Cat on the hunt

Out at lunchtime with an oddity- a 600mm "solid cat" lens. Got it off ebay a while back. Its a mirror lens, although only around 100mm long uses internal mirrors to bounce the light back and forward internally, so that it's focal length is 600mm. fixed F8 aperture, so on a D3 simply use auto ISO with shutter priority to sort out the exposure. Dial in DX crop with the 1.5 mag factor and it's a 900mm. Just about hand holdable too, a really sneaky long lens. Oh- "solid" because no spaces between the glass bits, and "cat" because it's a catadioptric lens. Made by Perkin-Elmer, who did the Hubble space telescope?