DESHIMA is a superconducting on-chip spectrometer using MKIDs in the sub-mm wavelength regime (240-720 GHz, 1:3 bandwidth). This thesis presents (1) an analysis of the quasi-optical system used for DESHIMA on the ASTE telescope in Chile (2) design of the room-temperature optics of this design and (3) a preliminary investigation into an ultra-wideband leaky-lens antenna suitable for multi-pixel, constant aperture efficiency operation to be used as a feed to high f-number (>2) reflectors. The designed and analyzed optics show good performance and tolerance for the fall 2017 campaign of DESHIMA-on-ASTE. The wideband investigation leads to constant, >70% optical efficiency over the 1:3 bandwidth for a single pixel design (f-num=3.7) or >55% optical efficiency with multiple pixels spaced at 2*\lambda*f-number (f-num=5). Future work must be done to optimize the radiation efficiency of the antenna over the whole bandwidth, which is now the largest obstacle to frequency-dispersion free aperture efficiency.