Edwin Hubble in the observer's cage at the top of the tube of the 100-inch telescope on Palomar Mountain. Astronomers can sit in the cage wih thier photographic plates to collect light reflected up from the main mirror 55 feet below. Alternatively, collected light can be directed out of the side of the telescope to a more convenient place near the bottom, but with loss of light for each additional reflection by a mirror redirecting the light.

Image courtesy Mount Wilson and Palomar Observatories.