The Occultation of a W=10.15 Star by the Asteroid (65803) Didymos

Wed eve Sept 18, 2024 at 10:08:42pm

OWc page

This is a very high value event, but weather is very iffy. A cold front is predicted to pass over this area at 2:30pm Wednesday, and bring showers. By 10pm and event time, there should be clearing, but still post storm clouds to deal with. The target is in Sagittarius, at only 11 degrees altitude, and the event lasts only 0.1s, We'd need very clean skies. The 98%- moon will also provide bright sky background.

The altitude gets worse as you go further east. But if you get too close to the coast, then fog threatens. I'm planning on trying it from the western side outskirts of Santa Rosa, in an orchard area. If we try it.

Yanzhe Liu did a test video of the target star tonight Monday.

Cloud forecast as of Tuesday 11pm. cloud forecast continues to worsen. If it doesn't get MUCH better, I'll be skipping this one.

C2A map with limiting to SAO stars.

Same scale, but now UCAC-4 stars as well. This area is very close to the core of the MW Galasy.

Results:

I'd just about given up on this one. Cloud forecasts were grim and getting worse during the day. But, Yanzhe's late email that the HRR cloud prediction showed clearing and improvement from past predictions, got me reconsidering. I checked the ECMWF model and it too was improved, for the Sebastapol / Santa Rosa area. I hurried to alert Kirk, got the last gear loaded, told him to meet outside my place at 5:40pm, and we took off up Hwy 17, to 280, over the Golden Gate Bridge, all the while watching the fog near the coast and the remaining storm clouds higher up. The outcome was unclear, and I guessed we have to be very lucky to get anything. At the Hwy 101/ Hwy 116 decision point whether to go for Sebatapol or to Calistoga, had us initially in fog and nothing visible, but right at the intersection, I saw stars off in the west, and quick grabbed the offramp. Sebatapol would be where we made our stand. I'd already selected a site on Wilson Rd, and on arrival, it looked perfect. I put Kirk here, and then I would go south, as the word had been that Ted and Yahzhe would go north of the centerline. As it turned out, Ted got fogged out, and Yanzhe ended up at 0.1km south, essentially between Kirk and I on the near south side of the centerline.

Richard Nolthenius

It looks like a fairly confident miss to me. The data, after calibrating with star=Ref1, and smoothing and offsetting to minimize the metric, shows decently good evidence of no event longer than 1/50 sec and no hint of a noisy event to be labored over. The target star was surprisingly easy to see at 11 degrees altitude, and the seeing was rather good. Clouds were present over most of the sky, but not over the low west/Southwest in Sagittarius (see post-event wide angle camera shots below). I had to scramble to find a site, as the other ideas I had did not pan out; high fences next to vineyards, brilliant lights, private homes and driveways... I then found a quiet dirt road into an apple orchard and took the only spot on that road that had a 11 deg horizon in the right direction.

Reductions: I set the Watec at 1/50s and adjusted gamma to 0.5, which seems a decent compromise, and a way, too, to get the sky histogram above the usual zero level and give it a Gaussian histogram as PyMovie assumed. Despite the bright sky pixels, the sky subtraction was effective and the target star showed a light curve consistently above 'sky'. I saw no events visually while recording, and none are evident in the light curve. Looks like a miss.

The 'appsum' version of my target light curve, so that any possible sky subtraction errors are not present. Still no evidence of an event. Looks like a miss

I will submit an IOTA report as a 'miss'. And submit to Lucky Star the raw files.

Kirk Bender

No apparent event, but too noisy to be sure. Much scintillation in target due to low 11.6 degree altitude. Scattered high clouds but looked clear in target area. Recorded on a Watec 910HX camera at 1/60 sec. integration because of short predicted max duration of 0.1 sec. Used gamma 0.5 setting on Watec. For attached files: In PyMovie, I processed in field mode and used static circular mask of size 4. In PyOTE, I normalized the target on a tracking star. Vertical line in graphs indicating predicted event time of 05:06:43 UT.
PyOTE detectability test reports an event of 0.5 sec or longer with mag drop of 3.9 is likely detectable. Occult Watcher Cloud indicated stars nearby to target: Predicted drop at target: 9.0m, at 4" 6.0m, at 8" 3.9m. I also tried a 12-stack of static apertures in PyMovie but still could not see an event in the light curves. My station was about 100m SE of the predicted centerline.
My avi movie file is on google drive here, size is 752MB, duration 3:17: https://drive.google.com/file/d/1DHpi4A7ir2G7wWypdxih6rerHva7lgcg
Also finder field attached.

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The "miss" results for this event look pretty solid, but the star's possibile duplicity is not settled, complicating the interpretation and the planning for the event 3 days later, on Sept 21 in Eastern Az and New Mexico. However, probabilities now are that Didymos is north of the s212 JPL orbital prediction, or else even farther is it south of that prediction. I expect that the centerline for the 9/21 event will not see an event. Link to the Hera mission back to Didymos and Oct 7 launch.

Here are images to supplement our efforts...

clouds to deal with, was the main problem

Kirk was co-pilot, updating ECMWF data cloud predictions, and trying to match up chords with observers, as scrambling to avoid clouds was the issue.

In heavy fog as we crossed the Golden Gate Bridge

I dropped Kirk at the centerline site then tried for a site farther south, ending up finally in an apple orchard. Clouds filled most of the sky, and the near full moon.

I decided I would be able to get the target between two trees if I set up under this apple tree.

Looking towards the spout of the Sagittarius teapot, just above the target. Clouds did not yet get to the target, here a few minutes after the event

And 18 minutes after the event, as the target went behind the tree at center, still no clouds over the target.

my gear set up. At this point, the apple orchard owner showed up, and I had some explaining to do. He was sympathetic and all was OK.