Analysis of a Lysistrata "Blink"
Robert L. Sandy
I was informed by e-mail on 11/17/03 about a possible occultation of the 6.6-mag, target star, SAO 144313 (Spectral Class K5--Orange) by the faint, and very fast-moving 15th-mag. asteroid Lysistrata. The occultation was predicted to occur at about 2:20 on 11/19/03 UTC, or 8:20 p.m. CST on the evening of the 18th.Κ This was a very favorable time for my home location at 94d 20m 42.3s west longitude and 39d 01m 40.2s north latitude.Κ Also the predictions showed that the target star/asteroid would be at an altitude of 23-degrees at azimuth 240-degrees for my location.Κ The 28% waning phase moon would not even have to be considered, since it would not rise till 2:a.m. the next morning.
The predictions supplied by Steve Preston indicated that I would be near the extreme southern edge of the path.Κ Also, very noteworthy, the Preston predictions showed that Lysistrata was "really moving out" at a shadow speed of one second of time per degree of longitude across the earth's surface in the direction southwest to northeast.
About a month before this event, I decided to purchase a twice used Celestron 8", f/10 GPS Nexstar SCT.Κ I found that the target star was on the Nexstar's menu of SAO numbered stars, --how good could it get?
Now another noteworthy item is that I came very close to moving straight east to a high hill instead of staying here at home in a mobile home park, since I had thought that the target star might be behind a tree to the southwest during the occultation period.Κ I'm very glad I did not, since, if I had, I most likely would have had a miss.
I observed using the 8-inch, f:10, Celestron Nexstar and recorded with the PC23C camera.Κ WWV was recorded on the audio channel.Κ I thought I saw a "blink" at one time and commented so on the audio recording.Κ On replay and single step I observed what I thought was a single frame occultation.
On the same occasion Walt Robinson reported a 1.1 second occultation.Κ Walt's location is about 2 miles north and 38 miles west of mine.Κ That seemed consistent with a path generally a bit north of his location and the 24 km diameter of the asteroid.Κ Intrigued by the "blink", Art Lucas volunteered to try to do a digital analysis of the tape.Κ
At Art's lab the tape was copied with arbitrary time insertion using the STVASTRO by Blackbox so as to tag each field.Κ Several seconds of the tape were copied to computer memory field-by-field using a Panasonic, 4-head VCR and a Pixelsmart frame grabber.Κ The several hundred images were titled and arranged for automatic analysis.Κ An analysis program was written in Microsoft QuickBasic 4.5.Κ The advantage of operating in this mode was that it provided for flexible "what if" of the result.Κ
The final analysis was done by placing a box around the star, finding its peak pixel in the box, shifting the box so that the peak pixel was in the center and summing the pixels in the box.Κ A ring of pixels around the box was used to subtract background signal.Κ
The value of the average brightness was recorded along with the peak brightness and the x,y location of the peak.Κ This provided some evaluation of the stability of the "seeing" as the peak moved about in the turbulent air.Κ Typically, the peak pixel moved by as much as 5 pixels.Κ The motion was relatively slow.Κ It seemed to have a period of approximately one second which would seem to be explicable in terms of air turbulence.Κ The motion in the x-direction was similar to that in the y-direction indicating that the motion was not correlated with electronic interference.
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Figure 1 shows the data in a 5 second range about the identified "blink".Κ A clear event is shown near 31 seconds.Κ These several seconds of data provide assurance that the data are real and not just a statistical anomaly.Κ Note that a similar, less credible, event occurs at about 28.4 seconds.Κ The shortness of that event along with its similarity to single, statistical, occurrences leaves one with not sufficient certainty to report it as an occultation.Κ Κ
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Figure 2 shows an expanded view of the 31 second event.Κ Note the sloping sides of the box indicating diffraction fringes similar to those described by Frank Anet at the 2003 IOTA meeting.Κ While the data are available field-by-field with resolution of 16 ms, the limiting resolution of the measurement is eventually established by these diffraction effects.
We conclude that the track of the occultation passed a bit north of Walt Robinson's location and just grazed my location.Κ The closeness of the absolute times of the measurements leaves little possibility that this was an observation of a satellite of the asteroid.Κ The numerical analysis provides clear, statistically valid, evidence that the "blink" was not an aberration or a "blink of the eye".