# 14.1.2 ruwe

This table contains the Renormalised Unit Weight Error (RUWE) associated to each source in gaia_source.
The RUWE is expected to be around 1.0 for sources where the single-star model provides a good fit to the astrometric observations. A value significantly greater than 1.0 (say, >1.4) could indicate that the source is non-single or otherwise problematic for the astrometric solution.
The desciption of how this parameter is calculated is described in the document ”Re-normalising the astrometric chi-square in Gaia DR2”, which can be downloaded from:
https://www.cosmos.esa.int/web/gaia/public-dpac-documents

Columns description:

source_id : Unique source identifier (unique within a particular Data Release) (long)

A unique numerical identifier of the source, encoding the approximate position of the source (roughly to the nearest arcmin), the provenance (data processing centre where it was created), a running number, and a component number.

The approximate equatorial (ICRS) position is encoded using the nested HEALPix scheme at level 12 (Nside = 4096), which divides the sky into $\simeq 200$ million pixels of about 0.7 arcmin${}^{2}$.

The source ID consists of a 64-bit integer, least significant bit = 1 and most significant bit = 64, comprising:

• a HEALPix index number (sky pixel) in bits 36 - 63; by definition the smallest HEALPix index number is zero.

• a 3-bit Data Processing Centre code in bits 33 - 35; for example MOD(source_id / 4294967296, 8) can be used to distinguish between sources initialised via the Initial Gaia Source List by the Torino DPC (code = 0) and sources otherwise detected and assigned by Gaia observations (code $>0$)

• a 25-bit plus 7 bit sequence number within the HEALPix pixel in bits 1 - 32 split into:

• a 25 bit running number in bits 8 - 32; the running numbers are defined to be positive, i.e. never zero

• a 7-bit component number in bits 1 - 7

This means that the HEALpix index level 12 of a given source is contained in the most significant bits. HEALpix index of 12 and lower levels can thus be retrieved as follows:

• HEALpix level 12 = source_id / 34359738368

• HEALpix level 11 = source_id / 137438953472

• HEALpix level 10 = source_id / 549755813888

• HEALpix level n = source_id / $2^{35}\times 4^{(12-{\rm level})}$

Additional details can be found in the Gaia DPAC public document Source Identifiers — Assignment and Usage throughout DPAC (document code GAIA–C3–TN–ARI–BAS–020) available from https://www.cosmos.esa.int/web/gaia/public-dpac-documents

ruwe : Renormalised unit weight error (float)

The Renormalised Unit Weight Error is computed as

 ${\tt ruwe}=\frac{\sqrt{{\tt chi2}(AL)/({\tt nObs}(AL)-m)}}{f(G,G_{\text{BP}}-G% _{\text{RP}})}$

where $m$ is the number of parameters solved (the number of set bits in paramsSolved) and $f$ is a renormalising function.

In practice $f$ is determined in an off-line statistical analysis of the secondary solutions (cf. described in “Re-normalising the astrometric chi-square in Gaia DR2”) and provided as a look-up table.

Please note that this value is set to null for sources with only a two-parameter solution, since this value would be difficult to interpret in such cases.