In coaxial lenses with a half-mirror inside, it is not physically possible to insert a half-mirror larger than the diameter of the lens.
To observe a wide area, one solution is to use a planar light-source coaxial illumination system, separate from the lens, and position it along the optical axis.
The above setup uses a low-magnification lens and a 25×25 mm planar light-source coaxial light.
With this configuration, it is possible to observe the entire surface of a 1-yen coin.
