However, unlike previous decontaminations, the February 3,1992 procedure caused the re-appearance of the measles contaminant. These permanent spot-like features, dubbed persistent measles, are generally a few pixels in radius; some have light cores, some have dark cores but all appear surrounded by one or more weaker rings. On an uniformly illuminated source (i.e., flatfield) they produce a 1.1% rms modulation with a 4 to 5% peak amplitude in their cores. Optical modelling finds them to be consistent with 10-15 µm sized particles on the field flattening windows. A likely cause of persistent measles is the usage of the pyramid motor without a sufficient cool down period between motions. This often occurred during the SV flatfield observations (fourth quarter of 1991); the waiting period between pyramid motions was increased after the problem was recognized.
Detectors WFC2 and PC6 were the least affected CCDs while PC7 and PC8 were probably the most affected. Figure 46.2 shows a 100 x 100 region of the PC6 and PC8 detectors (divided by a flat taken before the measles problem appeared) at 4 epochs. The third from the left is after an HST safing event and illustrates the post-safing measles which the decontamination procedures regularly removed. The display range in Figure 46.2 is about +/- 10% about the mean.
Precision photometry and astrometry of images with measles will be degraded. Applying an appropriate delta flat may partially correct for the measles; however, because the measles are out-of-focus features, a delta flat will not completely remove their effect. In addition, image deconvolution can actually enhance the measles, since they are roughly the same size as the PSF. In this case, delta flats may prove useful as a template, to allow differentiation of target features and measles.