The Naval Observatory Merged Astrometric Dataset (NOMAD)
README File Description of the NOMAD, version 1
Naval Observatory Merged Astrometric Dataset (NOMAD) version 1
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"readme" file, last update: 2005 Jan 10
1) Introduction
2) Catalogs used
3) Data structure
4) Remarks
5) Flags & Statistics
6) References
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1) Introduction
---------------
The U.S. Naval Observatory (USNO) announces the release of the first
version of the Naval Observatory Merged Astrometric Dataset
(NOMAD). The almost 100 GB dataset contains astrometric and
photometric data for about 1.1 billion stars derived from the
Hipparcos, Tycho-2, UCAC2, Yellow-Blue 6, and USNO-B catalogs for
astrometry and optical photometry, supplemented by 2MASS near-infrared
photometry. For each unique star the "best" astrometric and
photometric data are chosen from the source catalogs and merged into a
single dataset. A sequence of priorities is followed and NOMAD
contains flags to identify the source catalogs and gives
cross-reference identifications.
This first release of NOMAD is not a compiled catalog; that is, if a
star is identified in more than 1 of the above mentioned catalogs,
only 1 catalog entry is chosen. Thus the local and global systematic
errors of the various source catalogs will be present in this version
of NOMAD. All source catalogs astrometric data are on the
International Celestial Reference System within the limitations of the
source catalogs.
For more information and data retrieval see our homepage
http://www.nofs.navy.mil/nomad .
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2) Catalogs used
----------------
Hipparcos Catalogue: Positions, proper motions and errors are used;
however, no parallaxes are included in NOMAD.
Tycho-2: is a compiled catalog with proper motions derived from the
combination of Hipparcos satellite measures (mainly its star tracker
data) and over 100 ground-based astrometric catalogs which provided
the early epoch data. Most of the Tycho B and V magnitudes (which
includes Hipparcos stars) went into NOMAD as well.
UCAC2: is also a compiled catalog, including all catalogs used for
Tycho-2 (thus also including Tycho and Hipparcos astrometric data),
plus the recent epoch ground-based observations of the UCAC project.
However, only 86\% of the sky are covered by UCAC2 (the north
celestial pole area is missing). For stars not in USNO-B, the UCAC2
magnitude has been used as "R" photometry value, although the UCAC2
bandpass is between V and R.
YB6: (Yellow-Blue catalog version 6) is unpublished data obtained from
complete scans of the NPM and SPM plates performed on the PMM at USNO,
Flagstaff Station. The limiting magnitude is about 18 and YB6 is the
major source of faint B and V magnitudes in NOMAD.
2MASS: the near IR photometry without errors or flags has been copied
into NOMAD. For those stars without optical counterparts, the 2MASS
astrometric information was used (no proper motions).
USNO-B: provides positions and proper motions for most faint stars in
NOMAD. Most R photometry in NOMAD comes from this catalog.
For more details, see the catalog introductions for each individual
catalog used in NOMAD.
NOMAD is not a compiled catalog, no average values are taken if a star
appears in more than 1 source catalog. Each astrometric and
photometric entry in NOMAD is taken from a specific source catalog.
The priority order is as follows:
astrometry brighter than 8 mag:
Hipparcos
Tycho-2
UCAC2
astrometry of fainter stars:
Hipparcos
UCAC2
Tycho-2
USNO-B
YB6
2MASS
photometry: optical
Tycho-2 (incl. Hipparcos stars)
YB6
USNO-B
UCAC2
photometry: near IR
2MASS
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3) Data structure
-----------------
The original NOMAD data are stored in binary, direct access files of
88 byte record length. Each record corresponds to 1 object. Each
file (*.cat) contains stars for a 0.1 degree wide zone. Stars within
a zone are sorted by ascending RA. Zones are ordered according
declination starting from the south celestial pole. Thus there are
1800 files for zone number 0 to 1799. 10 such files are combined into
a separate directory. For each zone data file there is also a *.acc
(acceleration) file, containing an index in RA in steps of 0.25
hours (text ASCII file).
- The NOMAD id numbers consist of a zone number (0 to 1799
in 1/10 degree steps of declination, starting from the
south celestial pole), and a running star number within that
zone (increasing along RA). The ids are given in the form
ZZZZ-NNNNNNN, where ZZZZ is the zone number, and NNNNNNN is the
record number.
The longest single file contains 1,329,904 stars, and the longest set
of 3 adjacent files contain 3,978,080 entries.
Each record contains 22 integers (4 byte). The schema is:
( 1) RA at 2000.0 in integer 0.001 arcsec
( 2) SPD at 2000.0 in integer 0.001 arcsec
( 3) std. dev. of RA*COS(dec) in integer 0.001 arcsec at central epoch
( 4) std. dev. of SPD in integer 0.001 arcsec at central epoch
( 5) proper motion of RA*COS(dec) in integer 0.0001 arcsec/year
( 6) proper motion of SPD in integer 0.0001 arcsec/year
( 7) std. dev. of (5) in integer 0.0001 arcsec/year
( 8) std. dev. of (6) in integer 0.0001 arcsec/year
( 9) central epoch of RA in integer 0.001 year
(10) central epoch of SPD in integer 0.001 year
(11) B magnitude in integer 0.001 mag
(12) V magnitude in integer 0.001 mag
(13) R magnitude in integer 0.001 mag
(14) J magnitude in integer 0.001 mag
(15) H magnitude in integer 0.001 mag
(16) K magnitude in integer 0.001 mag
(17) USNO-B1.0 ID integer
(18) 2MASS ID integer
(19) YB6 ID integer
(20) UCAC-2 ID integer
(21) Tycho2 ID integer
(22) flags integer
--- web access:
slighly different units + additional columns (if requested)
....
