VIII/51             the FIRST Survey, version 1998Feb   (White+ 1998)
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The FIRST Survey Catalog of 1.4GHz radio sources
     White R.L., Becker R.H., Helfand D.J., Gregg M.D.
    <Astrophys. J. 475, 479 (1998)>
    =1997ApJ...475..479W
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ADC_Keywords: Radio sources ; Surveys

Description:
    The  FIRST survey to produce  Faint Images of the  Radio Sky at Twenty
    centimeters began in 1993. It  uses  the  VLA  (Very  Large  Array,  a
    facility  of the  National Radio Observatory (NRAO)) at a frequency of
    1.4GHz,  and it is slated  to 10,000 deg^2^ to  a sensitivity of about
    1mJy with  an angular resolution of about  5". The co-added images are
    available  on   the   Internet   (see   the   FIRST   home   page   at
    http://sundog.stsci.edu/ for details). The source catalogue is derived
    from the images.

    This version of the FIRST Survey is derived from the 1993 through 1997
    observations, and contains 382,892 sources for the north Galactic cap,
    and 54,537  sources for the  south Galactic cap.  The northern catalog
    covers about 4150 square degrees of sky, including most of the area:

        7h20m < RA(2000) < 17h20m     22.2{deg} < Dec < 57.6{deg}

    The southern catalog covers about 610 square degrees of sky, including
    two narrow strips in the area

       21h20m < RA(2000) <  3h20m    -11.5{deg} < Dec < 1.6{deg}

File Summary:
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 FileName    Lrecl    Records    Explanations
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ReadMe          80          .    This file
north.dat      124     382892    North Galactic Cap
                                   (7h20<RA<17h20, +22.2{deg}<Dec<+57.6{deg})
south.dat      124      54537    South Galactic Cap
                                   (21h20<RA<3h20, -11.5{deg}<Dec<+01.6{deg})
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See also:
    http://sundog.stsci.edu/ : home page of the VLA FIRST Survey
    http://sundog.stsci.edu/first/catalogs/history.html : Version History
    http://sundog.stsci.edu/cgi-bin/searchfirst : the FIRST search engine
    http://third.llnl.gov/cgi-bin/firstcutout : the FIRST Cutout Server.
    http://www.cv.nrao.edu/first/ : FIRST page at NRAO

Byte-by-byte Description of file: *.dat
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   Bytes Format Units   Label     Explanations
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   1- 16  A16   ---     FIRST    *FIRST Source designation
  18- 19  I2    h       RAh      *Right Ascension J2000 (hours)
  21- 22  I2    min     RAm      *Right Ascension J2000 (minutes)
  24- 29  F6.3  s       RAs      *Right Ascension J2000 (seconds)
      31  A1    ---     DE-      *Declination J2000 (sign)
  32- 33  I2    deg     DEd      *Declination J2000 (degrees)
  35- 36  I2    arcmin  DEm      *Declination J2000 (minutes)
  38- 42  F5.2  arcsec  DEs      *Declination J2000 (seconds)
      44  A1    ---     Wflag    *[W] Warning flag
  46- 53  F8.2  mJy     Fpeak    *Peak flux density at 1.4GHz
  56- 63  F8.2  mJy     Fint     *Integrated flux density at 1.4GHz
  66- 71  F6.3  mJy     Rms      *Local noise estimate
  73- 78  F6.2  arcsec  MajAxis  *Major axis (FWHM)
  80- 85  F6.2  arcsec  MinAxis  *Minor axis (FWHM)
  87- 91  F5.1  deg     PA       *Position angle
  93- 98  F6.2  arcsec  fMaj     *Fitted MajAxis before deconvolution
 100-105  F6.2  arcsec  fMin     *Fitted MinAxis before deconvolution
 107-111  F5.1  deg     fPA      *Fitted PA before deconvolution
 113-124  A12   ---     Field    *Name of the coadded image containing
                                  the source
--------------------------------------------------------------------------------

Note on FIRST:
    This column (not  part of  the original  catalog) contains  the source
    name  built from the rule registered at IAU ('J' followed by truncated
    J2000-position)

Note on RAh, RAm, RAs, DE-, DEd, DEm, DEs:
    Position  position (J2000) of the source. The accuracy of the position
    depends  on the brightness and size of the source and the noise in the
    map. Point  sources  at  the  detection  limit  of  the  catalog  have
    positions accurate to  better than 1  arcsec at 90%  confidence; 2 mJy
    point sources  in typically noisy  regions have positions  good to 0.5
    arcsec. An empirical expression for the positional accuracy is

        unc(90% confidence) = Size  (1/SNR + 1/20)  arcsec

    where Size is  either the  major or  minor axis  fitted FWHM  (fMaj or
    fMin) as given in the  catalog  and  SNR  is  the  peak  flux  density
    signal-to-noise ratio:

        SNR = (Fpeak-0.25) / Rms

    (The  positional uncertainty  is of course  elliptical for  elliptical
    sources.) The best possible positional uncertainty is limited to about
    0.1  arcsec by our  ability to fit  source positions in  maps with 1.8
    arcsec   pixels  and  by  various  random  calibration  uncertainties.
    Systematic errors in the positions are smaller than 0.05 arcsec.

