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The Naval Observatory Merged Astrometric Dataset (NOMAD)

For a basic description of the NOMAD, see the ReadMe file.

DESCRIPTION OF THE FIELDS IN THE NOMAD1 CATALOGUE:

The ASCII version of the extracted subset of the NOMAD1 catalogue contains some (or all) of the following fields:

1) id -
identification number. This number is unique for object in the catalogue and is of the form ZZZZ-NNNNNNN. ZZZZ is the 4 digit SPD zone number that the object is in. NNNNNNN is the record number of the object in that zone file.
2) altid -
alternate identification number(s). Identification indices from the individual catalogues that went into NOMAD. These include USNO-B1, 2MASS, YB6, UCAC2, Tycho2 and Hipparcos. The index is just the record number in the catalogue, and NOT the id field from that catalogue. The one exception is that Hipparcos stars are listed by their official Hipparcos IDs (they will show up in the Tycho2 column). To distinguish a Hipparcos ID from a Tycho record number it will be necessary to check the flagbits.
3) RA -
the Right Ascension of the object in hours or degrees. The epoch and equinox are those given in the file header. The default is equinox and epoch J2000.0.
4) DEC -
the Declination of the object in degrees. The epoch and equinox are those given in the file header. The default is equinox and epoch J2000.0.
5) sra -
the error in the RA position. This is given in integer milli-arcseconds. The error is given at the RA Mean Epoch.
6) sde -
the error in the DEC position. This is given in integer milli-arcseconds. The error is given at the DEC Mean Epoch.
7) sRaEp -
the error in the RA position at the epoch of the given catalogue position. This is given in integer milli-arcseconds.
8) sDeEp -
the error in the DEC position at the epoch of the given catalogue position. This is given in integer milli-arcseconds.
9) MuRa -
the proper motion in RA. This is given in angular velocity units of milli-arcseconds per year. [This equals MuRA = d(RA)/dt x cos(DEC)]
10) MuDec -
the proper motion in DEC. This is given in angular velocity units of milli-arcseconds per year.
11) sMuRA -
the error in the proper motion in RA. Given in angular velocity units of milli-arcseconds per year.
12) sMuDE -
the error in the proper motion in DEC. Given in angular velocity units of milli-arcseconds per year.
13) RA Epoch -
this is the mean epoch of all the observations that went into determining the RA position of this object. This is given in decimal years.
14) DEC Epoch -
this is the mean epoch of all the observations that went into determining the DEC position of this object. This is given in decimal years.
15) Flags -
this column merges many bits of information. They are indicated as individual bits in this integer. N.B: The value list in this column is given in HEXIDECIMAL. The first 12 bits are divided up into sets of 3 bits to indicate the source of the astrometry and photometry of the entry. The combination of the three bit sets for astrometry and photometry can be decoded as follows:
	   1 = USNOB
	   2 = 2MASS
	   3 = YB6
	   4 = UCAC2
	   5 = TYCHO2
	   6 = HIPPARCOS

