The Dominion Astrophysical Observatory - from the south
Course of light in telescope from star to focus
For direct photography or visual observations at the focus of the 72-inch mirror, the reflected cone of star light from the mirror B, is intercepted by a plane mirror also silvered on the front surface, 19·5 inches diameter and 3·25 inches thick placed at 45°. This form of reflecting telescope was first used by Newton, hence the name. The focus is then formed, as shown, at the side of the tube, and if a plate is placed there and accurately guided by small eyepieces with cross wires, photographs of any desired small region in the sky can be obtained, or visual observations may be made. The oculars can easily be reached from the observing platform for any position of the telescope.