Quality Control

Transcript

1. 1.

   Quality Control Antonia Rowlinson and Gijs Molenaar

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   Making bad images is easy...

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

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

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

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    None