Photometric uncertainties of the NACO K_s and L′ observations of the Arches cluster for Fields 1 through 5 (top to bottom). The left panels show the first epoch and the middle panels the second epoch K_s uncertainty for the two epochs from which proper motions were derived. The photometric uncertainties of the first epoch are daophot PSF fitting uncertainties, and are not derived from multiple measurements. As in the case of the Quintuplet cluster, L′ photometric uncertainties (right panels) indicate the differences in performance in the L′ observations due to thermal background variations. The sensitivity limits show more consistency than in the Quintuplet, with a detection threshold of L′ ~ 14.5−15.0 mag. The larger scatter in Fields 3 and 5 is caused by background fluctuations.