BrokenQuill/BeamofLightSolution

The Solution

Quite rightly you have guessed that the first thing you have seen is light. When light is it's path's beginning it is indeed dawn. The picture's twin is of course the mirror.

So the characters will have to put the mirror in the path of the light, by placing the pole into the hole in the middle of the room. They have to clean the concave side of it and place this side towards the east, so that time does not pass unfilled.

The Effect

When the characters place the mirror correctly and await the next dawn, the morning light will strike the the mirror and is reflected to a single rune on the wall. If the characters press this rune the central slab will open. In my setting the slab's sectors will move down to form a spiral stairway to the hidden central room of the manse.