A volume of atmosphere near the tangent point is seen first in the leading field of view, then a few minutes later in the trailing field. By combining the radial components measured in the two fields, the horizontal wind vector can be obtained. A conventional interferometer with the resolving power required for wind measurements has a very small field of view and is not suitable for imaging. A plate of refractive material is placed in the long-path arm, and the position of the mirror in the other arm is adjusted so the virtual images of the mirrors in the two arms, as seen from the front of the Michelson, are coincident. The effect of this pseudo-symmetry about the beamsplitter is to produce a very large central fringe, even at a large path difference.
More extensive optimizations are possible using more than one type of glass.