ACDS V10.0 - Sat Apr 27 06:40:53 CEST 2019
|  | Detailed Description of II/246 :
 2MASS All-Sky Catalog of Point Sources (Cutri+ 2003)
 |  | 
Note : this is the description file of the original catalog.
This HTML page corresponds well to the original description, but some minor
changes in the format may have been introduced in the FITS output
files in order to allow for arithmetic operations on quantities
such as coordinates and times. h:m:s and d:m:s units are normally
converted into degrees, and YYYY-MM-DD into julian days.
Similarly, some slight changes may have been introduced in the
units; the value given in the HTML files supersedes the value indicated in the ReadMe
file for this catalogue.
II/246              2MASS All-Sky Catalog of Point Sources     (Cutri+ 2003)
================================================================================
The 2MASS All-Sky Catalog of Point Sources
     Cutri R.M., Skrutskie M.F., Van Dyk S., Beichman C.A., Carpenter J.M.,
     Chester T., Cambresy L., Evans T., Fowler J., Gizis J., Howard E.,
     Huchra J., Jarrett T., Kopan E.L., Kirkpatrick J.D., Light R.M,
     Marsh K.A., McCallon H., Schneider S., Stiening R., Sykes M.,
     Weinberg M., Wheaton W.A., Wheelock S., Zacarias N.
    <University of Massachusetts and Infrared Processing and Analysis Center
     (IPAC/California Institute of Technology) (2003)>
    =2003yCat.2246....0C
================================================================================
ADC_Keywords: Infrared sources ; Photometry, infrared ; Surveys
Description:
    The Two Micron All Sky Survey (2MASS) project is designed to close the
    gap between our current technical capability and our knowledge of the
    near-infrared sky. In addition to providing a context for the
    interpretation of results obtained at infrared and other wavelengths,
    2MASS will provide direct answers to immediate questions on the
    large-scale structure of the Milky Way and the Local Universe.
    To achieve these goals, 2MASS is uniformly scanning the entire sky in
    three near-infrared bands to detect and characterize point sources
    brighter than about 1 mJy in each band, with signal-to-noise ratio
    (SNR) greater than 10, using a pixel size of 2.0". This will achieve
    an 80,000-fold improvement in sensitivity relative to earlier surveys.
    2MASS uses two new, highly-automated 1.3-m telescopes, one at Mt.
    Hopkins, AZ, and one at CTIO, Chile. Each telescope is equipped with a
    three-channel camera, each channel consisting of a 256x256 array of
    HgCdTe detectors, capable of observing the sky simultaneously at J
    (1.25 µm), H (1.65 µm), and Ks (2.17 µm), to a 3{sigma}
    limiting sensitivity of 17.1, 16.4 and 15.3mag in the three bands.
    The 2MASS arrays image the sky while the telescopes scan smoothly in
    declination at a rate of ~1' per second. The 2MASS data "tiles" are 6
    deg. long in the declination direction and one camera frame (8.5')
    wide. The camera field-of-view shifts by ~1/6 of a frame in
    declination from frame-to-frame. The camera images each point on the
    sky six times for a total integration time of 7.8 s, with sub-pixel
    "dithering", which improves the ultimate spatial resolution of the
    final Atlas Images.
    The University of Massachusetts (UMass) is responsible for the overall
    management of the project, and for developing the infrared cameras and
    on-site computing systems at both facilities. The Infrared Processing
    and Analysis Center (IPAC) is responsible for all data processing
    through the Production Pipeline, and construction and distribution of
    the data products. The 2MASS project involves the participation of
    members of the Science Team from several different institutions. The
    2MASS project is funding by the National Aeronautics and Space
    Administration (NASA) and the National Science Foundation (NSF).
Acknowledging 2MASS in publications:
    Please include the following in any published material that makes use
    of the 2MASS data products:
    "This publication makes use of data products from the Two Micron All
    Sky Survey, which is a joint project of the University of Massachusetts
    and the Infrared Processing and Analysis Center/California Institute of
    Technology, funded by the National Aeronautics and Space Administration
    and the National Science Foundation."
