LSST Crowded Fields photometry

Abstract

A report on the performance of current LSST Stack pipelines in crowded stellar fields. We use the DECAPS data to define the photometry and astrometry quality assurance metrics.

In the top 10% region, where DECAPS detects 200 000 sources per sq.deg., the mean LSST-DECAPS completeness in 18-20 mag is 80%, and it drops to 50% at 21.5 mag. For the same visit, the DECAPS 5σ limiting depth is 23 mag.

For a top 2% region, within the exclusion zone, in which DECAPS detects 500 000 sources per sq.deg., the mean completeness in 18-20 mag of LSST to DECAPS source-by-source is 78%, and it drops to 50% at 20.2 mag. For the same visit, the DECAPS 5σ limiting depth is 23.2 mag.

The systematic offset in photometry (the difference between the median photometric uncertainty and the measure of internal photomtric repeatability) at 21 mag for the density of 200 000 sources per sq.deg. is 0.06 mag.

The LSST photometry is consistent with DECAPS. Above 19th mag, LSST and DECAPS are in systematics-dominated regime, consistent at 0.02 mag level. At fainter magnitudes, the scatter between LSST and DECAPS is less than the photometric uncertainty.

The spread of astrometric repeatability for LSST epoch-to-epoch is at the level of 10-30 miliarcsec, and is not strongly dependent on stellar crowdedness.