# 5.1 Introduction

Author(s): Giorgia Busso

With respect to Gaia DR2, the photometric catalogue in Gaia EDR3 will be similar in content but of significantly higher quality. Photometric data in the $G$ broad band and integrated $G_{\rm BP}$ and $G_{\rm RP}$ bands from the Blue and Red Photometers will be provided. The better photometric quality is due to several factors:

• the increase in the amount of data used to obtain the mean photometry from 22 to 34 months;

• the significant improvements in the AGIS (see Section 4.4.2) and IDU (see Section 3.4.2) processing, especially the use of a PSF/LSF model depending on time, colour, AC motion (see Section 3.3.5);

• the improvements in the processing of the BP/RP spectra, including a higher-resolution background treatment (see Section 5.3.1) and the evaluation of the crowding level of all transits (used to filter out contaminated data from the calibration process, see Section 5.3.2);

• the adoption of a selection of sources to initialise the photometric calibration (see Section 5.4.1);

• the use of the Spectral Shape Coefficients (see Section 5.1.1) calculated directly from calibrated spectra, allowing an overall more stable photometric system.

A direct comparison of the distribution of the uncertainties between Gaia DR2 and Gaia EDR3 (see Section 5.4.2 clearly shows the higher quality of the Gaia EDR3 photometry. Users are discouraged from doing per-source comparisons between the two catalogues. It is difficult to extract useful information from this sort of analysis due to the different data that may have contributed to the mean photometry associated to a given sourceId and to the many changes occurred in the processing of the satellite observations in the latest processing cycle. Furthermore the externally calibrated photometry in the two releases refer to different passbands.

Figure 5.1 shows an overview of the photometry and low-resolution spectroscopy processing, the elements of which are further described in the following sections.