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Observing With Bolometer Arrays

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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 4 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 5 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 9 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 13 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 18 Noise Power Spectrum Residual drifts 19 Noise Power Spectrum Refrigerator Residual drifts 20 Noise Power Spectrum astronomy signal (scanning telescope) 21 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 23 Large Scale Sunyaev-Zel’dovich Effect Decrement (Model) --> Passed through MUSTANG transfer function MUSTANG FOV 24 ... with real data (green contours) MUSTANG FOV 25 ... with real data (green contours) MUSTANG FOV MUSTANG-2 FOV 26 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 27 Sky Brightness How do Structures on the Sky appear in Bolometer Time-Stream Data? x v 28 Noise Power Spectrum 8’’ @ 1’/sec 29 Noise Power Spectrum 5’ @ 1’/sec 8’’ @ 1’/sec 30 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 ... 35 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) 36 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 37 38 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” 39 How Sensitive is a Bolometer? For a well-designed bolometer, detector noise is less than the background noise “Background Limited Performance” or “BLIP” 40 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