Sourcefinding Strategy in MSSS, the first LOFAR survey

Dario Carbone

Transcript

1. Sourcefinding strategy in
   MSSS, the first LOFAR
   survey
   DARIO CARBONE
   TKP meeting, 10 January 2014
   

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2. Collaboration
   Job done in collaboration with:
   ●
   the LOFAR Sourcefinder WG (led by Chiara
   Ferrari)
   http://www.lofar.org/operations/doku.php?id=commi
   ●
   the MSSS team (led by George Heald)
   http://msss.astron.nl
   Dario Carbone
   

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3. Outlines
   ●
   PySE & PyBDSM
   – ASKAP Data Challenge
   – Performance
   – Parameters
   ●
   MSSS
   – MVF
   – First catalog
   Dario Carbone
   

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4. 4
   Sourcefinder tools in LOFAR
   ●
   Python Blob
   Detection and Source
   Measurement
   (PyBDSM)
   ●
   Designed by EoR
   group (Niruj Mohan
   Ramanujam), for an
   accurate detection of
   extended emission
   ●
   Get rid of all
   foreground signal
   ●
   Python Source
   Extractor (PySE)
   ●
   Designed by
   Transient group
   (Hanno Spreeuw) to
   be sensitive to point-
   like sources
   ●
   Detect radio transient
   

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5. 5
   PySE vs PyBDSM
   PySE is not able to detect extended
   sources
   Dario Carbone
   

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6. 6
   Focus on point-like sources
   ●
   Radio transients are
   point-like sources
   ●
   PySE was developed
   to find point-like
   sources
   ●
   Speed requirements
   ●
   Implemented in the
   TraP
   Dario Carbone
   

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7. 7
   PySE
   1. Fit and subtract a variable background across
   the image
   2. Fit the RMS noise across the image
   3. Identify islands of contiguous pixels which
   appear above some multiple of the RMS
   4. “Deblend” multi-component islands into the
   constituent parts
   5. Perform initial estimation of source parameters
   6. Fit each component with an elliptical Gaussian
   and return the result
   

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8. 8
   ASKAP Data Challenge
   Three simulated images including point sources and
   complex structures.
   14 different sourcefinder tools participated.
   

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9. 9
   Challenge 2 – point sources only
   σRA σDec
   PySE 0.39” 0.39”
   PyBDSM 0.44” 0.43”
   Credits to
   M. Whiting
   Dario Carbone
   

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10. 10
    Challenge 2 – PySE performance
    PySE completeness,
    reliability and flux recovery
    seems very good.
    ●
    Force-beam option was
    turned on.
    Credits to
    M. Whiting
    Dario Carbone
    

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11. 11
    Challenge 3 – extended sources
    PySE isn't detecting
    extended sources, but isn't
    detecting fake ones.
    ●
    Force-beam option was
    turned on.
    Credits to
    M. Whiting
    Dario Carbone
    

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12. 12
    Parameters for PySE
    Best parameters for source extraction with
    PySE:
    ●
    grid size = 50 pixels
    ●
    deblending turned on
    ●
    deblending threshold = 90
    ●
    detection threshold = 8 σ
    ●
    analysis threshold = 3 σ
    ●
    Force-beam option turned on Dario Carbone
    

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13. 13
    Force-beam option
    ●
    Avoid big ellipses
    collecting high flux
    ●
    Only point-sources
    Dario Carbone
    

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14. 14
    The extracted flux can change just because the
    ellipse shape changed
    Flux
    Time
    Flux
    Time
    No force-beam With force-beam
    Force-beam option
    Dario Carbone
    

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15. 15
    ●
    Very important for the transient pipeline
    ●
    Source could not to be associated
    ●
    Tests on 70 images of 10 fields, with a pixel
    size of 45 arcsec.
    Systematic position errors
    Dario Carbone
    

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16. 16
    Systematic position errors
    ●
    ●
    PyBDSM loses 13.3% PySE 3.4%
    R
    DR
    = 5.68
    Credits to A. Van der Horst
    Dario Carbone
    

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17. 17
    Systematic position errors
    ●
    Systematic position error of 20 arcsec
    ●
    PyBDSM loses 2.0% PySE 1.0%
    Credits to A. Van der Horst
    R
    DR
    = 5.68
    Dario Carbone
    

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18. 18
    MSSS intro
    ●
    LBA: 30 − 75 MHz;
    8x2 MHz band
    ●
    HBA: 119 − 158 MHz;
    8x2 MHz band
    ●
    Angular resolution ∼
    2′
    ●
    Sensitivity ∼ 10 mJy
    beam−1
    http://msss.astron.nl
    Dario Carbone
    

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19. 19
    MSSS recap
    Heald et al., in prep.
    Dario Carbone
    

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20. 20
    MSSS Verification Field (MVF)
    ●
    MSSS Verification Field (MVF): 100 square degree
    including no bright, complicated sources and distant
    from A-team sources.
    ●
    Coordinates of the center: RA = 15 h; dec = +69 deg
    Heald et al., in prep.
    Dario Carbone
    

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21. 21
    MSSS Verification Field (MVF)
    LBA combined image of MVF
    Heald et al., in prep.
    HBA combined image of MVF
    Dario Carbone
    

