ADC/CDS Standard Document for Catalog:/external/photom/E2005/
ADC/CDS Standard Document for Catalog:
E/II/5 The 2MASS database (IPAC/UMass, 2000)
================================================================================
The Two Micron All Sky Survey
Skrutskie M.F., Schneider S.E., Stiening R., Strom S.E., Weinberg M.D.,
Beichman C., Chester T., Cutri R., Lonsdale C.,
Elias J., Elston R., Capps R., Carpenter J., Huchra J.,
Liebert J., Monet D., Price S., Seitzer P.
<Proc. Workshop "The Impact of Large Scale Near-IR Sky Surveys", 25 (1997)>
================================================================================
ADC_Keywords: Infrared sources ; Photometry, infrared ; Surveys;
External catalog
Location: http://www.ipac.caltech.edu/2mass/overview/access.html
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 {mu}m), H (1.65 {mu}m), and Ks (2.17 {mu}m), to a 3{sigma}
limiting sensivity of 17.1, 16.4 and 1.3mag in thge 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).
File Summary:
--------------------------------------------------------------------------------
FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
out.sam 292 1000 2MASS Point Source Catalogue, sample output,
incremental data release 2 (March 2000).
--------------------------------------------------------------------------------
See also:
http://www.ipac.caltech.edu/2mass/ : 2MASS documentation
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 HHMMSSS+/-DDMMSS, where HH are the
hours of right ascension, MM the RA minutes and SSS are the
deci-seconds of RA; DD are the degrees in declination, MM are DEC
minutes and SS the declination seconds. Note that both the RA
deci-seconds of time and DEC seconds of arc are truncated rather than
rounded, per the IAU recommendation.
It was discovered that about 100,000 objects have non-unique source
designations; a column f_Name was added to flag those objects which
name appears more than once in the survey.
Byte-by-byte Description of output: out.sam
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 10 F10.6 deg RAdeg Right ascension (J2000)
12- 21 F10.6 deg DEdeg Declination (J2000)
23- 26 F4.2 arcsec MajAxis ? Major axis of position error ellipse
28- 31 F4.2 arcsec MinAxis ? Minor axis of position error ellipse
34- 36 I3 deg Theta [0,180]? Position angle of error ellipse
major axis (E of N)
38- 38 A1 --- f_Name [* ] A '*' flags a non-unique source
designation
39- 52 A14 --- Name Source designation
--------------------------------------------------------------------------------
54- 60 F7.3 mag Jmag ?=-99.999 J selected "default" magnitude (1)
62- 66 F5.3 mag e_Jmag_d ? J "default" magnitude uncertainty (2)
68- 72 F5.3 mag e_Jmag ? J "total" magnitude uncertainty (3)
74- 80 F7.3 mag Hmag ?=-99.999 H selected "default" magnitude (1)
82- 86 F5.3 mag e_Hmag_d ? H "default" magnitude uncertainty (2)
88- 92 F5.3 mag e_Hmag ? H "total" magnitude uncertainty (3)
94-100 F7.3 mag Kmag ?=-99.999 K selected "default" magnitude (1)
102-106 F5.3 mag e_Kmag_d ? Ks "default" magnitude uncertainty (2)
108-112 F5.3 mag e_Kmag ? Ks "total" magnitude uncertainty (3)
--------------------------------------------------------------------------------
114-116 A3 --- rd_flg [012348] Source of JHK "default" mag (4)
119-121 A3 --- bl_flg [012] JHK components fit to source (5)
124-126 A3 --- cc_flg [UNPCDSRGB0] Artifact contamin./confusion (6)
131 I1 --- extd_flg [0,2]? Extended source contamination (7)
133 I1 --- mp_flg [0,1]? Association with asteroid or comet (8)
--------------------------------------------------------------------------------
138 A1 --- Id_opt [UT] Associated optical source (9)
140-144 F5.2 mag Bmag ? Blue mag of associated optical source
146-150 F5.2 mag Rmag ? Red mag of associated optical source
153-156 F4.2 arcsec Dist_opt ? Distance to associated optical source
159-161 I3 deg PAopt [0,360]? position angle to optical source
165-166 I2 --- Nopt_mchs ? Number of optical sources within 5 arcsec
--------------------------------------------------------------------------------
