Skip to main content

Correction of the atmospheric phase noise in interferometric radio-astronomy by water vapour radiometry

Final Activity Report Summary - INTERFEROMETRY, IRAM (Correction of the atmospheric phase noise in interferometric radio-astronomy by water vapour radiometry)

Interferometry is a powerful technique whereby a number of smaller telescopes are combined electronically to simulate a larger telescope capable of astronomical images of extremely high detail. The principle behind the technique is that light from an astronomical source reaches the different elements of the interferometric array at different times, depending on which direction it is coming from. Precise measurements of these time delays can be used to pinpoint the origin of the light accurately, thus increasing the resolution of the obtained image. The Institut de Radio Astronomie Millimetrique (IRAM) in Grenoble, France, operates such an interferometric array in the millimeter wavelength at a high altitude site on the French Alps.

At millimeter wavelengths, additional delays are introduced by refraction of the incoming radiation inside the Earth's atmosphere. The effect results in rays of light not travelling in a straight line, and the excess path introduces an artificial delay which results in a degradation of the image quality. In particular, it is bubbles of water vapour in the atmosphere which are responsible for this effect. Fortunately, water vapour also emits radio waves, the intensity of which is proportional to the amount of water vapour present. A direct measurement of the intensity of the water vapour radiation therefore should indicate the amount of excess path in the propagation of the astronomical signal towards the telescope.

The objective of this research proposal was to use receivers, sensitive to water vapour emission (called water vapour radiometers) to counter the effects of the atmosphere on astronomical imaging. The IRAM interferometer consists of 6 antennas, each of which has been equipped with a water vapour radiometer. The main goal achieved during this project was to develop a calibration scheme for these receivers, in other words a way of translating the output signal into an absolute quantity of water vapour. This was done by closely monitoring the performance of the system and understanding the intrinsic variability in the sensitivity of these receivers. The new calibration scheme utilises a set of sky measurements by the radiometers at different elevations, to estimate the level of noise intrinsic to the receivers themselves.

These measurements are now performed at regular intervals with the purpose of updating the parameters that define the radiometer performance. Each of the 6 radiometers now provides a second by second measurement of the temperature of the sky, which combined with a sophisticated atmospheric model results in an independent measurement of the water vapour in the line of sight of each telescope of the array. The scientific consequence is that as the interferometric array expands to higher frequencies and covers a wider area, observations under a variety of atmospheric conditions yield better quality images. As all the next generation radio telescopes will be interferometric arrays, developments in this field are of extreme interest.