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4) Remarks
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- NOMAD id numbers consist of a zone number (0 to 1799 in 1/10 degree
steps of declination, starting from the south celestial pole), and a
running star number within that zone (increasing along RA).
- Positions are on the International Celestial Reference System
(ICRF), which is consistent with a "J2000 equinox". The epoch of
the positions (update for proper motions) is for J2000.
- A zero in the proper motion field indicates no significant proper
motion. Thus the given position is the same as for the original
epoch in these cases.
- Flags field: see next section
- 30.000 in any magnitude field indicates "no data".
- 30.001 = "no data" but ... see explanation from Dave (need to copy
this in here)
- bright stars: for 2 stars with negative optical magnitude photometry
was extracted directly from the Hipparcos Catalogue. The brightest
magnitude are close to -5.0 mag (IR).
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5) Flags & Statistics
---------------------
Data field (22) contains a 4-byte long flag field. In hex code,
individual bits are set corresponding to the following tables:
The combination of the three bit sets for the source of each object's
astrometry and photometry can be decoded as follows:
Table 1:
--------
hex code = flag name and explanation
---------------------------------------
0x00000001 = Astrometry source bit 0
0x00000002 = Astrometry source bit 1
0x00000004 = Astrometry source bit 2
0x00000008 = Blue photometry source bit 0
0x00000010 = Blue photometry source bit 1
0x00000020 = Blue photometry source bit 2
0x00000040 = Visual photometry source bit 0
0x00000080 = Visual photometry source bit 1
0x00000100 = Visual photometry source bit 2
0x00000200 = Red photometry source bit 0
0x00000400 = Red photometry source bit 1
0x00000800 = Red photometry source bit 2
Where the sum of each of the 3 bit sets indicates the catalogue which
was the source of the information, and the values decode as:
1 = USNOB
2 = 2MASS
3 = YB6
4 = UCAC2
5 = TYCHO2
6 = HIPPARCOS
Table 2:
--------
hex code number stars flag name and explanation
---------------------------------------------------------------------
0x00001000 469314198 UBBIT Fails Blaise's test for USNO-B1.0 star
0x00002000 123508070 TMBIT Fails Roc's test for clean 2MASS star
0x00004000 0 YBBIT (unused)
0x00008000 0 UCBIT (unused)
0x00010000 484333 TYBIT Astrometry comes from Tycho2 cat
0x00020000 2289 XRBIT Alt correlations for same (RA,Dec)
0x00040000 0 ITMBIT Alt correlations for same 2MASS ID
0x00080000 19 IUCBIT Alt correlations for same UCAC-2 ID
0x00100000 45 ITYBIT Alt correlations for same Tycho2 ID
0x00200000 45066488 OMAGBIT Blue magnitude from O (not J) plate
0x00400000 87873547 EMAGBIT Red magnitude from E (not F) plate
0x00800000 61787163 TMONLY Object found only in 2MASS cat
0x01000000 120242 HIPAST Ast from Hipparcos (not Tycho2) cat
0x02000000 18031305 SPIKE USNO-B1.0 diffraction spike bit set
0x04000000 4468 TYCONF Tycho2 confusion flag
0x08000000 1076296 BSCONF Bright star has nearby faint source
0x10000000 161579 BSART Faint source is bright star artifact
0x20000000 16117123 USEME Recommended astrometric standard
0x40000000 0 EXCAT External, non-astrometric object
0x80000000 0
The following number of stars have the corresponding astrometric (AST)
and photometric (B,V,R magnitudes) data taken from the individual
source catalogs used for NOMAD:
Source=none AST= 0 B= 205051492 V= 850210552 R= 116921500
Source=USNOB AST= 992508499 B= 675416340 V= 0 R= 999780649
Source=2MASS AST= 61787163 B= 0 V= 0 R= 0
Source= YB6 AST= 14613180 B= 234598772 V= 264856052 R= 0
Source=UCAC2 AST= 48219557 B= 0 V= 0 R= 910583
Source=Tycho AST= 364091 B= 2546128 V= 2546128 R= 0
Source=Hipp AST= 120242 B= 0 V= 0 R= 0
Source=----- AST= 0 B= 0 V= 0 R= 0
Thus all stars do have astrometric data, while not all stars have a
complete set of (optical) photometric data. Only stars identified as
2MASS stars have near IR photometry.
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6) References
-------------
AAS 205, San Diego meeting, January 2005, poster paper
Zacharias,N., Monet,D.G., Levine,S.E., Urban,S.E., Gaume,R.,
& Wycoff,G.L., 2004, BAAS, ...
Press release: Tue, Jan 11, 2005 (in conjunction with AAS meeting)
Hipparcos Catalogue:
The Hipparcos and Tycho Catalogues 1997, ESA SP-1200
Tycho-2 Catalogue:
Hoeg et al. 2000, A&A, 355, L27
UCAC2:
Zacharias, N. et al. 2004, AJ, 127, 3043
NPM:
Klemola, A.R., Jones, B.F., & Hanson, R.B. 1987, AJ, 94, 501
SPM:
Platais, I. et al. 1998, AJ, 116, 2556
YB6 = Yello-Blue all-sky catalog, version 6:
Monet, D.G. 2004, complete scan of NPM and SPM plates
2MASS:
http://www.ipac.caltech.edu/2mass/releases/allsky/
USNO-B:
Monet, D.~G.~et al.~2003, AJ, 125, 984
NOMAD:
http://www.navy.mil/nomad.html
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