[Left] The primary mirror before realuminising, in its shipping box about to be sent to the aluminising plant. [Right] The primary mirror after realuminisation, in the mirror cell about to be refitted to the telescope. The difference in surface finish is notable. Images © 2024 Chris Mottram.

The LT was offline for three weeks recently (22 Aug to 13 Sep) for a major bout of scheduled maintenance.

Top of the long list of tasks was realuminising the 2-metre primary mirror using the nearby aluminising plant at the William Herschel Telescope (WHT) facility.

This is no small feat — the mirror would have to be hoisted out of the top of the enclosure through a narrow gap, and despite weighing 1.35 tonnes it can still be blown by the wind.

Despite a few hitches the procedure was successful. The mirror was carefully craned into its transporting box, and then onto a special mirror transport truck to go to the aluminising plant. Over the next two days WHT personnel cleaned and stripped the mirror, then evaporated a new layer of aluminium onto its surface. It was returned to the LT facility on 5th September.

Once the mirror and instruments were refitted, recommissioning could begin. The telescope was recollimated, the instruments realigned and recalibrated, and on 13th September routine science observations resumed.

The realuminising improved the primary's reflectivity by over 100% across the optical spectrum. This, plus cleaning of the science fold (tertiary) mirror, caused throughput to the workhorse imager IO:O to increase by nearly a magnitude in some bands.

Some of the other tasks performed over these three weeks included –

• improving the vacuum in IO:O's cryostat by vacuum pumping it while at room temperature
• performing maintenance on LIRIC's water cooling system
• replacing a motor, gearbox and encoder set for the cassegrain axis derotator
• prepping all components for the new weather mast, to be installed later this year
• reinstalling SkyCam Z – mirrors were cleaned, recollimated, and a new camera fitted
• two new enclosure hydraulic cylinders installed
• science fold (tertiary) mirror cleaned

Lead for this maintenance project, and present throughout, was LT Engineering Manager Stuart Bates. Also present at various times were Joao Da Silva Bento (NRT DevOps Engineer), Ali Ranjbar (NRT Mechanical Engineer), Adam Garner (NRT Control & Automation Engineer), Chris Mottram (LT DevOps Engineer) and Chloe Miossec (NRT Instrument Scientist). Robert Smith (LT Project Scientist) provided essential remote support during recommissioning.

Overall, the three-week event went well. The vast majority of maintenance tasks were completed successfully, ensuring the telescope will continue to provide time-domain and rapid-response reactive data for the astronomical community.