The Occultation by (182) Elsa, and a Cold, Brisk Trail Adventure at Laguna Mountain

Dec 25/26, 2015

The cosmos take little note of human holidays... on Christmas Night, the little 45km asteroid (182) Elsa was predicted to occult a 10.5 magnitude star in Aquarius along a path crossing King City, Laguna Mountain, and places northeast. I had decided I was too tired, from a big trail run earlier, to do anything big and was going to skip this event. Instead, I was just going to do an easy bike ride up the coast... but 8 miles into that ride, I changed my mind, turned tail, and pedaled home, tossing the necessities into the car, and driving south to King City. The Elsa event was of surprisingly high rank for such a tiny asteroid. The tiny asteroids are of particular interest to map, because these seem to so far be primarily made of rock, as shown by the spin/mass scatter plot, and not held together by gravity. Elsa is above this borderline, but still pretty small and with very weak gravity, and so getting a good profile would be important. The path passed also over Laguna Mtn, but the fastest way to the path (after my late change-of-plans), was King City, and a spot (Jolon Rd and Oasis Rd) I knew well because I drive by there every time I go to Lake San Antonio for the Wildflower triathlon.

The Preston page and Western US path map


I set up on Oasis drive, about a mile west of King City, in ag fields on a quiet road. Weather was clear, dry, and no wind.

My circa '80's triathlon over-pants had almost all their waistband stretch gone. I was, shall we say, properly attired for a solo trip into the middle of nowhere

The Q70 eyepiece circular field of view. Finder chart for the target star in Aquarius (image is properly left-right reversed due to the prism diagonal in the optics

The smaller field of view of the LCD screen on the Canon ZR camcorder, shown by the small box

My site, the shoulder of Oasis Drive, which had no traffic during the hour I was there. GoogleEarth shows the elevation as 364 ft at the level of the telescope OTA. This is less than a km south of the predicted centerline.

I set the Watec 910hx video camera to 4x (2-frame integration) to give a decent signal on the small LCD screen to see the star. This is 1/15 second time resolution. Here is the analysis of the video record... LiMovie to analyze the raw video and generate the .csv photometry file. Then to Occular to analyze the photometry to determine the disappearance and reappearance timings. In Occular, the block average box was given "2", and the frame offset was determined to be "1". The LiMovie .csv file is here

The photometry was done in LiMovie; aperture photometry, with 5/7/25 pixel photometry circles for comparison star (pink) and target (blue). Both stars were set to "drift", since the occultation was very brief, and I didn't want to introduce noise by having linked tracking where it wasn't really necessary. I verified visually that the aperture circles remained on target during the 1/3 second occultation.

The same, zoomed in on the occultation.

The LiMovie panel, showing the parameters used in the reduction

In Occular, I entered the star and asteroid magnitude and color indices from the Preston prediction pages, block average=2, offset=1, ran 100 Monte Carlo trials to find the highest probability D and R times

The Occular analysis and T-statistic, with the highest Figure of Merit solution for D and R highlighted in green. The confidence in the event was 100%.

For 100 trials, the distribution of D and R times, in terms of frame number

The frequency distribution of the timing determinations

The noise distribution in the timings, compared to a pure Gaussian, which is a decent fit.

The Occular Final Report, with timings (not including the video camera corrections)


The IOTA report containing the analysis and timings was sent to Brad Timerson at 11:54pm Dec 30. Here's the single-chord plot of the occultation. Clearly I just barely nicked the edge of the asteroid, either at the northern or southern limit.

After the occultation, I drove in to King City to the Taco Bell and grabbed 4 chicken tacos for dinner, and the drive to Mt Laguna via Bitterwater Rd. Got to Mt Laguna about 9pm, set up my tent and sleeping gear on the hilltop campsite, got into my down jacket and down sleeping bag, opened up "The Snow Leopard" and read a couple of pages before comfortably falling asleep - very early for me. So, woke up at 4:30am and couldn't fall back asleep. At 5am, I decided I wanted to get up and go for a walk even though it would be dark for another 2 hrs. The full moon and stars and rising planets of the eastern pre-dawn sky made the idea inviting. It was well below freezing, somewhere around 20F I'd guess, and the muddy ground from all the rains was frozen solid and crunchy to walk on. This was only my third visit to Laguna Mountain campground and I'd not yet hiked to the mountain itself, which is a 6 mile round trip. I decided I'd at least head up the fire road till I decided to turn around. But in fact, the crisp night was so stimulating that I got more and more into my brisk walk, and decided I'd go ahead and climb to the top of the mountain (4,400 ft, about 2,000 ft of total climbing from the campground.)

a happy camper, out for a 5 am hike

The ice-crunchy ground was perfect to walk on

Below the mountain, as pre-dawn light came, this frosty greenery

Just before sunrise, the Earth's shadow and red sunlight illuminating aerosols in the atmosphere, below the setting full moon. Laguna Mountain's slopes are on the left. I didn't actually find the primitive trail from the fire road to the mountain top, so the last quarter mile remained unclimbed.

A few minutes later, sky light and moonlight became more equalized, permitting detail on the moon's surface, together with chapparal a quarter mile away


First light catches these distant twin peaks, beneath the full moon


The sun peeks over the horizon sending the long shadow of Laguna Mountain out into space

Frost on an old tree and its shed bark, Golden grass highlight the blue cast lent by the dawn overhead sky.

Ice-entombed chaparral leaf cluster. It has the look of late December about it

I wonder for how long these two Miocene Epoch clams had been together here, and was it true love? These two were not in rock, but yet were lying together in this frozen ground 3,000 ft above today's sea level.


There was a half mile of trail that had many ancient sea bottom mudstones with all manner of ancient shells.

Breakfast; 5 slices of camptoast soaked in olive oil, a half dozen eggs, and an orange. The cold and the hike stoked a good appetite - it was delicious!

Then, I headed off again, towards Laguna Falls.

Oak leaves and new sprouts, just beyond Laguna Creek.

After several trials that almost spilled me into the ice-covered stream, I managed to get the camera 10s delay and rock-hop over to this tree for a contemplative moment

Hoar frost on this oak leaf.... something about this shot reminds me of the film "The Martian"