DarkhanT, Yuki Inoue (KAGRA), RickS (SudarshanK assisting with image analysis over the phone - until the phone died).
Today, we went to Yend to investigate the apparent clipping that EvanG had discovered (see entry 30827).
We found that both beams were off centered on the aperture of the Rx module integrating sphere, with the Outer (lower) beam clipping on the aperture (see attached image). Blocking one beam and then the other revealed that the increased noise in the Rx power sensor spectrum was all due to the clipping of the Outer beam.
Camera images revealed that the beams had moved on the ETM as well (exonerating movement of the ETM as the cause of the movement on the Rx side).
Analyzing Pcal camera images we found the beam position offsets from the optimal locations were (x, y): Upper (-1.5, 1.8); Lower (-6.9, -2.5) (millimeters) Usual x,y convention: positive x, right; positive y, up.
Not knowing what had moved, we used the output steering mirrors in the Tx module to center the beams on the Rx intetrating sphere aperture (see attached photo).
After moving the beams, analysis of camera images (see attached) gave the following beam locations (x, y): Upper (-1.4, -0.9); Lower (-1.3, -1.4) (millimeters).
These are within our 2 mm tolerance.
We installed apertures at the output of the Tx module and at the input (just upstream of the steering mirrors) in the Rx module.
We will repeat the Yend calibration using the Working Standard as soon as possible.
Spectra look much better too! Attached are times from before Oct 11 maintenance work (blue), just after Oct 18 maintenance work (green), and this morning after the clipping had been fixed (red). Note that the amplitude of the 331.9 Hz has returned to the O1 amplitude. A trend of the mean power level over the last 10 days is also attached. The calibration discrepancy between TX and RX PD is likely now reduced. Previously, as identified by Shivaraj (LHO aLOG 30845), the discrepancy was ~4%. Now the discrepancy is less than ~1% as the end of the trend indicates.
