Deformation of an Aluminum Mirror coated of Ni

Diamond turning method can produce aspheric aluminum mirrors that can be useful for infrared optics. Usually nickel plating is applied to the aluminum substrate to protect it to environment degradation and to harden the surface so that it can be worked with better cosmetic results. This coating could be a problem for optics working at cryogenic temperatures: the different thermal contraction of the two metals between room and cryogenic temperature could critically deform the mirror. To evaluate this problem we perform a set of measures of a plane aluminum mirror of 50x50x8 mm coated with a nickel layer of about 10 micron.

 

Figure 1 - Set up of the measure

 

Figure 2 Results (deformation in micron)