The Occultation by Henrietta

Oct 20, 2014

This was one of the highest probability events for Cabrillo of 2014, and the brightness was not bad, at V=11.3. While rain had been predicted, the brief shower came several hours early, and by sunset it was clear. The occultation was at 9:22pm, which unfortunately was only 22 minutes after the end of my Astro 7 "Planetary Climate Science" class. I had hoped that I'd be able to get it opened up and set before I arrived, watched over by Becky or Ann or Gene.... but I only had time for a quick plea in the morning, and then teaching late afternoon and evening and I was out of contact. Anyway, it was going to be a solo attempt in the end. I was able to wrap up my lecture, give the quiz, go over the answers, and release the class by 8:35. I got to the observatory at 8:50pm, quickly opened up for a try at a star-trail image to extract an approximate timing. I was able to get oriented and find the target star in about 15 minutes. I used the NIST time widget to put time on the computer display. I got the target star in the proper spot on the frame, started a 50 second "grab", then toggled off "tracking" at 9:21:56.2pm on my watch, which was 4:21:58.4 UT +- 0.4 seconds. There was about 8 seconds of tracking on the image before I switched off the tracking, so I had a solid point to measure from.

No miss this time! The path indeed went over Aptos. The ~7 second (?) drop out in the light curve is obvious below

Forgot to bring the little camera for "hero" shots.

Here's the data shot. The target star started at the bottom center, burned in for a few seconds, then trailed upward. the occultation gap (drop to the 13th mag of the asteroid) is obvious.

To get the calibration for the rate the stars trails, I shot this which was at the same declination and same altitude.


calib star on calibration frame (brightest star) is 7 pixels wide, so must be 3.5 pixels in radius, so back in 3.5 pixels from the absolute edge of the star trail to get the rate
therefore: start pixel is 1033, and end pixel at 251 = 782 pixels/50 sec or 15.64 pixels/sec . Try again on the dimmer star to the right... 1043-261 = 782 pixels/50 sec or same rate.

My timings reduction:
I'm at track 36N, predicted center of occultation at 4:22:14
end of tracking = target burn-in start at pixel y=1139 centroid. Using star trail width and taking the centroid of the D and R.... measured as:
D at pixel 945
R at pixel 825

So duration is 7.67 seconds
D is at 194 pixels = 12.40 seconds after end of tracking = 4:21:58.4 + 12.40 = 4:22:10.80 UT
R is at 314 pixels = 20.07 seconds after end of tracking = 4:21:58.4 + 20.07 = 4:22:18.47 UT so center was at
pixel of time accurate to +-1 pixel, or call it 0.1 second. Add in quadrature the error in the end of tracking, and get a final acccuracy of +-0.5 second for the absolute start of the occultation. The interval of occultation is accurate at 0.17 sec.

Commentary: According to the sites data, I should have been 1 sec after Derek (rounded to nearest second). Derek's center of event is at 4:22:12.75. Mine was at 14.64 or 1.9 seconds later.



Derek Breit in Morgan Hill got a great recording on video and his reductions are here: from the "sites" page, he was at 26N and center of event at 4:22:13

Event time in UTC

D: 4 22 9.16 ± 0.03

R: 4 22 16.33 ± 0.08