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Specific description of the contents of the NOMAD1 catalog. This also contains a short description of the sources used to build NOMAD, and tables for decoding the bit flags in the catalog.

The Naval Observatory Merged Astrometric Dataset (NOMAD)

README File Description of the NOMAD, version 1

Naval Observatory Merged Astrometric Dataset  (NOMAD) version 1

"readme" file,  last update: 2005 Jan 10

1) Introduction
2) Catalogs used
3) Data structure
4) Remarks
5) Flags & Statistics
6) References


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 .


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:

astrometry of fainter stars:

photometry: optical
   Tycho-2 (incl. Hipparcos stars)  

photometry: near IR


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)


4) Remarks

- 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).


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

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.


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 

  Zacharias, N. et al. 2004, AJ, 127, 3043  

  Klemola, A.R., Jones, B.F., & Hanson, R.B. 1987, AJ, 94, 501

  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 


  Monet, D.~G.~et al.~2003, AJ, 125, 984 



Last updated: 2008Dec12 by sel