We report on an ongoing study where different optical configurations for a multi-beam limb-viewing (four to eight receiver channels at 340 and two channels at 625 GHz) spaceborne instrument for climate research are presented and compared. The optical configurations are analyzed in terms of optical performance (gain, side lobe levels, beam efficiency etc.), weight and size of the overall instrument envelope. Using ideal fundamental Gaussian beam modes and numerical tools relying on ray-tracing and physical optics methods, the different configurations are designed and evaluated. Preliminary results indicate that a 1.3 m x 0.65 m primary reflector can be used in a configuration that includes a relay optics system having two to four elements. In addition to the limb-viewing instrument, there will be an additional instrument operating at 640 GHz for observing clouds in nadir mode.