<title>APT: an astrometric and photometric telescope</title>

Abstract

Helioseismic and precise solar photometric measurements reveal that the Sun varies globally as a start during the source of an 11 year solar cycle. To understand the physical mechanisms of the magnetic cycle in the solar interior we must learn how to measure the tiny changes in the Sun's global properties, like its radius, internal temperature distribution and surface luminosity. The SoHO/MDI experimental has proven that exceedingly small solar shape fluctuations are measurable from outside our atmosphere. We describe here an instrument which will not only measure limb shape oscillations with unprecedented accuracy, but it will also detect solar radius changes with heretofore unachieved accuracy and precision. Variations in these parameters are caused by physical changes, both in the photosphere and the deep solar interior. Solar radius and shape observations will teach us how the Sun's convective envelope responds to emergent energy fluctuations. The determination of this outer boundary condition is essential to understand the solar total irradiance and luminosity variations.