J. Kissel, B. Weaver

Picking up from the recommendations from LHO aLOG 60494 and LHO aLOG 60498 regarding what to do with the HXTSs H1 SUS SRM and H1 SUS SR3 OSEMs "issues" respectively, Betsy and I followed the recommendations for both SUS today, and centered all OSEM values such that they're at mid-range when the SUS are at the position they are when in-vacuum, during nominal low noise.

For SR3, this was *very easy*, and just involved re-centering the M1 SD OSEM. Since SD is not involved in any alignment control, we merely need the flag position to sit in the middle of the calibrated range. So, Besty backed out the OSEM from its previous position at -76 [um], to ~0 [um].

For SRM, this was more of a process, but the process outlined in LHO:60494 actually worked really well.

To the end user, though it may *appear* as though alignment of H1 SUS SRM has changed, if one solely looks at the DC position of OSEM sensors to reflect mechanical alignment, but this is not the case. Today's exercise proves (albeit crudely and imprecisely) that the actuation strength of the coil-magnet systems of the BOSEMs is roughly constant across/within the range the of the LED / PD shadow sensor system, and assuming such we successfully centered the OSEM sensors around the position of the flags when the coil magnet systems are steering the SUS into the place that the IFO is happy during nominal low-noise.

I attach detailed notes of all the steps along the way, and I'll add a comment to this log that has the "in vacuum, during nominal low noise" trends that I used to inform where we want the SUS to be. The trends were taken from a ~24 hour stretch in May 2021, *after* we swapped ITMY and the IFO was aligned around a "point absorber free" ITMY.
Note, because there is signficant low frequency DAC request from the global length and alignment loops when in nominal low noise, I recreated that additional requested DC position adjustment (beyond just the slider values) from global control by adding OFFSETs in 

Also of interest during this process was gathering the second ever open light current values for the M1 BOSEMs. As mentioned in LHO:60494, I dug up the first and only measurements from 2014 in LHO:60477, and we now have a definite measure of the decay of these 6 BOSEMS over 7.5 years. In short, there is ~20% decay in open light current. But, that doesn't mean I want to replace them -- my gut tells me "we still have time."
Full data listed in the table below, with the new open light current (OLC) value in bold.
              OLC 2014    OLC 2014      OLC 2021   OLC 2021     2014-2021    2014-2021    (2014-2021)/2014     
              [ADC ct]    [PD uA]       [ADC ct]   [PD uA]      [ADC ct]     [uA]         [%]
SRM M1 T1     26908       68.431        20585       52.35         6323       16.08        23.499
SRM M1 T2     25472       64.779        18996       48.309        6476       16.469       25.424
SRM M1 T3     28670       72.912        22119       56.252        6551       16.66        22.85
SRM M1 LF     27764       70.608        20831       52.976        6933       17.632       24.971
SRM M1 RT     26272       66.813        21110       53.686        5162       13.128       19.648
SRM M1 SD     25516       64.891        18715       47.595        6801       17.296       26.654

Note: even though we *measured* new open light current values, we did *not* update the compensating offset or gain.
I'll need to think a bit more about that that means in terms of systematic error in the OSEM displacement calibration.