The Occultation by the Asteroid Hancock

Apr 24, 2013

This was a tough one, being in bright evening twilight, with the sun at an altitude of -7.9 degrees. Fog had come in all week and was expected to stay. Worse, I'd left my video gear at Chris Kitting's after last weekend's graze and so would have to merge Chris Angelos' gear with the observatory 10". But the toughest - the magnitude of the image was only 10.9 and there would be only a few minutes at best that I'd even be able to see it before the occultation. The rank was only 44, but Cabrillo Observatory was right on the centerline, so it was definitel worth a try. Derek had promised to try it as well, so we had a good shot at getting at least one and maybe two chords.

I did a swim workout with Ferrell at 6pm, got to the observatory gate, saw no Chris A, unlocked the gate, drove up to my office to print the charts and prediction sheets, and as I ran to the door, heard someone running behind me... it was Chris. He'd parked down on Porter Gulch and walked up to find me. He already had printed the predictions, so we were good to go, and immediately drove back to the observatory. Driving up the dirt road, there was this girl... it was Becky! Didn't know she'd gotten my emails and was able to join. Great - it's always more interesting to try and get these events as part of a team.

I quickly got the 10" set up while Chris brought out his video/VTI/monitor combo. We studied the charts, found what looked like it might be a star cluster on the finder chart, sent Becky in to the main dome to fire up the computer and see if we could identify the star cluster near the target star - it would make it easier to aim the scope. But, after using Aldebaren and Procyon as my two-star alignment, we got on the target star perfectly by simple RA/Dec entry (no doubt because the target was only a few degrees away from the 2nd alignment star Procyon). 10 minutes to go, and it was still too bright to see the target star even visually. That ruled out doing the event by video. I fired up the WWV radio and my digital tape recorder. 5 minutes to go... and the star began to be visible above the darkening twilight. At event time - 8:32:09 pm PDT - the target star was definite and almost could be followed without averted vision, but not quite. That would make it harder to keep at the optimum averted angle. And, in fact, when the "D" happened I wasn't sure enough to shout, thinking I might have lost it by having it not at the perfect averted angle. But within a fraction of a second I realized it was indeed gone, and waited for the "R" to shout. I didn't want to comment on the missing D because the predicted event duration was only 2.1 seconds for a central event. I did shout "R", but even that was 1.5 seconds late (estimated), and the event was estimated at 2.03 seconds duration +- 0.4 seconds.

10" pier: Lat=36 59 33.95, Long=-121 55 26.49, elev 211 ft WGS84 on Google Earth

Began: 3:30:30 UT
D 3:32:06.45 by eye/ear, and estimated accuracy on the interval of +-0.3 s, or 0.50 s net accuracy of the time of D
R 3:32:08.47 with 0.66s RT, and estimated accuracy +- 0.3 s
End 3:32:40 UT

for the accuracy of the D timing, add in quadrature the 0.3s on the R and the 0.3s on the D-R interval, and get 0.5s accuracy

Here's Steve Preston's page on the event.

Here's the formal .xls report filled out

Derek had a good timing of the event, with video and VTI, much shorter than mine. It appears the asteroid was pretty much right on target, albeit about a second early. Here's the OCCULT reduced profile on Brad Timerson's page.