The Bella Occultation

Mar 18/19, 2016

This decent ranked event (Preston page) crosses right over Santa Cruz, and nicely offset from Derek Breit's site, so we can hopefully get a good asteroid profile.

The altitude will be 51 degrees, under the head of Hydra. Odds of an event at Cabrillo Observatory is 43.8%, odds on the centerline are only 47.3%, so it's not worth making a trip just to up the odds. However, fog is a threat and that may force me up the mountain to somewhere around Loma Prieta School perhaps.

 

   

Event Night:

Cirrus was streaming over a "dirty ridge" in the atmosphere, giving poor prospects. Fog rolled in early the previous evening. First foggy night of the spring season. Not good. I made plans to escape the fog by driving up into the Summit Rd area, but the marine layer deepened according to the latest weather service notice, to over 2,000 ft and it wasn't obvious I'd be able to escape the fog at all, nor the cirrus. I decided it'd either be an attempt at Cabrillo College Observatory, or bail. I walked outside at 7:50 pm and saw pretty clear skies, and decided I'd go for it, despite the ClearSkyClocks showing a blob of thick clouds between 9pm and 10pm, centered on event time. First stop was Home Depot, where I figured I needed a long steel pipe as leverage to get open the stiff doors of the steel shipping container shed. Got to the observatory at 8:45pm and now that I'm practiced at my occultation procedure, I felt confident all would go well, if the weather held. My first 2-star alignment didn't give good pointing, so I did a second, which worked better on locating Messier objects, and so I then went to the target, and ID'd it easily. Swapped in the Watec camera, got it refocused, changed the integration to 8x field = 4x frames or 1/4 second time resolution to get the faint 11.8 magnitude target in the bright moonlit sky only 20 odd degrees away, to be steadily visible. There was a band of cloud that was drifting in and looked like it would cross the target and probably be just past by event time. Which, is what happened. The event record showed no evidence of variable extinction with clouds.

Snapshot of the camcorder LCD screen

There was an old, spreading jet contrail that drifted over the moon and target a few minutes before the event, but was past by event time. This was shot a few minutes before the event, the contrail was moving left (north-east)

My "OccBox" in its current form. Not as perfectly streamlines and cusomized at e.g Ted Swifts; I just haven't had time to make my own yet. But this has served me well for years now.

 

Reductions

There were only two observers for this event - me and Bob Jones near L.A. Both of us saw distinct visual and light curve evidence that the star was a close double star, and so IOTA's Brad Timerson and also Tony George wanted to do their own set of fits to try and determine the best parameters of the system. Unfortunately, the reduction software (R-OTE, Occular) do not have built-in binary star identification and reduction features, so it requires truncating the light curve and fitting. I wonder if one can't do better just by eyeballing the light curve, knowing the frame integration, and then applying the time corrections manually. There was some issue with R-OTE finding a slight drop remaining for almost 2 seconds after the R, but I believe this was noise, and in fact the solution doesn't work well at all unless the steps are what is in fact most obvious.

Final solution with my roughly text'd in D's, R's in Photoshop, from Tony's reduction

My own aperture photometry light curve in LiMovie, using 5/7/25 photometry circles and drift rather than linked tracking. Linked tracking was a bit worse, I felt, and the asteroid was bright enough to still give counts enough for decent drift tracking

Comparison star (bright one south of target), in pink, target in blue. From Tony's reduction.

   

 

The full 350Meg .avi file

The .csv file from LiMovie, using aperture photometry with circles 5,7,25 pixels.

Final (?) determined times below...
D1 4:33:20.6994
D2 4:33:21.3762
R1 4:33:27.5919
R2 4:33:27.9923

Here is the preliminary solution to the shape of the asteriod as determined by Bob's and my timings with the "shelves" interpretted as a binary star. The ellipticity is large for an asteroid 100km across, which hurts confidence a bit.