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 Density (Jy) ZSPEC (Caltech Submillimeter Observatory) Rotational Transitions Of CO Thermal emission from dust CCH, HCN…

M82 See Condon (1992, ARA&A) Stellar death Magnetic fields Stellar birth dust 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~ 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) Euclidean:

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) Euclidean:

The 5σ extragalactic confusion limits for Arecibo (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.

NRAO VLA Sky Survey (NVSS): 1.4 GHz GB6: old 300’+7-beam receiver, 5 GHz BUT!

A Simple Picture of Gain Fluctuations Extra Noise Term (Contributes to baseline ripple for S.L.) Suppose T SRC << T RX

Flux Density (Jy) A Simple Picture of Gain Fluctuations DC Offset (e.g., Tsys)

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 How fast do G(t), T ATM (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 νkνk Gaussian, uncorrelated Noise (Radiometer Equation): White (flat) power spectrum Atmosphere & gain Fluctuations: 1/f Power spectrum

How does k depend on bandwidth? νkνk νkνk Less bandwidth More bandwidth Eliminate bad fluctuating signals by independently measuring the bad signal on some timescale  or shorter. Use  < 1/(2πν k ): Dicke switching, chopping, multi pixel, etc..

Characteristic Timescales for Broadband Measurements Gain fluct. For coherent amplifiers: 100+ Hz – receiver architecture incoherent (bolometer) detectors: 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”) For T SRC <<T SYS gain fluctuations don’t contribute significantly to the noise

Dicke-Switching Receiver Rapidly alternate between feed horns to achieve theoretical noise performance Penalty: –Differential sky measurement –2x higher thermal noise

?

Dicke-Switching Receiver What happens if the feeds see slightly different things? Components before the switch must be as identical + well matched as possible.

An Easier Way?

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

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)

Bolometers θ adjacent pixels 4″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) s 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

MUSTANG GBT 3.3mm Bolometer Array Prototype for a 1000-pixel class camera Maping Speed >15x ALMA Thermal Dust + Free-Free Emission In Orion KL/Integral Filament Region.

MUSTANG GBT 3.3mm Bolometer Array Prototype for a 1000-pixel class camera SZE in RXJ User instruments Maping Speed >15x ALMA

END

Performance Measures From before: An effective aperture of 2760 m 2 is required to give a sensitivity of 1.0 K/Jy. Gain Units of 1/Janskys. You sometimes see The System-Equivalent Flux Density, SEFD (small is good) “Mapping Speed” commonly defined as: These are single-pixel measures. For mapping, increase by Nfeeds.

Gain Effects: deviations from linearity Input Power Output Power Ideal Gain compression/ saturation T Rx Integrated power is what usually matters (RFI)

Gain Effects: deviations from linearity Input Power Output Power Ideal Gain compression/ saturation Be aware of the limitations of the instrument You’re using & calibrate at a similar total power Level to what your science observations will see. T Rx Integrated power is what usually matters (RFI)

Gain Effects: fluctuations Gain drifts over the course of an observing Session(s) easily removed with instrumental Calibrators (e.g., noise diodes) Short term gain fluctuations can be more Problematic. (see continuum lecture)

Sampling/Quantization & Dynamic Range: Postdetection Diode or square law detector: Turns E-field into some output Proportional to power (E 2 ) A/D 2 N levels (N~14) Common for continuum systems. Robust & simple (large dynamic range) A/D sample time ~1 level

Sampling/Quantization & Dynamic Range: Predetection Sample 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:

BOOMERAnG map of CMB Anisotropies