  We present a concept of a millimeter wavefront sensor that allows real-time
sensing of the surface of a ground-based millimeter/submillimeter telescope. It
is becoming important for ground-based millimeter/submillimeter astronomy to
make telescopes larger with keeping their surface accurate. To establish
`millimetric adaptive optics (MAO)' that instantaneously corrects the wavefront
degradation induced by deformation of telescope optics, our wavefront sensor
based on radio interferometry measures changes in excess path lengths from
characteristic positions on the primary mirror surface to the focal plane. This
plays a fundamental role in planned 50-m class submillimeter telescopes such as
LST and AtLAST.