File Summary:
File Summary:
       FileName      Lrecl  Records   Explanations
×                   80        .   This file
× ReadMe            80        .   This file
× out.sam          382     1000   2MASS Point Source Catalogue, sample output
                                (on a total of 470,992,970 sources)
See also:
    http://www.ipac.caltech.edu/2mass/ : 2MASS documentation
    VII/233 : The Two Micron All-Sky Survey: Extended sources   (Cutri+ 2003)
Nomenclature Notes:
    As specified by the IAU recommendations on source nomenclature, the
    object designation is derived from the J2000 sexagesimal coordinates
    of the source and has the form HHMMSSss±DDMMSSs, where HH are the
    hours of right ascension, MM the RA minutes and SSss are the
    centi-seconds of RA. DD are the degrees in declination, MM are DEC
    minutes and SSs the declination deci-seconds. Note that both the RA
    centi-seconds of time and DEC deci-seconds of arc are truncated rather
    than rounded, per the IAU recommendation.
    Note that when several sources have the same identifier, they are
    distinguished by a trailing letter: A, B, C, etc...
Remark on the Byte-by-byte Description:
    In the Byte-by-byte Description below, the column names differ
    slightly compared to the original description, mainly to follow the
    conventions used in the CDS Standaridzed description of catalogues.
    The original names are written (in parentheses) in the Explanations.
Byte-by-byte Description of file:  out.sam
   Bytes Format Units    Label    Explanations
   1- 10  F10.6 deg      RAdeg    (ra) Right ascension (J2000)
  12- 21  F10.6 deg      DEdeg    (dec) Declination (J2000) (dec)
  23- 26  F4.2  arcsec   errMaj   (err_maj) Semi-major axis of position 
                                  error ellipse
  28- 31  F4.2  arcsec   errMin   (err_min) Semi-minor axis of position 
                                  error ellipse
  33- 35  I3    deg      errPA    [0,180] (err_ang) Position angle of error
                                         ellipse major axis (E of N)
  37- 53  A17   ---      2MASS    (designation) Source designation (1)
--------------------------------------------------------------------------------
  55- 60  F6.3  mag      Jmag     ?(j_m) J selected default magnitude (2)
  62- 66  F5.3  mag      Jcmsig   ?(j_cmsig) J default magnitude uncertainty (3)
  68- 72  F5.3  mag      e_Jmag   ?(j_msigcom) J total magnitude uncertainty (4)
  74- 83  F10.1 ---      Jsnr     ?(j_snr) J Signal-to-noise ratio
  85- 90  F6.3  mag      Hmag     ?(h_m) H selected default magnitude (2)
  92- 96  F5.3  mag      Hcmsig   ?(h_cmsig) H default magnitude uncertainty (3)
  98-102  F5.3  mag      e_Hmag   ?(h_msigcom) H total magnitude uncertainty (4)
 104-113  F10.1 ---      Hsnr     ?(h_snr) H Signal-to-noise ratio
 115-120  F6.3  mag      Kmag     ?(k_m) K selected default magnitude (2)
 122-126  F5.3  mag      Kcmsig   ?(k_cmsig) K default magnitude uncertainty (3)
 128-132  F5.3  mag      e_Kmag   ?(k_msigcom) K total magnitude uncertainty (4)
 134-143  F10.1 ---      Ksnr     ?(k_snr) K Signal-to-noise ratio
--------------------------------------------------------------------------------
 145-147  A3    ---      Qflg     (ph_qual) JHK Photometric quality flag (5)
 149-151  A3    ---      Rflg     (rd_flg) Source of JHK default mag (6)
 153-155  A3    ---      Bflg     (bl_flg) JHK components fit to source (7)
 157-159  A3    ---      Cflg     (cc_flg) Artifact contamination, confusion (8)
 161-166  A6    ---      Ndet     [0-9] (ndet) Number of aperture measurements
                                       (jjhhkk) (9)
--------------------------------------------------------------------------------
 168-172  F5.1  arcsec   prox     Distance between source and nearest neighbour
 174-176  I3    deg      pxPA     ? (pxpa) Position angle of vector from source
                                   to nearest neighbour (E of N)
 178-187  I10   ---      pxCntr   (pxcntr) Sequence number of nearest neighbour
     189  I1    ---      Xflg     [0,2] (gal_contam) Extended source
