Transcript Observational Astronomy

Observational Astronomy
Astronomical
interferometers
Part Deux
7 July 2015
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Radio interferometers
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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)
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Radio path difference
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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
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Very Long Baseline
Interferometry (VLBI)
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Widely
separated
antennae not
connected by
cables
Data
recorded
along with
very
accurate
time signals
& correlated
later
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Connected interferometers
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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
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Radio telescopes
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(terminology)
Primary mirror  main dish
Secondary mirror  subreflector
PSF  far-field beam shape
Noise  antenna temperature
Scattered light  side lobes
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Radio detectors
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High-frequencies:
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Amplifiers
Bandpass filters
Local oscillators
Mixers
Bolometers
Low frequencies
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Amplifiers
Bandpass filters
ADC
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First observations with
APEX/LABOCA
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ALMA science
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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
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ALMA outline
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Fifty 12m antennas
Two types of antennas:
Variable separation from 15m to 15 km
Compact array (twelve 7m
antennas):
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ALMA bands between the
atmospheric H2O absorption
1.5
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0.75
0.5
0.375
λ in mm
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ALMA “fringes”
ALMA Correlator Specifications
 64 antennas
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8 frequency bands per antenna
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4 Gsamples/sec per frequency band, 2 bits/sample correlated
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1024 lead + 1024 lag correlations per baseline
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30 km maximum baseline delay range
 Full polarization capability plus autocorrelation
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Digital filter for bandwidths <2 GHz
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Switch modes in less than 1.5 seconds
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This requires 4,194,304 multiply-and-add correlators at 4 GHz
rate
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Total computation rate is 1.7 X 1016 multiply-and-add
operations/sec
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ALMA
Changing array configuration
Compact size array
Intermediate size array
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ALMA
Site development: signal is mixed down to
megaHerz range, digitized and sent via optical fibers to the
correlator
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ALMA receivers
ALMA dewar
ALMA receiver cartridge
ALMA quasi-optics
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ALMA: observing
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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
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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
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