A fixed spherical reflector tracking absorber solar collector, popularly known as Fixed Mirror Distributed Focus (FMDF) collector, is described. An analysis has been performed to study the variations in optical concentration along the absorber. Optical concentrations have been determined for cases when the sun rays strike the reflector obliquely and have been compared with the values obtained for a noon condition when the sun rays would be normal to the reflector. An elemental ring on the reflector surface is taken up for analysis to determine the useful area of the reflector that would reflect the incident rays towards the absorber. The area of the sector of the elemental ring which illuminates the absorber is determined. The effect of the reflectivity on the optical concentration is also studied. The predicted values are indirectly compared with experimentally measured surface temperatures along the absorber of a laboratory FMDF unit. © 1986.