1. A multi-spectral weapon sight optical system, comprising: a first catadioptric optical system having a first optical axis and comprising a first primary concave mirror that substantially reflects first radiation having a first wavelength band and that substantially transmits second radiation having a second wavelength band different from the first wavelength band, the first catadioptric optical system forming a first image at a first image plane on the first optical axis using the first radiation; and a second catadioptric optical system having a second optical axis and arranged optically downstream from and in line with the first catadioptric optical system, with the second optical axis being coaxial with the first optical axis, the second catadioptric optical system comprising a second primary concave mirror that receives and reflects the transmitted second radiation and that forms from the transmitted second radiation a second image at a second image plane on the optical axis using the second radiation.