International Occultation Timing Association
European Section
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International Occultation Timing Association European Section |
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Occultations by |
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Moon |
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Asteroid |
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Planet |
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Pluto, TNOs |
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Astrometry |
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Sol.Eclipse |
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Contact us :
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Occultations by Trans-Neptunian Objects including Pluto, Charon, Hydra and Nix
For the year 2012...
High precision predictions
The observation of occultation of stars by Trans-Neptunion Objects (TNO) is one of the most challenging astronomical projects in these times. Occultation Astronomy is the only precise method, to determine the diameter and shape of these far away objects as well as to detect possible gas envelopes of these objects such as Pluto's atmosphere.
The precision of pre-event astrometry has been increased in the past few years so far, that successful observations can be carried out right now. Pluto's orbit has been pinpointed with an accuracy much better than its radius, so that planning of expeditions can be done far ahead of the event.
However, last minute astrometry is still needed urgently, such as it is for standard asteroids occultation work. The required precision however, has to be much better, around 10 milliarcsec.
We have to be very thankful to the group around Marcelo Assafin in Brazil (the RIO Team), who does astrometry with large instruments from Pico dos Diaz observatory in Brazil as well as from ESO sites in Chile. He is analyzing these high precision images with his software package PRAIA. Bruno Sicardy in Paris/Meudon Observatory calculates the predictions for the objects. IOTA-ES can have access to these predictions as soon as they are provided.
Because of the growing number of occultations predicted, IOTA-ES presents on its site only an excerpt of all predictions, for stars brighter then 17m5 and only for those events, where a reasonable area of visibility on earth exists or with a special point of interest.
Keep in mind, that recently a small telescope of only 38cm diameter was able to record a star of 17m in 1 second exposure time with a camera as simple as the QHY6. And for many objects, even occultation observations with a time resolution of 2 seconds are very valuable research data. The diameter of the TNOs can be pinpointed with a precision of around 5% in many cases!!
With best regards from Munich
Wolfgang Beisker
How are the predictions done?
A good description of the process of predictions as has been done for Pluto and its satellites can be found in A&A 515, A32 (2010), "Precise predictions of stellar occultations by Pluto, Charon, Nix, and Hydra for 2008–2015" by M. Assafin et al. .
To summarize, an astrometric star catalog of the path of the Pluto system has been generated in 2007 with the ESO2p2/WFI instrument for the years 2008 to 2015. From these images, using the PRAIA software (Platform for Reduction of Astronomical Images Automatically) positions have been determined in the UCAC2 frame with errors of 50mas for stars up to R = 19mag, and 25mas up to R=17mag.. 2252 predictions of stellar occultations have been predicted for the Pluto system for 2008 to 2015. New proper motions have been evaluated with 2MASS and USNO B1.0 catalog. Pluto's offset has been determined by using the occultations of the years 2005 - 2008.
For other TNO's however, no occultations are available yet, therefore dedicated astrometric observations from other sources have been used (M. Assafin wrote: " the offsets were derived from dedicated astrometric observations with small to mid-sized telescopes from collaborators, not from previous (inexistent) occultations as in the case of Pluto."). Otherwise, the procedure is similar to that of the Pluto system.
Therefore it is to believe, that the predictions are the best predictions, we can get for the moment. As soon as better data are coming in, they will be presented here.
