next up previous contents
Next: 8.3.4 Checks Against Other Up: 8.3 High-Dispersion Wavelength Calibration Previous: 8.3.2 Calculation of the

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).

Karen Levay