This is a high rank event, a "sure thing" from Santa Cruz and the SC Mtns. It's in the Scutum Star Cloud, so plenty of nearby stars to challenge identification. Altitude 12 degrees Az=118 in the south east. That's pretty low, making it necessary to find a good horizon and get on target while the target is even lower. Very few such places up in the mountains above the fog. The event lasts roughly 13.4 sec with a dip of 0.4 and will require some significant integration at that low altitude. Heavy aerosol layering may make it too hard to do, as too little integration and you won't see the 0.4 drop, but too much integration and the moonlit sky will wash out the view. From Sunlit Lane it should be clear enough I'm guessing from doing some azimuth lines in GoogleEarth and double-checking "streetview". But fog has been rising high this June.
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Fog threatened mightily... but at the last go/no-go moment of decision, less than an hour before the event, I decided to end the tension by just going for it. I threw the equipment in the RAV4 and went for it, at Sunlit Lane and Bonny Doon airport at 2,000 ft elevation. It worked! No fog up there yet, and good conditions. I set the gain way down to about 20, and sharpness =4 (default) and integrated at 16x, to keep good S/N for the target star, given the full moon only15 degrees away and the low altitude too. I believe I saw the event visually on-screen.
Karl noted fog hit his place before the event and didn't set up. Kirk did not attempt it.
Occultation analysis Pyote Log File (note that I tried a failed analysis after the one I found most successful, to get the timings on the IOTA report file. So don't just look at the last analysis on the PyOTE log file.)
I arrived at Sunlit Lane and it was far enough above the last fog that I encountered, that I judged it would be fine for the next half hour till the event. I'd already used GoogleEarth streetview to know exactly where to park to clear the distant trees. The target was higher than the moon but only by a degree or so, and only 12 degrees above the horizon - one of the lowest events I've tried. I was careful in my 2 star alignment to take up gear slack from the same 2 directions, and that resulted in a perfect go-to the target. I initially got on target and set integration to my guessed best of 16x. But the nearby moon and low altitude combined to make the sky so bright it was nearly completely white-washed out the field and the star was too contaminiated to get optimal data. I could dial down the integration, or I could lower the gain. Given the long duration of 13.4 seconds, I felt I could afford to minimize sky time-dimension noise by longer integration and instead use lowered gain. Lowering the gain from the usual max=41 down to about 20, made for the target showing itself decently above the background. Sharpness set at =4, although it didn't appear to make much difference in the appearance of the field stars. It appeared to my vision that lowering gain darkened the sky more than it darkened the target star, so that apparent S/N was going to be better. This is not something I've tried before; typically if there's a sky brightness problem I'll just lower the integration to get it darker.
I used PyMovie 3.7.65 for my first attempt at analyzing. But I didn't create a folder to hold PyMovie's results, and so while I could see the occultation on the plots, I needed to start over. Anyway, here's some images from the first, the 3.7.65 reduction, using aperture photometry and not the new NRE option. I thought with such a thick star field with high scintillation, fitting to a PSF would buy more noise than desirable. At least on Kirk's Quaoar data, NRE did not improve the signal, and it was designed to do best on faint targets.
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I then downloaded PyMovie 3.7.68x and used that for my completed analysis. I had a little trouble in early attempts , when using 10 apertures and glacially slow, and choosing a tracking star that drifted too close to the top of the chip and ruined the tracking. I instead settled on simple aperture photometry with a single 5px for the target. I used 5 pixel apertures for the sky, ref, and target, and 4 pixel dynamic apertures for the two tracking stars. The tracking was good, the occulation was found, no chance of a false positive, it fit the predicted time window and also the predicted light drop of 0.4 magnitude. But the timing errors are large; lots of scintillation in a crowded star field.
PyMovie screen capture, with the shown aperture on the target star. |
The target star, with the occultation in the middle of the light curve. |
PyMovie composite light curves. |
The PyOTE reductions showed a comfortably confident occultation. Zero chance of a false positive. The depth was 0.43 magnitude, just as predicted, and the duration was 14 seconds, in good agreement with the predicted 13.1 seconds. |
The PyOTE solution and error bars. I used a 20 point smoothing on the reference star and this improved the visual significance of the occultation dip. |
And, zoomed in. Pretty noisy data at that altitude. It's possible there was convective turbulence in the clear air above the fog. Low stratus fog is created by radiative cooling from the cloud tops that exceeds the convective stability limit. |
I set up on the air-field side of Sunlit Lane. |
That's the moon above the telescope |
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I wondered if the timing accuracy could be improved by using "appsum". But the results were far worse; using the similar but brighter star above/left of the target as my reference, in hopes of seeing correlated seeing scintillation. Didn't work, noise only increased. The best reduction was the one above.
After finishing this reduction, I downloaded version 3.7.68z, the latest beta. I did not use it on this event, but will on Quaoar May 13 event, reductions to happen later this week.
