Caroline C and I went down to EX... again.... (Sorry EY, I know it was your turn, but next time I promise I'll measure you... )
We took the PS4 Working Standard (WS) integrating sphere down to EX.
We found EX's PCAL system is covered in caution tape, like it was in the center of a foreboding crime scene!  

Unfortunately the red Current Limit LED was Lit again, and I trended the channel H1:CAL-PCALX_LASERDIODECURRENT back to find it has been lit up since last Tuesday. Supisiously close to the beckhoff restart time. I'm not sure which channel is best to check for beckhoff restarts yet.  I was able to just move the voltage down a little and back up to rectify this issue before starting the measurement.

We took some pictures of the beam spots before we started to prove our innocence.
Followed instructions on T1500062 and followed it without any deviations from our part. The SEI system was put in ISI Damped HEPI offline for this measurement for other work though. 
Everything went as smooth as it can while dodging lifts, scaffolding, and caution tape.
Took some After measurement beam spot pics to prove our innocence again. 

Overall I don't think the GV20 work had any real impact on PCAL pointing or performance. Shoutout to the GV20 team for doing such a great job! 


Wanna see the Absolute Calibration details of this specific measurement and the commands input and output? :

@cdsws25: python3 generate_measurement_data.py --WS PS4 --date 2026-10-13
Reading in config file from python file in scripts
../../../Common/O4PSparams.yaml
PS4 rho, kappa, u_rel on 2026-10-13 corrected to ES temperature 299.8 K :
-4.700380443762894 -0.0002694340454223 0.0007505543347185126
Copying the scripts into tD directory...
Connected to h1daqnds1
martel run
reading data at start_time:  1475945650
reading data at start_time:  1475946100
reading data at start_time:  1475946460
reading data at start_time:  1475947100
reading data at start_time:  1475947444
reading data at start_time:  1475947820
reading data at start_time:  1475947998
reading data at start_time:  1475948640
reading data at start_time:  1475949030
Ratios: -0.45984106211593845 -0.46714267909102675
writing nds2 data to files
finishing writing
Background Values:
bg1 =        9.805339; Background of TX when WS is at TX
bg2 =        5.167711; Background of WS when WS is at TX
bg3 =        9.886270; Background of TX when WS is at RX
bg4 =        5.195956; Background of WS when WS is at RX
bg5 =        9.755134; Background of TX
bg6 =        0.367068; Background of RX

The uncertainty reported below are Relative Standard Deviation in percent 

Intermediate Ratios
RatioWS_TX_it      = -0.459841;
RatioWS_TX_ot      = -0.467143;
RatioWS_TX_ir      = -0.454541;
RatioWS_TX_or      = -0.462195;
RatioWS_TX_it_unc  = 0.068338;
RatioWS_TX_ot_unc  = 0.070460;
RatioWS_TX_ir_unc  = 0.074890;
RatioWS_TX_or_unc  = 0.065120;
Optical Efficiency
OE_Inner_beam                      = 0.988231;
OE_Outer_beam                      = 0.989349;
Weighted_Optical_Efficiency        = 0.988790;

OE_Inner_beam_unc                  = 0.046695;
OE_Outer_beam_unc                  = 0.045578;
Weighted_Optical_Efficiency_unc    = 0.065252;

Martel Voltage fit:
Gradient      = 1636.946737;
Intercept     = 0.081815;

 Power Imbalance = 0.984370;

Endstation Power sensors to WS ratios::
Ratio_WS_TX                        = -1.078768;
Ratio_WS_RX                        = -1.391922;

Ratio_WS_TX_unc                    = 0.042184;
Ratio_WS_RX_unc                    = 0.040097;

=============================================================
============= Values for Force Coefficients =================
=============================================================

Key Pcal Values :
GS           =      -5.135100; Gold Standard Value in (V/W)             
WS           =      -4.700380; Working Standard Value             

costheta     =      0.988362; Angle of incidence
c            =      299792458.000000; Speed of Light
             
End Station Values : 
TXWS         =        -1.078768; Tx to WS Rel responsivity (V/V)
sigma_TXWS   =        0.000455; Uncertainity of Tx to WS Rel responsivity (V/V)
RXWS         =        -1.391922; Rx to WS Rel responsivity (V/V)
sigma_RXWS   =        0.000558; Uncertainity of Rx to WS Rel responsivity (V/V)

e            =        0.988790; Optical Efficiency
sigma_e      =        0.000645; Uncertainity in Optical Efficiency

Martel Voltage fit : 
Martel_gradient         =        1636.946737; Martel to output channel (C/V)
Martel_intercept   =        0.081815; Intercept of fit of     Martel to output (C/V)

Power Loss Apportion : 
beta          =        0.998895; Ratio between input and output (Beta)  
E_T          =        0.993829; TX Optical efficiency 
sigma_E_T          =        0.000324; Uncertainity in TX Optical efficiency 
E_R          =        0.994929; RX Optical Efficiency 
sigma_E_R          =        0.000325; Uncertainity in RX Optical efficiency 

Force Coefficients : 
FC_TxPD          =        7.894807e-13; TxPD Force Coefficient 
FC_RxPD          =        6.188002e-13; RxPD Force Coefficient 
sigma_FC_TxPD          =        3.610367e-24; TxPD Force Coefficient 
sigma_FC_RxPD          =        2.777621e-24; RxPD Force Coefficient 

data written to CDS: /ligo/gitcommon/Calibration/pcal/O4/ES/measurements/LHO_EndX/tD20261013

That's cool and all but perhaps you wanna see that data with relation to the End X's history, I gotchu: 
You can see the trends here or go to the git page for this measurement.