8.3.3 Time and THDA Dependence of the Wavelength Zeropoint

  Once the mean dispersion solutions were derived from typically several hundred images, the wavelength zeropoint motion was determined as a function of time and THDA. Mean time- and THDA-dependent coefficients were determined, as in low-dispersion, permitting predicted zeropoint shifts (one value for each order of an image) to be evaluated from these relations. Typically fourth-order polynomials are used to represent the variation in the wavelength system as a function of time, while the THDA dependence is represented by a linear function. The time- and THDA-predicted zeropoint shifts in the wavelength direction are applied to every image. The time- and THDA-predicted zeropoint shifts in the spatial direction are applied only in cases where empirically-measured registration shifts cannot be successfully determined by the order registration algorithm within the RAW_SCREEN function (see Chapter  4.9).