Staghorn fern

We have an old and large fern hanging under a tree. The leaves are looking yellower than usual, and a few have rather thick gray/white dust on their undersides. generally, the plant looks scruffier than it has, and we are worrying.

Watering it by the method you suggest above is not possible; we have to do it with a hose. How much water should it be getting, and how often? We suspect that it may be getting more sun than it likes at this point, but there are not many alternatives on that front. Any suggestions would be appreciated, as we do not want to lose this gorgeous plant.