The Siwa Occultation - Aug 8/9, 2009

San Lucas, California

On the late evening of Saturday night, Aug 8, the large asteroid Siwa (m=10.7) was predicted to occult a m=11.2 star (red magnitude = 10.7) in Capricorn from a wide path stretching from Greenfield to about Gorman. The rank was 98 and the predicted drop was 0.5 magnitudes, so it was a "sure thing" and worth a trip - especially since I was just recovering from the flu and pneumonia and couldn't do much else of interest this weekend - no racing for me for the moment. I drove to Cabrillo, packed up the gear, went to my office and generated the hardcopy of the predictions and maps, and headed south to my planned site at San Lucas, south of King City. Weather was murky, with fog and a low inversion layer that trapped lots of hazy air. I reserved the possibility of driving east from San Lucas to get into a bit higher elevations if the sky conditions warranted. When I arrived, it looked like I was losing close to a magnitude of extinction at around 20 degrees elevation. The event was at 31 degrees elevation, and I hadn't checked exactly how much elevation I could hope to gain, but I figured it was only another 1000 feet at most, and I wasn't sure how deep the murky layer was. In the end, I settled on my original site - the entrance to Wilson Organics farms off Oasis Road just west of the Salinas River.

Set up was uneventful, alignment went well; I chose Polaris and Antares to get a wide calibration. I slewed to M55 not far from the target and it came in bang on-center in the 22mm eyepiece. I slewed to the J2000 coords of the target star, thought I had the right asterism, but then couldn't get the other fainter stars to work out right. I had an hour though, and used it to patiently star hop from a bright double / double star north of the target. The star and asteroid were clearly separated but quite close together at this time.

The sky was hazy, and there was a light wind from the northwest, and a 90% moon 47 degrees away. Seeing was mediocre, but there were no clouds or hint of fog anywhere. Temperature about 60 degrees F.

Notes: The rectangular Canon ZR45mc LCD screen is about 13 arcmin wide with the f/3.3 and PC164cEX2 at Cass focus. You twist the focus knob 1.3 revolutions clockwise after removing the 22mm Nagler/Diagonal combo and inserting the video camera and 1.25" sleeve all the way, to get the stars in focus on the LCD screen.

I'm at the entrance to the wide dirt road - Wilson Organics - is on the gate. I'm about a quarter mile from the Salinas River just off the top of the GoogleEarth shot.

This first plot didn't look good. I thought it might be that the red circle was too tight around the target and the inner sky circle too close and picking up some wandering star PSF. Wanted to show it for reference. The star was not well focused, even at best moments.

Much better with the inner sky circle expanded to 13. Also tried changing 'drift' to 'anchor' - gave bad wandering light curve. Make sure 'drift' is set for both target and (yellow) comparison stars in the future.

The Occular 4.0 solution from the .csv file. Limovie mistook a few 8's for 6's on the time stamp. Manually fixing made for a 'pass' on data integrity. I zoomed in on the occultation light curve, trimming some at the begin and end here.

The errors about the solution look nicely Gaussian

There's a slightly suspicious dropout at frame ~966, which is probably statistical noise but was the most suspicious point for a secondary "moonlet".

Clicking "Final Report" it gives 5 red, 6 gray, and 89 green solutions

Tony George used my .csv file to generate an Occular final report, below, with slightly wider data range and different wing size assumption. Thanks, Tony!

The Darth Vader helmut shadow is from my lens hood

My hero shot


Magellan GPS says: Long = 121deg 02.04' Lat=36deg 06.65'. Topo map says elevation of telescope was 400 ft. WGS84.

Garmin GPS hooked to Kiwi says: Long=121 02m 02.538s, Lat=36 06' 38.68" WGS84

I've also got a couple of pix of the site, I was 61.4km north of the centerline.

Brad Timerson's reduction of all the observations of this event; Patrick Wiggins' timings are preliminary. My Excel IOTA report.