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Wavelength Scale

The wavelength scale was determined using known positions of narrow emission lines in the symbiotic star spectra listed above. Aside from small offsets necessary to correct for telescope pointing errors along the dispersion direction, the results for different objects proved consistent within measurement error. These measurements revealed deviations from a linear wavelength scale by as much as 1.5 Å. The data were fit with a four-term Legendre polynomial, resulting in an RMS error of 0.236 Å. Additional terms did not significantly reduce the overall RMS error of the fit. Comparison of laboratory and in-flight positions of Lyman-, Lyman-, and the calibration lamp lines shows agreement to within 1 pixel (0.51 Å), indicating no disturbance of the internal spectrograph alignment during launch.

Consistency checks of the wavelength scale were made using measurements of spectra in which the slit was uniformly illuminated and which, therefore, do not require telescope pointing corrections to the wavelength scale. The extended sources chosen for this purpose were a daytime airglow spectrum and a lunar (essentially solar) spectrum. Although the RMS deviations about the adopted wavelength scale are larger due to the increased linewidth and blending, the data indicate that the mean offset of the wavelength scale is Å.