I acquired a Sigma 600mm mirror lens this weekend. I don't know if I will give it much use but it looks like a fun thing to own. Has anyone else got such a thing or has used one in the past? It comes with a set of rear mount filters and is currently fitted with a clear 'filter'. I recall reading that it needs a clear piece of glass in the filter position if no other filter is being used in order to maintain focus. Is this correct? Steve.