7.2.2 Wiggle Corrections

The LWP and LWR cameras are known to exhibit localized discontinuities in the spectral format which are believed to be due to slight misalignments between neighboring fiber optic bundles in the output stage of the ultraviolet converter (UVC) portion of the cameras. These misalignments are seen as abrupt changes in the spatial centroid of the spectrum of up to 1 pixel. In addition to these abrupt changes, there also exist more slowly varying ``wiggles'' in the spectral format. These wiggles can have peak-to-peak amplitudes of 1 pixel or more within the space of a few dozen pixels along the dispersion direction. Figures 7.2 and 7.3 show the locations of the spectral centroid as a function of wavelength for the LWP and LWR cameras, respectively. Note the sharp discontinuities at $\sim$3290Å in the large aperture (the feature at $\sim$3100Å is a reseau) and at $\sim$2780Å and $\sim$3290Å in the small aperture for the LWP. The LWR shows similar discontinuities at $\sim$2450Å and $\sim$2830Å in the large aperture. The existence of these wiggles complicates the task of spectral extraction since it is necessary to have the extraction slit follow the changing spectral centroid in order to ensure that all of the spectral flux is extracted at a given wavelength.

To compensate for these wiggles, the spectral centroid as a function of wavelength has been measured from approximately 30 low-dispersion SI for each of the LWP and LWR cameras and the average deviations have been incorporated into the resampling step of the image processing system as an additional geometric correction vector. These corrections affect only the spatial dimension of the low-dispersion SI. Separate deviation vectors are applied for the large- and small-aperture regions of the LWP and LWR low-dispersion SI. As Figures 7.2 and 7.3 show, the deviation vectors for the two apertures are qualitatively similar, but differ in detail, due to the fact that they are imaged onto different portions of the camera faceplate. Figures 7.4 and 7.5 show examples of the measured centroid locations for ``dewiggled'' versions of LWP and LWR images.

Figure 7.2:  Spectrum centroid location in LWP low-dispersion SI data.

Figure 7.3:  Spectrum centroid location in LWR low-dispersion SI data.

Figure 7.4:  Spectrum centroid location in ``dewiggled'' LWP low-dispersion SI data.

Figure 7.5:  Spectrum centroid location in ``dewiggled'' LWR low-dispersion SI data.