Observing with Bolometer Arrays - Arecibo...

Observing with Bolometer Arrays

Brian Mason (NRAO)15jul137th NAIC/NRAO single dish school

What is a Bolometer?

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C: “Heat Capacity” [Joules/Kelvin]G: “Thermal Conductance” [Joules/sec/Kelvin]

Destroys EM-wave phase information instantly!Measures intensity very sensitively.

What is a Bolometer?

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Lowest possible noise for a phase-preserving amplifier:

Minimum noise gets *added* to the intrinsic Tsky noise

Bolometer Sensitivity

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Bolometers avoid this limit completely

Their sensitivity is set by:1) electrical & phonon noise in the detectors


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Their sensitivity is set by:1) electrical & phonon noise in the detectors Bolometer cameras

want to be cold!

few 100 mK vs 15K forcoherent (HEMT) amplifiers


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2) photon noise in the signal

Bolometer cameraswant to be cold!


shot-noiseradiometer equation

well designed bolometer camera has photon noise > intrinsic detector noise

“Background Limited Performance” (BLIP)

e.g., Richards (1994)


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2) photon noise in the signal

Bolometer cameraswant to be cold!


shot-noiseradiometer equation

well designed bolometer camera has photon noise > intrinsic detector noise

“Background Limited Performance” (BLIP)

e.g., Richards (1994) increasing bandwidth is cheap and easy!

TES

MUSTANG

SQUID MUX readout chips

Bolocam• 1-2 mm• 144 pixels• CSO (10m), APEX -> LMT

Also IRAM 30m (MAMBO)

MillimeterACT (6m) , SPT (10M)• 1-2 mm• Large Area Surveys (SZ) - 1000s of deg^2• few 1000 detectors

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

Observing & Data Analysis Challenges

• Systematics– fluctuations in atmospheric emission– receiver instabilities (e.g., cryogenic)– gain or offset drifts (1/f)

• Observing– scan strategy

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Bolometer Arrays measure broad-band (continuum) radiation

θ

angle distance from gbt surface

adjacent pixels 4″ 86 km

Across array 40″ 9 km

Atmospheric Emission Across the Array

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 ...

i=N

Sources with size << FOV don’t contribute much to CM

Noise Power Spectrum

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Residual drifts


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Residual drifts

Refrigerator


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astronomy signal (scanning telescope)

Penalty of Common Mode Subtraction

Detector Array:

i=1 i=2 ...

i=N

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.

If you care about diffuse, extended structure you want a large FOV!

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Large Scale Sunyaev-Zel’dovich Effect Decrement (Model)

MUSTANGFOV

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Large Scale Sunyaev-Zel’dovich Effect Decrement (Model)--> Passed through MUSTANG transfer function

MUSTANGFOV

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... with real data (green contours)

MUSTANGFOV

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... with real data (green contours)

MUSTANGFOV

MUSTANG-2FOV

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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How do Structures on the Sky appear in Bolometer Time-Stream Data?

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x

v

Sky

Brig

htne

ss


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8’’ @ 1’/sec


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5’ @ 1’/sec

8’’ @ 1’/sec

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Peak in sky intensity

Trough in sky intensity

Peak

Trough...

Well-Constrained Sky Structure

Telescope Trajectory

Appears at a reasonable frequency in the data stream

Period or angular scale of structure isn’t all that matters: orientation relative to your scan pattern matters too!

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Peak in sky intensity

Trough in sky intensity

Peak

Trough...

Well-Constrained Sky Structure


Appears at a reasonable frequency in the data stream

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Poorly-Constrained Sky Structure


Appears at a very low frequency in the data stream (dominated by drifts)

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Poorly-Constrained Sky Structure


Appears at a very low frequency in the data stream (dominated by drifts)

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Scan the other way!

cross-linking,basketweaving ...

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?

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For a well-designed bolometer, detector noise is less than the background noise “Background Limited Performance” or “BLIP”

Best possible for a phase-preserving amplifier:

How Sensitive is a Bolometer?

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For a well-designed bolometer, detector noise is less than the background noise “Background Limited Performance” or “BLIP”

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timeEM

wav

e am

plitu

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timeEM

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Bolometer

for coherent amplifier ~ picosecondfor bolometer ~ millisecond

Bolometers evade this limit completely!

Observing with Bolometer Arrays - Arecibo...

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