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Scientific paper
Jul 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008hst..prop11917m&link_type=abstract
HST Proposal ID #11917. Cycle 17
Other
Scientific paper
This program is an experimental path finder for Cycle 18 calibration. {The WFC3 UVIS version of this is program 11914 and contains additional detail in its description}. Infrared-wavelength flat fields will be obtained by observing the dark side of the Earth during periods of full moon illumination. The observations will consist of full-frame streaked WFC3 IR imagery: per single "dark-sky" orbit, we anticipate achieving Poisson S/N > 100 per pixel in each of three to five exposures, depending on sample sequence {SPARS25 or SPARS50}. Why not use the Sunlit Earth? It is too bright for WFC3 IR full-frame minimum exposure time of 3 sec. Similarly, for NICMOS the sunlit-Earth is too bright which saturates the detector too quickly and/or induces abnormal behaviors such as super-shading {Gilmore 1998, NIC 098-011}. In the narrowband IR filters the sunlit earth sometimes is faint enough to not saturate immediately, but based upon predictions {Cox et al. 1987 "Standard Astronomical Sources for HST: 6. Spatially Flat Fields."} and observations {Gilmore 1998}, we consider sunlit Earth unlikely to be successful unless it is twilight. Other possibilities? Cox et al.'s Section II.D addresses many other possible sources for flat fields, rejecting them for a variety of reasons. A remaining possibility would be the totally eclipsed moon. Such eclipses provide approximately 2 hours {1 HST orbit} of opportunity per year, so they are too rare to be generically useful. An advantage of the moon over the Earth is that the moon subtends less than 0.25 square degree, whereas the Earth subtends a steradian or more, so scattered light and light prior to the unshuttered exposure presents additional problems for the Earth. Also, we're unsure if HST can point 180 deg from the Sun.;
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