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A.A.S. Gulbis, J.L. Elliot, M.J. Person (MIT), B.A. Babcock, J.M. Pasachoff, S.P. Souza (Williams College)
Occultations, eclipses, and transits can produce data of the highest spatial resolution for any Earth-based observing method and are thus used to determine planetary diameters and probe atmospheric profiles. Observing these events requires precise geographic and temporal information. For occultations, the size of the shadow on the Earth is a function of the occulting body's size and distance. This shadow is significantly smaller than the Earth's angular diameter for objects in which we are particularly interested (Triton, Pluto, Charon, and Kuiper belt objects). Therefore, instruments capable of traveling to a predicted shadow path increase the opportunities for observing events. Having multiple systems is also beneficial, since multiple chords must be observed to derive a body's shape.
We have constructed four portable observing systems (POETS; Portable Occultation Eclipse and Transit Systems), which can be transported as carry-on luggage and attached to portable or fixed telescopes. The cameras have E2V CCD97 sensors: a 512 x 512 array of 16 micron pixels, back illuminated, with > 90% QE. The CCDs are thermoelectrically cooled to ~80 degrees C in air. Readout modes are 1, 3, 5 and 10 MHz, with a maximum data rate of 32 full frames per second. Binning and subframes increase the rate to a few hundred frames per second. The lowest achievable read noise in conventional mode is approximately 6 electrons. One of the two amplifiers employs electron multiplying gain, which effectively reduces the read noise to sub-electron levels and allows the cameras to be used for counting photons. Event timing is done using a state-of-the-art GPS receiver to trigger images. We present details of the systems, an analysis of the use of photon counting in the field of small body occultations, and sample occultation data.
Funding for this work is provided by NASA Planetary Astronomy grants NNG04GE48G, NNG04GF25G, and NNH04ZSS001N.
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Bulletin of the American Astronomical Society, 37 #3
© 2004. The American Astronomical Soceity.