Abstract: The tilted optical flat was added to the Cassegrain system to achieve the reception of visible light and the emission of infrared light at the coaxial transmission end. The Cassegrain system had an aperture of 400 mm, a focal length of 4 000 mm and a field of view of ±1.5′, which could have greater aberration after adding the tilted optical flat. In order to meet the designed requirements, the angular resolution less than 1″ and the modulation transfer function (MTF) greater than 0.6 at 35.7 lp/mm, we analyzed the aberration caused by tilted optical flat and compared the effects of plates with different materials and thicknesses on the aberration. The aberration was corrected by adding the deflected double cemented lenses, but this method made the exit pupil position changed. Hence an alternative method was proposed to fix the pupil position by adding the deflected cemented triplet lenses.Subsequently, the tolerance analysis for the added lenses was done, and finally the design target was achieved.