                                       contamination (10)
     191  I1    ---      Aflg     [0,1] (mp_flg) Association with asteroid
                                       or comet (11)
--------------------------------------------------------------------------------
 193-202  I10   ---      Cntr     (pts_key) Unique source identifier in catalogue
     204  A1    ---      Hemis    [ns] (hemis) Hemisphere of observation
 206-215  A10   ---      Date     (date) Observation date
 217-219  I3    ---      Scan     (scan) Scan number (within date)
 221-227  F7.3  deg      GLON     (glon) Galactic longitude (12)
 229-235  F7.3  deg      GLAT     (glat) Galactic latitude (12)
 237-242  F6.1  arcsec   Xscan    (x_scan) Distance of source from focal plane
                                         centerline
 244-255  F12.4 d        JD       (jdate) Julian date of source measurement (13)
--------------------------------------------------------------------------------
 257-262  F6.2  ---      Jpsfchi  ?(j_psfchi) J band reduced chi2 value of fit
 264-269  F6.2  ---      Hpsfchi  ?(h_psfchi) H band reduced chi2 value of fit
 271-276  F6.2  ---      Kpsfchi  ?(k_psfchi) K band reduced chi2 value of fit
 278-283  F6.3  mag      Jstdap   ?(jmstdap) J standard aperture magnitude (18)
 285-289  F5.3  mag     e_Jstdap  ?(jmsigstdap) error on Jstdap
 291-296  F6.3  mag      Hstdap   ?(hmstdap) H standard aperture magnitude (18)
 298-302  F5.3  mag     e_Hstdap  ?(hmsigstdap) error on Hstdap
 304-309  F6.3  mag      Kstdap   ?(kmstdap) K standard aperture magnitude (18)
 311-315  F5.3  mag     e_Kstdap  ?(kmsigstdap) error on Kstdap
--------------------------------------------------------------------------------
 317-321  I5    arcsec   edgeNS   (distedgens) Distance from the source to the
                                  nearest North or South scan edge
 323-325  I3    arcsec   edgeEW   (distedgeew) Distance from the source to the
                                  nearest East or West scan edge
 327-328  A2    ---      edge     [nsew] (distedgeflg) flag indicating to which
                                  edges the edgeNS and edgeEW values refer
     330  I1    ---      dup      (dup_src) Flag indicating duplicate source (14)
     332  I1    ---      use      (use_src) Use source flag (15)
--------------------------------------------------------------------------------
     334  A1    ---      opt      [0UT] (a) Associated optical source (16)
 336-338  F3.1  arcsec   Dopt     ?(dist_opt) Distance to associated optical
                                   source
 340-342  I3    deg      PAopt    [0,360]? (phi_opt) position angle from optical
                                   source to the 2MASS source position
 344-348  F5.2  mag      Bmag     ?(bmopt) Blue magnitude of associated
                                   optical source
 350-354  F5.2  mag      Rmag     ?(vrmopt) Visual or red mag of associated
                                   optical source
     356  I1    ---      Nopt     (nopt_mchs) Number of optical sources
                                   within 5arcsec (17)
--------------------------------------------------------------------------------
 358-364  I7    ---      extKey   ?(ext_key) Record Identifier in XSC
 366-370  I5    ---      scanKey  (scan_key) Record Identifier in the
                                   Scan Information Table
 372-378  I7    ---     coaddKey  (coadd_key) Record Identifier in the
                                   Atlas Image Data Table
 380-382  I3    ---      coadd    (coadd) Sequence number of the Atlas Image
Note (1):  Sexagesimal, equatorial position-based source name in the form:
           hhmmssss+ddmmsss[ABC...].
    The full naming convention for 2MASS All-Sky Release PSC sources has
    the form "2MASS Jhhmmssss+ddmmsss[ABC...]", where "2MASS" indicates
    that the source is from the All-Sky PSC, "J" indicates the position is
    J2000, and the optional trailing letters are used to distinguish
    sources with otherwise duplicate names. The "2MASS J" prefix is not
    listed explicitly in the designation column.
