Also known as a
Scotch Mount, a '
Barn Door' is used by practitioners of
astrophotography to keep a
camera synchronised with the movement of
the night sky and prevent those pesky
star trails.
A good list of links to guides to barn door and other type of camera mount construction can be found here:
http://www98.pair.com/rfields/cgi-bin/DIY/barndoor/links.html