  High throughput optical system is defined to possess the features of both
large field of view (FOV) and high resolution. However, it is full of challenge
to design such a telescope with the two conflicting specifications at the same
time. In this paper, we propose a method to design a high throughput telescope
based on the classical off-axis Three-Mirror Anastigmat (TMA) configuration by
introducing a scanning mechanism. We derive the optimum initial design for the
TMA system with no primary aberrations through characteristic ray tracing.
During the design process, a real exit pupil is necessitated to accommodate the
scanning mirror. By gradually increasing the system's FOV during the
optimization procedure, we finally obtained a high throughput telescope design
with an F-number of 6, a FOV of 60$^{\circ}$*1.5$^{\circ}$, and a long focal
length of 876mm. In addition, the tolerance analysis is also conducted to
demonstrate the instrumentation feasibility. We believe that this kind of large
rectangle FOV telescope with high resolution has broad future applications in
the optical remote sensing field.
