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A 18.49 second occultation was recorded by John Broughton, using drift scan technique.
View the updated prediction.
Observers: 1 J Broughton,Reedy Creek, QLD, AU 2(P) Predicted 26 Apr
Discussion:
The circle above is plotted at the expected 216 km diameter of Ursula. With only one chord it is not possible to determine whether John was north or south of the central line of the event. However, assuming the updated prediction was approximately correct, and the time very close to maximum expected suggests that the chord might lie close to the central portion of the asteroid.
The image below shows the light curve as measured by John's Scanalyser software.
Observational Data:
Observation details for MP (375 ) Ursula By Star UCAC2 12060928 On 2008-05-27 at 16:16:13.0 _______________________________ Observer John Broughton Location Reedy Creek, QLD, AU Longitude +153:23:52.8 Latitude -28:06:30.3 Altitude 66 m Datum WGS84 Telelscope Newtonian Apperture 51 cm Seeing Steady Thin cloud Timing Tape Recorder + time signal + shutter Drift scan Disappearance 16:15:07.00 Reappearance 16:15:25.49 This was my 4th high-probability asteroid occultation in a 24-hour period! The other 3 were clouded out and this was close to suffering the same fate. At 10km from the predicted centerline, the probability was 98%. I measured the magnitudes of the asteroid and star as 11.77 and 12.53 from an image taken before the occultation. That represents a magnitude drop of 0.44 or a 33% drop in brigtness. : In the analysis, the first problem I encountered was a half-second discrepancy between my manual shutter timing and that calculated from the tape recording. It was eventually established that my stopwatch had correctly been syncronised to UTC before the occultation but after the event when I recorded that 10mhz shortwave signal on tape for rigorous timing, it had changed from a faint WWVH UTC to a loud chinese UT1 broadcast. To remedy this, I applied a correction appropriate for May 27UTC=UT1+0.42. : The second problem involved variable attenuation due to the influence of high-altitude haze, making the identification of D rather tricky. As the atmospheric effect is smoothly varying, it should be possible to discriminate the more abrupt real events. Althought the low point in the occultation trail coincides with that of other trails, it is deeper and there is a clear indication of reappearance. At first sight of the image, disappearance appears to occur about mid way along the trail, however the profile in Scanalyzer doesn't show any abrupt 33% drop near there. The earlier location for D I eventually chose does have an abrupt light drop apparent in the raw plot and the results do fit well with prediction.