Late maturity

The method has been known for a long time; it seems simple and presents the excellent advantage of being inexpensive. “By way of comparison, the ILMT project whose primary mirror will have a diameter of 4 meters will cost 20 to 50 times less than the construction and maintenance of a traditional telescope of the same size”, explains François Finet. At this accessible price, a telescope can be constructed and optimized for a particular scientific application, where the observation time with classical telescopes are astronomical in terms of cost and disputed by teams of researchers from all over the world.

And yet the first tests for this type of technology for scientific observation puposes only date back to the beginning of the 1980s and it was largely due to the combined research of Paul Hickson in Vancouver (who was also part of  François Finet’s thesis committee) and Ermanno Borra at the University of Laval (in Québec) that this technology came to fruition.

Up to now this kind of technology was subject to two main limitations. First, the technology of the sensor at the focal point of the telescope. The mirror, which in order to acquire its parabolic form uses the local gravity, can only point straight at the Zenith. It is therefore not possible to point in a given direction and track an object as it moves across the sky due to the Earth rotation as can be done with a traditional telescope.
Due to the Earth rotation, the region of the sky accessible at the Zenith changes constantly. It was only after the advent of the CCD camera, at the beginning of the 1980s, that electronic imaging of the sky became possible and made it possible to track objects electronically as they moved across the Zenith. Thanks to the electronic surveillance of the CCD camera and Earth the rotation, the telescope has access to a strip of sky that it can image night after night.

Another limitation was due to the liquid character of the mirror. “Any vibration or disturbance transferred to the rotating dish results in the formation of wavelets on the surface of the liquid which diffuse the light reflected by the mirror and degrade the quality of the image formed by the telescope, as bumps or scratches in a conventional mirror. The reflected light will then be more or less diffused according to the quality of the mirror and affect the quality of the image which will appear blurred”, explains the researcher

Several causes can be attributed to the formation of these wavelets. First, there are so-called transitory phenomena. “Someone falls and knocks against the plate, for example”, jokes the astrophysicist, “ or a fly lands on the mercury”.  But there are also other stationary phenomena. “These can be vibrations transmitted by the rotation system or lack of stability in the speed of rotation. In reality, only a relative variation of the period of the order of 10-6 can be allowed”.  For example, the rotation period of the ILMT is 8 seconds and cannot vary by more than 8 microseconds which represents a high level of precision. It was therefore necessary to await the advent of systems which allowed for a very stable rotation and without the transmission of vibrations. In order to avoid these vibrations, the rotating part of the motor is kept suspended in a pressurized air bearing.

Another type of wavelet can also affect the quality of the mirror: spiral waves. These are due to the instability phenomena arising in the air layer above the mercury, because of a too-high relative velocity between the rotating mercury and the air. “This problem particularly concerns the outer edge of the parabola as it rotates faster than its center. The mercury rotates with the dish and is thus in motion with respect to the air just above the mirror. If the speed of the mercury (relative to the air) is too high, zones of turbulence are generated in the air which in turn impact on the shape of the surface of the mercury therefore causing deterioration in the quality of the mirror”.  These turbulences could be a limiting factor for the construction of primary mirrors with a verylarge diameter.

It also goes without saying that a liquid mirror telescope must be placed in a stable and calm environment.  In India, the telescope will be assembled in a the early Himalayan mountains at an altitude of more than 2,400 meters. 

It is because of this series of technological problems (the need for a stable rotation system and the possibility of electronically tracking the objects moving across the sky) that liquid mirror telescopes were originally left by the wayside in the realm of star-gazing. But they have started to make a comeback again over the last thirty years thanks to the resolution of these technical issues and thanks to their low cost are perfect candidates for carrying out research complementary to that of traditional telescopes. It is in this context that François Finet participated in the design of the ILMT.