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Preamble:
The updated path of this event swept across the north Queensland coast and then onto the southern North Island of New Zealand. The accuracy of the updated path was expected to be moderate. The updated track is available here.
The star was magnitude 10.9 and the event occurred at a convenient time of night on a Tuesday evening, leading to a major effort being mounted in New Zealand. Reports were eventually received from 16 observers. A number of observers were new to occultation events and several travelled significant distances to participate. Of the 16, three observers were clouded out, seven saw a miss, and six chords were obtained across the asteroid.
Two plot are presented below, the first showing the locations of all the observers, and the second an enlargement of the region around the asteroid. It will be noted that many of the chords are offset relative to adjacent chords. The reasons for this are discussed extensively in the Discussion section below.
Observers:
1(M) D. Watson, Whakatane, NZ
2(M) D. Brock, Hamilton, NZ
3 I. Cooper, Kimbolton, NZ
4 S. Suter, New Plymouth, NZ
5 G. Blow, Cheltenham, NZ
6 R. Wright, Brunswick, Wanganui, NZ
7 R. Skilton, Tayforth, Wanganui, NZ
8(C) T. Whelan, Cape Egmont, NZ
9 N. Munford, Longburn, NZ
10(C) G. Hudson, Shannon, NZ
11(M) J. Field, Waitarere Beach, NZ
12(M) R. Dippie, Hokio Beach, NZ
13(M) T. Butt, Te Horo, NZ
14(C) R. Dodd, Martinborough, NZ
15(M) R. Hudson, Pukerua Bay, NZ
16(M) I. Porritt, Whitby, Wellington, NZ
NOTE: C = Cloud
M = Miss
All observers plotted:
Enlarged region surrounding the asteroid:
(Plot generated using WinOCCULT. Disappearances on the left, Reappearances on the right)
DISCUSSION:
Firstly, all the observers who participated in this event are to be congratulated on their efforts. Six occultation chords across an asteroid is one of the better results obtained within New Zealand, and the degree of interest shown in the Vincentina event bodes well for this type of observation in the future. The results of this occultation also define the position of Vincentina to very high accuracy, thereby providing a valuable data point in further refining the orbit of this asteroid. The actual track appears to have moved north by a small amount - perhaps 1/5th of a track width - relative to the initially updated prediction.
Fitting the Data:
However, while the Win-OCCULT software used to produce the above fit has converged to axes of about 133 x 51 km, the convergence is poor, with the software cycling between several other results within the same ballpark. As such, the best we can say is that Vincentina is probably more cigar-shaped than spherical (in keeping with many other asteroids in this size range).
The poor convergence is due to a couple of factors:
To alleviate the former we need better observational coverage, particularly in the central and eastern North Island. As regards the latter, differences in chord length and relative position are not due to the physical properties of the asteroid but rather show up the limitations of timing these events by purely visual means.
The role of reaction times:
In this event a number of observers of differing levels of experience took part. Studies have shown that the accuracy of visual occultation timings is severely affected by the reaction times of observers, and that reaction time is a function of:
It is known that first time observers or those with little experience can take surprisingly long to react to an occultation event - sometimes in excess of 2 seconds, and between five and ten times longer than they think they took to react. Even experienced observers have difficulty with reaction times. For example, studies by the International Lunar Occultation Centre and others show that an average reaction time for an experienced observer at a lunar occultation is about 0.4 seconds for a disappearance (and 0.6 seconds for a reappearance). In lunar occultations of course one knows that the star will disappear within a band of usually about 10 seconds. In a minor planet occultation event, because event paths (while considerably more accurate than a few years ago) are still not exact, one cannot be certain that one will see the event at all, let alone within a narrow time window. Thus average reaction times even for experienced observers at these events are likely to be higher than for lunar occultations. Another factor to be considered is that reaction times for disappearances and reappearances can differ markedly, and of course these factors will be different for each observer contributing results. In summation, observing occultations visually does have significant limitations. (However some of these can be alleviated somewhat - see following discussion).
In the case of Vincentina all the effects described above are at play. For example, noting that disappearances occur on the left side of the chords, the three most experienced observers (chords 3, 5 and 9) all had disappearance times significantly earlier than the other less experienced observers. That said, within themselves the chord lengths of observers 3, 5 and 9 are not particularly consistent (within the context of a probably elliptical asteroid), probably due to reappearance reaction time issues rather than the physical characteristics of the asteroid. Disappearance times for the other observers are all significantly delayed with respect to chords 3, 5 and 9, indicating that significant reaction time issues are likely to be at play. If one considers for example that the length of chord 5 was only 6.2 seconds, the offset with respect to this for some adjacent chords shows the magnitude of the relative reaction times. [NB: For chord 4 the duration of the occultation was timed while the UT times of the event were determined only approximately].
While our report forms do provide places for observers to estimate and record their reaction times, observers often don't do so - simply because (quite understandably) it is so difficult to estimate a meaningful value. Under such circumstances some have advocated improving the fit by sliding each chord along its length so that the midpoints "line up". However we have generally resisted the temptation to do this on the basis that tampering with the data in this way is unsustainable in a scientific sense.
