The Washingtonia Occultation - Nov 24/25, 2013

This was a faint and low-drop event; magnitude 12.0 combined image dropping to magnitude 12.8 for 7 seconds max. Fortunately weather was perfect; clear, no moon, 79 degree altitude. I planned to swap out the Owl 0.5x from the Orion Shorttube on the 12" scope to the 10" for the occultation, to take advantage of the concentration of the star image onto the small pixels. I also planned to get a 1-minute time exposure of the star to verify an occultation happened, and perhaps get crude timings as well.

Got the 12" on the target first, and set it tracking. Then went out to set up the 10" scope. When it came time to turn on the camcorder... trouble. I'd deliberately left the camcorder at the observatory after the Jenny event a week earlier, to simply make sure I didn't forget it. However, it was quite cold tonight, in the low 40's, and I also had not charged the battery in a while. Fatal! The battery died before the event started and too late for me to drive home to get the charger. So, I had to settle for just the star trails...

I got a few shots before the occultation, to verify, check the asteroid color, and see the track it would follow. I succeeded in getting a 60 second image cenetered on the expected occultation time. The target+ asteroid showed up reasonably well, considering their combined 12.0 magnitude. The large .tif file of the event is here.

Central occultation was predicted for 7:40:07 for me, and 7:40:08 for Rick Baldridge in Campbell. Sites list here.

About an hour before the event, from the 12" with ST2000xcm CCD, 5 second exposure

5 second exposure, 11 minutes before the event.

The occultation: 60 second star trail exposure.



There's no 7 second clear dropout, but the deepest drop in light occurs for 1.59 seconds at

begin 7:39:40
D 7:40:18.21 +- 0.15 sec
R 7:40:19.80
+- 0.15 sec
end 7:40:40

using the histogram function in CCDOPS on the .tif file. It's just to the left of the blue 'color blotch' about 2/3 of the way from the bottom of the star trail to the top. I did dash to the 10" scope and watched visually, with my eyes recovering enough night vision (after looking at the LCD monitor) to be able to see the target about 10 seconds before the events. I saw the target star, then had the impression it was gone for a few seconds, before the impression of an R, but I did not have my tape recorder and did not make any timings. From listening to WWV, this corresponded to about 17 seconds or so after the minute mark; 7:40:17. Here's the Excel report sent to Brad Timerson on Nov 28, 2013. Brad then sent me this reduction profile (right), which indicated a bad timing... That prompted me to take a second look at the photo.

Second Reduction
Since the asteroid is clearly redder than the target star, and since the video camera is more sensitive to red light than V light, the true drop would likely be less than 0.8 on the video screen. I'd chosen my initial D and R on two assumptions
1. The south shift in the asteroid path was small, so that my chord was likely the shortest of our trio of observers.
2. The drop in magnitude was deep and nearly complete.

Both assumptions, on looking at the pre-occultation photos, and the timings of Derek and Rick Baldridge, were clearly false. I then made a second version of the TIFF file, processed in Photoshop to enhance color, reduce sky noise, adjust levels using "curves" and the eyedropper to select levels best suited to the non-occulted and occulted startrail levels, and then redid the CCDOPS analysis using the crosshairs function once again, from scratch. New TIFF file

the sky trailed objects from the bottom to the top of these posted images. The R seems quite obvious and unambiguous to the eye, and it is the D which then is in question. Allowing the asteroid alone to be reasonably visible and above sky, I find another point which seems to correspond to a drop in brightness by ~1/2 (vs. near complete).

Trail speed = 13.76666 pixels/second
Begin at 7:39:40UT = pixel Y value =1039
D at 7:40:13.85 = pixel Y value = 573
R at 7:40:20.00 = pixel Y value = 488
end at 7:40:40 = pixel Y value = 213

Photoshopped version; enhance color saturation, Tools/Noise/Despeckle 3 times, AstroTools 'space noise reduction' 2x, Curves twice, then measured as a tiff file in CCDOPS. Converted finally to .jpg for posting here. The D and R labels are close but not exact, due to the vertical spacing of the text tool in photoshop. They give a good idea of the points I decided were the D and R points.

John Broughton kindly ran the tiff file through MaxImDL and Scanalyzer to produce this light curve along the star trail. The derived timings agree well with my by-eye measurements with CCDOPS.

The profile reduction for the new timings, makes a rounder asteroid - to be expected for a reasonably large (97 km) asteroid.

Predicted path. My track is in gray, or at least fairly close.

The Scanalyzer light curve analysis on the un-photoshop'd TIFF file, from Broughton gives
D = 7:40:13.80 +- 0.32
R = 7:40:19.78 +- 0.32


Other Results:

Rick Baldridge reported a positive from Mtn View area, Derek Breit had a 5 second event in Morgan Hill, nominal predicted central occultation 2 sec before me.