Note (2): This is the selected "default" magnitude for each band, [JHK].
    If the source is not detected in the band, this is the 95% confidence
    upper limit derived from a 4" radius aperture measurement taken at the
    position of the source on the Atlas Image. The origin of the default
    magnitude is given by the first character of the Rflg value
    (Rflg). This column is null if the source is nominally detected in
    the band, but no useful brightness estimate could be made
    (Rflg="9").
Note (3): This is the corrected photometric uncertainty [JHK]cmsig for
    the default magnitude. If Rflg="2", this is the measurement error
    from the profile-fitting procedure, corrected to be consistent with
    observed repeatability statistics. If Rflg="1" or "4", this is the
    RMS of the brightness measured in aperture photometry on the
    individual frames. If Rflg="3", the uncertainty is derived from the
    residuals to the 1-d radial profile fit to the unsaturated wings of
    the bright star.
    If the value is >8.0, it is a flag value indicating that a meaningful
    uncertainty could not be determined for the source. This column is
    null if the default magnitude is a 95% confidence upper limit
    (Rflg="0" or "6") or if the source is not measurable (Rflg="9").
Note (4): Combined, or total photometric uncertainty [JHK]msigcom
    for the default magnitude in that band. The combined uncertainty is
    derived from the following relation:
      e_[JHK]mag = sqrt([JHK]cmsig2 + [JHK]zperr2
                 + fferr2 + [r1normrms2])
    where
    cmsig = Corrected band photometric uncertainty
    zperr = Nightly photometric zero point uncertainty = 0.011 mag
    fferr = Flat-fielding residual error = 0.005 mags
    r1normrms = R1 normalization uncertainty = 0.012 mags
          (applied only for sources with Rflg="1")
    This column is null if the default magnitude is a 95% confidence upper
    limit (i.e. the source is not detected, or inconsistently deblended in
    the band).
Note (5): Three character flag, one character per band [JHK],
    that provides a summary of the net quality of the default photometry
    in each band, as derived from the Read Flag (Rflg), measurement
    uncertainties ([jhk]cmsig), scan signal-to-noise ratios ([jhk]snr),
    frame-detection statistics (Ndet), and profile-fit reduced chi-squared
    values ([jhk]psfchi). The value for Qflg is set for a band
    according to the precedence of the table below. For example, a source
    that is tested and meets the conditions for category "X" is not tested
    for subsequent qualities.
    X = There is a detection at this location, but no valid brightness
        estimate can be extracted using any algorithm. Rflg="9" and
        default magnitude is null.
    U = Upper limit on magnitude. Source is not detected in this band
        (Rflg="0"), or it is detected, but not resolved in a consistent
        fashion with other bands (Rflg="6"). A value of Qflg="U"
        does not necessarily mean that there is no flux detected in this
        band at the location. Whether or not flux has been detected can
        be determined from the value of Rflg. When Rflg="0", no flux
        has been detected. When Rflg="6", flux has been detected at the
        location where the images were not deblended consistently in all
        three bands (JHKs).
    F = This category includes Rflg="1" or Rflg="3" sources where a
        reliable estimate of the photometric error, [jhk]cmsig, could
        not be determined. The uncertainties reported for these sources
        in [jhk]cmsig and e_[jhk]mag are flags and have numeric
        values >8.0.
    E = This category includes detections where the goodness-of-fit
        quality of the profile-fit photometry was very poor (Rflg=2 and
        [jhk]psfchi>10.0), or detections where psf fit photometry did
        not converge and an aperture magnitude is reported (Rflg=4), or
        detections where the number of frames was too small in relation
        to the number of frames in which a detection was geometrically
        possible (Rflg="1" or Rflg="2").
    A = Detections in any brightness regime where valid measurements
        were made (Rflg="1","2" or "3") with
          [jhk]snr>10 AND [jhk]cmsig<0.10857.
    B = Detections in any brightness regime where valid measurements
        were made (Rflg="1","2" or "3") with
          [jhk]snr>7 AND [jhk]cmsig<0.15510.
