In a parallel paper, we have outlined a methodology for assessing the comparative scientific performance of large basic research facilities (and their associated user groups) working in the same specialty, and applied this method of `converging partial indicators' to an evaluation of the contributions to science made by a number of radio telescopes. In this paper, we employ this methodology to evaluate the scientific performance of various optical telescopes — in particular, the 2.5-metre Isaac Newton Telescope, operated as a central facility by the Royal Greenwich Observatory in South-East England. For several years, this was Britain's only major optical telescope, as well as being the largest such instrument in Europe. We compare its performance over the last decade with that of three American telescopes of similar size. This paper has three aims: first, to ascertain whether the method of converging partial indicators, originally applied to radio astronomy, provides a more general policy tool that can be extended to other specialties; second, to determine just how successful each optical telescope has been in producing new astronomical knowledge over the past decade; and, third, to discuss whether our results on the comparative scientific performance of the Isaac Newton Telescope may have any implications for British astronomy policy in general.