In order to decrease the levelized cost of energy (LCOE) of solar power towers, efficiencies of all components have to be analyzed and improved. Among them is receiver absorptivity, which depends on the angle of incidence of solar radiation. While cylindrical receivers consist of arrays of vertical tubes, they are usually represented as flat surfaces for the optical part of simulations, imposing different incidence angles. Ray tracing and analytical algorithms for both primary and secondary absorption are used to compare absorptivity values for a detailed tube geometry with the simplified approach. Moreover the effect of horizontal aim point shifting on absorptivity is investigated. Results show that assuming flat surfaces leads to an underprediction of absorptivity of several tenths of a percentage point. A smaller overprediction of the same order of magnitude is reached for assuming entirely perpendicular incidence. Furthermore it is found that spillage losses due to horizontal aim point shifting exceed the effect of implied lower incidence angles by more than one order of magnitude.