	   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
The bits that follow are all single bit flags:
	   0x00001000 = UBBIT   Fails Blaise's test for USNO-B1.0 star
	   0x00002000 = TMBIT   Fails Roc's test for clean 2MASS source
	   0x00004000 = YBBIT   (unused)
	   0x00008000 = UCBIT   (unused)
	   0x00010000 = TYBIT   Astrometry comes from Tycho2 cat
	   0x00020000 = XRBIT   Alt correlations for same (RA,Dec)
	   0x00040000 = ITMBIT  Alt correlations for same 2MASS ID
	   0x00080000 = IUCBIT  Alt correlations for same UCAC-2 ID
	   0x00100000 = ITYBIT  Alt correlations for same Tycho2 ID
	   0x00200000 = OMAGBIT Blue magnitude from O (not J) plate
	   0x00400000 = EMAGBIT Red magnitude from E (not F) plate
	   0x00800000 = TMONLY  Object found only in 2MASS cat
	   0x01000000 = HIPAST  Ast from Hipparcos (not Tycho2) cat
	   0x02000000 = SPIKE   USNO-B1.0 diffraction spike bit set
	   0x04000000 = TYCONF  Tycho2 confusion flag
	   0x08000000 = BSCONF  Bright star has nearby faint source
	   0x10000000 = BSART   Faint source is bright star artifact
	   0x20000000 = USEME   Recommended astrometric standard
	   0x40000000 = EXCAT   External, non-astrometric object
	   0x80000000 =         (unused)
16) B Magnitude -
Blue magnitude. This is some sort of B magnitude. It is necessary to check the blue photometry source bits in the flag to know exactly what kind of B magnitude. In order of decreasing brightness, the B magnitude comes from Tycho-2, YB6 and USNO-B. If it is from Tycho, then it is the Tycho BT magnitude. From YB6 or USNO-B1, then it is a photographic B magnitude. The YB6 magnitude is derived from the Lick and Yale/El Leoncito blue astrograph plates. The USNO-B magnitude is derived from the POSS-II IIIaJ, SERC-EJ, and SERC-J plates. The valid magnitudes should span 0 to 22 magnitude. The formal valid range is 0.00 to 99.99. A value of 30.00 indicates that NO B magnitude was available for this object.
17) V Magnitude -
Visual magnitude. This is some sort of V magnitude. It is necessary to check the visual photometry source bits in the flag to know exactly what kind of V magnitude. In order of decreasing brightness, the V magnitude comes from Tycho-2, and YB6. If it is from Tycho, then it is the Tycho VT magnitude. From YB6, then it is a photographic V magnitude. The YB6 magnitude is derived from the Lick and Yale/El Leoncito yellow astrograph plates. The valid magnitudes should span 0 to 22 magnitude. The formal valid range is 0.00 to 99.99. A value of 30.00 indicates that NO V magnitude was available for this object.
18) R Magnitude -
Red magnitude. This is some sort of R magnitude. It is necessary to check the red photometry source bits in the flag to know exactly what kind of R magnitude. For almost everything that has an R magnitude, it is the PHOTOGRAPHIC R magnitude from USNO-B1. The USNO-B magnitude is derived from the POSS-II IIIaF, SERC-ER and AAO-R plates. In a most number of cases, the UCAC-2 magnitude has been used. This is a very broad band red CCD magnitude. The valid magnitudes should span 0 to 22 magnitude. The formal valid range is 0.00 to 99.99. A value of 30.00 indicates that NO R magnitude was available for this object.
19) J Magnitude -
Near IR J magnitude. This is the J magnitude from the 2MASS survey. The valid magnitudes should span -5 to 16 magnitude. The formal valid range is 0.00 to 99.99. A value of 30.00 indicates that NO J magnitude was available for this object.
20) H Magnitude -
Near IR H magnitude. This is the H magnitude from the 2MASS survey. The valid magnitudes should span -5 to 16 magnitude. The formal valid range is 0.00 to 99.99. A value of 30.00 indicates that NO H magnitude was available for this object.
21) Ks Magnitude -
Near IR Ks magnitude. This is the K magnitude from the 2MASS survey. The valid magnitudes should span -5 to 16 magnitude. The formal valid range is 0.00 to 99.99. A value of 30.00 indicates that NO K magnitude was available for this object.
22) Xi (computed quantity) -
the standard coordinate Xi, in the tangent plane projection about the requested region center. This is only computed for rectangular or circular regions. The units are decimal arcseconds.
23) Eta (computed quantity) -
the standard coordinate Eta, in the tangent plane projection about the requested region center. This is only computed for rectangular or circular regions. The units are decimal arcseconds.
24) DistCtr (computed quantity) -
distance from this object to the extraction center in arcseconds.
25) Gal_L (computed quantity) -
computed galactic longitude, in decimal degrees.
26) Gal_B (computed quantity) -
computed galactic latitude, in decimal degrees.

Comments, questions, etc., write to S. Levine
Last updated: 08dec2009 by sel