| 摘要 |
The apparatus includes first and second prisms (10, 11) each of small apex angle and mounted for rotation about a common optical axis (12) passing through the centres of the prisms. A third prism (13) is made from a material having a lower refractive index and has a smaller apex angle than either of the first and second prisms, and is located for rotation about the same optical axis (12). Drive means (21, 24, 27) are provided for rotating each prism independently about the common axis (12) and pick-off means (22, 25, 28) are operable to determine the angular position of each prism relative to a datum direction. Control means (29) control the positions of each prism and ensures that the third prism is positioned so that the angular deflection which it produces is opposite to the vector sum of the deflections produced by the first and second prisms. <IMAGE> |
