Single-Dish Continuum

Brian Mason (NRAO)NRAO/NAIC Single-Dish Summer School

July 13, 2009

• Basics• Issues

• Confusion• gain fluctuations• atmosphere

• Receiver architectures & observing strategies•The future: large arrays

Flux D

ensi

ty (

Jy)

ZSPEC (Caltech Submillimeter Observatory)

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ΔT =Tsys

tΔνRotational Transitions

Of CO

Thermal emission from

dust

CCH, HCN…

M82

See Condon (1992, ARA&A)

Stellar deathMagnetic fields

Stellar birthdust

synchrotron

Thermal BB

Free-Free

Dust emission from galaxies at at high redshift

HDF-- Hughes et al. (1998)

• Discovered an abundant population of submm galaxies at z~1-3

• 50 hr map with the JCMT at 850 m, confusion limited

The Sky Isn’t Empty: Confusion

NVSS (45 arcsec FWHM) grayscale

GB6 300’ (12 arcmin FWHM) contours

The confusion amplitude P(D) distribution [for n(s) = dN/ds=kS-2.1]

D = image brightness (e.g., Jy/beam)Condon (1974); Scheuer (1956)

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γ=−2.5Euclidean:

The confusion amplitude P(D) distribution [for n(s) = dN/ds=kS-2.1]

D = 0 mean is not a good baseline; use running median instead Long tail --> use at least 5 sigma threshold (src/30 beams)

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γ=−2.5Euclidean:

The 5σ extragalactic confusion limits forArecibo (d = 220 m) and the GBT (d = 100 m).

Gets much weaker with increasing frequency:

The noise level won’t go below this if you integrate for longer.

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θFWHM ∝1

ν

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S ≈ ν −0.8

NRAO VLA Sky Survey (NVSS): 1.4 GHz

GB6: old 300’+7-beam receiver, 5 GHz

BUT!

A Simple Picture of Gain Fluctuations

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G(t1){TSRC + TRX + TATM }

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G(t2){TRX + TATM }

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On −Off =G(t1)TSRC + ΔG(TRX + TSKY )

Extra Noise Term (Contributes to baseline ripple for S.L.)

Suppose TSRC << TRX

Flux D

ensi

ty (

Jy)

A Simple Picture of Gain Fluctuations

DC Offset (e.g., Tsys)

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ΔGG<ΔT

T=

1

Δντ≈ 3×10−5

Stability requirement in order that gain fluctuations cause smaller changes in the output than thermal noise:

(1 sec, 1 GHz bw) - too small to be accurately measured each second --> mitigate with instrument design

Why not calibrate the fluctuations away?

Atmospheric Emission

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G{TSRC + TRX + TATM (t1)}

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G{TRX + TATM (t2)}

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On −Off =GΔTSKY

How fast do G(t), TATM(t) vary?

Sky emission often adds noise to continuum measurements, so you need to measure your noise instead of assuming the radiometer equation or formal uncertainties!

Postdetection power spectrum of the receiver output

νk

Gaussian, uncorrelated Noise (Radiometer Equation):White (flat) power spectrum

Atmosphere & gainFluctuations: 1/f Power spectrum

How does kdepend on bandwidth?

νk

νk

Less bandwidth

More bandwidth

Eliminate bad fluctuating signals by independently measuring the bad signal on some timescale or shorter. Use < 1/(2πνk): Dicke switching, chopping, multi pixel, etc..

Characteristic Timescales for Broadband Measurements

Gain fluct. For coherent amplifiers: 100+ Hz– receiver architecture

•incoherent (bolometer) detectors: 0.1-10 Hz•Atmosphere 0.1-few Hz

– chopping or rapid scanning– common mode subtraction for imaging arrays

Dicke-Switching Receiver

Rapidly alternate between feed horns to achieve theoretical noise performance. (old-fashioned receiver architecture but good illustration, & same principle as “chopping”)

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ΔP =G(V12 −V2

2)

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G{TSRC + TRX + TATM }

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G{TRX + TATM }

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On −Off =GTSRC + (ΔG = 0)(TRX + TSKY )

For TSRC<<TSYS gain fluctuations don’t contribute significantly to the noise

Dicke-Switching Receiver

Rapidly alternate between feed horns to achieve theoretical noise performance

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ΔP =G(V12 −V2

2)

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ΔT = T1 −T2

RMS(ΔT) = 2 × RMS(T1)

= 2T1Δντ 1

=2T1Δντ Tot

Penalty:–Differential sky measurement–2x higher thermal noise

?

Dicke-Switching Receiver

What happens if the feeds seeslightly different things?

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ΔP =G(V12 −V2

2)

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G {TSRC + TRX + TATM }

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G {TRX + TATM + TOFF}

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On −Off =G TSRC +GTOFF

Components before the switch must be as identical + well matched as possible.

An Easier Way?

An Easier Way?

