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.
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YB6: (Yellow-Blue catalog version 6) is unpublished data obtained from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
