Some astronomical performance advantages of off-axis telescopes

Measurements requiring very high photometric dynamic range, like astronomic al observations of faint objects near bright sources (e.g., extrasolar plan et detection), are often limited by the scattered light characteristics of the telescope. Although the light-gathering power of recently built telesco pes has increased dramatically, their scattered light performance has not. We compare models and measurements of telescope scattered light and discuss some of the scientific and technical issues that suggest how a low scatter ed light design could extend the scientific capabilities of moderate (4 m a perture) telescopes.