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8 Wavelength Calibration (TTDC)


High- and low-dispersion small-aperture spectra of the on-board hollow cathode platinum-neon (Pt-Ne) calibration lamp are used to determine wavelength as a function of position in IUE images. These wavelength calibration (WAVECAL) exposures were obtained once a month for each camera and are usually a combination of the calibration spectrum and a tungsten flood lamp (TFLOOD) exposure. The TFLOOD exposure was originally added to raise the DN level of the fainter emission lines and was also used to allow reseau marks to be located on the low-dispersion WAVECAL images; these reseau positions were used by IUESIPS to perform geometric corrections, but are not needed in the NEWSIPS system. Since approximately mid-1992 WAVECAL images have been obtained without the superimposed TFLOOD exposures, as it was found that the NEWSIPS wavelength calibration analysis was more accurate without them. Instead, the TFLOOD was taken as a separate exposure.


Karen Levay