filter cavity input mirror wedge

The input mirror wedge sign was studied with an autocollimator callibrated with the reflection of the front surface (by superposing the reticle and the front face reflection).
To understand which reflection was the one from the front surface, we looked at the reflections due to a roof light. When this light hitted only the front surface, its reflection was red. When it was hitting both surfaces, we could see two shifted reflections, one green and one red. So the red reflection corresponds to the front surface reflection.

The callibration of the autocollimator means that the incident beam and the front surface reflection are on the same direction. Then, depending on the wedge sign, the back surface reflection will either be seen on the top or on the bottom of the front surface reflection.

By looking at the autocollimator, we were able to tell that the wedge sign corresponds to the second configuration on the scheme. This means that the "#4" sign on the mirror corresponds to the lower part of the wedge.