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Published on Saturday 11 June 2011
Since 2010 I have been involved in the observation of occultations of TNOs (Trans Neptunian objects).
Very little is known on these far away asteroids, their orbits, for some which have satellite(s) their mass, but they are so far away that their diameters can not be determined directly.
The observations of occultations of very small objects (like the ones in the main belt between Mars and Jupiter) require a high density of telescopes on the ground. So while I did observe MBA's occultations while I was in France, I have never intended to observe any from Chile because there are almost no amateur astronomers able to provide other chords on the asteroid's shadow and you can not get observing time on professional telescopes for such events concerning "ordinary" asteroids..
Under the impultion of Bruno Sicardy, who maintains a web page for such events, I have observed successfully several of these TNO occultations. Felipe Braga Ribas of the same team, also has a similar page. One of the member of the group, Mario Assafin from Rio de Janeiro has published an exhaustive study of the future occultations involving the Pluto system in Assafin, M., Camargo, J.I.B., Vieira Martins, R., Andrei, A.H., Sicardy, B., Young, L., da Silva Neto, D.N., and Braga-Ribas, F., Precise predictions of stellar occultations by Pluto, Charon, Nix, and Hydra for 2008-2015, Astronomy and Astrophysics, Volume 515, id.A32, 2010.
Of course they typicallly involve rather faint stars, therefore not of the utmost astrometric accuracy, and the same is true for most of these TNOs, which have relatively short arcs (compared to long numbered MBAs), so there is an important work of high quality astrometry in order to predict the reality of a probable occultation. For the November 4th 2010 event for example, predictions for the shadow were somewhere between Alaska and the middle of Chile. Then the object is 14 billions of kilometers away, so ten milli arc second at this distance is 68 kilometers on the ground. The group working in collaboration with Bruno are observers from Brazil (Mario Assafin et al...) and Spain (Jose Luis Ortiz and his team). While the observations here are normally performed by one or two persons, it is the result of a pretty large collaboration (the last letter to Nature on the Eris occultation has 65 coauthors).
The observations have been performed in the past with a 40cm Ritchey Chrétien belonging to Campo Catino (occultation of Charon in 2006 ?), but mostly with Caisey Harlingten's 50cm Planewave telescope, which is equipped with a thinned CCD Apogee U42 camera which we normally used in binning 2x2 with a subframe of 100x100 pixels. When I can, I do the observations, when I can't (mainly during the beginning of the night when I work doing tours), somebody else is replacing me (thank you Sebastian and Nicolas).
There were 6 occultations of TNOs observed in the world in recent times and all 6 were observed successfully from our observatory.
Caisey's scope is the black one in the foreground on the left of the image, 2 domes further back, the ASH2 40cm ASA telescope, normally used to search for large transneptunian objects in the southern hemisphere.
136472 Makemake, April 23rd 2011
50000 Quaoar, May 4th 2011
All these events were or are going to be published, first in IAU circulars or electronic circulars, the Eris event paper has been submitted by Bruno Sicardy to Nature, the others will take time before all the reduction and analysis can be done (with these astronomers, one year is typical :) ). At this rate, a lot more data than previously available on TNO physical properties will be available in a few years, we live exciting times.
To be continued...