The volcanic ash rarely gets up to that altitude. The reason the telescopes are sited on Mauna Kea is because they are generally above the tropical inversion la

The volcanic ash rarely gets up to that altitude. The reason the telescopes are sited on Mauna Kea is because they are generally above the tropical inversion layer (leading to clear, dry skies). That same inversion layer generally keeps the volcanic emissions down at a lower altitude (not to mention that trade winds generally push the plume in a different direction). When the inversion layer breaks down, there is not much observing anyway as the telescopes are then in the clouds.