Femtosecond laser cleaning the surface of reflective mirror in telescope

Abstract

Metallic mirror is the core component of the imaging system in the reflective astronomical telescope. However, because the telescopesare usually located in the wild, they are often contaminated by dust and other pollutants which will greatly reduce the imaging quality. Therefore, the surface of reflective mirror in telescope should be cleaned routinely in order to recover high performance of telescope. Currently, the artificial ice cleaning is the most common method employed in the cleaning of mirror surface, which is complicated and costly. Besides, the micron particles adhered on the surface of the mirror are especially difficult to be removed by dry ice flow when their diameters are less than 20 μ m because of the Van der Waals force between the particles and metal mirror. Moreover, an ordinary method with wipe and cleanser is also used in which the friction between wipe and mirror may damage the telescope. To overcome these shortcomings, this work proposed a laser cleaning strategy in which amplified femtosecond pulsed lasers was used to clean the reflecting mirror surface. As a new cleaning method, laser cleaning technology which is non-contact and frictionless has been applied successfully in the cleaning of industrial molds, buildings, precision machinery components. For the advantages of femtosecond laser, such as short pulse duration, small thermal effects, high pulse energy, it has been used in the cleaning of cultural relics and has good achievement in recent years. This article firstly studied the interaction between the femtosecond laser and the reflective aluminum mirror. The laser damage threshold of aluminum mirror was measured about 60 mJ/cm2, which would reduce to 57 mJ/cm2 for dirty mirror because of the microparticles adhered on the surface of reflector mirror. Then, by optimizing laser parameters including the scan rate, the scan pitch, and the laser energy, it could be found that when the laser energy density ranges from 30 to 55 mJ/cm2, micron dust particles with diameters less than 20 μ m on mirror surface could be cleaned with excellent clean effect and the reflectivity in the visible light region has been greatly improved. Finally, through the dust particles and aluminum mirror substrate spectrum analysis, we ruled out that the removed microparticles were thermal melted by laser energy. The results indicated that thermal expansion plays the key role in laser cleaning of metal mirror. This work shows that femtosecond laser cleaning has a good effect on the removing of micron particles. As a simple and easy way, femtosecond laser cleaning has a good prospect in cleaning of astronomical telescope.