J. Kissel, K. Izumi, S. Ballmer


Inspired by the presentation Suresh gave today on the ISC / Integration Call, (see T1400311, which includes data originally taken from LLO aLOG 11518), I looked at the spot positions on the IMC WFS using a 1 hour trend over the past 30 days (Suresh used 20 days). See first attachment. While LLO's spot position oscillates roughly +/- 0.5 [fractional beam diameters] (though it's tough to quantify without y-axis labels or gridlines), LHO's WFS DC signals fluctuate about an order-of-magnitude less over these time-scales. Though we haven't cross-referenced in detail, we believe the discrete jumps seen correspond to the MCWFS alignment mystery starting and te subsequent recovery (see LHO aLOG 11481).

I also attach the amplitude spectra of MC2 Trans, calibrated* into [W/rtHz] at of intra-cavity power noise, as Suresh has done on pg 7 of T1400311. One can see that the (equivalent) transmitted power noise floor is higher at LHO, (which we guess it QPD electronics noise), so at this point we're not able to resolve noise features at the same level, so it's unclear if we have similar problems with WFS centering creating intensity noise.


* Calibration of H1:IMC-MC2_TRANS_SUM_IN1_DQ into intra-cavity power:
The SUM is already calibrated into [uW] at the MC2 TRANS QPD as per LHO aLOG 9716. 
From T1300386 and E1300756, the two non-suspended optics between the QPD and MC2 are AROM RHS (a high-reflector), and ROM RH13 (a R=90% beam splitter). From the galaxy optics page and E1200463, our MC2 is IMCC02, with a transmission of 5.1 ppm. So, the extinction ratio between the HR surface of MC2 and MC2 trans is
     5.1e-6 [MC2 AR / MC2 HR] * 0.1 [ROM RH13 AR / MC2 AR] = 5.1e-7 [ROM RH13 AR / MC2 HR]
(where I've ignored the loss from MC2 AR surface, since it's small w.r.t. the 10% loss from the ROM RH13). Therefore 1 [uW] at MC2 TRANS QPD is (1 / 5.1e-7) = 1.96e6 [uW] = 1.96 [W] of intra-cavity power. Currently, the calibrated MC2 TRANS reports 433 [uW] at DC, i.e. 848 [W] of intra-cavity power. This is roughly consistent with the 5.7 [W] measured at the input at PSL periscope * Finesse/pi = 936 [W] of intra-cavity power (with IMC Finesse as 516).