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Asteroid (38476) 1999 TA91 occulting star 4U 444-161

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About this observation
Observer
Alex Pratt
Time of observation
05/01/2017 - 17:42
Object
Asteroid (38476) 1999 TA91 and star 4U 444-161
Observing location
Z92
Equipment
C11 and f/3.3 focal reducer
Watec 910 video camera
IOTA-VTI GPS time inserter
Exposure
1/50th sec (0.02sec exposures)

The mag 20 asteroid (38476) 1999 TA91 was predicted to occult the mag 8.6 star 4U 444-161 on 2017 January 5 at 17:42:25 UT +/- 5s, as seen from my location. The asteroid is small, producing a ground track shadow only 8km in diameter, and the maximum duration of the occultation would be 0.3s with a probability of success of about 2%.

The star is bright, so the Watec camera was set to run at 50 fields per second, 0.02s exposures. I was most fortunate to record the target star blink out for 0.3s as it was covered by the asteroid (see the light curve created using Limovie). I must have been on the centreline of the ground track. A short extract of the video is currently available on the BAA ARPS website.

The reduced timings are:-

17:42:22.54 +/- 0.02s     Disappearance

17:42:22.84 +/- 0.01s     Reappearance

Multiple observations of occultations by larger asteroids produce chords across the ground shadow of the body, allowing us to create a shape profile of the asteroid.

Accurate timings of asteroidal occultations also give an astrometric measurement of the position of the asteroid, of particular value now that Gaia is delivering a star catalogue of unprecedented accuracy.

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