A device that uses two movable mirrors to reflect sunlight along a particular
path – for instance, down a fixed telescope tube – so that the
image at the other end of the telescope doesn't appear to move or rotate
when the Sun moves across the sky. One of the
mirrors rotates around an axis parallel to Earth's axis of rotation, at
half the solar rate. The orientation of the other mirror is adjustable in
all directions. The coelostat was invented by Gabriel Lippmann.
| The coelostat at Mt. Wilson Observatory,
A coelostat (pronounced "seelostat") is similar to a heliostat,
but has a more complex design and, unlike a heliostat, gives an image in
a fixed orientation.