My Date with Jenny (the asteroid)

Nov 16/17, 2013

Couldn't resist playing with the title, sorry! This asteroid occultation was predicted to pass north-to-south through the Bay Area and the northwestern Santa Cruz Mountains, and miss Aptos. We were on the eastern 1-sigma error line. But odds were still 18% and the fact is, I needed to spend the night at Cabrillo Observatory anyway. Why? Rick Ferrell, Dave Deluchi and I were on plan for doing the Big Sur Half Marathon, which went off at sunrise in Monterey. It required my being at Rick's at 5:15am. Also, Comet ISON was brightening dramatically and I planned to drive to the top of Aptos School Rd and Fern Flat, which has a nice SE horizon, and take some photos before continuing on to Rick's. The Jenny occultation was at 9:38pm Saturday night - convenient! Here's the prediction page.

The star was 11.3 magnitude and the drop was 3.5 magnitudes, so it sounded like a good event to get a star trail image of the occultation. This is something I've not succeeded at since the Pallas occultation of 1 Vulpeculae in1983, and tonight was the night I hoped to succeed. Challenges awaited, it turned out. I emailed Chris Angelos and Gene and invited them to join me at the observatory and help with the astrophotography while I tried to get the Jenny event on video with the 10" scope. Gene was game, and showed up at 6:30pm. This was good; right after I'd done some grading outside of Verve downtown, then picked up new running shoes at Fleet Feet, got a burrito at Sofia's, swung by my office and printed up finder charts and prediction sheets for Jenny, and then arrived at 6:30pm at the observatory. Gene worked on polishing up an image of a nebula he'd taken while I fired up the 12" and did some scouting on exactly where the star would rise. As soon as I started thinking about this, I realized we may have a big issue. 25 degrees altitude in the northeast is the worst horizon we have. It required being meticulous in finding a star on C2A with the same declination as the target and seeing where it would be at 25 degrees altitude. I thought I did a decent job of that task, deciding we'd have 16 minutes of clearance on the 12" scope for Gene, and that if I trucked the 10" scope down the road 100 yards I'd be able to get it on video with a similar amount of time to spare.

Getting the star trail required getting on target, putting the target at the bottom of the CCD chip, then in the Gemini software, turning off tracking at the right instant, and starting a 'grab' exposure in CCDOPS of known length - I decided on 60 seconds as this would make a trail about 65% of the height of the frame. Gene needed to be there to initiate the exposure at 5:37:46 UT and we and then he, practiced how to do this till it seemed do-able. The problem was, when 15 minutes before the event I sent the scope to the target, it was dead in the middle of the biggest tallest tree on the biggest tallest hill within our horizons, and I had a strong suspicion it wouldn't clear at all. I focused all my attention on the 10" setup. The nearby 6.9 mag star cleared the bushes in the distant hill with 12 minutes to go, and 3 minutes later the target cleared... I thought. But it then proceeded to creep up the spine of the tallest spindly pine tree visible only in the telescope.Doh! I gave up trying to get it on video. The full moon 40 odd degrees away and the low altitude made the 11.3 star just too tough to try and video record with my equipment, so I would do it visually.

At longitude 122 W, the mid-occultation time was 05:38:17UT, and so when 05:38:28 arrived and still no occultation, I figured I had a miss. Not surprising, as it was only 18% odds the path would shift over me. I relaxed just a bit, and missed the actual D, shouting it weakly 2 seconds too late. I was ready for the R however, when it came 6 seconds later. The star was dim and only visible with averted vision, but fairly confidently if you kept your eye in the right place. Here's the Excel report.

Begin: 5:37:10
D 5:38:30.3 +- 1.3 RT=2.4
R 5:38:36.6 +- 0.3 RT=0.5
end 5:40:00

To get the 25 deg altitude event from the observatory, I needed to set up the 10" at the red circle with long/lat shown


Several misses observed, together with my occultation, are plotted here.