Origin of far sidelobes. The “SR spillover” (for secondary reflector spillover; the lowest set of rays on the left of the figure) arrives at the focal plane from outside the secondary mirror, directly from the sky. The “PR spillover” (for primary reflector spillover), arrives at the focal plane from above the primary mirror and reflects off the secondary to arrive at the focal plane. The set of rays between these two contributions represents the main beam. The “baffle” contribution, light reaching the focal plane after reflecting from the inner sides of the baffles, is not shown here. It is often included as part of the SR Spillover. Adopted from Tauber et al. (2010).