Lecture Summaries


Radio Astronomy

Willem Baan

  • What type of research at what frequency range?
  • Single dish studies
  • Interferometry
  • Special strengths of Radio Astronomy:
    • sensitivity,
    • resolution,
    • frequency range
  • Comparison with observational properties at other wavelengths
  • What kind of physics do we look at ?
  • Examples:
    • the large and the small
    • the weak and the strong

Practical Interferometry

Robert Laing

  • What is an interferometer?
    • How does an array make images?
    • Building up an image from individual interferometer baseline responses.
  • Basic 2D Fourier Transform relation between visibility and image planes
    • Examples of FT's
    • Choosing the correct baseline range
  • Antennas and the primary beam
  • Basic receiver concepts:
    • amplifier,
    • mixer,
    • bandwidth,
    • IF conversion,
    • digitisation
  • What is a correlator?
    • How are spectra produced
  • Sampling;
    • dirty beam as the FT of the sampling function;
    • gridding
  • Deconvolution
  • Introduction to existing and near-future arrays from 10 MHz - 1000 GHz.

Calibration of radio data

Neal Jackson

  • Need for calibration.
  • Quantities to be calibrated:
    • flux scale,
    • elevation,
    • phase calibration,
    • bandpasses,
    • pointing,
    • polarization,
    • fringe finders.
  • The enemies:
    • troposphere,
    • ionosphere,
    • correlators,
    • severity of the problems,
    • effect on different types of array.
  • Smearing:
    • choice of bandwidth and integration time.
  • Useful calibration sources for each type of calibration.
  • Calibration strategies.
    • Some examples for different interferometers, frequencies, types of observations.
    • Self-calibration.
  • Calibration software.
    • Brief description of calibration strategies in AIPS.
    • Very brief description of calibration in CASA.


Tom Muxlow

  • Radio imaging
    • a few relevant points
    • interferometry etc...
  • Review of basic imaging
    • within AIPS
    • phase & gain calibration, de-convolution
  • Non-coplanar baselines and multi-faceted images
  • Wide-field combination imaging
  • Bandwidth smearing (Chromatic aberration) and spectral-line mode
  • Time-averaging smearing
  • Primary beam response
  • Confusion
  • In-beam self-calibration
  • Wide-field imaging: Multi-Frequency Synthesis
    • e-MERLIN and the EVLA
  • Mosiacing
    • an example field from the GMRT at 610 and 1400 MHz
  • High dynamic range imaging and implications for future instruments like the SKA

Common data formats and packages

Anita Richards

  • Don't lose sight of the sky
    • The physical relationships between interferometry data and astrophysical emission mechanisms
  • All interferometry data packages support similar operations
    • Removing bad data
    • Comparing visibility data with models to correct for atmospheric and other errors
    • Fourier Transforming the visibility data and removing artefacts from the resulting image
    • Visualisation and measurement of the resulting data cubes etc.
  • How to choose a package
    • Most packages originally developed for specific instruments
    • Some have been greatly extended - what does what you might want in future as well as now?
    • Ease of scripting/pipelining
    • Support
  • AIPS and CASA
    • Outlines of packages
    • Data formats - FITS and MS
  • Where to get packages and help
    • Limitations and specificity
  • A few basic utilities
    • Getting data in and out
    • What have I got?
    • What's in the data?
    • What did that command just do?
      • Where did it put the result?
    • How do I look at that?
  • Structure of the forthcoming tutorials

Software installation/checking

Ian Heywood with Dickson, Bourke, Petry, Purcell

  • AIPS - essential
  • CASA - highly recommended
  • ParselTongue - recommended, especially for VLBI
  • MEQTrees - useful for low frequency/wide-field imaging
See http://astrowiki.physics.ox.ac.uk/ERIS2009/WebHome#Computing_requirements Computing Requirements


Jackson with Muxlow, Purcell, Venturi

see http://astrowiki.physics.ox.ac.uk/ERIS2009/WebHome#Links_to_data_and_scripts First Calibration and Imaging Tutorial


Spectral Line Radio Interferometry

Rob Beswick

Throughout, this lecture will primarily concentrate on Centimetric radio interferometry examples.

