1.5m Telescope Adaptive Optics Images of Vesta

Details 1.5m Telescope Adaptive Optics Images of Vesta 1.5m Telescope Adaptive Optics Images of Vesta

Sep 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996dps....28.1022d&link_type=abstract

American Astronomical Society, DPS meeting #28, #10.22; Bulletin of the American Astronomical Society, Vol. 28, p.1100

Physics

Optics

2

Scientific paper

Full high order adaptive optics images of Vesta were obtained on May 11, 1996, with the 1.5m telescope at the Starfire Optical Range under exceptionally good weather conditions, with loop performance occasionally yielding PSFs better than the 0.12" diffraction limit at 0.85microns, giving 5-6 resolution elements across the diameter of the asteroid. From 14 images over a complete rotation, by following Vesta's changing elliptical outline, we derived a rotational pole of RA=320;Dec=+44, and triaxial ellipsoid dimensions of 555 x 535 x 428 km, with a fitting error of 3km. This compares nicely to the 560 x 544 x 454 dimensions adopted primarily from the control point solution applied to HST images (Thomas et al, Icarus, in press, 1996). Their elliptical outline analysis yielded a smaller c diameter of 426km, similar to our results. From two previous sets of visible speckle observations, two sets of IR speckle observations(McCarthy et al, Icarus 108, p285, 1994), and three sets of images obtained at our facility, we have found a correlation with the latitude of the sub-earth point on the asteroid during the observations and the size of the derived short axis dimension. (No such correlation is found for axes a and b). A simple equation, good for sub-latitudes of 0 to +/- 15 degrees (the extremes observed), describing the correction to the measured c is: c(true)=c(obs) + 54 - 2293 [1 - cos(x)] , where x is the cosine of the sub-earth latitude. The correction ranges from +55km (12% of true) when the earth is over Vesta's equator and the smallest c is seen with these techniques, to -24km when the earth is 15 degrees from Vesta's equator and c is derived to be larger than its 'true' (HST) diameter. This phenomenon can be attributed to polar limb darkening or a feature(s) at high latitudes that distorts the c derived from the assumption of a smooth ellipsoid figure for Vesta. A movie of 'spots' transiting the face of Vesta will be presented, as derived from images obtained with the smallest telescope to ever successfully detect such features.

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