Study why the Cobra arm length were different between Sep 2021 and June 2022.

In the June 2021 run, the Cobra arm lengths were measured (INSTRM-1532). The result showed that the arm lengths were different from those measured in September 2021. Since the measurement on the test stand (e.g. Feb and Mar 2022, see the report in INSTRM-2532 for the details) were the same as each other, the reason for the difference on the telescope needs to be studied.

At the ICS/PFI+MCS telecon on 8 July, the following action were suggested:

• Check the variation of the arm lengths themselves across the cobras, rather than the difference between the two measurement
• Re-anlyse the 2021-Sep data with the latest software and compare the results