  • Why do spectral line (multi-channel) observations?
    • Brief outline of the science drivers
  • Types of spectral line observations
    • All future interferometry data will be spectral line data
  • Planning & observations
    • What do we need to consider
    • Proposals & plannings
    • Different spectral lines
    • Velocity conventions (inc doppler tracking)
    • Line experiment set-ups
    • Identifing potential problems (& planning solutions)
    • Resolution & brigtness sensitivity
  • Data Analysis (specific to spectral line observations)
    • Spectral bandpass calibration (theory & practice)
    • Identification of RFI
    • Gibbs Ringing
    • Continuum subtratction techniques
    • Deconvolution of spectral line data
    • Anaylsis of line data (cubes)
      • extracting spectra, moment analysis etc

e-MERLIN - Garrington


Muxlow with Fenech, Laing, Purcell

see http://astrowiki.physics.ox.ac.uk/ERIS2009/WebHome#Links_to_data_and_scripts First Calibration and Imaging Tutorial

Image Analysis

Robert Laing

  • Recognising errors in the image plane
    • Expectations: how to decide whether an image is as good as it needs to be/can be.
    • Predicting the off-source noise level.
    • Signatures of phase and amplitude errors:
      • additive and multiplicative errors;
      • long and short timescales;
      • subtracting a model
    • Examples:
      • bad antennas, baselines, data-points,spectral channels;
      • interference;
      • confusion;
      • deconvolution problems (polarization errors in other lecture)
    • Remedial action
  • Image Analysis
    • The importance of matched resolution and spatial frequency coverage
    • Image manipulation and combination; error propagation (1, 2 and many images)
    • Interpolation
    • Convolution
    • Deriving parameters from images:
      • (Gaussian) fitting and error estimation
      • Automated source-finding algorithms
      • Integrated flux densities; resolution, short-spacing and zero-level issues; summing clean components, integrating images, ....
    • Time variability; component-fitting and cross-correlation techniques for measuring motion
    • More sophisticated model-fitting (jet models as an example)
    • Getting data out of the standard packages and into your own code

Image Analysis

Venturi With Purcell, Kramer, Baan
  • Evaluation of the image quality
    • overall look at the image and estimate of the rms (TVSTAT)
    • standard plots (CNTR/KNTR)
  • Observational measurements
    • Flux density measurements of a compact component (IMFIT/JMFIT)
    • Flux density measurements of an extended component (TVSTAT)
    • Total spectral index
    • Slices along the structure (SLICE/SL2PL/TKPL)
  • Going from flux densities to monochromatic radio powers
  • Spectral index
    • Matching uv-coverage
    • Matching images
    • Spectral index images (COMB)
See http://astrowiki.physics.ox.ac.uk/ERIS2009/WebHome#Links_to_data_and_scripts Image Analysis

Spectral Line

Beswick with Richards, Kramer, Etoka

The demo concentrates upon the anylsis of HI line data, including continuum subtraction, deconvolution, RFI identification, moment anaylsis etc.

See http://astrowiki.physics.ox.ac.uk/ERIS2009/WebHome#Links_to_data_and_scripts Spectral Line

VLBI considerations

T. Venturi

  • Need for mas resolution
  • u-v coverage of VLBI arrays and related issues
  • Calibration of VLBI observations
  • Present VLBI arrays and their performances
    • available bands
    • sensitivity
    • resolution
  • Status of eVLBI
  • VLBI science
  • Future Space VLBI missions

VLBI Calibration

Stephen Bourke with Baan, Beswick, Venturi

  1. Tom Muxlow: imaging and self-calibration.
  2. Stephen Bourke: how to do bandpass calibration.
  3. Rick Perley: how to do polarisation calibration.
Part 1: Presentation (~5 minute, ~5 slides)

