Note: U and BVRI (Harris set) and ugriz (Sloan) are calibrated from Landolt standards in SA93 and SA95 that we observed on 12/01/05. We will add calibrations for other filters upon request.
Selecting a filter from the pull-down menu automatically fills in the appropriate sky counts and magnitude zero-point. Or, you can enter them manually if your favorite is not among the choices. You set the aperture size -- roughly double the expected FWHM should be close to optimal. Then set two from the bottom set of parameters and click compute to get the third.