The Winchester Occultation

Dec 23/24, 2015

This event was by a well-observed large asteroid, mapped on earlier occultations, and had a very high rank. The northern limit missed Santa Cruz, and came ashore at Moss Landing, angling down towards Nogales, AZ. Pretty much all of California south of that northern limt was inside the path. I wasn't sure it was scientifically valuable enough to make a significant effort for, especially since there was only one other observer - Greg Lyzenga in LA - who had signed on. However, LA was forecast for 80% cloud cover. I'd earlier thought of driving to Carmel, where odds of a hit were 75%, while only 34% from Cabrillo Observatory. I settled on thinking I'd pack the 10" and go up the coast just a couple of miles, where the northern limit got closest to the SF Peninsula and odds went up to 39%. I did a beach run, felt great afterwards, and ready to just prepare and do this thing.... but once I got on Hwy 1 to Cabrillo traffic was a crawl. I got to Cabrillo late enough to instead switch to staying at Cabrillo Observatory. Temperatures dropped (and I'd forgot my jacket)... 42F at event time, and very damp. Clouds came in at 6:20pm and I went in to get warm, till a bit later and skies cleared. Found the target with no trouble, just 20 degrees from the 99% moon, got a good recording, and then packed up as fast as I could, hands numb.

Here's the sites list, the path map.

The Q70 eyepiece field of view

The camcorder LCD field of view

The magnitude drop of only 0.34 (asteroid at magnitude 10.5, asteroid at 11.5) meant I could use drift tracking on both the target and the comparison star.

To access the target, only 29 degrees up in the southeast, I set up outside the door of the dome building, rather than the usual concrete pier. An exposure in natural light, plus my red headlamp, of the video/telescope gear set up

The event was 21 seconds long or so, from the centerline, but for me would almost certainly be only a couple of seconds or less. The target photometry here is blue, the comparison star light curve pink. I see no occultation here. This was using PSF photometry. I decided to try again on the raw video, using aperture photometry.

The same full video, 3 minutes long, but using aperture photometry. Target lightcurve in blue. Again, looks like a clean miss. An occultation would have dropped to 73% of normal brightness

Aperture photometry light cuve, zoomed in onto the predicted occultation time in the center. Again, looks like a clean miss.