We quantify the performance of the LSST pipeline for processing crowded fields, using images obtained from DECam and comparing to a specialized crowded field analysis performed as part of the DECAPS survey.
Considering single-visit LSST depth, in an example field of roughly the highest density seen in the LSST Wide-Fast-Deep area, DECAPS detects 200 000 sources per sq. deg. to a limiting depth of 23rd magnitude. At this source density the mean LSST-DECAPS completeness between 18th and 20th mag is 80%, and it drops to 50% at 21.5 mag.
For fields inside the Galactic plane cadence zone, source density rapidly increases. For instance, in a field in which DECAPS detects 500 000 sources per sq. deg. (5σ depth of 23.2), the mean completeness between 18th and 20th mag is 78%, and it drops to 50% at 20.2 mag.
In terms of photometric repeatability, above 19th mag LSST and DECAPS are in a systematics-dominated regime, and there is only a slow dependence on source density. At fainter magnitudes, the scatter between LSST and DECAPS is less than the uncertainty from photon noise for source densities up to 100 000 per sq. deg, but the scatter grows to twice the photon noise at densities of 300 000 per sq. deg. and above.
For repeat measurements of the same field with LSST, the astrometric scatter per source is at the level of 10-30 milliarcseconds for bright stars (g < 19), and is not strongly dependent on stellar crowdedness.