Evaluation of difference of thermal expansion coefficient of two alloys of aluminium (Al6061 and Al6082) between room and liquid nitrogen temperature.

In the next infrared instruments we plan to use two types of aluminium for the mirrors (Al 6061) and for their supports and optical bench (Al 6082). It's important verify that these two materials have the same coefficient of thermal expansion when the temperature changes from the ambient one (where we set the optics) to the liquid nitrogen one (where the optics works).

This verification was made in laboratory using two bars of aluminium (one from block used for the mirrors and the other from the block used for the supports) with one end fixed together and the other (free) end used to measure small difference of length when this apparatus was immersed in liquid nitrogen as showed in pictures above.

The relative difference between the two thermal contraction is of 2.0 +/- 3.8 10^-5 (that is comparable with zero).