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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).

*Karen Levay*

*12/4/1997*