The Liverpool Telescope, March 2022. Image ©2022 Iain Steele.

Summary

• Many long overdue mechanical, hydraulic, pneumatic and electrical maintenance tasks, to ensure continued reliability.
• Horizon limit lowered from 35° to 26° elevation
• Mirrors cleaned, increasing zeropoint by 0.5 magnitudes
• New photometric shutter reduces minimum recommended integration with IO:O from 20s to 0.01s

Due to travel restrictions imposed by a global pandemic and a volcano on La Palma, it had been two years since the last maintenance trip from Liverpool to the LT. Fortunately due to the remote efforts of LT staff and on-site support by Dirk Raback, the telescope continued to observe over that time.

By early this year a large list of things to do had nonetheless built up and a maintenance trip was very welcome. Travel restrictions eased in March, and on the 10th telescope director Iain Steele and engineering manager Stuart Bates flew to La Palma for two weeks to address the most pressing items.

Part of the telescope's altitude axis encoder tape had become contaminated with "calima" dust from the Sahara, preventing observations below an altitude of 35°. This tape was cleaned, restoring observations down to 26° altitude.

The telescope focuses by adjusting the position of the secondary mirror, and the machine control system that governs this needed replacing. This wasn't a simple task as it sits inside the secondary mirror enclosure at the top of the telescope. By erecting scaffolding inside the enclosure and tilting the telescope down to meet it, the control unit was replaced.

Primary mirror being cleaned by Stuart Bates.
Image © 2022 Iain Steele.

Over two years the primary mirror had built up a coating of calima that reduced reflectivity to 55%. It wasn't possible to strip and recoat the mirror with a fresh layer of aluminium, so it was washed instead. "Washing" actually consisted of very carefully dabbing the surface with cotton wool soaked with a cleaning solution.

The Acquisition and Guidance ("A&G") box, the structure that the instruments are mounted on, had been removed along with the instruments in anticipation of removing the primary mirror for cleaning. However a cold snap at the mountaintop meant the roads were too icy for the crane to drive up from sea level, so the mirror had to be cleaned in-situ. This made the task more difficult but not impossible. It took two days, but reflectivity was increased to 76%. While the A&G box was off the telescope, the science fold (tertiary) mirror was also cleaned, improving its reflectivity from 63% to 83%.

IO:O's iris shutter was replaced with a travelling-curtain shutter. As well as providing more even illumination for short exposures, integration times as small as 1 millisecond are now possible (10ms recommended for photometry). This means targets much brighter than before are now potentially observable with IO:O. MOPTOP's cameras were replaced with more robust models that should cope better with the dusty calima environment.

There are more maintenance tasks to do, and they will be addressed in future site visits through the year.