I checked the difference in position of the Imx mirrors before and after the chamber was pumped. I used the vacuum pressure sensor channel (HVE-LY:Y1_120BTORR) to find the times were the pumped was performed. Then, to check the difference in position I used the times: Before: 2014/08/05 09:00:00 UTC After: 2014/08/07 10:00:00 UTC The table shows the count difference (before and after the pump): Difference L (counts) P (counts) Y (counts) IM1 10 7.42 -7.76 IM2 -5 -10.6 38.8 IM3 -27 -21.2 9.7 IM4 4 31.8 38.8 The DAC is 18 bit , which gives us a maximum of +- 131072 counts (2^18 /2 = 262144/2) in OSEMS basis (UL, LL, UR, LR). Right now, the maximum number of used counts is 116600 (H1:FEC-104_DAC_OUTPUT_0_5), which give us a free range of 14472 more. UL LL UR LR IM1 133.0 5.4 -0.4 -128.0 IM2 -425.8 -243.6 241.1 423.3 IM3 -272.3 92.1 -105.6 258.8 IM4 -59.2 -605.7 607.7 61.2 To recover the position had it before the pump, we need a maximum of 425.8 counts. The next table shows, in counts, how much is needed to recover the original position. I am attaching the spreadsheet used for the calculations and the times series plots.