How the Green Bank Telescope (GBT) works

Incoming rays are reflected first off the mirror surface and then off the subreflector. They are focused to a point at the receiver room. Feeds attached to different receivers, covering different frequency (wavelength) ranges, can be moved into position at the focal point because they sit on a rotating turntable in the receiver room.

Notice that none of the incoming rays are blocked or diffracted by anything as they are reflected on the telescope structure. This "unblocked aperture" is a unique feature of the GBT design. Note too that although oriented upright in this diagram, the GBT would actually be observing a source off to the right.

The GBT will be fully steerable. It can move by tilting, and also by rotating.