Obtaining profiles of attenuated backscatter coefficient from the signal-to-noise ratio measured by a pulsed coherent Doppler lidar requires knowledge of the instrument telescope focus function, which is often unknown. Here, we present a methodology for deriving the telescope focus function from horizontally-pointing pulsed coherent Doppler lidar measurements without the use of an external reference. The method relies on the assumption that the horizontal profile of attenuated backscatter coefficient is close to homogeneous, which is tested for using a convolution filter. The methodology was applied to measurements from eight co-located Doppler lidars and the results compared to a methodology for deriving the telescope focus function using vertically-pointing Doppler lidar measurements using an elastic backscatter lidar as reference. The telescope focus functions obtained with the two methodologies show good agreement even for measurement periods as short as two days.