#169-178 A10 "MM/DD/YYYY" Date Observation date
169-178 A10 --- Date Observation date
183 A1 --- Hemis [ns] Hemisphere of observation
186-188 I3 --- Scan ? Scan number (within date)
190-195 I6 --- Id Source id number (within scan)
197-206 F10.6 deg GLON Galactic longitude
208-217 F10.6 deg GLAT Galactic latitude
219-224 F6.2 --- Jpsfchi ? J band reduced chi2 value of fit
226-231 F6.2 --- Hpsfchi ? H band reduced chi2 value of fit
233-238 F6.2 --- Kpsfchi ? K band reduced chi2 value of fit
--------------------------------------------------------------------------------
240-246 F7.3 mag Jm_stdap ? J standard or BF aperture magnitude (10)
248-253 F6.3 mag e_Jm_stdap ? error on J_m_stdap
255-261 F7.3 mag Hm_stdap ? H standard or BF aperture magnitude (10)
263-268 F6.3 mag e_Hm_stdap ? error on H_m_stdap
270-276 F7.3 mag Km_stdap ? K standard or BF aperture magnitude (10)
278-283 F6.3 mag e_Km_stdap ? error on K_m_stdap
--------------------------------------------------------------------------------
287-292 A6 --- Ndet_flg [0-9 ] Number of aperture measurements
(jjhhkk) (11)
--------------------------------------------------------------------------------
Note (1): This is the selected "default" magnitude for each band
(centered at J: 1.25, H: 1.65 and Ks: 2.17{mu}m).
For most sources, the [jhk]_m values are the PSF fit mags for each
band (rd_flg=2). If the source has a null PSF-fit magnitude but a
valid std. aperture magnitude in a band, the [jhk]_m value for that
band is the std. aperture magnitude (rd_flg=4). If the source is
saturated in R2-R1 in a band, but not saturated in the R1 exposure,
the default magnitude is the R1 aperture magnitude (rd_flg=1). If the
source is saturated in even the R1 exposure (rd_flg=3) or if the
source is a bright star placeholder (rd_flg=8), the default magnitude
is given as -99.999. Finally, if a source is band-filled in a band,
the value of [jhk]_m for that band is the band-fill 95%-confidence
upper limit mag (rd_flg=0).
This value is computed as follows:
Lim95 = integrated flux + (2 * noise)
where the noise in this case means the "local noise", computed as
follows (using the DAOPHOT algorithm):
noise = sqrt(error1 + error2 + error3)
where
error1 = variance in the local sky
= npix * skyvar
npix = number of pixels in the aperture
skyvar = the variance of the sky brightness
error2 = photon noise
= int. flux * Gain
error3 = (skyvar/nsky) * (npix ** 2)
nsky = number pixels in sky annulus
(skyvar/nsky) = square of the standard error of the mean sky brightness
For the case in which the integrated flux is negative, the upper limit
is
Lim95 = 2 * noise
where
noise = sqrt(error1 + error3)
Note (2): The mag sigma values corresponding to the selected "default"
magnitude for each band are put in the [jhk]_msig fields. For
non-detections (band-fills), the mag sigmas are null. An unphysically
large magnitude sigma value (e.g. >0.36) indicates that the source was
detected in that band, but either the brightness measurement
encountered problems due to confusion or other contamination, or the
detection is of very low confidence. Objects that are saturated in R1
in any band have magnitude sigmas of 9.999.
Note (3): The "combined" uncertainty in the quoted magnitude for each band
that provides an estimate of all random errors that contribute to the
photometric uncertainty. This uncertainty is the quadratic combination
of the extraction uncertainites ([jhk]_msig), and the photometric zero
point offset uncertainties from the night during which the source was
observed ([jhk]_zperr from the "Scan Data" table), and the residual
flat-fielding errors (estimated to be 0.005 mags for all bands). For
R1 sources, the quadratic sum includes the RMS uncertainty in the
normalization of the R1 photometry to the R2-R1 photometry
([jhk]_r1normrms from the "Scan Data" table).
Note (4): 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. The values are as
follows:
0: source is not detected in this band (is band-filled)
1: default mag is R1 aperture
2: default mag is R2-R1 profile-fit
3: default magnitude is -99.999. Source is saturated in R1.
