1. Observational Astronomy
Astronomical interferometers
Part Deux
30 November 2018

2. Radio interferometers
Do not have many of the problems that optical counterparts have
Wavelengths are longer (by a factor ) 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

3. 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

4. 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

5. 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

6. 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

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

8. First observations with APEX/LABOCA
30 November 2018

9. 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

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

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

12. ALMA “fringes” ALMA Correlator Specifications 64 antennas
8 frequency bands per antenna
4 Gsamples/sec per frequency band, 2 bits/sample correlated
1024 lead 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

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

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

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

16. 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

17. 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