The 21 cm hydrogen line was discovered during the telescope's construction; the telescope was subsequently redesigned so that it could observe at that frequency. Using this line emission, hydrogen clouds both in the Milky Way galaxy and in other galaxies can be observed; for example, the telescope discovered a large cloud around the M81 and M82 galaxies. The motion of these clouds either towards or away from us either redshifts or blueshifts the line, allowing the velocity to the cloud to be measured. This provides a probe of the internal dynamics of galaxies, and can also provide a measurement of the rate of expansion of the universe.
