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Over exposed? Webb observations of bright solar system targetsSince the Webb’s L2 orbit avoids Earth occultations, there are no shutters in its instruments. Instead, the IR detectors use multiple readouts with the change in signal between reads or among a number of reads to yield the target flux. For the brightest targets, a shortest single exposure can take between tens of milliseconds to twenty seconds depending on the readout mode and the size of the array being sampled. Since typical full wells are 75,000-200,000 e, the detectors become saturated for wide-band NIR & MIR imaging of large and nearby planets in the visible/near-infrared (Mars, Jupiter, and Saturn). Imaging or moderate resolution spectroscopy of small portions of such planets can reduce the saturation effects by reducing the exposure times (number of pixels per read) and dispersing the light. Narrow band and neutral density filters can also help.The STScI and the Science Working Group has completed a study of these effects and concludes that wide-band imaging and moderate spectral resolution spectroscopy of Uranus and Neptune, KBOs, and the satellites of Jupiter and Saturn are feasible in the NIR and MIR (1-28 microns). The NIRSpec can be used to obtain NIR spectroscopy of regions on Mars and the MIRI integral field unit with high resolution gratings will work on Jupiter (5-10 microns) and Saturn (5-28 microns). The Tunable Filter Imager (TFI) can image all these planets using a neutral density filter and a spectral resolution of R ~ 100. The STScI and the science instrument teams are considering additional operational modes that may extend Webb’s capabilities for these brighter planets. In the future, the Call for Proposals, Webb Instrument Handbooks and Astronomers Proposal Tools will provide the brightness constraints and acquisition methods for observing solar system objects in the future. Links:
Updated:October 2009 |
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