The Vesta Occultation of March 29, 2006.
John
Broughton
Expanded view of the raw plot
Timing results after smoothing
4 Vesta was last seen to occult a star 15 years ago. The main-belt's second largest and brightest asteroid traditionally requires a star of equivalent or brighter magnitude for an event to be spotted visually and that is a relatively rare occurrence. Nowadays armed with low-light video or CCD cameras and software to analyse their output, amateur observers finally have the tools to detect events involving stars as much as 3 magnitudes fainter than an asteroid, as long as the duration is not too short for the subtle dip in brightness to be lost amongst the dense forest of seeing variations.
Eager for a crack at such an event, an
opportunity arose March 29 when the 7.7 magnitude asteroid was predicted to
occult a 10th magnitude star for my east coast Australian location. The profile of my CCD drift image as viewed
in SCANALYSER clearly shows a deeper event than the 0.11 drop expected and this
contributed to measurements of reasonable accuracy. Had the vertical seeing
variations of the occultation overlapped that of pre-occultation, then the
measurement accuracy would have degraded by as much as several tenths of a
second, so it was fortunate the star appeared brighter than expected.
The 39.11-second duration represents
a chord length of 559 ±3km - in close agreement with results of a 1989
occultation and current knowledge from HST images of the wide axis of this
pumpkin shaped asteroid. Apparently the
occultation path shifted approximately 150km north of prediction to near my
location for this to have occurred.