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Occultations by Trans-Neptunian Objects including Pluto, Charon, Hydra and Nix

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.

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. Occultations by Varuna and Eris have been observed so far. Pluto's orbit has been pinpointed with an accuracy much better than its radius, so that planning of expeditions can be done far ahead the event.

However, last minute astrometry is still needed urgently, such as it is for standard asteroids occultation work.

We have to be very thankful to the group around Marcelo Assafin in Brazil, 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 packages and due to the cooperation of IOTA-ES with him and B. Sicardy in Paris we can have access to the data as soon as they are provided.

A software has been developed by Hristo Pavlov with help of Dave Herald in Australia, to transfer the astrometry tables into a star catalog, which can be read into the OCCULT software system. The output of the OCCULT program is semi-automatically transferred into a website, which is presented here to the community.

Because of the large interest in these occultations, the full data set is presented here, up to the 21st magnitude. This seems to be a lot too faint for amateurs, but keep in mind, that even amateurs have access to telescopes in the 1-m class. and due to the cooperation with astronomers all over the world, I feel the obligation, to present all the data here, even for extremely faint stars. With an 1m instrument and a dark sky, its possible to observe occultation events as faint as 18th to 19th mag.

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!!

From day to day, more and more predictions will be available, and technologies will be explained here on these pages, so everybody should have a look from time to time at http://www.iota-es.de/tno_header.html .

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.