I measured the spot position on PRM by using the usual angular dither technique (see for example LLO alog 5010) with PRY locked. It turned out to be pretty good.

• measured vertical off-centering = too low by 2.0 mm
• measured horizontal off-centering = off toward PR3 by 1.0 mm

(some settings):

• ITMX and SRM are intentionally misaligned.
• PRY is locked on the carrier light by REFL_A_RF45_I with a demod phase of 6.9 deg
• No whitening gain, but whitening stage 1 and 2 are engaged to obtain better signal-to-noise ratio.
• Feedback on PRM and PR2 as usual
• servo gain in LSC = -50 which should give a UGF of 30-60 Hz.
• Excitation in SUS-PRM_LOCK_P(Y) with an amplitude of 1000 counts at 10.1 Hz
• I used dtt to take a transfer function of (REFL_A_RF45_I) / (PRM_LOCK_P(Y)) 
• Before the measurement, PRM was tweaked to maximize the intracavity power

(results)

See the plot below.

(some notes)

• At the beginning of the measurement, the beam on REFL_A was at the edge of REFL_A. This resulted in bogus data because it introduced some kind of wave-front-sensing effect as the PRM angle was modulated.
  • This was fixed by moving the tip-tilt mirrors in HAM1, namely RM1 and RM2.
• I used a conversion factor of 42.2 mm for converting the imbalance alpha (see 40m elog 2863 for the definition) into the position of the rotational nodes.
  • According to D1101377-v2,  I used the following numbers to derive the conversion factor:
    • radius of the mirror: R = 75 mm
    • mirror thickness: L = 75 mm
    • magnet separation D = 95.45 mm