The zeropoint is given as the number of detected photons per second from a zero magnitude star on the alpha-Lyrae system. The throughput measurements and background estimate are here . Read the material here as well. This represents a gain in system throughput of 1.7. Z is not available at the DuPont since the tip-tilt dichrioc wavelength split is at 9000ang.
Sky brightness can vary substantially during the night and with season.
At the Dupont, Ks will be available be z shall not. Exposure Time Do not confuse exposure time and total observation time, the latter being a sum of exposure time and overheads in the telescope and instrument. Further information on this will follow.