I measured again the red beam when passing through the pinhole and extracted waist size and position (from power transmitted by a circular aperture) and the propagation directions (fig.1 in red).

The goal is to have the ir beam crossing the red beam at their waists with angle between their propagation directions of 0.1 rad (in fig 1 it corresponds to Y direction).

The corresponding ir propagation direction is shown in blue in figure 1.

I placed the pinhole at 2 positions (extracted from the figure) and acted on the 2 lenses after the periscope to maximize the power transmitted by the pinhole.

However, the ir beam was quickly clipped on the last lens before the translation stage.

Also, I wanted to use a small rail to more precisely control the first lens position and therefore the waist position.

However, the lens mount is too large and it is not possible to use the rail with this mount.