Details of ongoing changes in geomorphology and structure of nested volcanic craters are lacking due to the difficult and hazardous access. In this study, we present a novel unoccupied aircraft system (UAS) dataset collected at Lascar, northern Chile, one of the most active volcanoes in the Central Volcanic Zone of the Andes. Lascar features nested craters, the deepest crater of which has experienced repeated lava dome emplacement and numerous violent explosions in recent decades. We performed UAS surveys in 2017 and 2020 to collect the optical and thermal imaging data of the active crater. By applying the Structure-from-Motion (SfM) method, we obtained centimeter-scale optical and thermal orthomosaics as well as digital terrain models. We quantify the spatial and volumetric changes that occurred during the observation period. The results show material removal from the crater wall and a significant accumulation of volcanic material on the crater floor. The thermal orthomosaic helps identify thermal anomalies and the spatial distribution of fumaroles. The highest thermal anomaly was found on the crater floor and delimited by ring structures. Both the optical orthomosaics and DEMs from 2017 and 2020 showed a consistent ring structure, which remained stable following two explosive events. Possible localized subsidence was observed within the ring structure. Consequently, we hypothesize that the ring structure represents the surface expression of underlying concentric fracture systems and constrains a narrow conduit top. Our results provide important insights into an active crater's morphological, structural, and thermal features, with implications for understanding the formation mechanism and evolution of volcanic craters.