The MICADO first light imager for the ELT: Proof of concept and performance testing of the Relay Optics alignment procedure

MICADO is the Multi-AO Imaging Camera for Deep Observations, the first light instrument for the Extremely Large Telescope (ELT). The instrument provides imaging, astrometric, spectroscopic and coronographic observing modes. MICADO will be assisted by a Single-Conjugate Adaptive Optics (SCAO) system and the Multiconjugate adaptive Optics Relay For ELT Observations (MORFEO). The instrument will provide a narrow (19") and a wide (51") Field of View. MICADO can operate in the so-called stand-alone mode in the absence of MORFEO with the SCAO correction alone. In this mode, the ELT focal plane is reimaged to the MICADO focal plane via the relay optics (RO). This subsystem consists of an optical bench made of carbon fiber reinforced plastic, the MICADO calibration assembly, a cover to protect all opto-mechanical components on top of the bench, and an optical assembly. The optical assembly of the RO consists of six mirrors, with diameters that go up to around 500 mm. Three of the mirrors are powered, and constitute a Three Mirror Anastigmat (TMA). To be compliant with wavefront error and pupil quality requirements, these mirrors must be aligned to within sub millimeter and sub arcminute tolerances. The remaining mirrors are flat motorized piston, tip-tilt mirrors for interface alignment. In this work, we present the procedure for the alignment of the optical elements of the RO. We present a proof of concept test using dummy mirrors within the already manufactured RO optics mounts, complemented with analyses that extends the results obtained from the test to evaluate the performance of the alignment, finding very promising results within expected tolerances.