Graze logo

OCCULTATION OF TYC 0239-00706-1 BY 585 Bilkis
2012 February 11

Two, just over 1 second occultations were recorded from the same telescope by John Broughton, using video OSD technique.

View the updated prediction. 

Observers:
  1    J Broughton,Reedy Creek, QLD, AU
  2    J Broughton,Reedy Creek, QLD, AU
  3(P) Prediction, 26 Jan

Bilkis occultation - 2012 February 11

Discussion:

The circle above is plotted at the expected 57 km diameter of Bilkis.
The two events could indicate as shown above a double star with separation of 36.1 milliarcseconds and PA of 58 degres. But that is alsmost exactly along the line of the chords and the stars have very similar magnitude. The probability of this is small.

Bilkis occultation - 2012 February 11

The other possibility is that we have a grazing event of an object with two lobes that resulted in 2 equal length dips. Again the probability is small.
Unfortunately there are no Damit or ISAM 3D models that could be used to see if the shape and orientation could support this option.

On the balance of probabilities it seems slightly more likely that the second option is is the right one, however in either case with these chords from only one site it is not possible to determine whether John was north or south of the central line of the event so the astrometric location will show the chord centered on the object.

The figure below shows the light curve of the occultation.

Bilkis occultation - 2012 February 11

Dave Herald writes:
I’ve had a look at the light curve for the Bilkis event, and the prediction. The following arise:
The predicted mag drop is 1.2. This corresponds to a light drop from 100% to 33%. The full light level is at around 2400 units – so the bottom light level is expected to be at around 700 units. The first light drop is consistent with this.
The 2nd light drop is to about 1200 units, or 50% light level.
If you interpret this as a double star, the light units from the two components would be:
A = 2400-700 = 1700
B = 2400-1200 = 1200
A+B = 2900
which is greater than the combined flux of the star and asteroid. Hence interpretation that the two dips correspond to different components of a close double star is untenable.
As to the explanation:
Star diameter is expected to be negligible –estimated to be 0.0000”
Fresnel diffraction – for a light drop if 2 mag the equivalent angular extent is 0.0002”, with fades of 0.04 secs on the centre expected.
Given the implied grazing situation, my interpretation is that the first dip was a full occultation, whereas the 2nd dip was only partial due to Fresnel diffraction.
My final thought – if Fresnel diffraction was significant (as would appear to be the case) the times to be extracted from the light curve are at the 25% of full light for the star. That is, at a light drop of 1275, or a light level of ~1100. This inevitably leads me to ask whether there is any need to revise the times.
JT notes:
I have looked at the light curve again and there is definitely some evidence of diffraction on both the D events. It is not so obvious on the Rs but there is a hint.
I feel the second D would maybe a few frames earlier if you took the 25% point.
It would have almost no effect on the astrometric position as that is stil +/- radius.

Observational Data: 

Observation details for MP ( 585 ) Bilkis
By Star              TYC 0239-00706-1
On 2011-02-11  at 18:02:41.0

_______________________________

Observer   John Broughton
Location   Reedy Creek, QLD, AU
Longitude  +153:23
Latitude   -28:06
Altitude   66 m
Datum      WGS84
Telescope SCT including Cass and Mak
Aperture   25 cm
Seeing     Steady Clear
Camera Type  WAT120N        PAL/CCIR
Camera Delay 0.09
Timing     GPS - time inserted Video with frame analysis
Disappearance  18:02:39.43
Reappearance   18:02:40.71
Comments: This appears to be a graze involving two 1-second disappearances
within the predicted duration.
D= 18:02:39.43    R= 18:02:40.71    D= 18:02:42.31    R= 18:02:43.43.

_______________________________

Observer   John Broughton
Location   Reedy Creek, QLD, AU
Longitude  +153:23
Latitude   -28:06
Altitude   66 m
Datum      WGS84
Telescope SCT including Cass and Mak
Aperture   25 cm
Seeing     Steady Clear
Camera Type  WAT120N        PAL/CCIR
Camera Delay 0.09
Timing     GPS - time inserted Video with frame analysis
Disappearance  18:02:42.31
Reappearance   18:02:43.43
Comments: This appears to be a graze involving two 1-second disappearances within
the predicted duration.
D= 18:02:39.43    R= 18:02:40.71    D= 18:02:42.31    R= 18:02:43.43.

Return to 2012 occultation results


[Site Map] [What's an Occultation?]
[Total Occultations] [Grazing Occultations] [Planetary Occultations] [Jovian Satellite Eclipses]
[Timing Occultations] [Reporting Observations] [Coming Events] [Software]
[About Us] [Publications] [Membership] [Links]

[Top of Page][Return to Home Page]