A method is outlined for performing out-of-field rejection predictions using hand calculator methods and small computer programs. This method is useful in getting quick rejection estimates in the formative stages of a design, when detailed baffle design is not complete. General methods of obtaining point source scattering results in a reimaging optical system are outlined. Useful approximations are described, along with their derivation. The integration of point source values to obtain earth background values is then described.
Albert D Warren,
"Simplified Techniques For Estimating Out-Of-Field Radiation", Proc. SPIE 0257, Radiation Scattering in Optical Systems, (3 March 1981); doi: 10.1117/12.959597; https://doi.org/10.1117/12.959597