Average KPNO Mosaic response in the UBRI passbands (solid lines). The dash‐dotted line represents the average QE of the eight Mosaic Loral CCDs. The transmission of each filter is given by the dashed lines. The total response is the combination of the filter transmission and QE. The U‐band response of the system used for our observations drops off rapidly blueward of ~3750 Å.