San Pedro de Atacama Celestial Explorations

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Occultation of asteroid 136199 Eris - November 6th 2010 2h15 TU

Publicado el 06 de noviembre 2010

Preparations :

This occultation was predicted by Marcelo Assafin from Brazil about a year ago. Many astronomers worked in order to improve the astrometry, but it was a difficult task. Eris is about 32 milliarc second of diameter as seen from the Earth. One can see on Bruno Sicardy's page that the predictions varied from Alaska to middle Chile according to whom made an astrometric reduction in order to know the position of the occulted star and that of Eris itself. In the last days though, it looked like it might be for Chile, so Bruno Sicardy alerted me on the 4th of November, asking if we could observe this event. The night before (from Nov 4th to 5th), after my tours, we acquired the field with Caisey's 20inch planewave telescope, and looked at what we could do. With the Apogee U42, we could readout a 100x100 pixel subframe with a readout time inferior to the second. We used Maxim DL for the acquisition and PRISM for the preprocessing. The PC was synchronized with the Dimension 4 software. We made several frames with different exposure time, and together with Bruno the following day, we decided that 3 seconds of integration was a good compromise. There is a very stupid thing in Maxim which is that if you have not calibrated the shutter latency, the time of exposure is stored in integer. It would not have costed too much to store the time in decimal fashion, even not taking care of the shutter latency (which we could have measured later anyway). Strange thing.

Our observations :

On the night of the occultation (which occured around 23h15 local time), we centered the star at dusk, and started recording, Sebastian making sure the star would stay centered during the observation. After the tours, I acquired a sequence of subframes for the dark image (30 individual frames). We substracted the median dark to the images, and saw at one moment during the processing the star dissapear. That was a good moment :). Occultation observation is always very uncertain (unless you get a numbered asteroid occulting a Hipparcos star). We had been lucky.


This is a small animation from the observation. While we were still looking at the images and preprocessing them, we took a Piña Colada to celebrate (as chileans we should have taken Pisco, but it was Piña Colada :)). We know then that Jose Luis's team had observed it too. Their telescope is 20 meters from the one we used. Emails were coming from other places which had either been clouded out (like in Brazil) or had not seen the event (like in France or in the USA). Emmanuel Jehin, with whom we had been in contact before reported his observation. This was nice, it meant a diameter for Eris could be derived.

The telescopes :

We sent preliminary results on a small list of astronomers who had been observing (or meaning to observe) the occultation, and it turned out there are only 3 positive reports, Jose Luis Ortiz who has a telescope here (we offer telescope hosting), Emmanuel Jehin using the TRAPPIST telescope at La Silla, and us. In San Pedro de Atacama, the occultation was longer than in La Silla, and since we used a shorter integration time than Jose Luis (they have a slow STL11000 camera) we have many more data points and more precision in the timing. ASH2 telescope is normally scanning the skies for bright TNOs in the southern sky. It is an ASA 16inch Newtonian with an STL11000 camera on an Astrophysics 1200 mount, and TRAPPIST is a telescope built by Astelco to follow up on exoplanets transits and the study of comets. Caisey's planewave (and another RC 20inch) is normally used to follow up exoplanet observations (mainly through the microfun project), but is used for other scientific purposes.


We decided that each group would publish an IAU circular, and that we will very likely publish the complete analysis in a letter to Nature.

Preliminary results (we cannot give here all the information we have so far, and we still have to work a bit better on the light curves) :

What we can already state is that Eris is smaller than Pluto. By how much ? Not much. A few percent smaller. But smaller. When you measure something, you always have error bars (the ones on which we are working right now). Eris is so many kilometers, plus or minus some kilometers. If you take "reasonable" values for your error bars, the largest Eris we can get from our observations is still smaller than the smallest Pluto...

A smaller Eris means a higher density (we know the mass from the rotation period of its satellite), which is not shocking. And, and this is a problem, it has to have a higher albedo than Pluto, which is like saying it is whiter than white (if you ever saw some stupid washing powder publicity), Pluto being already as white as snow. So let's see how the next days will develop.

We can't say anything about a potential atmosphere. If you look at the animation, you will see that when the star dissapear, you still see some light, it is Eris itself (magnitude 19). In a way, reaching 19th magnitude in only 3 seconds of exposure with a 50 centimeter exposure is already quite impressive. We can only regret that no big telescopes could be used for this observation. And there are big telescopes in Chile. For other occultations Bruno's group has had access to the NTT and the VLT. Big telescopes, big administrations... Also nobody observed at Cerro Tololo or Las Campanas :(. We could have done better, and could have known more. More chords, better precision on the diameter (eventhough it is already quite good), more signal to noise, deeper we can set a minimum pressure of the possible atmosphere of Eris, etc... So not good for science, but good for our small telescopes.

About SPACE (San Pedro de Atacama Celestial Explorations)

The first activity of SPACE is to show the sky to the tourists who come in the heart of the Atacama desert, i.e. San Pedro de Atacama (one of the 3 most visited sites in Chile, with Easter Island and Torres del Paine national park). We receive about 12000 visitors per year. We also have several lodges which we rent to amateur astronomers, to whom we rent visual and photographic telescopes up to 60cm diameter (24 inch). And we have a robotic telescope farm, hosting and maintaining several telescopes (9 right now, 2 more being installed in the coming months). These telescopes either belong to some institutions or private amateurs who either pursue scientific programs (like the ASH2 telescope) or just do astrophotography. The site has an average 300 clear nights per year. Since the beginning of September, October 11th has been the only night clouded out. For now we don't accept more installations. I have to finish the installation of my own 90cm (36") telescope, which I have postponed month after month since 2007. Then I will see.

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