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Quality Control

transientskp
December 04, 2012

Quality Control

Antonia Rowlinson

transientskp

December 04, 2012
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  1. Quality Control
    Antonia Rowlinson and Gijs Molenaar

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  2. Making bad images is easy...

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  3. ...but ruin transient searches
    An extreme example with the lightcurve of 3C295
    before and after removal of bad images...
    Time Time

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  4. Bad images cause:
    Inaccurate source
    fluxes
    Large position offsets
    Elliptical restoring
    beams which can
    make source
    association difficult
    Artifacts in images

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  5. Problem: Too many images to
    check by eye...
    Need automated
    quality control
    within the TraP
    that can quickly
    identify bad
    images and skip
    over them in
    the following
    processing steps

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  6. Pointing direction issues
    Problems may occur if pointing direction is near:
    A-team sources - need demixing
    Sun - e.g. solar flares
    Galactic plane - source confusion
    Other things that might be important:
    Time of day - particularly for LBA
    Low elevation - prone to more RFI
    Lack of sources in skymodel - not an issue if using a calibrator
    = Version 1
    First identified and studied by
    A. Stewart & R. Breton (MSSS
    report week 41 and talk
    yesterday)

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  7. Flagging
    Too much data is flagged:
    total - likely to be suffering from severe RFI
    particular baselines - causes poor uv coverage
    particular station types - causes poor uv coverage
    = Version 1
    See work by Y. Cendes
    for more details about
    flagging and transient
    searches

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  8. Calibration
    Primary beam problems
    Amplitude spikes
    Zero amplitude baselines
    Bad weight spectrum values
    Features in images such as stripes due to bad calibration -
    possibly use the stripy image detection script developed for
    MSSS
    = Version 1
    MSSS report Week 42:
    R. Breton, D. Carbone, V. Heeson

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  9. Imaging
    RMS noise in centre of image:
    RMS significantly above/below the theoretical noise
    Restoring beam:
    Beam undersampled or significantly oversampled
    Beam highly elliptical
    = Version 1
    Already
    Im
    plem
    ented
    in
    TraP
    Over sampled
    Under sampled
    Normal

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  10. Flux scale and positions
    Use the brightest catalogued sources to check flux
    scale and position offsets
    Extract brightest sources from image
    Basic source association using the beam FWHM and sky model
    Calculate average position offset - systematic position errors
    Calculate average flux ratio - incorrect flux scale in image
    = Version 1
    MSSS report Week 28:
    A. Rowlinson

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  11. Source extraction
    Source finder problems include:
    Large errors on gaussian fits
    FWHM of fit is smaller than restoring beam
    Spectrum unrealistic
    Not flagging sources in the catalogue but when they
    are extracted, therefore thresholds are all user
    defined
    = Version 1

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  12. Transient Specific
    Relative to bright sources:
    Specific transient is very close to a bright source
    Over density of transients close to bright sources - further
    study required
    At peaks in the PSF for bright sources
    Systematic effects:
    Specific transient is very close to image edge
    Over density of transients near image edge or with specific RA/
    Dec - further study required
    = Version 1

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  13. These tests should mean all our
    images will look like:

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  14. Contribute to discussion online:
    http:/
    /www.lofar.org/operations/doku.php?
    id=tkp:qualitycontrol

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  15. http:/
    /www.lofar.org/operations/doku.php?
    id=tkp:qualitycontrol

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  16. View Slide