Selecting a filter & instrument combination from the pull-down menu automatically fills in the appropriate pixel size, read noise, sky counts, and magnitude zero-point. Or, you can enter them manually if your favorite is not among the choices. For point sources you set the aperture size -- roughly 1.3 times the FWHM should be close to optimal. Then set two of the bottom set of parameters and click compute to get the third. We assume magnitudes on the 2MASS (Vega) system; for AB mags you must add 0.6 in Y, 0.9 in J, 1.4 in H, and 1.9 in K.