The dispersion coefficients as actually applied to science images are
determined by adding to the mean zeropoint terms corrections appropriate
for the observation date and THDA of the particular image. The
correction terms and *W*_{s}, which represent the offset
that is added to the mean wavelength and spatial zeropoints, are defined
by the following general expressions:

*W*_{s} = *W*_{1s} + *W*_{2s}*t* + *W*_{3s}*T* + *W*_{4s}*t ^{2}* +

A first-order (i.e., linear) fit is sufficient to characterize the
correlation between THDA and zeropoint for all cameras. The correlation
between time and zeropoint, however, has second- and third-order
dependencies for some cameras. The and *W*_{s} coefficients
in use for the low-dispersion mode for pre-1990 LWP and SWP images are
listed in Table 8.2. Coefficients appropriate for more recent
LWP and SWP images are discussed in Chapter 8.2.3. The
database of exposures used to generate the coefficients in
Table 8.2 include WAVECAL images obtained through mid-1993 for
the LWP and mid-1991 for the SWP. The respective coefficients for the
LWR camera are listed in Table 8.3. In the case of the LWR,
a separate set of coefficients are tabulated for the ITF A and ITF B
calibrations. In addition, each ITF has an early and late epoch. This is
due to the fact that the wavelength zeropoint shift was dramatically
different between these two epochs. The cutoff date for the early and
late epochs is 1980.1 for ITF A and 1979.9 for ITF B. The shift of the
spectrum center does not exhibit this trend, so the spatial coefficients
are identical for each epoch. The zeropoint shifts and corresponding
polynomial fits are illustrated graphically for each camera in
Figures 8.2-8.5.