    C = Detections in any brightness regime where valid measurements
        were made (Rflg="1","2" or "3") with
          [jhk]snr>5 AND [jhk]cmsig<0.21714.
    D = Detections in any brightness regime where valid measurements
        were made (Rflg="1","2" or "3") with
          no [jhk]snr OR [jhk]cmsig requirement.
Note (6): The read flag indicates the source of J,H,K "default" magnitudes
    as a 3-digit integer, where the 1st digit corresponds to the J band,
    the 2nd to the H band, and the 3rd to the Ks band. Rflg values of
    "1", "2" or "3" generally indicate the best quality detections,
    photometry and astrometry (although other quality flags must be
    considered). Values of "0", "4", "6" and "9" in a band indicate
    either non-detections, or generally poor quality photometry and
    positions. The definitions of the Rflg values are:
    0 = Source is not detected in this band. The default magnitude
        is the 95% confidence upper limit derived from a 4" radius
        aperture measurement taken at the position of the source on the
        Atlas Image. The sky background is estimated in an annular region
        with inner radius of 14" and outer radius of 20".
    1 = The default magnitude is derived from aperture photometry
        measurements on the 51 ms "Read_1" exposures. The aperture radius
        is 4", with the sky background measured in an annulus with an
        inner radius of 14" and an outer radius of 20". Used for sources
        that saturate one or more of the 1.3s "Read_2" exposures, but are
        not saturated on at least one of the 51 ms "Read_1" frames.
    2 = The default magnitude is derived from a profile-fitting
        measurement made on the 1.3 sec "Read_2" exposures. The
        profile-fit magnitudes are normalized to curve-of-growth
        corrected aperture magnitudes. This is the most common type in
        the PSC, and is used for sources that have no saturated pixels in
        any of the 1.3 sec exposures.
    3 = The default magnitude is derived from a 1-d radial profile
        fitting measurement made on the 51 ms "Read_1" exposures. Used
        for very bright sources that saturate all of the 51 ms "Read 1"
        exposures.
    4 = The default magnitude is derived from curve-of-growth-corrected
        4" radius aperture photometry measurements on the 1.3 s "Read_2"
        exposures. This is used for sources that are not saturated in any
        of the Read_2 frames, but where the profile-fitting measurements
        fail to converge to a solution. These magnitudes are the same as
        the standard aperture magnitudes (Jstdap, Hstdap, Kstdap), but
        when they are the default magnitudes, it generally implies that
        they are low quality measurements.
    6 = The default magnitude is the 95% confidence upper limit derived
        from a 4" radius aperture measurement taken at the position of the
        source on the Atlas Image. The sky background is estimated in an
        annular region with inner radius of 14" and outer radius of 20".
        This is used for pairs of sources which are detected and resolved
        in another band, but are detected and not resolved in this band.
        This differs from a Rflg="0" because in this case there is a
        detection of the source in this band, but it is not consistently
        resolved across all bands.
    9 = The default magnitude is the 95% confidence upper limit derived
        from a 4" radius aperture measurement taken at the position of the
        source on the Atlas Image. The sky background is estimated in an
        annular region with inner radius of 14" and outer radius of 20".
        This is used for sources that were nominally detected in this
        band, but which could not have a useful brightness measurement
        from either profile fitting or aperture photometry. This often
        occurs in highly confused regions, or very near Tile edges where
        a significant fraction of the measurement aperture of sky annulus
        falls off the focal plane.
Note (7): Blend flag. Three character flag, one character per band [JHK],
    that indicates the number of components that were fit simultaneously
    when estimating the brightness of a source:
    0 = Source is not detected, or is inconsistently deblended in that
        band.
    1 = One component was fit to the source in R_2 profile-fitting
        photometry (Rflg="2"), or default magnitudes are from aperture
        photometry (Rflg="1" or "4") or saturated star 1-d radial
        profile-fitting (Rflg="3").
