Differential path considerations in optical stellar interferometry.

A formulation of the differential-air-path problem for a large-baseline optical interferometer is presented. Because air is a dispersive medium, each wavelength has a different optical path length. This can be corrected to a large extent if an extra piece of glass is placed in each arm of the interferometer. The problem then becomes analogous to the doublet-achromat design, in which, in this case, the air path takes the place of one piece of glass. Atmospheric refraction and field-of-view considerations also amount to a change in the differential paths within the interferometer. All three effects can therefore be considered to be aspects of the same problem. The focus of this work has been for a proposed array to be developed by the Center for High Angular Resolution Astronomy (CHARAArray), although the results are applicable to any optical array.