The S functions described above apply only to point
source observations acquired through the large aperture. The
small-aperture and trailed observing modes are known to have different
relative responses as compared to large-aperture point spectra (e.g.,
Harris and Cassatella 1985, Bohlin 1986, Crenshaw and Park 1989). This
effect has been ascribed to several sources, including vignetting
effects of the entrance apertures in trailed and small-aperture spectra
and spatial inhomogeneities in the UVC efficiency (Cassatella 1990).
Application of the large-aperture point source calibration to spectra
obtained either in trail mode or through the small aperture will
introduce photometric errors of at least 5-7% in the regions near the
detector edge and of 1-2% in regions where the camera response is
flatter.
Calibration corrections for small-aperture observations and for large-aperture trailed observations have been derived and are applied by the processing system when necessary. IUE standard-star observations from the 1984-1985 epoch have been used to calculate the wavelength-dependent flux ratios of small- and large-aperture point sources, and of large-aperture point source and trail mode observations. Observations from the 1984-1985 epoch were chosen so as to match the large-aperture point source calibration epoch. Also, by limiting the range of time over which the observations were obtained, effects due to sensitivity degradation in the cameras are minimized.
Optimal exposure level observations of the four TD1 standard stars
BD+28 4211, HD 93521, HD 60753, and BD+75
325 were used to
compute the small-to-large aperture (S/L) ratios and the large-aperture
trailed-to-point source (T/L) ratios. The effective exposure times of
the trailed observations were determined using a value of 21.48
arcseconds for the SWP major-axis trail path length (Garhart 1992b),
22.55 arcseconds for the LWR major-axis trail path length, and 21.84
arcseconds for the LWP major-axis trail path length. The ratios of pairs
of observations of the same object obtained through the small and large
apertures, or in point and trail modes were averaged together to
determine the mean S/L and T/L spectral ratios. Approximately 20 pairs
of spectra were used to determine each of the two response ratios. The
mean spectral ratios were resampled into the bin size of the appropriate
inverse sensitivity function and a spline fit to the binned ratios was
calculated. Tabulated values of the spline fits for each camera are
listed in Tables 11.5-11.8.
Because centering errors in the small aperture can lead to large variations in the overall observed flux level for individual spectra, it is impossible to determine an absolute S/L ratio. Therefore, the average of S/L over all wavelengths is normalized to unity. As a result, the small-aperture fluxes are known in a relative sense but not in an absolute one. The relative small- and absolute large-aperture inverse sensitivities are related by