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gaia early data release 3 documentation

6.5 Quality assessment and validation

6.5.1 Verification

Verification of the processing steps

The verification of the processing steps is done automatically via the Automated Verification workflows in charge of producing diagnostic plots which permit to verify the results of the associated processing workflow (Figure 6.2).

The verification of the Ingestion workflow (AVPP) consists mostly on the monitoring, per calibration unit, of the number and of some properties of SpectroObservations entering the pipeline after the filters applied in Section 6.2.1 (each SpectroObservation corresponds to a transit and contains 3 spectra of the star, one per CCD). The total number of SpectroObservations with 1D windows is 91 255 288 and with 2D windows is 1 677 114, corresponding to a total number of spectra of 278 797 206.

The verification of the Calibration workflow (AVCAL) mostly consists in counting the calibration stars and the standard stars used in each calibration unit to verify they are in sufficient number. The trending plots of the calibration models obtained are also provided for visual inspection (Figure 6.7).

Figure 6.7: Wavelength calibration model: the trending function of the coefficient C00, representing the wavelength-calibration zero point, is shown for FoV0 (black line) on the left and for FoV1 (black line) on the right, and for the CCD in row 5 strip 16. The trending functions are the final results of the wavelength calibration and are used to calibrate all the spectra acquired at any time and at any position. The time scale, on the abscissa, is expressed in OBMT. The blue points are the values of C00 obtained at each calibration unit. The most important discontinuities are due to optics decontamination, indicated by the red arrows, at OBMT 1317 and 2330.6, and to the re-focusing events, indicated by the blue arrows, at 1443.9 and 2574.6. The trending epochs described in Figure 6.1 are indicated. Part of this figure was also presented in Sartoretti et al. (2018).

The verification of the FullExtraction workflow (AVFE, similar to AVEXT) mostly consists of counting the total number and plotting some of the spectra that have been rejected from the data flow because of bad quality (i.e. one of the reasons in Section 6.4.4); The total number of rejected spectra is approximately 29 million, the large majority (25 million) is excluded because of negative flux, due to the fact that the straylight background was overestimated. Automated Verification also counts the number of cosmic rays and saturated samples detected and the number of spectra per various transit properties.

There are various automated diagnostics in charge of the verification of the STAMTA workflow (AVSTA and AVMTA). The good quality of the final results is also an indirect proof of the correct behaviour of the upstream workflows. The STA and MTA results are compared with external radial velocity catalogues obtained with ground-based observations (see Sartoretti et al. 2018).

Ground-based radial velocity catalogues used for validation

The radial velocity catalogues used for validation are listed in Table 6.3, where the stars expected to be stable have been selected.

Table 6.3: The external radial velocity catalogues.
Cat Name nb stars nb transits σ⁢(Vrad) pipeline use
CU6GB-cal 2568 34630 <0.1 calibration
XHip 5729 83366 <1.0 calibration
CU6GB-val 1819 16819 <0.3 validation
RAVE 9542 86035 <1.5 validation
APOGEE 8584 76177 <0.5 validation
SIM 650 4885 <0.1 validation
GES 187 1233 <0.24 validation
Notes. The number of stars (nb stars) per catalogue that have been processed in the pipeline is listed. The total number of valid observations (nb transits) per catalogue is also listed. The first two catalogues are used also in wavelength calibration and are described in Section 6.2.3. The other catalogues are used to verify the RVS pipeline results. σ⁢(Vrad), in  km s-1, is the uncertainty associated to the radial velocity measurements provided by each catalogue. Except for GES, where only single observations are available, the stars selected are those not showing radial velocity variability. The catalogue CU6GB is described in Soubiran et al. (2018).