Sensitivity Bolometers avoid this limit completely Their sensitivity is set by: 1) electrical & phonon noise in the detectors
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Bolometer Sensitivity Bolometers avoid this limit completely Their sensitivity is set by: 1) electrical & phonon noise in the detectors
Bolometer cameras want to be cold! few 100 mK vs 15K for coherent (HEMT) amplifiers
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Bolometer Sensitivity Bolometers avoid this limit completely Their sensitivity is set by: 1) electrical & phonon noise in the detectors
few 100 mK vs 15K for coherent (HEMT) amplifiers
2) photon noise in the signal e.g., Richards (1994)
shot-noise
Bolometer cameras want to be cold!
radiometer equation well designed bolometer camera has photon noise > intrinsic detector noise
“Background Limited Performance” (BLIP) 6
Bolometer Sensitivity Bolometers avoid this limit completely Their sensitivity is set by: 1) electrical & phonon noise in the detectors
2) photon noise in the signal e.g., Richards (1994)
shot-noise
Bolometer cameras want to be cold!
few 100 mK vs 15K for coherent (HEMT) increasing bandwidth amplifiers is cheap and easy! radiometer equation
well designed bolometer camera has photon noise > intrinsic detector noise
“Background Limited Performance” (BLIP) 7
TES
SQUID MUX readout chips
MUSTANG
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Millimeter Bolocam • 1-2 mm • 144 pixels • CSO (10m), APEX -> LMT
ACT (6m) , SPT (10M) • 1-2 mm • Large Area Surveys (SZ) - 1000s of deg^2 • few 1000 detectors
Also IRAM 30m (MAMBO)
Sub-millimeter SCUBA-2 • JCMT (15m) • ~10,000 pixels! • SQUID-MUX’d TES bolometers (CCD-like)
Also: SHARC-II on CSO, 384 pixels at 350 um
Sub-millimeter SCUBA-2 • JCMT (15m) • ~10,000 pixels! • SQUID-MUX’d TES bolometers (CCD-like)
Also: SHARC-II on CSO, 384 pixels at 350 um
Observing & Data Analysis Challenges Bolometer Arrays measure broad-band (continuum) radiation • Systematics – fluctuations in atmospheric emission – receiver instabilities (e.g., cryogenic) – gain or offset drifts (1/f) • Observing – scan strategy
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Atmospheric Emission Across the Array θ angle
distance from gbt surface
adjacent pixels
4″
86 km
Across array
40″
9 km
Scale Height of Water Vapor ~ 1-2 km
Simplified Picture: •3D •power in sidelobes •Near field (D2/λ)
Common Mode Subtraction Data (“time-stream” or “time-ordered data”): i: detector
j: integration number
Detector Array: i=1 i=2
...
Sources with size << FOV don’t contribute much to CM i=N
Noise Power Spectrum
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Noise Power Spectrum
Residual drifts
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Noise Power Spectrum
Refrigerator
Residual drifts
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Noise Power Spectrum
astronomy signal (scanning telescope)
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Penalty of Common Mode Subtraction Removes structure larger than the array field-of-view (FOV) • Can usually be retrieved if extended signal is bright • not generally feasible if the extended signal is faint.
Detector Array: i=1 i=2
...
If you care about diffuse, extended structure you want a large FOV! i=N
Large Scale Sunyaev-Zel’dovich Effect Decrement (Model) MUSTANG FOV
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Large Scale Sunyaev-Zel’dovich Effect Decrement (Model) --> Passed through MUSTANG transfer function MUSTANG FOV
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... with real data (green contours) MUSTANG FOV
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... with real data (green contours) MUSTANG FOV
MUSTANG-2 FOV
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How you generally observe with a bolometer array: On-the-Fly Mapping • “OTF” • Slew the telescope around – record samples (10 Hz to 1 kHz) of: • telescope pointing (R.A., Dec.) • each bolometer’s reading (total power) How should you scan? (speed, pattern, depth, etc.)
Noise is key
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Sky Brightness
How do Structures on the Sky appear in Bolometer Time-Stream Data? x
v
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Noise Power Spectrum
8’’ @ 1’/sec
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Noise Power Spectrum
5’ @ 1’/sec
8’’ @ 1’/sec
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Well-Constrained Sky Structure Period or angular scale of structure isn’t all that matters: orientation relative to your scan pattern matters too! Telescope Trajectory Peak in sky intensity
Trough in sky intensity Peak
Trough...
Appears at a reasonable frequency in the data stream 31
Well-Constrained Sky Structure Telescope Trajectory Peak in sky intensity
Trough in sky intensity Peak
Trough...
Appears at a reasonable frequency in the data stream 32
Poorly-Constrained Sky Structure Telescope Trajectory
Appears at a very low frequency in the data stream (dominated by drifts) 33
Poorly-Constrained Sky Structure Telescope Trajectory
Appears at a very low frequency in the data stream (dominated by drifts) 34
Scan the other way!
cross-linking, basketweaving ...
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So how *should* you scan the telescope? • As fast as possible! – telescope limits are often the limiting factor: • slew speed • acceleration limits • servo bandwidth limits • backend minimum integration time • Sample in different directions (interlock) – e.g.:“Basket Weaving” for rectangular areas • Scans should be as long and uninterrupted as feasible – on the GBT, observing system overheads limit this (grow with length of scan)
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Summary • Bolometer cameras are excellent tools to carry out sensitive continuum observations of large areas of sky – mm to sub-mm wavelength – can achieve sky-background limited sensitivity – very broad band – 100s to 1000s of pixels (feeds) – must be very cold! (~0.1 K) • Multiple pixels give powerful way to greatly reduce systematics – cryogenic, atmospheric – penalty: filters sky signal -> large field of view is good! • Typically observe by means of “on-the-fly mapping” (OTF) – scan the telescope or subreflector as fast as feasible
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How Sensitive is a Bolometer?
Best possible for a phasepreserving amplifier:
For a well-designed bolometer, detector noise is less than the background noise “Background Limited Performance” or “BLIP”
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How Sensitive is a Bolometer?
For a well-designed bolometer, detector noise is less than the background noise “Background Limited Performance” or “BLIP”
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EM wave amplitude
time
Lowest possible noise for a phase-preserving amplifier:
C: “Heat Capacity” [Joules/Kelvin] Minimum Conductance” noise gets *added* to the intrinsic G: “Thermal [Joules/sec/Kelvin] Tsky noise
Destroys EM-wave phase information instantly! Measures intensity very sensitively. 41
EM wave amplitude
time
Bolometer
Lowest possible noise for a phase-preserving amplifier: for coherent amplifier ~ picosecond for bolometer ~ millisecond Bolometers evade this limit completely!
C: “Heat Capacity” [Joules/Kelvin] Minimum Conductance” noise gets *added* to the intrinsic G: “Thermal [Joules/sec/Kelvin] Tsky noise
Destroys EM-wave phase information instantly! Measures intensity very sensitively. 42
