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gaia early data release 3 documentation

5.3 BP/RP spectroscopic processing

5.3.2 Crowding Evaluation

Author(s): Francesca De Angeli

In Cycle 03 for the first time a module assessing the crowding status of each transit was introduced. This was based on the pre-computation of predicted observing time and AC coordinate in the focal plane for all objects in the source catalogue based on the satellite attitude as provide by AGIS and covering the entire range covered by the Cycle 03 processing. Of course this included predicted observations for both FoVs. This is called the scene, and each predicted observation is called scene item.

For each transit then the scene was interrogated to assess whether some of the surrounding scene items were blending or contaminating the current transit. We define blending when two or more sources are inside the same window, and contamination when a transit is affected by the flux from another source outside the window. The blending assessment is simply based on the relative distance between the transit window and the scene items: for Gaia EDR3 we consider a window slightly bigger than the acquired BP/RP window, adding 5 pixels each side in the AL direction and 2 pixels each side in the AC direction. The contamination assessment instead requires some knowledge of the distribution of the contaminating light surrounding a source: if the contamination flux is bigger than 1% of the total flux, then the transit is flagged as contaminated.

Black-listed transits around bright sources (transits that were not cross-matched to any source because believed to be spurious detections in the surroundings of bright objects) were used to characterise the AL and AC profiles of the contamination. The AL profile turned out to be quite complicated and was fitted using splines, while in the AC direction a linear interpolation in magnitude was deemed sufficient.