Femtosecond laser cleaning the surfaceof reflective mirror in telescope
2016
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.
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