  We present a simple analytical formula for the geometric concentration ratio
of a circular trough optical concentrator coupled to a flat (planar) receiver.
The receiver is sized to achieve full collection (no spillage) for any
combination of acceptance and rim angle. The development reveals the existence
of two concentration ratio regimes, one controlled by the edge ray caustics,
and the other controlled by the intersection of the edge rays from the rim
alone. Notably, we show that in the latter rim ray regime, the circular trough
achieves the exact same geometric concentration ratio as a parabolic trough.
The provided formula fills a gap towards the development of a complete
catalogue of concentration ratio formulas for different concentrator/receiver
geometries.