   >1 = More than one component was fit simultaneously during R2
        profile-fit photometry, where the value of the field is the
        number of components simultaneously fit. The maximum number of
        components is 7 in any band for the PSC, so this Bflg is always
        a three character flag. Multi-component fitting occurs only for
        profile-fitting, and only when more than one detection is found
        within ~5". Single detections that are not well-fit by a single
        PSF are not split.
Note (8): Contamination and confusion flag. Three character flag, one
    character per band [JHKs], that indicates that the photometry and/or
    position measurements of a source may be contaminated or biased due to
    proximity to an image artifact or nearby source of equal or greater
    brightness. The Cflg in each band is set in hierarchical order
    according to the following table, in the event a source is affected by
    more than one artifact or condition:
    p = Persistence. Source may be contaminated by a latent image left
        by a nearby bright star.
    c = Photometric Confusion. Source photometry is biased by a nearby
        star that has contaminated the background estimation. This is
        very common in high source density regions.
    d = Diffraction spike confusion. Source may be contaminated by a
        diffraction spike from a nearby star.
    s = Electronic stripe. Source measurement may be contaminated by a
        stripe from a nearby bright star.
    b = Bandmerge confusion. In the process of merging detections in
        the different bands for this source, there was more than one
        possible match between the different band components. This occurs
        in regions of very high source density, or when multiple sources
        were split in one band but not another.
    0 = Source is unaffected by known artifacts, or is not detected in
        the band.
    Non-zero values the Cflg in any band is an indicator that the
    measurements of that source may be contaminated.
Note (9): ndet is a six-character flag, two characters per band [JJHHKsKs],
    that indicates the number frames on which a source was detected, N,
    and the number of frames on which the source could have been measured,
    M. The first digit in the pair for each band, N, gives the number of
    frames on which there were >3 sigma aperture photometry detections of
    the source. The second digit for each band, M, gives the number of
    frames on which aperture measurements were possible. Normally, M=6,
    (occasionally M=7 because the scanning step size was slightly less
    than 1/6 of the frame size). M can be <6 if frames are "lost" because:
    a) there were masked pixels within the source aperture due to cosmic
    ray detections, noisy pixels, meteor trails, etc, or b) there were
    saturated pixels within the source aperture. Values of M<6 are not
    uncommon, but do indicate an effective loss of coverage and
    sensitivity for a source.
    The values of N and M for a band always pertain to the aperture
    photometry on the 51 ms "Read_1" exposures if Rflg="1", and to the
    aperture photometry on the 1.3 s "Read_2" exposures if Rflg="2" or
    "4". Sources that are saturated on all of the Read_1 frames
    (Rflg="3") have N=0 because no aperture photometry was possible.
    Sources with signal-to-noise ratios >8-9 should be detected on all
    available frames. Values of N < M for such sources are an indication
    of possible loss of measurement integrity. Values of N < M, or even
    N=0, are normal for fainter sources because they are too faint on
    individual frames to yield a >3 sigma detection.
Note (10): Extended source "contamination" flag. A value of
    Xflg="2" indicates that this point source falls within the elliptical
    boundary of an XSC source defined by the semi-major axis and
    ellipticity of the 20 mag.arcsec-2 Ks-band elliptical isophote
    (r_k20fe)+10%, for XSC sources where r_k20fe is >10''. Sources so
    indicated are often foreground stars superimposed on background
    galaxies, or sometimes extractions of pieces of the galaxy or nebula.
    The point source photometry for these sources is probably contaminated
    by the surrounding structured extended emission. Point sources can
    still be superimposed on extended sources and remain unflagged if the
    XSC source shape is not well-described by an ellipse.
    This flag also denotes (Xflg="1") PSC sources that are equivalent to
    sources in the XSC, but only for XSC sources with r_k20fe is >10''.
    Since most XSC sources have semi-major axes <10'', Xflg does not
    identify all PSC sources with exact XSC counterparts. Use the extKey
    to identify all PSC sources that were found to be resolved relative to
    a single point-spread function.
    0 = Source does not fall within the elliptical profile of an extended
        source with semi-major axis >10'', or it is not identified
        exactly with an XSC source with semi-major axis >10''. However,
        the source may correspond exactly to a smaller XSC source.