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22. 22
    Source extraction strategy
    ●
    Combined non-primary-beam-corrected image as
    detection image.
    ●
    Run PyBDSM and PySE separately on each band and
    then combine the results.
    ●
    For each sourcefinder, the detected sources are
    associated across the bands using the nearest neighbor
    criterion.
    ●
    Catalogues from PyBDSM and PySE are then cross-
    matched.
    ●
    Positions and fluxes are taken from PyBDSM outputs (if
    available).
    Dario Carbone
    

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23. 23
    Parameters for PySE
    ●
    grid size = 50 pixels
    ●
    deblending turned on
    ●
    deblending threshold = 90
    ●
    detection threshold = 10 σ
    ●
    analysis threshold = 5 σ
    ●
    Force-beam option turned on
    Parameter settings based on real maps and
    simulations and the requirement to be
    conservative for this first catalogue.
    Dario Carbone
    

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24. 24
    Comparison
    ●
    697 sources are detected in
    the area of interest.
    ●
    We compared HBA MSSS
    catalogue to the 7C survey
    @ 151 MHz.
    ●
    We compared LBA MSSS
    catalogue to the VLSS @
    74 MHz and the 8C
    catalogue @ 38 MHz.
    Heald et al., in prep.
    Dario Carbone
    

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25. 25
    Column in MSSS catalogue
    ●
    ID; Source ID, formed as Jhhmmss+ddmmss
    using the IAU convention.
    ●
    RA & eRA; Source J2000 Right Ascension, in
    decimal degrees & error on Right Ascension, in
    seconds of time.
    ●
    DEC & eDEC; Source J2000 Declination, in
    decimal degrees & error Declination, in
    arcseconds.
    ●
    SFFLAG000; Flag indicating how many
    sourcefinders detected the source (0 means it
    was detected in both; 1 only in PyBDSM; 2 only
    in PYSE.)
    ●
    Sint000 & eSint000; Source integrated flux at
    000 MHz, in Jy & error on integrated flux, in Jy.
    ●
    Spk000 & eSpk000; Source peak flux at
    000 MHz, in Jy & error on peak flux, in Jy.
    ●
    SLBA & eSLBA; LBA flux density (from
    combined map). Effective frequency is 000 MHz
    & error on LBA flux density.
    ●
    SHBA & eSHBA; HBA flux density (from
    combined map). Effective frequency is 000 MHz
    & error on HBA flux density.
    ●
    SPIX & eSPIX; Spectral index & error on
    spectral index.
    ●
    SCURV & eSCURV; Spectral curvature & error
    on spectral curvature.
    ●
    MAJAX000 & eMAJAX000; Major axis of fitted
    ellipse at 000 MHz & error on major axis.
    ●
    MINAX000 & eMINAX000; Minor axis of fitted
    ellipse at 000 MHz & error on minor axis.
    ●
    PA000 & ePA000; Position angle of fitted
    ellipse at 000 MHz, in degrees & error on
    position angle.
    Dario Carbone
    

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26. 26
    Column in MSSS catalogue
    ●
    ID; Source ID, formed as Jhhmmss+ddmmss
    using the IAU convention.
    ●
    RA & eRA; Source J2000 Right Ascension, in
    decimal degrees & error on Right Ascension, in
    seconds of time.
    ●
    DEC & eDEC; Source J2000 Declination, in
    decimal degrees & error Declination, in
    arcseconds.
    ●
    SFFLAG000; Flag indicating how many
    sourcefinders detected the source (0 means it
    was detected in both; 1 only in PyBDSM; 2 only
    in PYSE.)
    ●
    Sint000 & eSint000; Source integrated flux at
    000 MHz, in Jy & error on integrated flux, in Jy.
    ●
    Spk000 & eSpk000; Source peak flux at
    000 MHz, in Jy & error on peak flux, in Jy.
    ●
    SLBA & eSLBA; LBA flux density (from
    combined map). Effective frequency is 000 MHz
    & error on LBA flux density.
    ●
    SHBA & eSHBA; HBA flux density (from
    combined map). Effective frequency is 000 MHz
    & error on HBA flux density.
    ●
    SPIX & eSPIX; Spectral index & error on
    spectral index.
    ●
    SCURV & eSCURV; Spectral curvature & error
    on spectral curvature.
    ●
    MAJAX000 & eMAJAX000; Major axis of fitted
    ellipse at 000 MHz & error on major axis.
    ●
    MINAX000 & eMINAX000; Minor axis of fitted
    ellipse at 000 MHz & error on minor axis.
    ●
    PA000 & ePA000; Position angle of fitted
    ellipse at 000 MHz, in degrees & error on
    position angle.
    This catalogue has 93 columns for the point sources,
    and 93 + 96 = 189 columns for extended sources!
    

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27. 27
    Conclusions
    ●
    First catalogue of 100 squared degrees of MSSS is ready
    to be released.
    ●
    This will contain a lot of data on several hundreds of
    sources.
    ●
    Compared to other existing surveys at similar frequencies
    MSSS is behaving as expected.
    ●
    Sourcefinding was done following a slightly more
    conservative approach than usual.
    ●
    PySE is behaving very well on the Data Challenge maps
    as completeness, reliability, and flux recovery.
    ●
    The force-beam option should be turned on as we are
    looking for point sources only.
    

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