2D distribution of ﹩\mathrm{tan}{z}_{a}\mathrm{sin}q=(\mathrm{cos}\delta ){\rm{\Delta }}\alpha /{\rm{\Delta }}{\bar{R}}_{45}﹩ and ﹩\mathrm{tan}{z}_{a}\mathrm{cos}q={\rm{\Delta }}\delta /{\rm{\Delta }}{\bar{R}}_{45}﹩ for one particular field in the LSST Operations Simulation Run 3.61 (Delgado et al. 2006). The distribution follows a curve due to geometric constraints. The centroid of the distribution is indicated by the red “×.”