4.4.3 Secondary source processing
Author(s): Jose Hernandez
The converged attitude, calibration and global parameters obtained in the primary solution were used to perform a source update on all sources which had at least one AstroElementary with valid cross matched AF transits. The solution was done performing a five- or six-parameter update (depending on the availability of an effective wavenumber for the source), and the full or covariance matrix was used to compute the formal astrometric errors and correlations. As for the primary source update all the secondary sources where treated as single stars and the reference epoch of the solution was J2016. The primary sources where also updated in the secondary update as this is easier for practical reasons and also to ensure that all sources in the catalogue were treated in the same way. The attitude used for the secondary update was already in the ICRF3 reference frame which automatically ensures that all the source positions for the secondary data set are also in the same reference frame.
As described in Section 4.3.4, a so-called six-parameter solution was used for sources that did not have reliable colour information in the form of an effective wavenumber derived from the BP and RP spectra in the spectrophotometric processing of the previous cycle (that is, for Gaia DR2). In the pre-processing (IDU) step, the image parameter determination for these sources was done using the calibrated LSF or PSF at the default wavenumber , and a correction to was estimated using the known chromaticity of the instrument. The resulting estimate of , designated , is called the pseudocolour and measured in m. In Gaia EDR3 the pseudocolour, and its uncertainty, is given along with the usual five astrometric parameters for all sources receiving a six-parameter solution. An approximate relation between or and the colour index is Equation 4.105.
Sources failing to meet the acceptance criteria for a full five- or six-parameter solution were relegated to a two-parameter solution instead (so just the position was determined), as described in Section 4.4 of Lindegren et al. (2020). The different kinds of solutions in the Archive can be distinguished using the field astrometric_params_solved which will take the values 31 (for a five-parameter solution), 95 (for a six-parameter solution), or and 3 (for a two-parameter solution).