WFPC2 Flatfields with Reduced Noise and an Anomaly of Filter FQCH4N-D

The transmission of the filter FQCH4N-D varies by 20 percent across the filter while the mean wavelength shifts by 3 nm. For objects with a flat spectrum across the main bandpass (889–897 nm), the flatfielding removes the spatial variations except for the outer corner, for which we give the necessary photometric correction. On the other hand, for objects with steep spectral features within the bandpass, such as Jupiter and Saturn, the spectral shift causes photometric variations of some 30 percent across the filter which are not taken out by flatfielding. We give the magnitude and direction of the shift to account for these variations. Flatfields with reduced noise are described in the Instrument Science Report WFPC2 200107 http://www.stsci.edu/instruments/wfpc2/Wfpc2 isr/wfpc2 isr0107.html and not repeated here, except for the abstract: We examine the noise contributed by the WFPC2 flatfields during normal calibration, and provide new low-noise flats for 41 filters. Highly exposed science images (> 20, 000 electrons per pixel) will show significant noise reduction if these new flats are used; this is especially true for images on the PC1 chip. For some ultraviolet filters a significant improvement occurs even for much lower exposure levels. Potential photometric issues are also discussed. The new flats are available in the HST data archive as calibrated science data (i.e., data which have already calibrated with the normal flatfields) to obtain the noise reduction. These corrections may be incorporated in the normal pipeline flatfields at some future date for selected filters.