Observations of Saturn's inner satellites during the May 1995 ring-plane crossing

Science

Volumn 272, Issue 5261, 1996, Pages 518-521

  Bosh, Amanda S ^a   Rivkin, Andrew S ^a,b  

^a LOWELL OBSERVATORY   (United States)

^b UNIVERSITY OF ARIZONA   (United States)

Author keywords

[No Author keywords available]

Indexed keywords

PLANETARY RING; RING STRUCTURE; SATELLITE;

SATURN;

EID: 0029748911     PISSN: 00368075     EISSN: None     Source Type: Journal    
DOI: 10.1126/science.272.5261.518     Document Type: Article

References (32)

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17. The three to four MSFs taken during a single HST orbit were loaded into separate image buffers, which were then displayed sequentially. Any stationary light sources remained fixed as we cycled through the buffers With four images to blink, we could follow objects across the field, verifying true satellite-like motion. Using this technique, we searched outside the disk of Saturn to the edges of the chip (5 Saturn radii), near the plane of the rings. Centers for sources on the MSFs were determined by the centroiding routine of the IRAF software package (23) (for brighter satellites) or "by eye" (for fainter ones). The "by eye" method was compared with center determination on isolated satellites by IRAF centroid results on MSFs and raw frames and was found to agree to within 0.25 pixel
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19. Mimas and Enceladus were not used to determine the center of Saturn, even though the orbit of Mimas agreed well with its ephemeris in May. Ground-based observers reported large offsets (from predictions) in the timing of eclipses of these satellites. The only reference satellite contained in data taken in the sixth and seventh HST orbits was Mimas; an average center for Saturn for these data was computed from the remaining data and used for these images Because of the excellent tracking of the HST, even across orbits, the location of the center of Saturn was extremely stable.
20. We accounted for field distortion across the chip using Eq 1 of J Holtzman et al., Publ. Astron Soc. Pac 107, 156 (1995).
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22. There is no systematic offset in a, indicating that there is probably no large error in the fixed value of pixel scale used. Because the WF pixel scale under-samples the point-spread function, center determination for satellite images is good to about 0 25 to 1 WF pixel (equivalent to 170 to 670 km at Saturn) for isolated satellites, more for smeared satellites or those with background contamination from other satellites or Saturn This value is the same magnitude as the typical fit rms; therefore, differences in a of this order cannot be considered real
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32. We acknowledge the invaluable help of A. Lubenow and A Storrs at the Space Telescope Science Institute in scheduling these observations We are also indebted to the HST High Speed Photometer (HSP) Instrument Definition Team, whose time was used to obtain these observations. We thank P. D. Nicholson for many enlightening discussions A S B. was supported by HSP GTO grant NASG5-1613; A S.R. was supported by NASA grant NAGW-1912. The Space Telescope Science Institute is operated by the Association of Universities for Research in Astronomy under NASA contract NAS5-26555