By José Antonio Araiza Durán, MAS and University of Andrés Bellos’s postdoctoral researcher

In the year 1989, the New Technology Telescope’s (NTT) first light occur and with this event a new concept of telescope design was successfully introduced in the astronomical instrumentation, this concept is called active optics. This technology allowed the NTT to concentrate 80 percent of its collected light into a bright spot of about 0.3 arcseconds. The results had a big impact in such a way that the telescopes that followed incorporate an active support system for the primary mirror (in most of the cases, some times the secondary mirror had it too) in order to support the mirror’s weight and to have the ability to introduce deformations in the mirror surface to compensate aberrations (small deformations, in nanometric units or microns).

Adaptive optics is a well known technology now a days because is one of the few ways to overcome the atmospheric turbulence. It is important to differentiate one concept from another, the primary difference its the time rate where the actuators configuration changes from one state to another. In the adaptive optics systems the changes are faster than the active system, considerably faster. Although in the late 80’s both concepts were under study active optics came in to action first. Active optics is a technology that allow us to compensate low order aberrations, misalignment, and unwanted mirror deformations due to gravitational effect, temperature or stress of the mirror. Its important to highlight the control system in this active support, this technology facilitates the telescope guiding with a high quality image (in principle).

The 2.1-m telescope from the National Astronomical Observatory in San Pedro Mártir had its first light in the year 1979. The telescope have a 2.12-m primary mirror with a 26cm width in the edge, and weights 2 tons. Its optical performance was acceptable for long time before a new technology of detectors appear and the active support systems came to action, it was then when the instrumentation department from the IA-UNAM considered an active support system in order to improve the performance of the telescope. In 1996, the 2.1-m primary mirror had its new active cell which consisted in a 18 actuators distributed in two concentric rings, twelve in the outside ring and six in the inner ring. With this actuator’s configuration the control system of the active cell had the possibility of introduce aberrations like astigmatism, comma, trefoil, and spherical aberration. The control system consisted in distributing a percentage of the mirror’s weight in to three load cells, the load cells act like the sensors that closes the loop to produce a response by the actuators in order to equally distribute the mirror’s weight in the three load cells.

 

        

This system work fine and was very efficient, however, the control system did not guaranteed that an actuator configuration could be introduced and maintained all over the telescope’s path because its main task together with the mercury belt was to compensate the gravitational effects. In order to improve the active cell’s control system and allow it to maintain the initial actuators configuration a new variable was included. In the year 2014, the active cell was intervened by sealing the space that carry the mirror so that a vacuum can provide a pull-component to the system. In this way the current active support system acted like a common-pull and multiple-push mechanism and successfully compensate the gravitational effect on the mirror’s surface.

The experiment proved that a seeing-limited image quality can be maintained with the new system and showed that large mirror deformations can be achieved and kept constant at large zenithal angles. To test the ability of the new system a considerable amount of astigmatism was induced onto the primary mirror, while observing at different angles of telescope inclination. The experiment with the induced astigmatism showed that the elliptical shape of the spot was constant along the telescope path. Followed by the ability to maintain the initial configuration of the actuators along the telescope path the upgrade include an extension of the dynamic range of the actuators.

This work was used for over six months in order to study its performance in a deeper way, and in the present time the instrumentation department is considering to build a new active cell that considers the common-pull multiple-push system. The actual active support system had minor changes in its original design in order to include the vacuum system control so a new cell could provide more efficiency, stability, and above all maintain a more secure environment.

This work resulted in a publication (https://www.osapublishing.org/ao/abstract.cfm?uri=ao-53-33-7979) in the year 2014, however i wanted to highlight this paper for its relevance in my career. Within the work that came later were my PhD thesis, a few papers in the image quality evaluation and wave-front sensing areas, and current interests that I have like aspherical surfaces design and atmospheric turbulence.

Main image: Telescope of the National Astronomical Observatory in San Pedro Mártir