The Occultation of a 10.2 Star by Alexandra

Aug 26, 2023 at 10:47:42pm

This event is nominally of high rank and includes Santa Cruz. But it's got a high RUWE=2.75 so the star positions is not great. I figure that then gives us maybe 60% odds at best. But it's a bright star, and worth the effort on a Saturday evening. Several other observers are signed up.

It's in Eastern Sagittarius, below/left of the "teaspoon". 30 degrees up at Az=185, almost due south.

The duration is unusually long; 33 seconds, The asteroid is likely near a stationary point. The magnitude drop is 1.0, so it'll drop from 10.2 to 11.2. This means we must be careful to not saturate pixels. No higher than 2x integration, I'm guessing.

Results:

Kirk and I drove up into Bonny Doon to escape the fog, and got 26s positives. Karl has power failure to the Watec and no data.

Richard Nolthenius

I initially drove to the bike crossing at upper meadow UCSC. I waited there to see how the fog would behave, as I got above it only above the main campus, but it was spotty. The air felt heavy, cool, and the moon nearby the target looked like it was brightening the sky more than would be true in a dry clear sky. I knew if I waited too long to change my location, I'd not allow enough time for a new set up. With 35 minutes till the event, I decided the risk of fog was too high and I continued driving up to Sunlit Lane. The map shows the path pretty much parallels Empire Grade, so we (Kirk and I) really didn't have much choice in trying to avoid each other's chords. Going east would get us into the other chords from SiValley and Karl. I figured, this was an excellent event to search for possible satellites, since the drop was to be large and the asteroid was moving very slow, and if we did happen to see a secondary drop we could use our proximity to confirm it. So, I chose Sunlit Lane. I had told Kirk to set up at the Eco Preserve, so that gave us a mile or so of cross-track difference.

I got deployed quickly and efficiently, chose Polaris and Nunki as my 2-star align. But after align (which it said "successful") and I sent it to the target, the star field could not be recognized. At first I was fooled by a close double star which was perhaps like the double star at far right on the eyepiece chart. But after a minute or two of failing to ID the field, I then told the scope to go to M22. I saw no globular cluster in the field. Then to go to M11, which was brighter and not near the moon. No cluster. Now, Nunki (?) was just above a tree and perhaps it was NOT Nunki? Anyway, it was clear I needed to do a new 2-star align. Polaris and Arcturus. Then to the target. It was bang-on and immediately recognizable. I got it set, there was minimal drift, put in the f3.3 and Watec, got it focused and on target, and it was not 10:43pm, time to start recording. I did start, but apparently there was a loose connection somewhere and at 3 points during early taping, I got the 'blue screen' on the camcorder. I snugged the input cord, it seemed to then behave OK for the rest of the recording. Strange. I'd never seen that before.

I was expecting to see a large 1 magnitude drop, usually dramatic enough to see easily. I did not see that, and so concluded I had a miss. I took a few pictures, packed up, went home. Early next morning I was up and rushing around to pack for my Sandman Triathlon competition... I was scheduled to be starting the swim at 8:10am! But I did see on my email that Kirk said he saw it fade and estimated he had a 26s occultation. So, maybe I DID have an event in that data? Yes, there's a very clear sharp 26s occultation, but the depth is only 36%, not more like 70% as a 1.0 mag drop would be. Color difference?? Not sure why it was so shallow. But it was on time, a reasonably to be expected duration, and quite clearly valid.

There was a fade early in the retained recording but it was seen on all stars, so was not an occultation. I attempted to normalize it, but early in the recording there was a saturated pixel that appeared in some of the integrations, and so the "over correction" by the reference star produced an erroneous spike in the target, which should be ignored. Towards the end, telescope drift had the target getting perilously close to the VTI numbers, so I had to move the star upward, which caused loss of tracking. I trimmed that part out of the PyOTE reductions.

This is the first reductions I've done using the latest update PyMovie 3.8.9, and PyOTE 5.4.0. I did use the new TME optimal mask option. I was surprised to see in the aperture matrix that it was listed as "10 pixels", when the radius of pixels looked to be more like 2 or 3 pixels. Perhaps this column now means the total number of pixels rather than the radius of the best-fit circular aperture of the same size. I did not use a flat field / dark option as I'm still trying to figure out how to use these. And, this data is good enough I believe it's unnecessary here.

Zoomed in.

"Red bar test" passed.

PyOTE log file timings are below. Mag drop only 0.49, not the 1.0 expected. The target star was perhaps rather blue and hence fainter than typical? Star was in the Milky Way of Sagittarius, so blue faint stars are uncommon, but not impossible.

magDrop report: percentDrop: 36.1 magDrop: 0.486 +/- 0.010 (0.95 ci)

DNR: 3.33

D time: [05:47:30.2074]
D: 0.6800 containment intervals: {+/- 0.0104} seconds
D: 0.9500 containment intervals: {+/- 0.0305} seconds
D: 0.9973 containment intervals: {+/- 0.1698} seconds

R time: [05:47:57.1009]
R: 0.6800 containment intervals: {+/- 0.0104} seconds
R: 0.9500 containment intervals: {+/- 0.0305} seconds
R: 0.9973 containment intervals: {+/- 0.1698} seconds

Duration (R - D): 26.8935 seconds
Duration: 0.6800 containment intervals: {+/- 0.0161} seconds
Duration: 0.9500 containment intervals: {+/- 0.0473} seconds
Duration: 0.9973 containment intervals: {+/- 0.2141} seconds

Kirk Bender

Set up at the Eco Preserve, about a mile or two southwest of me, at a little lower elevation. Had a good success