Observational Astronomy Astronomical interferometers Part Deux 30 November 2018

Radio interferometers 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) VLA New Mexico 27 antennas 25m diameter each. Maximum baseline 36 km VLBA Combination of several VLA-type observatories from Hawaii to Virgin Islands. Max baseline is 8000 km VLBI technique and consortium 30 November 2018

Radio path difference 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 30 November 2018

Very Long Baseline Interferometry (VLBI) Widely separated antennae not connected by cables Data recorded along with very accurate time signals & correlated later 30 November 2018

Connected interferometers 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 30 November 2018

Radio telescopes (terminology) Primary mirror  main dish Secondary mirror  subreflector PSF  far-field beam shape Noise  antenna temperature Scattered light  side lobes 30 November 2018

Radio detectors High-frequencies: Low frequencies Amplifiers Bandpass filters Local oscillators Mixers Bolometers Low frequencies ADC 30 November 2018

First observations with APEX/LABOCA 30 November 2018

ALMA science Imaging kinematics and chemical stratification in protoplanetary disks within 140pc Detecting CO emission lines in normal galaxies up to z=3 Imaging dust emission in evolving galaxies at z up to 10 30 November 2018

ALMA outline Fifty 12m antennas Two types of antennas: Variable separation from 15m to 15 km Compact array (twelve 7m antennas): 30 November 2018

ALMA bands between the atmospheric H2O absorption 1.5 0.75 0.5 0.375 λ in mm 30 November 2018

ALMA “fringes” ALMA Correlator Specifications 64 antennas 8 frequency bands per antenna 4 Gsamples/sec per frequency band, 2 bits/sample correlated 1024 lead + 1024 lag correlations per baseline 30 km maximum baseline delay range Full polarization capability plus autocorrelation Digital filter for bandwidths <2 GHz Switch modes in less than 1.5 seconds This requires 4,194,304 multiply-and-add correlators at 4 GHz rate Total computation rate is 1.7 X 1016 multiply-and-add operations/sec 30 November 2018

ALMA Changing array configuration Compact size array Intermediate size array 30 November 2018

ALMA Site development: signal is mixed down to megaHerz range, digitized and sent via optical fibers to the correlator 30 November 2018

ALMA receivers ALMA dewar ALMA receiver cartridge ALMA quasi-optics 30 November 2018

ALMA: observing 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 30 November 2018

ALMA: getting the Time… Phase I: Proposals are submitted using ALMA Observing Tool ALMA issues calls, provides documentation, proposal preparation and submission help, as well as coordinating refereeing process Regional Program Review Committee ranks proposals (~HST & Spitzer) Phase II: Successful PIs submit observing program using the Observing Tool ALMA SC helps with observation planning and verifies observing schedule 30 November 2018