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E. Karkoschka (U. Arizona)
The Wide Field Planetary Camera 2 (WFPC2) of the Hubble Space Telescope (HST) has a CCD designed for planetary imaging, the Planetary Camera (PC1). Many planetary exposures taken on the PC1 during the last eight years have better signal-to-noise than standard flatfields. Since the signal-to-noise ratios of flatfielded images has been limited by the noisier flatfields, the finest details have not been seen in these images.
This year, I investigated PC1 flatfields in more than 40 filters. I found that the flatfields have a number of features which change with wavelength, but the change is small for nearby wavelengths. Thus, by averaging flatfields of nearby wavelengths, one can decrease the noise without introducing significant erroneos features. For each filter, I searched for the "best" averaging method and created an improved flatfield.
None of the original flatfields, but most of the improved flatfields have a signal-to-noise ratio of 220 which is attainable in a single exposure. For the ultraviolet F336W filter, one of the 10 most frequently used filters, the signal-to-noise ratio increased from 54 to 240. Greater improvements occured for other ultraviolet filters.
I took some planetary images from the HST archive and tested, how much they improved by applying the new flatfields. For some images, especially images of jovian planets, the improvement is very obvious. It seems that the resolution improved since the smallest detail is only visible in the new version. Positional and photometric measurements on the surfaces or the atmospheres of planets can be made more reliably.
Applying the improved flatfields to previous and future WFPC2 images has been made simple for all users of HST. Results are published in the WFPC2 Instrument Science Report 2001-7, available at: http://www.stsci.edu/instruments/wfpc2/Wfpc2_isr/wfpc2_isr0107.html
This work was supported by STScI grant HST-GO-00108.03-A.