One thesis, two contributions

The first benefit that can be attributed to François Finet is his contribution to the construction of the telescope. This was designed and tested in the AMOS workshop. It was then dismantled and sent to India where it will be assembled and calibrated during the second part of 2013.  The device is a very interesting instrument. For example, the thinner the layer of mercury, the smaller the amplitude of the waves formed on its surface. In the case of the ILMT, the layer of mercury is only two millimeters thick. “The core of the plate is made of Styrofoam and therefore very light which means that a very powerful motor is not required. This core is cut into the shape of a sphere and covered with a layer of carbon fiber in order to solidify it. Then the rotation of the dish is switched on and a resin containing two compounds which remain in a liquid form for a period of time before solidifying is poured on it. In its liquid form, this resin is subject to the same force as the mercury and will form a parabola that will remain perfectly smooth once it has solidified. A two-millimeter-thick layer of mercury is then all that is required to create a parabolic mirror”.

But the major contribution of François Finet, is the development of a new procedure for detecting wavelets in order to test the quality of the mirror and, where possible, to be able to come up with solutions for improving it. “Traditionally, the different testing methods for a mirror are made at the center of curvature of the mirror, a point which is twice as high as the focal point”, explains the young researcher. The center of curvature has an interesting particularity. When the rays emitted by a light source from this point are reflected by the mirror, they return to the same point like a boomerang. By placing a source of light there and analyzing the light reflected, it is then possible to see if the mirror is smooth or if there is a propagation of waves (which can be of the order of one micron). 

Traditional testing method presented a slight problem. “In our case, the focal point was eight meters above the center of the parabola. The center of curvature was therefore sixteen meters high. There were two solutions available to us. Either we constructed a fixed and stable 16-meter-high structure making a hole in the ceiling of the AMOS warehouse in order to install the structure (a prospect that terrified the owners of the premises; editor’s note), or we found another technique for testing the optical quality of the mirror”.

The alternative method that was developed was based on the idea of the reflection of a laser beam on the mirror and analysis of the reflected beam. The idea was simple and worked well, a laser on one side of the mirror and a detector on the other. In the case where wavelets propagated on the surface of the mercury, they changed the slope at the point of impact of the laser. In this way, the direction of the reflected rays was changed and the reflected beam oscillated. By measuring the oscillation of the reflected beam, it becomes possible to retrieve the characteristics of the wavelets at the point of impact of the laser on the mirror. After what, one may choose the solution for reducing the wavelets according to their nature.