Example of a difference image and the outcome of our candidacy tests for some examples of the detected sources. Left panel: Typical difference image produced by TUVOpipe (science image: ObsID 00034748064, extension 2, u filter; template image: ObsID 00035028027, extension 1, u filter). In this difference image, 38 sources were detected. Only one passed all three tests (Sect. 3.3.3) and was retained as a candidate transient (source d – this is a real transient, the BL Lac prototype system, which is often monitored by Swift). The labels indicate the example sources displayed in the right panel. Right panel: examples of candidate transients detected in the difference image that did not pass one of the tests (a−c), and the only source that passed all tests (d). The elliptical regions are the source regions extracted by the source detection software (in this case uvotdetect).