Miss/occultation on Pic du Midi
>From J.Lecacheux, Meudon
Dear Planoccult list manager,
I have prepared a very detailed report about our
Kleopatra semi-positive observation. You might find it
verbose. In this case it could be reduced to the
second part ("Epilogue") describing data analysis.
Best regards.
copy to Jerome Berthier, Bureau des Longitudes (IMCCE), Paris
=============================================================
>From Jean Lecacheux, Meudon
I have just ended the full reduction of our CCD images taken at
Pic du Midi during the (216) KLEOPATRA / GSC 673 1438 miss three
weeks ago, and I obtain a very surprising result :
Actually this miss ... was NOT A MISS :
a grazing occultation probably occurred at Pic du Midi !
Remember that the predicted track passed between Portugal and
Canary Islands, then should cross Morocco at 1200 km from Pic du
Midi. Although no update had been published by J.Manek, we felt
very relaxed when preparing this observation at the 1.05-m
telescope. I was assisted by many people : F. Colas (Bureau des
Longitudes, Paris), P. Laques (a retired Pic-du-Midi astronomer),
D. Bardin and F. Bardin, astronomers from Marseille, and
Ph. Rousselot from Besancon University.
The consensus was that only astrometric results could be expected,
as the predicted asteroid/star flux ratio (of about 3) was very
unfavourable. Such a ratio meant a 0.3 magnitude occultation drop,
precluding tape recording at video cadency, and also precluding
visual survey with the 16-cm guide refractor. Morever the star was
especially faint, of V ~ 12.6 according preliminary measurements.
However KLEOPATRA is a very important asteroid, a target of
priority 0 for all occultation observers. This is a very elongated
body (axis ratio a/b =2.6 b/c=1.3), somewhat like (433) EROS, but
far larger than EROS, since its major axis lies in the 200 km
range.
KLEOPATRA is a possible contact binary asteroid (unlike Eros),
according very recent results obtained at the ESO 3.6-m with the
"Adonis" adaptative optics (see IAUC 7308).
I contacted the day before occultation J.Berthier and D.Hestroffer
at Paris Observatory, who were members of the successful ESO team,
and who possess a binary 3-D model of Kleopatra. So I knew in
advance the probable attitude of the asteroid at appulse time.
Also on March 6 I sent a special warning on Planoccult, with the
hope to mobilize CCD observers in Portugal, southern Spain or at
Teide Observatory.
On March 07 evening during the Kleopatra approach, we had clear
sky at Pic du Midi, not exactly of the best photometric quality,
but very convenient for differential photometry or accurate
astrometry. The appulse was expected near to 22:35:30 UT at 0".6
SSE according E.Goffin's map (from EAON). We used, as usual for
occultation work, the full 17.8-meter focal length of the
telescope, and our pixel size was 23 microns or 0".2644.
Kleopatra's measured brightness attained its maximum at 22:02 UT,
as expected, 1.26 magnitude brighter than the GSC star, then began
to decline. We watched on the computer screen the large asteroid
bright spot approaching the faint stellar spot, and the consensus
was that Kleopatra will pass a bit farther than expected, perhaps
at 0".8 south, in any case too north for Canary Islands and too
south for Portuguese observers ! Moreover south-Portugal appeared
clouded over according the last Meteosat infrared map.
What follows shows that we were mislaided by our cathodic display.
To easily perceive the star we had tuned brightness and contrast
at maximum levels, with the probable consequence that Kleopatra's
apparent centroid was shifted from true position by more than
1 mm. But only the real time display had this anomaly. The numeric
images of our CCD camera (used later for astrometry) did not
exceed a small fraction of the permissible dynamic.
3 minutes before appulse, as asteroid and star were quickly
merging, I switched to our fast CCD mode. Only a small 19" x 17"
window centered on occultation target was numerised by the CCD
controller and was saved on disk. As the predicted maximum duration
of the occultation was only 4.6 sec. according E.Goffin (or 6 or
7 sec. according the Hestroffer-Berthier model), I choosed a 0.80
second integration time. So we obtained a 1.15 sec. measurement
cycle : each 0.8 sec. exposure was followed by a 0.35 sec lapse
reserved to CCD lecture and signal numerisation.
I had clear conscience that 0.80 sec. was a too short duration to
clearly extract from the noise any short occultation feature. Our
prime intention was to remove the red filter used for astrometry,
but I decided it was something too risked at H - 2.5 minutes.
