Simulation of a cold optics misalignment in NIKA2, and the way to recover from it with the external optics

On December 8 we have checked the alignment of the cold pupil inside the NIKA2 cryostat, using an ecosorb slab, sliding it from the edge of the window toward the center and marking the position when it had an effect on the detectors response. Doing it from 4 sides allowed to determine that the cold pupil was misaligned on the window by 2mm horizontally and 10mm vertically.
We don't have enough margins on the cryostat air-compressed feet to compensate this misalignment, we have to do it with the external optics. Removing the M3 mirror and using the telescope laser on the elevation axis, we tilted the M6 mirror so that the laser spot on the window rose by 10mm, in order to match the telescope pupil with the cold aperture stop footprint on the window, hence aligning the external optics with the internal optics.

The most probable explanation for the cold pupil (aperture stop) misalignment is an error of adjustment of the shims which are used to fix the internal optics and various temperature screens concentrically within each other. In other words, it's not the sole cold pupil which is misaligned, but the whole cold optics inside the cryostat.

The images below show a simulation of this problem and the solution we chose to compensate this misalignment of the optics with the M6 mirror.

1) Nominal optics: everything well aligned.

2) Tilt of the internal optics: 10mm too high at window, 0.35 degree tilt downward to that plate near array in transmission through polariser stay at same height.

3) Extra tilt of M6 to compensate for the internal optics tilt and thus align external optics with internal optics.