Kevin, Craig

Today we measured the DARM offset and contrast defect by sweeping the DARM offset, similar to September 1, 2022 alog 64974.
The main difference between then and now is 
48 W vs 57 W input power,
52.7 vs 51.5 PRG,
4.0 W vs 2.7 W CO2X,
1.8 W vs 0.0 W CO2Y,
and EX and IX ring heater settings were changed (IX RH on, EX RH off).

The nominal DARM offset and homodyne angle for 60 W requested input are reported below.

Results

DARM offset = 6.83 pm

Contrast defect light = 4.8 mW
Total DCPD light = 23.3 mW
DARM offset light = 18.6 mW

Homodyne angle = 26.7 deg

X0_OFFSET nominal = 7.74 cts
X0_OFFSET true zero = 1.83 cts
X0_OFFSET to DARM offset calibration = 1.156 pm/cts


Method
This time, we added a notch at 255.0 Hz in the DARM loop (DARM2 FM1 ESB255), put all the PCALX power in a sine wave at 255.0 Hz, 
and put all the PCALY power at 410.3 Hz and made sure that the DARM loop notch at 410.3 Hz (DARM2 FM2 ESB410) was on.
The notches ensure the DARM loop suppression changing around does not affect our results.
Then we stepped the DARM offset around.

Afterwards we gathered the data, took some PSDs, and looked at the 255 and 410.3 Hz lines.
From this we backed out the above results.

Code
DARM offset step code and analysis code in
/ligo/gitcommon/labutils/darm_offset_step
There is a README that tells you how to run the test.