Potential Solutions:
The reaction time of an observer can be reduced significantly with practice. i.e. the more occultation events timed, the more the observer becomes relaxed and au fait with what to expect, and the smaller the reaction time. However as minor planet occultations crossing any one observer's location are still relatively rare, sufficient practice is unlikely to be obtained by observing these events alone. Lunar occultations however occur frequently and can be timed with the same simple equipment. Thus, the adoption of a regular program of lunar occultation observing is a viable means of helping minimise the reaction time issues discussed above.
In the longer term though, significant increases in the accuracy of timings are only likely to be achieved by the adoption of methods of observing which cut out the human component entirely. Such methods include video, CCD "drift-scan" techniques, and photometry. In the past these have not been viable for many amateur observers owing to cost and other considerations. However the advent in the last few years of very cheap, extremely low light-level surveillance cameras which simply slot into a telescope in place of the eyepiece, is revolutionising observing. Similarly, the advent of relatively cheap astronomical CCD cameras (although not as cheap or easy to use as video) has also made the CCD "drift-scan" technique available to serious amateurs.
For information on video and CCD observing techniques follow these links:
In Summary:
The Vincentina occultation has yielded one of the best results obtained in New Zealand from a minor planet occultation. A very accurate position for the planet at the time of the event can be derived, while the results indicate that (366) Vincentina is markedly non-spherical. All the participants in this event are to be congratulated for their efforts.
Observational Data:
CHORD 3:
Observer's Name : Ian Cooper
Aperture (cm) : 20
Focal length (cm) : 100
Type (e.g. SCT; Newtonian) : Newt
Magnification : 40
Observing site name : Old SH 54 0.7km sth of Dicks Rd, Kimbolton
Longitude (DD MM SS ; East +ve) : +175 48 36.4
Latitude (DD MM SS ; South -ve) : -40 00 37.3
Height above Sealevel (metres) : 500m
Geodetic Datum (e.g.WGS84,NZ1949): NZ1949
Height Datum (if known) : -
Sky Transparency (Delete two) : Good
Star Image Stability (Delete two): Fair
Time Source (e.g. WWVH, GPS) : WWVH
Recording method (e.g. tape) : stopwatch
Could you see the Asteroid? : No
Approx. Limiting Magnitude : -
| Estimated |
Universal Time | Reaction | Accuracy, Remarks
h m s | Time (sec) |
Started Observing : 08.38.00
Disappearance At : 08.39.33.44 0.2
Reappearance At : 08.39.40.28
Stopped Observing : 08.39.45.00
Was your reaction time (also known as Personal Equation) subtracted from
any of the above timings? :No
List all Interruptions to Observing:
Break 1: Pulled over by the Law for exceeding the required speed limit for an observer
Break 2: Took time out to consider the effects of $170 fine on finances!
ADDITIONAL COMMENTS:
This observation was observed at the minimum rate of $25.00/sec or $90,000.00/hr.
Who said I wasn't a professional astronomer?
CHORD 4:
Observer's Name : Sharon Suter
Aperture (cm) : 20
Focal length (cm) : 100
Type (e.g. SCT; Newtonian) : Dob Newtonian
Magnification : 100
Observing site name : Home (New Plymouth)
Longitude (DD MM SS ; East +ve) : 174 02 14.4
Latitude (DD MM SS ; South -ve) : -39 04 43.7
Height above Sealevel (metres) : 50
Geodetic Datum (e.g.WGS84,NZ1949): WGS84
Height Datum (if known) : MSL
Sky Transparency (Delete two) : Good
Star Image Stability (Delete two): Good
Time Source (e.g. WWVH, GPS) : WWVH
Recording method (e.g. tape) : Stopwatch
Could you see the Asteroid? : No
Approx. Limiting Magnitude : -
| Estimated |
Universal Time | Reaction | Accuracy, Remarks
h m s | Time (sec) |
Started Observing : 08:15
Disappearance At : 08:39:24 0.5 sec
Reappearance At : 08:39:30
Stopped Observing : 08:50
Was your reaction time (also known as Personal Equation) subtracted from
any of the above timings? : No
ADDITIONAL COMMENTS:
Clear most of evening - except for small patch of high cirrus cloud that appeared
(typically!) at 8.29pm. This cleared off at 8:35pm. Good before and after.
I had set my minute hand watch and a portable clock to the atomic voice clock at IRL.
I used this to determine the time. The stop watch was only used to time the actual duration
- along with listening to WWVH 10.00mhz. I then double checked the time with IRL after
the event. I see I'll have to find something more accurate now I've got the 'bug'!
