The `` 40-foot telescope '' proved very cumbersome , and most of his observations were done with a smaller 18.5-inch ( 47 cm ) 20-foot-focal-length ( 6.1 m ) reflector .
However , the 40-foot telescope was awkward to use , and most of his observations were done with a smaller 18.5-inch ( 47 cm ) 20-foot-focal-length ( 6.1 m ) reflector .