The dome that covers the 200-inch telescope weighs approximately 1000
tons, but it rotates smoothly on a series of railroad car wheels. As
the telescope moves about to point at different positions on the sky,
the dome follows automatically to keep the 'dome slit' aligned to the
telescope azimuth. This animation (Quicktime, 2.0 Mb)
shows a time-lapse sequence of the dome rotating through 180 degrees.
In reality, this rotation would take a minute or two.