  • How to calibrate the VLBI continuum data?
    • Provide a flowchart.
  • Useful references.
Part 2: Hands-on Tutorial (~40 minute)

  • Prepared data: N09L2, a latest EVN Network Monitoring experiment.
  • Reference document: http://www.evlbi.org/user_guide/guide/userguide.html EVN Data Analysis Guide
  • Main steps:
    1. Load the data into AIPS (FITLD, MSORT, INDXR, LISTR).
    2. Calibrate amplitude (UVFLG, ANTAB, SNEDT, APCAL, CLCAL, SNPLT).
    3. Fringe-fitting (VLBAPANG, FRING, CLCAL, SNPLT).
    4. *Polarization calibrations (See the later tutorial by Perley).
    5. Bandpass calibration (BPASS/CPASS).
    6. Split the data (SPLAT, FITTP).
    7. *Imaging and self-calibration (See the earlier tutorial by Muxlow).
See http://astrowiki.physics.ox.ac.uk/ERIS2009/WebHome#Links_to_data_and_scripts Continuum: VLBI



Bojan Nikolic


Robert Laing

  • What is ALMA?
  • Emission mechanisms in the mm/sub-mm band ("The Cool Universe")
  • Key science:
    • Planet formation and protoplanetary disks
    • Low- and high-mass star formation
    • Galaxy mass assembly
    • Starburst/sub-mm/ULIR galaxies
    • The first galaxies
    • Non-thermal astrophysics (jets and SZ)
    • ALMA as a VLBI station
  • Technology:
    • Antennas and configurations
    • Receiver bands
    • Calibration and phase correction strategies
    • Correlators and spectral configurations
    • Software: overview, OT, pipeline
  • Current status


Willem Baan

  • Low-frequency interferometry - largely uncharted territory
  • Frequency range 30-80 MHz and 130-210 MHz
  • Hierarchical core - extended structure
  • 50+ stations - centered in NL with stations in DE, SE, UK, FR, PL, IT, +
  • Definition of Key Science projects
  • A little about observing procedures
    • LOFAR will be an automated telescope and much of the data reduction will be done on-line.
    • A little about LOFAR computing structure
    • What does the user need to do with the data ?

Wide Field, Wide Band Imaging

Fenech with Muxlow, Laing

Wide-field images are commonly classed as images having large numbers of resolution elements (i.e. beamwidths) across them. They are usually required when surveying large regions of the sky at once, whether for multiple objects or large single objects. There are several issues that need to be addressed when producing wide-field images, which include:
  • Bandwidth smearing
  • Time-average smearing
  • Non-coplanar arrays
  • Confusing sources
  • Primary beam response
See http://astrowiki.physics.ox.ac.uk/ERIS2009/WebHome#Links_to_data_and_scripts Continuum: MERLIN Wide-field, wide band imaging


The Polarization Lecture:


  • Showing why polarization observations are important
  • Defining polarization ellipse, etc.
  • Defining Stokes parameters (including physical interpretation)
  • Discussing 'real' vs. 'perfect' outputs from radio telescope (Jones matrix)
  • Explaining the 4 x 4 complex matrix from cross-correlation (so-called Mueller matrix)
  • Demonstrating special cases to aid understanding. (Circular basis and Linear basis).


Perley with Laing, Kramer, Richards

See http://astrowiki.physics.ox.ac.uk/ERIS2009/WebHome#Links_to_data_and_scripts Continuum: VLA: Polarization Tutorial


Zwaan with Petry, Richards, Etoka

See http://astrowiki.physics.ox.ac.uk/ERIS2009/WebHome#Links_to_data_and_scripts CASA examples

The first examples will be NGC 5931 (Spectral Line, VLA) and Jupiter (Spectral line, continuum) but suitable data and examples are provided for other types of data also.

