As of 2018 Celestron seems to have replaced the optical system of the diagonal with a mirror. It is a very decent mirror for the price. The body is of quite rugged plastic and the connectors are metal. The diagonal is well collimated and shows no distortions.
It is not a prism, though. Prisms can be preferable for planetary viewing with high f/ achromats, as they have high transmitivity (>99.5%) which is comparable to the best dielectrics, yet show no scattering.
Mirrors such as this are standard equipment in faster instruments. This is a nice, well built accessory.