    1 = Source is resolved by 2MASS, and is equivalent to a source in the
        XSC that has a semi-major axis >10'' in size. Users must refer to
        the extKey to identify all PSC sources with XSC counterparts.
    2 = Source falls within the elliptical boundary of an XSC source that
        has a semi-major axis >10'' in size.
Note (11): Minor Planet Flag. Indicates if this source is associated
    with the predicted position of a known minor planet, comet, planet or
    planetary satellite. This association does not guarantee that the PSC
    source is a detection of the solar system object.
    0 = Source is not associated with a known solar system object
    1 = Source is associated with the predicted position of a known
        solar system object. The name and orbital data for the associated
        objects are given in the Known Asteroid Detection List, Known
        Comet Detection List, and Planet and Planetary Satellite
        Detection List. The associated PSC source is cross-referenced in
        those Lists via the value of ptsKey.
Note (12):  Galactic coordinate for 2MASS source derived by precessing
    reconstructed ra and dec to B1950, and performing rotational
    transformation into lII, bII coordinate system. This coordinate should
    not be used as an astrometric reference because it has been rounded to
    0.001 deg.
Note (13): The Julian Date of the source measurement is accurate to
    ±30seconds. This value is extrapolated from the start time of the
    Survey scan using the difference between the declinations of the
    source and the first row in the Tile divided by the scanning rate of
    the telescope (approximately 5sec). The scanning rate of the two 2MASS
    telescopes was slightly different because of the need to optimize the
    dithering of images on the arrays.
Note (14): Used in conjunction with the use flag, this numerical flag
    indicates whether the source falls in a Tile overlap region, and if
    so, if it was detected multiple times.
    0 = The source does not fall within a Tile overlap region
        (use="1"), or it falls in an overlap region and there were no
        sources detected within 2" of its position in the overlapping
        Tile (use="0").
    1 = The source falls within a Tile overlap region, and there is one
        source detected within 2" of its position in the overlapping Tile.
   >1 = The source falls within a Tile overlap region, and there are
        multiple sources detected within 2" of its position in the
        overlapping Tile, and/or within its own Tile. This implies
        confusion in the multiple source resolution procedure.
Note (15): Used in conjunction with the dup flag, this numerical flag
    indicates if a source falls within a Tile overlap region, and whether
    or not it satisfies the unbiased selection rules for multiple source
    resolution.
    1 = The source does not fall in a Tile overlap region
        (dup="0"), or it falls in an overlap region, but lies farther
        from its scan edge than any apparition of the source in another
        Tile. A source must have use="1" to be considered part of the
        sub-Catalog that satisfies the 2MASS Level 1 Science Requirements
    0 = The source falls in a Tile overlap region, but does not have
        a detection within 2" of its position in any overlapping Tiles.
        PSC sources with use="0" will always have dup="0". Such
        objects are usually faint, and thus toggling above and below the
        detection limits, or may have moved between the scans of the
        different Tiles.
Note (16): The catalog ID and other association parameters are given for
    the closest optical match found within 5 arcsec. The ID consists of a
    single-letter abbreviation for the catalog (e.g., 'T' for Tycho 2, 'U'
    for USNO-A2.0). '0' means no optical source was found.
Note (17): The optical association information is unreliable for sources
    with |dec|>86°, because of an error in the 2MASS/USNO-A2.0 position
    correlation procedure that caused associations to be missed in the
    vicinity of the equatorial poles. If the source has |dec|>86°, a
    value of a="0" does not necessarily mean that there is not an optical
    association within 5 arcsec of the 2MASS position.
Note (18): the standard aperture magnitude is the curve-of-growth corrected
    average brightness measured in a 4" radius aperture centered on the
    source position on each of the available 1.3 s "Read_2" frames. The
    column is empty if the source is not detected in the band (Rflg="0"),
    or if it is saturated in all of the 1.3 s "Read 2" exposures for the band.
History:
  * 10-Jun-2003: this ReadMe document was prepared from the documents at
    http://www.ipac.caltech.edu/2mass/releases/allsky/doc/sec2_2a.html
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(End)                 Sebastien Derriere, Francois Ochsenbein [CDS]  18-Jun-2003