Given $\alpha \in \left[ \frac { \pi } { 6 } , \frac { \pi } { 3 } \right]$, and the terminal sides of $\alpha$ and $\beta$ are symmetric about the origin, then the maximum value of $\cos \beta$ is \_\_\_\_.
Given $\alpha \in \left[ \frac { \pi } { 6 } , \frac { \pi } { 3 } \right]$, and the terminal sides of $\alpha$ and $\beta$ are symmetric about the origin, then the maximum value of $\cos \beta$ is \_\_\_\_.