29th September 2023

This month marks an important milestone for the NRT: we have our first mirror for the telescope! The science fold mirror is the smallest of the NRT mirrors but is important because it allows light to be distributed to the different instruments at the base of the telescope structure. The mirror is an elliptical shape due to the tight space it has to fit into and the shape of the beam of light that hits its surface. The machined mirror has now been delivered to the Istituto Nazionale di Astrofisica (INAF) in Italy, ready to have support pads glued onto the rear of the mirror and to undergo the first initial surface measurements prior entering the polishing process.

The machined science fold mirror at the INAF facilities

The final support pads to be glued onto the surface are still being manufactured: these will be made out of Invar material which does not change shape with temperature changes (only tiny fractions of a milimetre can impact the mirror surface). Therefore, aluminium pads are being used as placeholders in the meantime as it is important we test the physical effects of the pads on the mirror surface and whether the glue for the pads will warp the surface of the mirror during polishing and measuring.

These temporary pads can be removed by placing the mirror back surface in an acetone bath which dissolves the glue bond. The team tested this procedure prior to gluing the aluminium pads to be sure they could be removed without force ready for gluing the final Invar pads.

Mirror back surface with temporary pads after gluing (plastic shims shown under the pads which set the glue thickness which is critical to bond strength)

The image above shows part of the pad alignment jig setup which was the result of weeks of iteration to find the best way to accurately position and glue the pads in place. Due to the sensitivity of the support design, even tiny misalignments in the pad position can reduce the performance of the mirror (distorting the surface in certain conditions; gravity, or thermal changes).

In a previous update we noted the temperature of the pads and mirror was a concern during the polishing process in the ion-beam chamber. The INAF team conducted a test using thermocouples mounted on the pads and showed the maximum temperature (only 60 degrees centigrade) during a polishing run was much lower than the limit provided by the glue manufacturer (120 degrees centigrade). Therefore, the team are confident to continue with this approach and that the glue bonds should be secure throughout the polishing process (multiple runs expected).

NRT mirror mounted in ion-beam chamber and thermocouples fixed to mirror pads

Plot showing how a single fold mirror pad heats up during the polishing process. On the x axis is time in minutes and the y axis shows temperature in degrees centigrade