Spin-Float production of thin paraboloidal segments in glass Piero Salinari Istituto Nazionale di Astrofisica (INAF) Florence, Italy

Background The initial motivation of the Spin Float production technique was that of producing thin glass-substrate mirrors for extremely large telescopes of the future. The Florence INAF group in the past years had developed a technology for producing “adaptive telescope mirrors”, already in use on the MMT Telescope in Arizona (the first ever adaptive secondary mirror), and adopted by the l argest current telescope in the world: the Large Binocular Telescope (a collaboration of European and USA astronomy Institutes). The essence of the “adaptive mirror” technology is that a very thin mirror (1-5 mm, depending on application) is deformed in real time at high frequency (~1 kHz) and with great accuracy (a few nm rms) to correct the negative effects due to the propagation of the light through the earth turbulent atmosphere. The result is that of obtaining images of the same sharpness obtainable from space, but using much larger and less expensive ground-based telescopes. The extension of this technique to the primary mirrors of much larger future telescope (example of such projects are in the following) is highly desirable for various reasons: - Increasing actuator number (there is more area on the primary, one can put there more actuators) and therefore extending the adaptive correction to shorter wavelengths - Controlling the shape of the primary (exposed to wind).

Background But requires new developments: - Segmentation of the mirror in pieces of practical size (1-2 m) - Drastic reduction of the cost of the thin mirrors (currently produced by optical polishing and subsequent thinning of very homogeneous low expansion ceramics (typically high quality Zerodur) The adaptive control of the thin mirror on one side allows reducing the optical tolerances, on the other one removes the need of very low expansion materials. The required mirror shape, for a telescope primary mirror, is paraboloidal (or VERY close to it), and it is well known that this is the shape assumed by a rotating liquid in the gravity field... The float technology uses a simpler field of forces (only gravity) but is well proven. We worked for some time on this basic combination (rotation and flotation on liquid Tin), that we called Spin-Float, before realizing that there are many important potential applications for paraboloidal segments beyond astronomy: Solar concentration (double curvature or point concentration, in the language of solar energy engineers Others, such as architectural or automotive glass These applications DO NOT require the extreme “slope” accuracy of even “reduced requirements adaptive astronomical mirrors” (~10 -5 ) but are at the accuracy level of only ~ for solar concentration ~ for architectural automotive and other Potentially EASY AND CHEAP!!

Telescopi di domani: OWL Progetto Europeo (ESO) Diam 100 m Primario > 6000 m 2 (sferica)