Note on Wflag:
    `W' is a warning flag indicating that the source may be a sidelobe of
    a nearby bright source. In the northern catalog, 14,039 sources (3.7%)
    are flagged as possible sidelobes;  in  the  southern  catalog,  1,943
    sources  (3.6%)  are  flagged  as  possible  sidelobes.  The  flagging
    algorithm is an artificial  intelligence  approach  based  on  oblique
    decision trees. Our  analysis  indicates  that  <10%  of  the  objects
    flagged  as sidelobes are real sources and that considerably less than
    1% of the  unflagged sources  in the  catalog are  sidelobes. Complete
    details of  the sidelobe identification procedure  are included in our
    catalog paper.

    NOTE:  The sidelobe flagging  in the  southern catalog  may not  be as
    reliable as  in the north because  the sidelobe characteristics change
    with   the  zenith  angle  of  the  observation.  We  are  working  on
    improvements  to the sidelobe-flagging, but for now it is important to
    be  cautious in studies  of individual southern  sources. We recommend
    checking the  images using  the FIRST  Cutout Server  if there  is any
    doubt  about the reality of particular sources (in either the north or
    the  south.) This is easily done when using the FIRST Search Engine to
    search the catalog, since  each source  selected in  the search  has a
    link to the Cutout Server.

Note on Fpeak, Fint:
    Fpeak  and Fint are the peak and integrated flux densities measured in
    mJy. They  are derived by fitting an  elliptical Gaussian model to the
    source. To correct  for the ``CLEAN  bias'' effect, 0.25  mJy has been
    added to the peak  flux density  and the  integrated flux  density has
    been multiplied by (1+0.25/Fpeak) (see our Astrophysical Journal paper
    and our catalog paper for more details.)

    The  uncertainty in  Fpeak is  given by  the rms  noise at  the source
    position,  while the uncertainty  in Fint can  be considerably greater
    depending on  the source size  and morphology. For  bright sources the
    accuracies of Fpeak and  Fint are  limited to  about 5%  by systematic
    effects. Note that for sources  that  are  not  well-described  by  an
    elliptical  Gaussian model,  Fint is  not an  accurate measure  of the
    integrated flux density.

Note on Rms:
    Rms  is a local noise estimate at the source position measured in mJy.
    Rms is computed by combining the measured noise from all grid pointing
    images contributing to  this  coadded  map  position.  Note  that  the
    significance of  detection  for  a  source  is  (Fpeak-0.25)/Rms,  not
    Fpeak/Rms, because of the CLEAN  bias  correction  to  the  peak  flux
    density. The catalog includes only sources brighter than 5 Rms.

    FITS  images giving the rms noise as a function of position on the sky
    are available  for the northern  and the southern  areas. These images
    give the  rms in  mJy/beam tabulated  on a  ~3 arcmin  grid in  RA and
    Declination. If there is no source in the catalog at a given position,
    the source  peak flux density  (before CLEAN bias  correction) is less
    than 5 times the coverage map rms value at that position.

Note on MajAxis, MinAxis, PA:
    MajAxis, MinAxis, and PA give  the  major  and  minor  axes  (FWHM  in
    arcsec) and  position angle (degrees  east of north)  derived from the
    elliptical Gaussian  model for  the source.  MajAxis and  MinAxis have
    been  deconvolved  to  remove  blurring  by  the  elliptical  Gaussian
    point-spread function. (The fitted parameters before deconvolution are
    given in  the fMaj, fMin, and  fPA columns.) In the  north the beam is
    circular 5.4 arcsec FWHM; in  the  south  it  is  elliptical,  6.4x5.4
    arcsec FWHM, with the major axis running north-south.

    Noise  can cause the fitted values of the major and minor axes (before
    deconvolution)   to  be  smaller  than  the  beam.  The  corresponding
    deconvolved size is given as zero in those cases.

    The  uncertainties  in  the  deconvolved  sizes  depend  on  both  the
    brightness and  the sizes. Objects  at the catalog  flux density limit
    have  uncertainties of about 2 arcsec in their sizes (so faint objects
    with Maj  < 2  arcsec are  consistent with  point sources.)  An simple
    empirical estimate of the uncertainty is

        Sigma(Size) = 10 arcsec  (1/SNR + 1/75)

    where SNR is the signal-to-noise ratio defined above.

Note on fMaj, fMin, fPA:
    fMaj, fMin, and fPA give the major and minor axes (FWHM in arcsec) and
    position angle  (degrees east  of north)  derived from  the elliptical
    Gaussian model for  the source.  These are  the fitted  sizes measured
    directly  from the image; the elliptical point-spread function has not
    been deconvolved.

Note on Field:
    The Field Name is the name of the coadded image containing the source.
    Note that the field  name encodes  the position  of the  field center:
    field  hhmmm+ddmmm is centered at RA=hh mm.m, Dec=+dd mm.m. The images
    are available from several  archives  and  through  the  FIRST  Cutout
    Server.

    All  field names in  the current catalog  end with E  or F, indicating
    that  they were constructed using the  imaging pipeline that applies a
    small  rotation and scale correction. See the catalog history page for
    more  information. The F fields are new in this catalog, while sources
    extracted from the E  fields are  identical to  those in  the previous
    version  of the catalog. Some E fields from the last catalog have been
    replaced by new  F fields  because there  were additional  grid images
    available for the coadding.
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History:
    The files (originally catalog_north_98feb04.bin and
    catalog_south_98feb04.bin) were copied from
    http://sundog.stsci.edu/first/catalogs/readme.html

    The preceding version (1997 April 24) was numbered <VIII/48> in
    CDS Archives.

    See http://sundog.stsci.edu/first/catalogs/history.html
    for the complete version history.
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(End)                   Richard L. White, rlw@stsci.edu  [StScI]     1998-Feb-04