# 5.1.4 Reference System

Author(s): Dafydd W. Evans

An essential aspect of the photometric data processing is the split between internal and external calibrations. The internal calibrations bring all observations onto the same reference system, while the external calibrations provide the transformation between this internal system and an absolute one that can be interpreted physically. This general principle is applied to both the flux photometry and the BP/RP spectra. The models used for the internal calibration are described in Section 5.3.5. The external calibration model is described in Section 5.3.6.

For the internal calibration of the fluxes ($G$-band, integrated BP/RP and extracted SSC fluxes, Section 5.1.3), the reference system needs to be set up. No external data is used in the generation of these reference fluxes. The reason for only using data from the satellite is that if ground-based data is used, seasonal and hemispheric systematic effects can be introduced into the system. Also, Gaia has the potential to provide data that has better uncertainties and sky coverage than any current survey.

The internal calibration is carried out in a bootstrap manner illustrated in Figure 5.4. Initially, the reference fluxes are generated for each source by accumulating all the raw (uncalibrated) fluxes and generating weighted mean values. Using these as an initial reference, calibrations are carried out. There then follows an iterative loop where the calibrations are used in accumulating calibrated fluxes to generate a better set of reference fluxes and the calibrations repeated.

This method converges since the observations for the sources have different calibrations applied to them and provided that each calibration is carried out with different sources. Given that there is good mixing between the calibrations and sources, i.e., more than half of the sources are observed in two or more configurations (CCD, Gate, FoV, …), this process should converge quickly. Conversely, if there is no mixing between sources and calibrations, multiple photometric systems could form, e.g., two sets of sources observed in different configurations, each needing their own set of calibrations, would result in two independent photometric systems.

In general, this is not the case with Gaia, but there are cases in which there is poor mixing, where additional calibrations are needed to speed up the convergence. These calibrations are referred to as link calibrations since they link the photometric systems between different configurations. The two identified link calibrations are:

• Time Link Calibration. During the early stages of the mission, Gaia observed in a configuration known as Ecliptic Pole Scanning Law (see Section 1.1.4). This was also the period when the contamination of the mirrors was at the highest level and with large dependency in time, thus creating a systematic effect in the data orders of magnitude larger than any expectations (Section 1.3.3). Including these data in the calibrations caused the early attempts at defining the reference system to show a linear trend with time in the residuals. The initial accumulation of the raw fluxes had imprinted the contamination signal into the reference system. The Time Link Calibration uses differential measurements to largely remove this signal from the raw data without the need for a large number of iterations.

• Gate/Window Class Link Calibration. The determination of the gate and window class used for an observation is made on-board using an instantaneous magnitude determination. If this is accurate and the source is constant, there will be very poor mixing since a source will almost always be observed with the same gate and window class configuration and thus be associated with the same set of calibrations. Although, the accuracy is poor (0.3–0.5 mag) at the bright end ($G$ $<12$), it is sufficiently good at fainter magnitudes to cause problems for setting up the photometric reference system. The Gate/Window Class Link Calibration uses only the sources that have been observed in several calibration units.

Further details on these link calibrations can be found in Section 5.3.3 and Section 5.3.4.

A similar scheme is applied for the instrument calibration of the BP/RP spectra in that there is an iterative loop between the instrument calibration and the source update process which creates the reference spectra.

For more details on the photometric reference system, see Carrasco et al. (2016).