| Subject | English | German |
| opt. | diagonal flat mirror | diagonaler Planspiegel |
| opt. | diagonal mirror | unter 45° geneigter Spiegel |
| opt. | diagonal mirror | Sekundärspiegel |
| opt. | diagonal mirror | Diagonalspiegel (e.g. in a Newtonian reflecting telescope) |
| opt. | diagonal mirror of the telescope | Sekundärspiegel des Teleskops |