I have been practicing to optically contact two flat mirrors together. I think now I get it.
Since I need to build my refcav by optically contacting mirrors to a spacer. I tried the procedure by contacting two flat blank mirrors together. I bought blank fused silica mirrors from Thorlabs, pf10-03. Here are the instructions:
fig1: On the right, fringe patter can be seen when two surfaces are put close together. On the left, after applying pressure the fringe should go away. The fringe pattern(purple-yellowing) on the left side indicates that these surface are not properly contacted.
fig2: These mirrors are optically contacted somehow, except the center. I don't know what happen here that cause white area in the center. I might be that the mirrors are not flat enough. But the rim seems to have a nice optical contact. I tried to remove the mirrors by hands but they are well stuck. I'll ask peter for more about what causes the white area here.
As I tried to do this, I got an idea of a fixture for optical contacting the spacer to mirrors. It will be a cap with a center hole for the mirror position. Here is a solidwork drawing. The part can be aluminum. I have to think about the tolerance of the piece, but from the calculation it can be an order of a cm (to keep the beam to go through the window). So the hole can be ~ .01 inch larger than the spacer and the mirror.