Study and test of alignment techniques for extremely large-scale telescopes' instrumentation

In the context of the INAF instrumentation development program for the MORFEO instrument at the Extremely Large Telescope (ELT), I tested an optical alignment procedure based exclusively on the ghost reflections, without introducing any external target into the system. MORFEO, the multi-conjugate adaptive optics system for the ELT, contains both natural and Laser Guide Star (LGS) wavefront sensors. The alignment technique has been proposed for the alignment of the Laser Guide Stars Objective (LGSO) to MORFEO. To assess the reliability of this technique, I built a laboratory prototype to study its advantages and criticalities. The system was made of 2 commercial lenses illuminated by a point-like source; the 1st lens was a collimator lens, while the 2nd was a converging lens. I used the Micro Alignment Telescope (MAT) to set the reference of the optical axis and observe the ghosts, analyzing them using Python codes. From the analysis of the position of the centroid of the ghosts, both lenses resulted aligned with ∼ 5 µm of residual error in decenter and ∼ 5 − 10” of residual error in tilt. I applied the results retrieved with the lab prototype to a ray tracing simulation environment with the MORFEO LGSO, demonstrating a potential alignment residual error of ∼ 10 µm in decenter and ∼ 2” in tilt. Comparing these values with the LGSO alignment requirements, decenter < 1 mm and tilt < 360”, the ghost alignment technique proves to be reliable and yields better results than other methods considered, such as laser tracker and back reflections techniques.