26th July 2023
As part of the on-going work preparing for the polishing of the first NRT mirror (the science fold), members of the LJMU team travelled to Merate, Italy to visit the Italian National Institute for Astrophysics (INAF)'s mirror polishing facility.
Adam Garner, Ali Ranjbar and David Copley took various pieces of equipment with them to demonstrate the procedure required to glue pads to the back of the mirror prior to polishing. The blank ceramic will be milled to the required shape (according to the engineering drawing) by a supplier to INAF, then the team at INAF will use the ion-beam machine to polish the optical surface to the required flatness (to 16 nanometres RMS; much narrower than a human hair!).
The INAF facility where the mirror was polished
The team had a tour of the facilities and were shown how the ion-beam polishing machine works. The machine operates under vacumm and fires a stream of ions at the target surface. This knocks off atoms according to the 'removal function' which is a height map generated from the mirror surface measurement. A key concern for the team was how hot the mirror would get during this process as the pads are glued to the back surface holding the mirror. Under vacumm the heat cannot be easilly removed from the pads so there was a risk that this would weaken the glue bond. The team agreed a plan to perform a short test run before the actual polishing to measure the temperature inside the chamber to record peak temperatures around where the pads will be.
Equally as important to the actual polishing machine is the metrology setup. The team reviewed this together. Measuring to such an extreme accuracy requires a very repeatable optical interferometry setup requiring a stable environment and an optical bench. The resolution of the setup needs to be better than the desired performance of the mirror, requiring a performance of single digit nanometres. A very flat reference mirror (lambda/20) is used in combination with an interferometer camera device and a beam expander to measure the path difference the light takes between the reference and the fold mirror under test.
Proposed metrology setup for fold mirror measurements between polishing runs
A major challenge for the NRT team and for INAF is removing the effect of gravity on the mirror. The specification provided by the NRT team for the mirror flatness is excluding the impact of gravity and this must be removed when processing the results. This is because during its operation, the fold mirror will be impacted by gravity from all different angles (as the telescope mount, tube and cassegrain focal station rotate around). Therefore, it is important to understand the mirror performance without gravity acting.
This was discussed during the trip and a plan was made to use Finite Element Analysis (FEA) to model the effect of gravity on the mirror surface. This can then be removed from the measurement data leaving just the surface heights due to the polishing procedure.
INAF mirror polishing lab
