CONCERTO’s Last Tune: An Upgrade and a Farewell
Before removing CONCERTO from the C-cabin, we wanted to upgrade the instrument to demonstrate improved performance in the last run of observations (for science and future instruments). The upgrades are:
1) Change the large (interferometer) polariser (to avoid the membrane vibrations)
2) Change the small (cold reference) polariser (to avoid the membrane vibrations)
3) Change the LF array or both arrays (to have one 1mm and one 2mm array)
4) Change the motors guides (to reduce the friction and improve the movement)
5) Fix the feet 6 leak (to allow longer scans)
6) Add helium to the pulse-tubes circuit (to reduce the risk of injection blocking)
7) remove oil from GHS (to reduce the risk of injection blocking)
8) Replace the “baby cryostat” window (to further improve the cold reference)
The major upgrade (1) consists of adding HDPE plates to rigidify the big 300K polariser.
For these upgrades, we had to remove the chassis from the C-cabin, craned it down to a pick-up and took it down to Sequitor. The team arrived on site on 2 February 2023.
On Feb. 13, CONCERTO is mounted again in the C-cabin. On Feb. 15, the team worked on the alignment of the whole optical chain with the laser, on measuring the offset of the fixed roof mirror to rescale the ZPD with the new configuration and on the installation of M3 mirror.
We managed to do the upgrades despite bad weather, a security alert on a possible eruption of the Lascar volcano (associated possibly with an earthquake), communication problems between Sequitor and APEX (radio link) which created some problems for cooling the cryostat, and the discovery of a leak somewhere in the injection circuit.
On Feb 23, we performed the first test observations with the upgraded instrument, with a series of focus and pointing on J1426-4206 and J1957-3845. These preliminary observations performed under very tricky conditions showed highly degraded beams on Mars and revealed several Mars on the focal plane!
On Feb. 26, CONCERTO is warming up, the injection is totally blocked. From that date up to the planned last observing run, we didn’t manage to make CONCERTO working properly again. On March 28, a brief power outage affected the compressor, leading to further degradation of the CONCERTO cryostat’s performance, which had already been struggling for weeks, with the antenna mostly kept in park. The “double” Mars and focus issues were traced to the curvature of the HDPE plates. In April, the team returned to Chile to remove the plates. However, the leak in the cryostat proved too severe, and it could not be repaired.
On April 15, we officially declared the end of CONCERTO at APEX.