4: default mag is R2-R1 standard aperture mag
8: default magnitude is -99.999. Source is "placeholder" for a very
bright star heavily saturated in R1. Position information derived
from Positions and Proper Motions Star Catalogue (Roser and Bastian,
1991, See Catalogues <I/146>, <I/193>, <I/208>).
Note (5): The "blend" flag indicates how many point source components
were fit to each band's source during the profile fitting process. It
is a 3-digit integer, where each digit can be
0: source not detected in band,
1: single profile fit to isolated source, or
2: multiple sources detected abd fit simultaneously
The 1st digit corresponds to the J band, the 2nd to the H band, and
the 3rd to the Ks band.
Note (6): The contamination/confusion flag indicates whether the
source's photometry and position may be affected by artifacts of
nearby bright stars or by confusion with other nearby sources. The
flag consists of a 3 character string, where the 1st character
corresponds to the J band, the 2nd to the H band, and the 3rd to the
Ks band.
Sources are tested for contamination and/or confusion in the order
given below, and once a flag is applied to the source, testing is
stopped in that band. The flag values indicate the source may be:
U: unreliable source. Most likely a spurious detection of a meteor
trail or a noise excursion
N: photometric normalization problem due to the influence of a very
bright star in the scan. Photometry in this band may be as much as
1 magnitude offset from the correct value
P: affected by a nearby persistence artifact
D: affected by a nearby diffraction spike
S: affected by a horizontal "stripe" artifact (due to a bright star)
R: affected by a nearby bright star reflection artifact
C: affected by confusion with another nearby source
B: affected by confusion in bandmerging
0: default value -- not contaminated or not detected in the band
Note (7): The "extended" flag is set to
0 if the point source is not associated with an extended source,
1 if it is identically associated with an object in the extended
source catalog (e.g. the nucleus of a galaxy), or
2 if it is simply contaminated by an extended source (e.g. a star
superimposed on the disk of a galaxy).
Note (8): This object is positionally associated with the location of
a known minor planet or comet at the time of the 2MASS observation.
See the table of known asteroid and comet detections for more
information.
Note (9): 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:
'T' for Tycho-ACT (see Cat. <I/246>)
'U' for USNO-A (see Cat. I/252>)
Note (10): If a source is not detected in a band (band-filled),
this field in that band contains the band-filled aperture-photometry
mag. It is calculated by measuring the brightness within an 8"
diameter aperture at the position of the source on the Atlas Image:
BFMag = ZERO_CAL - 2.5 log10(flux)
where ZERO_CAL is the zero calibration magnitude. Note that this field
may have to represent a negative flux; in such a case, the value is:
BFMag = 99.0 - [ZERO_CAL - 2.5 log10(|flux|)]
This encoding requires fluxes close to zero to be clipped. The
clipping distance is set to 0.01 DN above zero for positive fluxes and
below zero for negative fluxes. For example, the positive flux 0.0005
DN would be clipped at 0.01 DN and encoded as:
BFMag = ZERO_CAL - 2.5 log10(0.01)
= 24.93 (for ZERO_CAL = 19.93)
For a negative flux of -0.0005 DN, the clipping would yield -0.01 DN,
and the encoding would be
BFMag = 99.0 - [ZERO_CAL - 2.5 log10(0.01)]
= 74.07 (for ZERO_CAL = 19.93)
BFMags near 50 cannot occur, so 50 marks the boundary between positive
and negative flux encoding.
Note (11): This flag consists of 6 single-digit numbers,
two digits per band in the order JJHHKK.
The 1st digit for each band indicates the number of frames with
>3-sigma aperture mag measurements, and
the 2nd digit indicates the number of frames in which aperture
measurements were possible. (This flag is AKA the
N-out-of-M statistic).
--------------------------------------------------------------------------------
Important Notice:
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, funded
by the National Aeronautics and Space Administration and the National
Science Foundation."
History:
* 07-Sep-2000: first version of 2nd Incremental Data Release
* 19-Mar-2001: added f_Name column which flags non-unique source names.
================================================================================
(End) Sebastien Derriere, Francois Ochsenbein [CDS] 07-Sep-2000