Free Astronomy Magazine March-April 2018

MARCH-APRIL 2018 I n the fore- ground, one of the domes hous- ing the ExTrA telescopes. The size of the en- trance is a clue to the small size of these structures: the telescope inside takes up almost all the available space. [ESO/Petr Horálek] that does not disperse any of the useful sig- nal, all the limitations imposed by our atmo- sphere remain. The two most significant of these are the efficiency with which humidity in the air breaks down infrared radiation from the red dwarfs (which are particularly bright in that domain of the electromag- netic spectrum); and the atmosphere’s chro- matic absorption, which unpredictably unbalances the colours of the comparison stars with respect to the target star, distort- ing the photometric measurements. So far, researchers have tackled these prob- lems by making observations from particu- larly dry sites with low air humidity, and by using narrow band-pass filters. If the first solution helps in some way, the second one thwarts it, because in this specific case the quality of the photometry improves with the reduction of the filter bandwidth. But the more you tighten the band, the more the light signal weakens, and the larger the telescope diameter must be to obtain a readable S/N ratio. To get around all these obstacles, the ExTrA designers have developed a new method that, in a sense, assigns the task of narrow band-pass filters to the aforementioned low-resolution multi- object spectrograph (R~200), that covers the range 0.85-1.55 microns of the near in- frared, in which there is the peak of sensi- tivity of the CCD camera joined with the instrument. This solution offers the double advantage of collecting much more incom- ing radiation and of being able to tune the colours of the target star and its comparison stars in specific spectral channels, solving the problem of stellar chromatism. By suit- ably managing individual channels, the bandwidth, the focusing of stars and the recording of the signal via CCD, the French team can produce light curves with a pho- tometric accuracy of 0.02% for measure- ment, and with a high S/N ratio. In theory, ExTrA should allow the discovery of planets with a minimum diameter ap- proximately the same as the Earth’s radius. This new method of research also has the great advantage of being much cheaper than the classical methods. !