Mosaicing in CASA

Zwaan with Laing, Etoka

See http://astrowiki.physics.ox.ac.uk/ERIS2009/WebHome#Links_to_data_and_scripts Mosaicing: BIMA

Transient/Variable Sources


Targets of opportunity

  • Known transient sources
  • Unknown transient sources
  • TOO policies & responses at different instruments
    • pre-approved allocations
    • balancing time requested against science requirements.
    • multiple requests from different groups
  • Observing considerations
    • consistent phase calibrators
    • variable structure vs variable uv coverage
    • brightness variations over observing windows (see practical session)
  • Specifying observing settings e.g VLA dynamic scheduling
  • Expect the unexpected?
  • Examples will be discussed

Variable sources

Rushton with Eyres and Beswick

The purpose of this tutorial is to investigate the effects of source variability on the image plane and produce a light curve from source visibilities. Your task will be to analyse MERLIN data of the X-ray binary Cygnus X-3.

A fundamental assumption of aperture synthesis is that the true sky brightness distribution does not intrinsically change during an observation. Such an assumption is not true for highly variable sources such as XRBs; these sources exhibit large variations in flux density on the time-scale of a few hours over many days.

The XRB Cygnus X-3 is known to vary between 0.1-1 Jy over a 12 hour period. Such rapid changes cannot be due to changes in the source structure and it can be assumed most emission originates from the central region. It is therefore desirable to study the light curve of the unresolved core so we can model the effects on the image plane.

see http://astrowiki.physics.ox.ac.uk/ERIS2009/WebHome#Links_to_data_and_scripts Variable and Transient sources

VLBI Spectral Line

Bourke with Beswick and Baan


Kloecker (Parseltongue)

Heywood (Meqtrees)

Petry (CASA)

Richards (Interoperability/choices)

  • Vast data volumes from new instruments have to be pipelined at source
  • The 3 types of user and the tools they need
    • Expert/developer
    • Specialist/multiple observations
    • Quick and simple
  • When to write/modify your own pipelines and scripts
  • Python, Parseltongue and interoperability
  • Ongoing development of algorithms

Pipelines in action

MEQtrees Heywood

ParselTongue Kloeckner with Bourke

CASA scripting Petry with Zwaan


Life Cycle of Data

Hans-Rainer Kloeckner

An overview of the Life Cycle of Data

  • genius idea
  • feasibility
  • proposal
  • observation
  • data reduction
  • data analysis
  • publication



Combining arrays

Richards with Beswick and Perley

See http://astrowiki.physics.ox.ac.uk/ERIS2009/WebHome#Links_to_data_and_scripts Combining Arrays

Designing an experiment

Laing with Heywood, Kloeckner, Etoka, Perley

  • Writing a good science case
    • Clear objectives (test some idea; identify an object in a new band; ...)
  • Basics
    • Frequency - particular spectral line(s); considerations affecting choice of continuum band (spectrum; variation of polarization with frequency).
    • Spatial resolution and maximum scale of structure. Wide-field issues (primary beam; need for mosaic; confusing sources)
    • Spectral configuration (one or more lines; resolution; continuum; ...)
    • Working out the surface brightness
    • Where can relevant archive data be found?
  • Choice of instrument
    • Which instrument(s)/configuration(s) are needed?
    • Single-dish data needed?
    • How much time?
    • Adequate coverage and signal to noise ratio?
    • Measurement accuracy?
  • Other issues for the technical justification
  • Calibration strategy
    • is self-calibration possible;
    • how important is good astrometry?
  • Timing constraints and conditions?
    • Observations simultaneous on different instruments?
    • Good observing conditions needed?
  • Proposal tools and where to find them


coordinator Heywood

incl. Reports from proposal designing groups

Future of radio astronomy

Steve Rawlings

-- AnitaRichards - 29 Aug 2009
Topic revision: r5 - 04 Sep 2009, AnitaRichards
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