CHORD 5:
Observer's Name : Graham Blow
Aperture (cm) : 20
Focal length (cm) : 200
Type (e.g. SCT; Newtonian) : SCT
Magnification : -
Observing site name : Cheltenham, NZ
Longitude (DD MM SS ; East +ve) : 175 38 14.8
Latitude (DD MM SS ; South -ve) : -40 06 54.5
Height above Sealevel (metres) : 210
Geodetic Datum (e.g.WGS84,NZ1949): WGS84
Height Datum (if known) : MSL
Sky Transparency (Delete two) : Good
Star Image Stability (Delete two): Good
Time Source (e.g. WWVH, GPS) : WWVH
Recording method (e.g. tape) : Tape
Could you see the Asteroid? : No
Approx. Limiting Magnitude : -
| Estimated |
Universal Time | Reaction | Accuracy, Remarks
h m s | Time (sec) |
Started Observing : 08:38
Disappearance At : 08:39:31.0
Reappearance At : 08:39:37.2
Stopped Observing : 08:45
Was your reaction time (also known as Personal Equation) subtracted from
any of the above timings? : No
CHORD 6:
Observer's Name : Ray Wright
Aperture (cm) : 25.4
Focal length (cm) : 101.6
Type (e.g. SCT; Newtonian) : SCT
Magnification : 39x
Observing site name : Wanganui
Longitude (DD MM SS ; East +ve) : +175 01 12.6
Latitude (DD MM SS ; South -ve) : -39 50 45.5
Height above Sealevel (metres) : 165
Geodetic Datum (e.g.WGS84,NZ1949): NZ1949
Height Datum (if known) : MSL
Sky Transparency (Delete two) : Good
Star Image Stability (Delete two): Good
Time Source (e.g. WWVH, GPS) : WWVH
Recording method (e.g. tape) : Tape
Could you see the Asteroid? : No
Approx. Limiting Magnitude : 13
| Estimated |
Universal Time | Reaction | Accuracy, Remarks
h m s | Time (sec) |
Started Observing : 08:20
Disappearance At : 08:39:28.2
Reappearance At : 08:39:34.6
Stopped Observing : 08:45
Was your reaction time (also known as Personal Equation) subtracted from
any of the above timings? : No
ADDITIONAL COMMENTS:
I was not happy with the quality of the time signals. I think the electronics on the
telescope might have caused distortion. I will try to correct this. Hope to better
next time.
CHORD 7:
Observer's Name : Ross Skilton
Aperture (cm) : 20
Focal length (cm) : 203
Type (e.g. SCT; Newtonian) : SCT
Magnification : 20mm Erfle=101x
Observing site name : Tayforth, Wanganui
Longitude (DD MM SS ; East +ve) : 174 59 55.2 East
Latitude (DD MM SS ; South -ve) : -39 54 55.3 South
Height above Sealevel (metres) : 56 m
Geodetic Datum (e.g.WGS84,NZ1949): WGS84
Height Datum (if known) : MSL
Sky Transparency (Delete two) : Good
Star Image Stability (Delete two): Good
Other Conditions : Gentle SE breeze, Clear sky. No Moon, Dark site
Time Source (e.g. WWVH, GPS) : WWVH
Recording method (e.g. tape) : Tape
Could you see the Asteroid? : No
Approx. Limiting Magnitude : 13
| Estimated |
Universal Time | Reaction | Accuracy, Remarks
h m s | Time (sec) |
Started Observing : 8:34:00
Disappearance At : 8:39:28.2 May have been a little slow as my eye was blurring with the tension
Reappearance At : 8:39:33.6 Seemed good
Stopped Observing : 8:41:00
Was your reaction time (also known as Personal Equation) subtracted from
any of the above timings? : No
ADDITIONAL COMMENTS:
First MP occ where I wasn't clouded out. Equipment performed well but don't know reaction time.
Average occultation time from tape is 5.43sec, agrees reasonably with stopwatch at scope of 5.49sec.
CHORD 9:
Observer's Name : Noel Munford
Aperture (cm) : 20
Focal length (cm) : 200
Type (e.g. SCT; Newtonian) : SCT
Magnification : 200X
Observing site name : Longburn, NZ
Longitude (DD MM SS ; East +ve) : +175: 32: 06.9
Latitude (DD MM SS ; South -ve) : -40: 23: 18.8
Height above Sealevel (metres) : 30
Geodetic Datum (e.g.WGS84,NZ1949): NZ1949
Height Datum (if known) : Mean Sea Level
Sky Transparency (Delete two) : Good
Star Image Stability (Delete two): Good
Other Conditions:
(Wind, Clouds, Lights, etc.): Clear, No Wind
Time Source (e.g. WWVH, GPS) : WWVH
Recording method (e.g. tape) : Stop watch
Could you see the Asteroid? : No
Approx. Limiting Magnitude : 13
| Estimated |
Universal Time | Reaction | Accuracy, Remarks
h m s | Time (sec) |
Started Observing : 08 37 UT
Disappearance At : 08 39 30.33
Reappearance At : 08 39 36.61
Stopped Observing : 08 41 00
Was your reaction time (also known as Personal Equation) subtracted from
any of the above timings? : Yes
If YES, state value : 0.2 sec
ADDITIONAL COMMENTS:
The observation was absolutely definite.
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