After all Kleopatra should pass over Africa... So we continued
recording with this red filter that blocked 60% of the light.
At H + 3 minutes Kleopatra began to separate from the star in the
19" x 17" window (of course no extinction had been noticed), and
we returned during 20 minutes to astrometric exposures, then we
closed observation.
There was one more miss in our rich collection, but we knew that
our astrometric data will permit to deduce the actual track
that Kleopatra had followed with +/- 10 km of accuracy.
EPILOGUE (3 weeks later)
150 astrometric images (among a total of 229) were selected and
measured. My final result for the closest approach is
a minimum angular distance of 0".042 +/- 0".004 (NNE)
at 22:35:36.5 +/- 00:00:01 UT
A fantastic finding !
1- Kleopatra did not cross Morocco, but south of France !
2- We were in immediate neighbour of the occultation.
According the Hestroffer-Berthier model the shadow of Kleopatra
should sweep a strip of width 140 km or 0".086 (projected on
sky). Thus Pic du Midi should pass at 2 +/- 7 kilometers INSIDE
the southern limb !!!
Consequently some occultation might have occurred at our site,
its probability being more than to 50% if the asteroid size is
correct in the model.
I have analyzed the sequence of images recorded in fast mode.
As we feared, there is strong noise. I find it of +/- 0.14
magnitude (standard deviation), with the following tentative
decomposition :
- photon noise : 0.05 magnitude
- scintillation noise (high frequency) : 0.05 magnitude
- transparency noise (low frequency) : 0.05 magnitude
- CCD (reading) noise : 0.11 magnitude
Remember that noise has to be composed quadratically :
(.05*.05)+(.05*.05)+(.05*.05)+(.11*.11) ~ (.14*.14)
Atmospheric contribution was stronger than usual as we
observed at low elevation (3.4 air masses). Clearly the
integration time was forced too short regarding the available
signal. Not removing the red filter was a blunder : without
filter CCD noise could be lowered at atmospheric noise level.
Among the data we remark two deviant values
-at 22:35:36.9, 2.6 standard deviations below the median line
-at 22:35:38.0, 3.2 standard deviations below the median line
It is a striking coincidence to find such a spike of low signal
just one second after the precise appulse time !
Moreover the model of Hestroffer and Berthier predict that
the southern limb passed 2 sec. after the center of mass :
* *
* *
* *
* * * *
* * * *
<---- * * *
24.25 km/s * * * *
* * *
*
* *
---------------------------------*-----*--------------
< 2 sec.>
We conclude that there is good time agreement.
It is interesting to compare the measured drop with the
expected drop. Indeed we have derived a very accurate
asteroid/star ratio of 3.06 (at appulse time) as by-product of
our 150 astrometric exposures. It results that any true
occultation should produce a 0.31 magnitude drop, corresponding
in our data to - 2.6 standard deviations below the median line.
One may conclude from a simple statistical point of view,
taking in account our low signal/noise ratio, that we were
unable to detect occultations of one second occuring at
unknown instant (S/N ~ 1), but that we were able to detect
occultations > 2 seconds occuring at a known instant (S/N > 3).
The picture above shows data with x1 and x2 binning.
The second curve appears a bit more convincing.
Finally I risk the following result :
A graze of 2.4 +/- 1.6 sec. at the southern limb of the
trailing lobe of Kleopatra occured at Pic-du-Midi. It was
centered at 22:35:37.5 +/- 0.5 sec. UTC.
From our photometry we passed between 2 an 30 km inside the
southern limb, a value to be compared to the 2 +/- 7 kilometers
deduced from astrometry (if the model is correct).
****
The final lesson is :
Do not rely on predictions computed with the GSC version 1.1,
when no update has been published ! These occultations are
exceedingly difficult to observe (implying very faint stars)
and often pass where nobody is waiting for.
A new GSC ajusted upon the Tycho-2 subset should be released
in the near future. Noticeable prediction improvements should
arise.
*****************************************************************************
* # STATION DE PLANETOLOGIE DES PYRENEES (S2P / Pic du Midi) *
* Telescope de 1 metre, Observatoire du Pic du Midi, La Mongie *
* 65200 Bagneres de Bigorre, FRANCE. Tel:(33) 05 62 93 34 06 *
* (42 56 12N, 0 0 34E, 2870m) Fax:(33) 05 62 93 41 65 *
* http://www.bdl.fr/s2p/ *
*****************************************************************************