The Antiope Occultation - July 19, 2011

This was judged to be the most important asteroid occultation ever, for IOTA. A significant part of the Annual IOTA Meeting at Rocklin, as well as the location, was built around this event. A possible binary star occulted by a known binary asteroid, with the possibility of doing detailed mapping of surface topography on such a unique and rare asteroid. We had roughly 50 observing chords planned, with multiple station deployments planned by Paul Maley, David Dunham, and Scotty Degenhardt. The star was 6.7 magnitude, red giant LQ Aquarii. The 83% waning moon would be 10 degrees above the target. The observer assignment list is on the Antiope occultation info page here.

Here's a cool animated .GIF of the orbiting asteroid pair at low resolution.

My planned site was at Locatelli Meadow at track -95, a site I knew well, and placing me just south of Walt Morgan (track -90) and Han Seleski (track -92) and just north of Dunham's northernmost remote station (track -99) The weather forecast was for a mixing out of the marine layer and indeed the day was crystal clean and clear and it looked like we might not have any fog at all. The balloon sounding out of Oakland showed a complex relative humidity profile, but the marine layer looked to be shallow. I thought things would go very well. At 9pm, though, I saw to my consternation that stratus was rolling through Santa Cruz. Rustled out of my complacency, I got on the internet and looked at the fog map, which showed fog around the entire Monterey Bay, but still clear in the mountains. But at the next update, fog was clearly moving inland, although Bonny Doon Mountain remained clear. I called Mary, who lives at the absolute high point of Bonny Doon at 2640 ft elevation, and she said it was cool and a bit moist, but clear, and promised to call me if clouds came in before she went to bed. Then I got on the email with Chris Kitting, who reported from Rick Baldridge that clouds were coming in over Oak Ridge. Some confusion there, and it didn't seem to make sense that it would be clear over Bonny Doon but cloudy at 2500 ft Oak Ridge farther inland. The fog picture was clearly getting complex, without a consistent top elevation. I decided to change my location from 2200 ft Locatelli Meadow to higher elevation, at Castle Rock State Park, where the road gets as high as 3100 ft. I noted GPS coords for possible sites in this heavily forested road with few turnouts using Derek Breit's maps. Then I drove off to my office at Cabrillo to print charts up, and then to the observatory to load up the 10" LX200 and video gear.

I returned home, had a final call to Chris Kitting, who decided the fog situation was too alarming to risk a sleep-ending desperate drive, and he had a proposal due hours after the occultation anyway. Rick Baldridge also got scuttled out of Oak Ridge by the weather. I thought I had a decent chance at 3100 ft elevation and drove up Hwy 17, where I was surprpised and encouraged to break into the clear even before arriving at Scotts Valley well below 1000 ft elevation. But as I got close to Summit Rd, fog reappeared, and I was in thick fog during the entire drive along Summit and Hwy 35. I briefly debated driving back down Bear Canyon Rd to Boulder Creek and on up to Bonny Doon, thinking perhaps the fog was coming in from the Bay side rather than the ocean. But decided to press on. I finally saw some breaks in the fog as I got to Oak Ridge, at 2500 ft. By the time I got close to Castle Rock I was in the clear. The fog tops here were about 2700 ft. But there were so few possible sites! I drove back and forth and stopped at several, getting out of the car and evaluating the sky's motion, where I could set up w/o getting run over, and the possibility of finding 2-star alignment stars, and rejected each site until I was out of buffer time and found actually a pretty good spot, pretty much on my designated track of -95km. I got everything done quickly and well, and had the star in the scope with 9 minutes to spare. But the nearby moon and faint field meant I had no other stars to confirm mine was really the target (I had put in the coords in the LX200 and it landed near a bright-ish red star - which is what I had expected. I spent the next 9 minutes star hopping and comparing the triangles of stars on my large and small scale charts.... confirming that I did have the right star, now could I just starhop my way back to it. I should've gotten there by coord input! Why didn't I?? Doh! I had the wrong triangle in my head when I hurried back to the target. I blew it, which dawned on me when I had a miss and then went back to re-confirm I was on the correct star, and found my mistake. I taped the wrong star! Sleep deprivation and lack of time, after doing the Donner Lake Triathlon the previous day, and hundreds of miles of driving, and little sleep the previous night... all added to the challenges.

20 minutes after the occultation, the rising fog hit my elevation at 3000 ft. I drove home down Hwy 9, and had intermittent clouds even at lower elevations. Orographic lifting probably accounted for fog at such high elevations as a weak front came through during the night. Bonny Doon mountain at the summit was clear, although perhaps not at my designated site. In fact, aside from setting up at Mary's place (would overlap with Dunham's fence) there were no acceptable sites above the fog level. Still, If I'd set up there, with therefore an additional hour or more, I'd have had time to get very sure I was on the right star.

The nominal prediction, not including the fact it's an orbiting pair. The lower limit of the southern member was expected to be about about -110 km, crossing near UCSC.

A self-portrait after realizing I'd taped the wrong star

A lunar "fog-bow" around the nearby moon, just minutes after the occultation and clouds over-ran my site.