Tiny mirrors for huge telescopes

Abstract

Observing the formation of early galaxies helps us understand our origins. The light coming from these faraway objects is very faint and shifted to the infrared. Multi-object spectroscopy (MOS) is the central method for studying many isolated objects simultaneously, using a slit mask in the focal plane of the telescope for blocking spoiling sources and background light. Today these masks are static perforated sheets or complex fiber-opticbased systems. In the future, microelectromechanical systems (MEMS) could provide a remote controllable, reconfigurable slit mask, increasing the scientific efficiency of MOS. In the framework of studies on the future European Extremely Large Telescope (E-ELT), we are developing a micromirror array (MMA)-based reflective slit mask. Another MEMS solution, a microshutter-based slit mask, is being developed for the James Webb Space Telescope by Moseley and colleagues at NASA.1 However, we believe that MMAs are an appropriate solution for extremely large telescopes (ELTs), as they have a large focal plane (in the meter range). Because we work with reflected light, the electric wiring and mounting of the MEMS can be realized under the MMA. These integrated MMA building blocks can be used as ‘paving stones’ to cover large surfaces nearly seamlessly. Figure 1 shows the basic concept of a MOS using an MMA as a slit mask. The mirrors at the locations of the objects of interest are tilted (ON state), and the light is sent toward the spectrograph. All the other mirrors remain unactuated (OFF state) and send the light of the spoiling sources back to the telescope. The ratio of wanted to unwanted light must be 3000:1 or better. Thus a mechanical tilt angle of at least 20◦ is needed.2 This tilt angle must be uniform across the whole array, so that the light of all selected objects goes through the entrance pupil of the spectrograph. In order not to disturb the wavefront of the reflected light, the mirror must remain optically flat (λ/20) during operation. For infrared applications, it must be in a cryogenic environment. Figure 1. In the multi-object spectroscopy microelectromechanical systems (MOS-MEMS) setup, the micromirror array in the focal plane of the telescope selects the light from the objects of interest and sends it to the spectrograph.