Observational Astronomy Astronomical interferometers Part Deux 5202021 1

Observational Astronomy Astronomical interferometers Part Deux 5/20/2021 1

Radio interferometers n n n Do not have many of the problems that optical counterparts have Wavelengths are longer (by a factor 103 -106) and tolerances are larger Individual mirrors are similar to optical but baselines are larger Effects of atmosphere are not important (coherence length is larger than antennas and coherence times are minutes) Can calibrate phase by looking at a reference source nearby In some cases we can digitize the signal and do the correlation (fringing) of many baselines in the computer (the so-called unconnected interferometer) 5/20/2021 2

Radio path difference n n n Path difference is not known in advanced Signal is usually down-converted to lower frequencies using heterodyne principle Lower frequencies can be digitized at each antenna Various path difference can be tried in the correlator thus guessing the exact phase Correlation reduces noise as the noise signal is generally uncorrelated 5/20/2021 3

Very Long Baseline Interferometry (VLBI) n n 5/20/2021 Widely separated antennae not connected by cables Data recorded along with very accurate time signals & correlated later 4

Connected interferometers n n n More complex to build (need fast analog or digital interconnect for all baselines) Solves the problem of knowing a priori the exact path difference (can be calibrated in real time) Example: ALMA 5/20/2021 5

Radio telescopes n n n (terminology) Primary mirror main dish Secondary mirror subreflector PSF far-field beam shape Noise antenna temperature Scattered light side lobes 5/20/2021 6

Radio detectors n High-frequencies: n n n Amplifiers Bandpass filters Local oscillators Mixers Bolometers Low frequencies n n n Amplifiers Bandpass filters ADC 5/20/2021 7

First observations with APEX/LABOCA 5/20/2021 8

ALMA science n n n Imaging kinematics and chemical stratification in protoplanetary disks within 140 pc Detecting CO emission lines in normal galaxies up to z=3 Imaging dust emission in evolving galaxies at z up to 10 5/20/2021 9

ALMA outline n n Fifty 12 m antennas Two types of antennas: Variable separation from 15 m to 15 km Compact array (twelve 7 m antennas): 5/20/2021 10

ALMA bands between the atmospheric H 2 O absorption 1. 5 5/20/2021 0. 75 0. 375 λ in mm 11

ALMA “fringes” ALMA Correlator Specifications n 64 antennas n 8 frequency bands per antenna n 4 Gsamples/sec per frequency band, 2 bits/sample correlated n 1024 lead + 1024 lag correlations per baseline n 30 km maximum baseline delay range n Full polarization capability plus autocorrelation n Digital filter for bandwidths <2 GHz n Switch modes in less than 1. 5 seconds n This requires 4, 194, 304 multiply-and-add correlators at 4 GHz rate n Total computation rate is 1. 7 X 10 16 multiply-and-add operations/sec 5/20/2021 12

ALMA Changing array configuration Compact size array Intermediate size array 5/20/2021 13

ALMA Site development: signal is mixed down to mega. Herz range, digitized and sent via optical fibers to the correlator 5/20/2021 14

ALMA receivers ALMA dewar ALMA receiver cartridge ALMA quasi-optics 5/20/2021 15

ALMA: observing n n n Submit a proposal If accepted: workout observing strategy Service observing Get the data reduced by pipeline (images and calibrated uv arrays) Observing modes: ALMA is an imaging instrument with spectral capabilities limited to narrow-band filters 5/20/2021 16

ALMA: getting the Time… v Phase I: Proposals are submitted using ALMA Observing Tool Ø ALMA issues calls, provides documentation, proposal preparation and submission help, as well as coordinating refereeing process v Regional Program Review Committee ranks proposals (~HST & Spitzer) v. Phase II: Successful PIs submit observing program using the Observing Tool Ø ALMA SC helps with observation planning and verifies observing schedule 5/20/2021 17