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A − B =G1TSRC + ΔG(TRX + TSKY )

Primarily you switch to suppress receiver effects.Switch must be before active (unstable) components

Correlation Receiver

Receiver noise is uncorrelated and drops out.

Always looking at both source & reference: only lose one sqrt(2) for differencing

Correlation Polarimeter

See talk by Carl Heiles

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I = L2 + R2

V = L2 − R2

Q = −Re(< LR >)

U = Im(< LR >)

Higher-Order Differences: Symmetric Nodding

• For sensitive photometry, one level of differencing is usually not enough– Gradient in sky emission, or with time– Dual-feed systems: Slight differences in feedhorn gains or

losses

Bolometers

Bolometer detectors• Incoherent; measure total power• Very broad bandwidths possible; approach quantum - limited noise.• Can be built in very large-format arrays!

MUSTANG (18 GHz)

Bolometersθ

adjacent pixels

4″ 98%

40km

Across array

32″

98%

5km

(principle also applies to dual pixel heterodyne receivers & FPAs)

MUSTANG (18 GHz)

Array Receivers

ALFA

ALFA on Arecibo 7-element λ = 21 cm coherent array, 3 arcmin resolution GALFACTS:full-Stokes , sensitive, high-resolution survey of 12,000 deg^2

Galactic magnetic fields, ISM, SNR, HII regions, molecular clouds …

Large Format Heterodyne Arrays QUIET

• 91 pixels• I,Q,U• 90 GHz• integrated, mass-producable “receiver on a chip” (MMIC)

T. Gaier (JPL)

Large Format Heterodyne Arrays QUIET

• 91 pixels• I,Q,U• 90 GHz• integrated, mass-producable “receiver on a chip” (MMIC)

OCRA• 1-cm Receiver Array• Under construction at Jodrell Bank, initially for Torun 30m telescope

• SZ surveys + imaging• 16 --> 100 pixels• correlation architecture T. Gaier (JPL)

Millimeter

Bolocam/AZTEC• 1-2 mm• 144 pixels• CSO (10m) & LMT (50m)

Also IRAM 30m, APEX

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

Sub-millimeter

SCUBA-2• JCMT (15m)• ~10,000 pixels!• SQUID-MUX’d TES bolometers (CCD-like)• Under commissioning now

Also: SHARC-II on CSO, 384 pixels at 350 um

MUSTANGGBT 3.3mm Bolometer Array

Prototype for a 1000-pixel class camera

Maping Speed >15x ALMA

Thermal Dust + Free-Free EmissionIn Orion KL/Integral FilamentRegion.

MUSTANGGBT 3.3mm Bolometer Array

Prototype for a 1000-pixel class camera

SZE in RXJ1347-1145

User instruments

Maping Speed >15x ALMA

END

Performance MeasuresFrom before:

An effective aperture of 2760 m2 is required to give a sensitivity of 1.0 K/Jy.

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TAnt =Aeff

2kSSrc →Γ ≡

TAnt

SSrc

=Aeff

2k Gain

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SNR =TAnt

ΔT∝Γ

TSys

Units of 1/Janskys. You sometimes seeThe System-Equivalent Flux Density,SEFD (small is good)

“Mapping Speed” commonly defined as:

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MappingSpeed =Area

(noise)2(time)

These are single-pixel measures. For mapping, increase by Nfeeds.

Gain Effects: deviations from linearity

Input Power

Outp

ut

Pow

erIdeal

Gain compression/saturation

TRx

Integrated power is what usually matters(RFI)

Gain Effects: deviations from linearity

Input Power

Outp

ut

Pow

erIdeal

Gain compression/saturation

Be aware of the limitations of the instrumentYou’re using & calibrate at a similar total power Level to what your science observations will see.

TRx

Integrated power is what usually matters(RFI)

Gain Effects: fluctuations

Gain drifts over the course of an observingSession(s) easily removed with instrumentalCalibrators (e.g., noise diodes)

Short term gain fluctuations can be more Problematic. (see continuum lecture)

Sampling/Quantization & DynamicRange: Postdetection

Diode or square law detector:Turns E-field into some outputProportional to power (E2)

A/D2N levels(N~14)

.

.

.

Common for continuum systems.

Robust & simple (large dynamic range)

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ΔP =P

Δντ

A/D sample time

~1 level

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≈2NΔP

Sampling/Quantization & DynamicRange: PredetectionSample the E-field itself.

Samplers must be much faster and sample much more corasely (typically just a few levels)

Dynamic range limitations more important One usually requires variable attenuators to get it right (“Balancing”).

Small # of levels increases the noise level. (K-factor in Radiometer equation)

More levels -> greater dynamic range (RFI robustness), greater sensitivity

Monitor levels through your observation.

Nonthermal Emission around Sgr A*

LaRosa, Brogan et al. (2005)

75 MHz, VLA:

330 MHz, GBT:

QuickTime™ and a decompressor

are needed to see this picture.

BOOMERAnG map of CMB Anisotropies