5HZULWDEOHSKRWRFKURPLFIRFDOSODQHPDVNV Emilio Molinari D , Chiara Bertarelli E , Andrea Bianco DE , Fabio Bortoletto F , Paolo Conconi D , Giuseppe Crimi D , Maria C. Gallazzi E , Enrico Giro F , Andrea Lucotti E , Claudio Pernechele F , Filippo Maria Zerbi D , Giuseppe Zerbi E D INAF - Osservatorio Astronomico di Brera – gOlem – Milano, Italy E Dip. di Chimica, Materiali e Ing. Chimica “G. Natta” - Politecnico di Milano, Italy F INAF - Osservatorio Astronomico di Padova, Padova, Italy $%675$&7 The application of organic photochromic materials in astronomy is opening new possibilities which we are investigating in order to design innovative devices for future instrumentation. The photochromic property of transparent/opaque transition (although in a limited wavelength range) and the changes in intrinsic refractive index have led our studies to application in astronomic spectrographs, both as focal plane mask (for MOS application) and as dispersive elements (volume phase holographic gratings, VPHG), respectively. In both cases the possibility to write and erase devices with suitable irradiation has revealed a new perspective for non-disposable and fully customizable items for spectroscopy. Pursuing this goal we have synthesized a series of novel photochromic materials belonging to the diarylethenes. They fulfill the requirements of thermal stability and fatigue resistance necessary to build functional devices. Prototypes of high contrast focal plane mask working in the H-alpha spectral region have been manufactured and characterized both in laboratory and with the AFOSC camera at Asiago telescope (1.8 m). A custom writing robot (ARATRO) which, taking imaging frames and with the aid of interactive mask design software and ad hoc control electronics, is able to write MOS masks, has been constructed. The design of the MOS masks allow the fitting in the AFOSC slit wheel. The overall set-up is ready for the sky tests. Keywords: Photochromism, diarylethenes, refractive index, optical focal plane masks, volume phase holographic gratings, spectrograph, infrared spectroscopy ,1727+(3+<6,&2&+(0,&$/3523(57,(6 A photochromic material undergoes a reversible change in color by suitable light exposure 1 . The transmission spectra in the two possible forms of the material show very different values in the UV-vis wavelength range, mimicking the behavior of opaque and transparent elements (Fig.1). The change in transmission spectra and consequently in color is ascribable to the fact that the molecule can take up two different structures. Moreover, this difference brings about the change in other physical properties such as the refractive index and oxidation potential. In general, the first transformation process takes place when the material is illuminated with a specific UV radiation giving a colored material, while the second one (reverse process) can be induced either by thermal energy (thermochromism) or by a radiation of different wavelength (generally in the visible). Among all the photochromic compounds, diarylethenes have been extensively studying because they belong to the latter class and their photochromic process can be totally controlled by suitable irradiation 2,3 .