Flat-fields in DASH interferometry.

When analyzing the fringe pattern of an interferogram to determine atmospheric wind velocities, inhomogeneities in the optical components and illumination can introduce uncertainty into the results. These variations in the image, which are generally characteristics of the measurement device, are commonly referred to as the "flat-field" of the system. In this work we discuss the effect of this flat-field on measurements made with a Doppler Asymmetric Spatial Heterodyne (DASH) spectrometer. It is found that the flat-field can have a significant effect on any single calculation of the fringe phase, but because the flat-field affects all measurements made with the same system, the uncertainty in the derived wind velocity, which is determined through a comparison of two interferogram fringe phases, typically remains small. Nonetheless, it is recommended to account for the flat-field when analyzing DASH data, if possible. To this end we discuss a method for determining the flat-field using only temperature variations of the system, which is particularly suitable for space-based instruments.