Fundamentals of mm Astronomy and Observing Tools ➢ fundamentals and instrumentation ➢ calibration, efficiencies, and observing modes ➢ pointing, refraction, and observing preparation ➢ polarization at mm wavelengths IRAM Observing School 2007 Clemens Thum IRAM,Grenoble, France

Lecture 1: Fundamentals and instrumentation ➢ peculiarities of millimeter astronomy ➢ the 30m telescope in context ➢ temperature and noise ➢ receivers ➢ backends IRAM Observing School 2007 Clemens Thum IRAM,Grenoble, France

principal peculiarities of mm astronomy ● terminology: temperatures ● detection: heterodyne receivers, correlators ● antenna: diffraction limit ● observing techniques: calibration; switching and mapping modes ● atmosphere ● science: cold universe (with exceptions)

C. Thum 1/2 The atmosphere at the site of the 30m telescope (altitude 2920 m) 3 main windows summer / winter mean opacities 3 < 2 < 1 < 0.8 mm sky temperatures: 100 GHz: 30 K 230 GHz: 70 K

C. Thum 1/2 Single dish telescopes operating at mm wavelengths (from Greve & Hily-Blant. 2002) Conclusion: - 30m is the largest single dish at short mm wavelengths - potential competitors: LMT, GBT

C. Thum 1/3 Temperature and noise available power w Nyquist relations (1928) = 4 kT R  example: T = 290 K = 10 kHz R = 10 k  ½ = 1.3  V in a linear passive circuit the noise is white concept readily extended to active circuits maximum power is transferred if Z is real w = k T [W/Hz] thermodynamics: electron gas has 2 degrees of freedom

Antenna temperature * formally: Ohmic R replaced by radiation resistance T is not the temperature of the antenna stucture box must be black * (c): Now increase the dimensions of the box T A = T b (brightness temperature, in simplest case) T A = temperature of equivalent resistor T b = temperature of equivalent blackbody * measure power at terminals: a thought experiment (Kraus): (a) w = k T (b) w = A ∬ B ( ,  ) cos  d  = = k T (antenna equation)using

C. Thum 1/2 receivers: basics

C. Thum 1/2 Receivers: mixer Historical note: (super) heterodyne = mixing Essential characteristic: a very non-linear resistance  I/V curve (current vs. voltage) of a SIS junction intermediate frequency (IF) lower and upper sideband

C. Thum 1/2 Receivers: calibration y-factor = voltage 2 voltage 1 T C + T Rx T H + T Rx = Aim: measure the noise temperature T rx of the receiver a receiver is good if : - its T rx < 4 h /k (h /k = 11 K at 230 GHz) - its T rx < T sky - it is stable - its instantaneous bandwidth ≥ 4 GHz - its sideband behavior is well defined

C. Thum 1/2 Receivers: signal processing

C. Thum 1/2 Receiver noise temperature T 1, T 2 noise temperature of component 1, 2 etc. G 1, G 2 gain of components 1, 2 etc Typically at mm wavelengths: component 1 is a mixer - mixers often have conversion loss of 3 dB then T rx = T mx + 2 T ampl - first amplifier must have low noise: located at low temperature - mixers are intrinsically sensitive to the 2 sidebands: signal and image formidable problem in the past now superseded: single sideband SIS Rxs, sideband separation mixers at 30m: all SIS receivers can be SSB tuned, some though with low g i - Rx design is easier if first stage is an amplifier (HEMT) Question: why can the IF signal be dis- tributed over many backends without loosing S/N ?

C. Thum 1/2 receivers: types in use at mm wavelengths Currently used at 30m telescope: 4 (single pixel,dual polarization) SIS heterodyne Rxs for line work SIS heterodyne 9 pixel array (HERA) bolometers with unfilled arrays for 1mm continuum receivers in preparation or planned for the 30m: ALMA- grade (single pixel) SIS heterodyne Rxs bolometer array with filled aperture (2mm) single pixel 3mm HEMT receiver HERA-type heterodyne array at low frequency MAMBO – 1 MPIfR

C. Thum 1/2 backends: basic types task: receiver provides v(t), observer wants P( ) Basic types: analog backends: filterbanks, acusto-optical spectrometers pro: loss-less, straightforward con: needs tuning, expensive, frozen resolution and bandwidth digital backends: correlators, FTS, digital filters,... pro: adjustable resolution and bandwidth con: small quantization loss, very demanding digital technology voltage time power frequency 

Filterbanks brute force approach long time work horse not built any longer superseded by digital technology filterbanks at 30m: * 1024 x 1 MHz * 9 x 256 x 4 MHz * 256 x 100 kHz (decommisssioned)

C. Thum 1/2 Wiener-Khinchin theorem: the power spectral density and the autocorrelation function of an ergodic random process are Fourier transform pairs digital backends FX versus XF types of digital backends: correlators (30m: VESPA, WILMA) Fourier transform spectrometers (30m: first tests with unit on loan from MPIfR) digital filters (30m: - )

C. Thum 1/12 VESPA – V ersatile S pectrum and P olarization A nalyzer 1 of 6 VESPA units  samplers  backplane  12 correlator boards of 256 delay channels resolution / bandwidth: 80 kHz / 40 MHz 18 bands 20 kHz / 20 MHz 18 bands 40 kHz / 20 MHz 6 bands auto + cross HCN (1-0) in a very cold cloud (Lapinov et al. 2002) connects to: 4 single pixel receivers or multibeam receiver resolutions: 3.3 kHz – 1.25 MHz (10 m/s – 4 3mm) modes: auto- and cross-correlation clock rate: 160 MHz VESPA creator Gabriel Paubert

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