Table of contents
Occultations by asteroids
1.0
Introduction
Information on observing occultations can
be found on the Introduction
and Circulars pages
on the EAON web site. The EAON site should be considered the prime source of
information on all matters relating to occultations. If there are differences
between this site and theirs then please assume they are correct.
From time to time during the course of its orbit around the Sun, an asteroid will appear to pass in front of a star or another Solar System body as seen from Earth. Such a passage is referred to as an occultation. As the asteroid passes in front of the star the latter will appear to dim or disappear altogether. The track width of such an occultation is very narrow – approximately the same width as the asteroid eg; no more than 100 kilometres or so. When the track intersects the Earth there will be only very few potential observers situated along it. Every observation whether positive or negative, is important in defining the limits of the track.
Occultations are the only way, apart from spacecraft missions to asteroids and radar observations of nearby objects, to determine the approximate size and shape of those bodies and are, of course, much cheaper.
The International Occultation Timing Association (IOTA) web site can be found here and that of the International Occultation Timing Association/European Section (IOTA/ES) web site can be found here.
2.0
Predictions
Predictions of possible occultations are available from the European Asteroidal Occultation Network (EAON) , Euraster and Steve Preston’s web site. Predictions are necessarily only approximate as the orbits of most asteroids and the positions of most stars are not known to the required level of precision to accurately define occultation tracks well ahead of time. The margin of uncertainty thanks to the Hipparcos astrometry is probably better than +/-50 km so that the probable area of visibility is accurate in general terms. But the uncertainty is now unlikely to be greater than +/-100 km in most cases now. The predicted times will be more accurate and the probable error is of the order of +/-2 minutes.
Astrometry is sometimes requested prior to the event to allow the predicted track and time of occultation to be refined.
An example of an EAON chart is shown below. It includes star and asteroid data, duration of occultation, drop in magnitude of the star, time of the occultation, the track and finder charts.
I would strongly recommend obtaining the latest orbital elements for the asteroid from the Minor Planet Center’s Minor Planet and Comet Ephemeris Service and plotting its predicted track on a planetarium program such as Megastar. Using a program with which you are familiar greatly reduces the chances of observing the wrong star. The example below shows the track of the asteroid from 10 – 15 Dec with the asteroid occulting the target star on 12 Dec.
3.0
Observing and timing the occultation
Spending several minutes staring at a star field through a telescope can be quite tiring and the eyes start to play tricks. This is particularly the case if the target star is difficult to see due to for example; its faintness, thin cloud or a bright sky background.
Practicing before the event is therefore to be recommended. You may find out that some obstacle is blocking your view of the event particularly if the target star is low down. It will also help you to identify the correct star field and, if the asteroid is bright enough, see it moving towards the target star. Such practice will also enable you to set the eyepiece in such a position that the event can be viewed from a comfortable sitting position if at all possible.
An alarm clock with two alarms is extremely helpful. Set the first alarm to one minute before the predicted time and the second for the end of the observing period. This way you can increase your concentration nearer the predicted time and know when to stop.
At the telescope, monitoring of the target star should commence some minutes before the predicted time and continue for at least the same period after it (the occultation prediction data will usually include a suggested time period). At the instant of occultation the brightness will reduce by the predicted amount, which depends on the relative brightness of the asteroid and star being occulted. Beware also when making an occultation observation of so-called secondary events which may happen some time either side of the primary occultation . Although termed `secondary', such an event may also produce a dip in brightness virtually identical in magnitude to that of the primary event. Any secondary event may represent an occultation by a satellite of the minor planet.
See EAON Circular #5, Parts two, three and four for detailed explanations of various methods of recording the time of the occultation.
4.0
Time keeping
A radio controlled clock which uses the broadcast time signals to update the display will indicate the time to the necessary degree of accuracy. One’s error, or Personal Equation (PE), can be calculated and the occultation timings modified accordingly.
See also EAON Circular #5, Part Five, Short notes about time-keeping sources.
4.1
Timing example
An example of timing using a stop watch (with lap timing feature) and a radio controlled clock and assuming a ‘Positive’ result.
|
Activity |
Stopwatch |
Stopwatch time |
Time of occultation |
Time with PE applied |
|
Star dims or winks out completely |
Press Start |
0 |
( |
20:59:47.7 ( |
|
Star returns to normal brightness |
Press Lap |
6.5 secs |
20:59:54.5 (20:59:48 +6.5 secs) |
20:59:54.2 (20:59:54.5 – 0.3) |
|
Radio controlled clock indicates a complete minute eg; 21:06:00 |
Press Stop |
6 mins 12 secs |
|
|
4.2
Personal Equation
One’s reaction time or Personal Equation (PE) should be calculated and subtracted from the times reported. The PE can be estimated using a stop watch as follows;
- cover the seconds, tenths and hundredths of second’s digits
- start the stopwatch
- press stop when the next full minute is indicated
The indicted seconds and fractions of seconds will be your reaction time. Repeat the exercise say thirty times and take the average (mine was 0.3 secs). See EAON circular #5, Part one, Personal equation and accuracy for a more detailed explanation with examples.
A simulation of an occultation allowing the observer to
determine their 'Personal Equation' can be found here. A more amusing example
can be found here.
5.0
Reporting
By joining the PLANOCCULT mailing list (use link on the Euraster ) observers can see the occultation reports submitted by other observers. Reports should be sent to this list and copied to Gilles Regherre at gillesregheere@yahoo.fr and Jan Manek at jan.manek@worldonline.cz . The ARPS Director is a member of PLANOCCULT and will thus see all reports submitted.
Information about the sizes, shapes and positions of asteroids comes from the occultation observations. Each positive observation gives a chord length across the minor planet and its precise position at that time. It is well worth attempting to observe the occultation if your site is close to but not on the predicted track. Such negative observations (ie; no occultation observed) help to set limits on the size of the asteroid. If poor weather prevents observation then a report should not be submitted.
Below is an example of how the approximate shape of an asteroid was derived from a number of observations. The importance of ‘Negative’ results are clearly illustrated. Observers 2 and 20 (and some others) didn’t see the occultation but their results put limits on the size of the asteroid.
A reporting form
is available from EAON . Report summaries can be
found on the EAON and Euraster web sites.
6.0 Results
European reports
can be found on the Euraster website. Mike Kretlow’s database can be found here.
This database was launched on
7.0 Conclusion
You have to be fairly dedicated to do this stuff. Remember
the ‘Rule of Ten’ ie; for every ten occultations you prepare for
the weather will wash out nine of them and for every ten you actually observe
you may get one ‘Positive’ . Remember however that, for those
observing close to the edge of the predicted track, ‘Negatives’ are
just as important as ‘Positives’.
Now that predictions are improving the ratio of ‘Positives’
to ‘Negatives’ may be improving so go out and prove me too
pessimistic!!!
Having put you off observing occultations for ever just remember that the greater the effort put in the more rewarding is the actual success when it comes – so do give it a try.