Stellar occultations by minor bodies have been widely used to measure
their sizes and constrain their shapes. The technique itself is quite simple
to implement; it is only necessary to measure the lightcurve of the
system minor body plus star before, during, and after the minor body
crosses the line of sight to the star. Measuring the duration of the
dimming in the lightcurve and knowing the speed at which the minor body
moves it is then possible to measure the length of that chord in
distance units. If many such chords could be measured of the same event
using different telescopes on Earth, then it is possible to obtain the
shape of the body.
To predict when and where such
events will occur is difficult as highly accurate positions of both the star
and the minor body are required, the latter being the main source of uncertainty.
For this reason, a large international campaing works to predict and observe
occultations by Trans-Neptunian Objects (TNOs) and Centaurs. TNOs are the most
primitive objects of the solar system and, due to their distance from the
Sun, the most difficult to measure reliable sizes via other
techniques (such as modeling of thermal data).
duration of the eclipsing events varies from a few to almost 30 seconds, depending on the size and distance from
the observer, and hence, high-speed cameras are necessary to obtain the data.
One such event was observed at the William Herschel Telescope during the
first hours of the night April 26, 2012 using ULTRACAM. ULTRACAM is a very fast camera able to obtain short
exposures with very short overheads, which is a critical instrumental constraint due to the high time resolution demanded.
The occulting body was (119951) 2002 KX14, a so-called
classical TNO in a low-inclination, low-eccentricity orbit with a
previous thermal estimate of its diameter of 455 km.
Using the superb
data provided by ULTRACAM on the WHT, astronomers were able to obtain a lightcurve with very sharp
edges, showing no evidence of atmosphere that could have refracted the
light coming from the star, and a total eclipse duration of 20.87±0.05 s, which
translates into a chord of 415±1 km, one of the most accurate measurements obtained so far of a chord.