The European Solar Telescope will have six mirrors, numbered according to their position in the optical path. M2 (or secondary mirror) is the second mirror that the light will encounter on its way towards the scientific instruments. This 800 mm deformable mirror has two correction mechanisms and it will be the main mirror of the adaptive optics system.
A telescope is just a combination of mirrors and lenses. A very simple yet sophisticated mechanism that has evolved over the years to provide better and better resolving power and optical quality.
EST will have six mirrors and two lenses, and the mirrors are numbered according to their position in the optical path. The primary mirror (M1) will gather sunlight and reflect it back to the secondary mirror (M2). This is a 800 mm mirror located after both the M1 focal point and the field stop (the configuration of EST is that of an on-axis Gregorian telescope), which means the amount of light reaching M2 is exactly that of the desired field of view: one and a half arc minutes. This is just a fraction of the full solar disk —which measures 30 arc minutes— but EST will be able to see that fraction in great detail (hence the EST sometimes being called “a solar microscope”).
The secondary mirror not only reflects the light, it also “corrects” the effect that turbulence exert on light. Indeed, atmospheric turbulence, wind and even the very own heat of the telescope can render the images blurry and shaky, and the secondary mirror has the ability to change its form to compensate for those effects (actually, all the elements in the telescope are designed to minimize turbulence or compensate for it).
Two correction mechanisms
The secondary mirror is equipped with two correction mechanisms. The first one minimises the vibrations produced by the wind with a fast, wide movement that positions the mirror to make the image as still as possible (that is, a rigid body motion).
This is important because EST will be the only 4-metre solar telescope not protected by a dome during observation time, which renders wind influence especially important for the better and the worse: “On the one hand, the wind will dissipate heat and turbulence more efficiently than in a closed telescope, hence improving refrigeration. On the other, it will also produce stronger vibrations throughout the structure and optical elements that need to be compensated", explains Miguel Núñez, EST system engineer.
The second mechanism deals with the blurring caused by atmospheric turbulence. This is achieved thanks to the ability of the mirror to deform itself; an ability made possible by dozens of actuators placed behind the mirror surface. These actuators exert force upon its base, allowing both longitudinal displacement and tilt-tip rotation of the surface of the mirror. This correction is and integral part of the adaptive optics system, of which the M2 will be the main mirror.
These inflicted deformations are essential for the image to appear sharp, yet they are so small that remain invisible to the naked eye (their amplitudes are measured in microns). “The reason we don’t need wider movement is because these systems compensate for waveform distortions [the changes in the speed of light produced by wind and temperature], and the maximum wavefront length for our field of view is between 500 nanometres and 2.2 microns", details Núñez.
This deformation mechanism was not part of the 2011 conceptual design, but technology developments in the last years made it feasible. Also, the inclusion of this mechanism allows to reduce the elements in the optical path from the initial 14 mirrors to 6 mirrors and two lenses, meaning less light is lost (no mirror offers a 100% reflectivity) and image quality will be better.