Instead, a default profile shape is used to compute the extracted fluxes. There are three default profiles for point sources, one for each camera. Each default profile has been derived from well-exposed observations of IUE standard stars, by averaging the profile fits for these observations over wavelength and renormalizing, thus producing a profile that is constant in wavelength. Since the spectral profile for IUE data is not repeatable from image to image, this average profile provides a rough weighting that is useful in very low signal images and provides a lower noise spectrum than attempting to fit the profile from the image. For extended and trailed sources, the default profile has a boxcar-like shape where all spatial image lines receive equal weights.
Warning messages appear in the processing history log when a default profile is used.
A warning message is also issued if the location of the spectrum centroid is found to be more than two image lines away from the predicted spectrum center. If this is the case, then the time and temperature-corrected dispersion constants that are computed for this particular image may be called into question. We have no information as to whether or not a similar shift may have occurred in the dispersion direction of the image.