Computer Science
Scientific paper
Apr 2005
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005wfc..rept...16b&link_type=abstract
Instrument Science Report WFC3 2005-16, 35 pages
Computer Science
Hubble Space Telescope, Hst, Space Telescope Science Institute, Wide Field Camera 3, Wfc3
Scientific paper
Several of the filters currently installed in WFC3 produce significant ghost artifacts, arising from reflections between optical surfaces within the filters. The ghosts are particularly severe in F225W, reaching as high as ˜15% of the source flux. Weaker ghosts are seen in F275W, F606W, and F656N. In all, a total of nearly two dozen filters have detectable ghosts. In all cases the structure of the ghosts changes significantly across the field of view, sometimes to a very large degree. The intensity of the ghosts also varies strongly with the spectral energy distribution of the target. At the request of the WFC3 SOC, we have carried out simulations of the impact of these ghosts on various kinds of scientific programs, and investigated several methods for mitigation of these effects. For uncomplicated scenes covering a small spatial extent, such as host galaxies surrounding isolated QSOs, the effects of ghosts are fairly minimal, even with F225W. PSF deconvolution algorithms, such as Lucy-Richardson, can remove such ghosts quite well. However, once the scene covers a significant field of view, the strong dependence of ghost structure on field position means that no existing software can deconvolve the image. In such cases, if the field is not too complex, images taken at different dither positions and/or different spacecraft roll angles can be combined in such a way as to remove the ghosts to a considerable extent. However, for extended targets such as nebulae or galaxies, especially those with a wide dynamic range, the ghosts cause irretrievable loss of information, especially in faint portions of the objects adjacent to bright regions. This can be true even with filters having faint ghosts, such as F656N. We also simulated the effects on crowded-field stellar photometry. The impact depends strongly on the nature of the stellar field (i.e., how crowded, how many bright stars), but in many cases, even in F225W, the effect on the derived color-magnitude diagrams is fairly modest. The effects can be reduced further by combining images taken with large dithers or changes in roll, but in general the slight improvement in the photometry will not be worth the loss in field or the additional observing time. Because of the significant impact of ghosts as severe as those of F225W on some types of scientific programs, we recommend a more detailed cost-benefit analysis of replacement of this filter with one that is ghost-free. If any of the filters discussed here are flown, it will be necessary to provide a significant level of support to scientists who are preparing observing proposals for WFC3. This advice will have to be provided before Phase I, since it affects the amount of observing time that must be requested to carry out each program.
Bond Howard E.
Brown Thomas M.
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