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VIS (SR3)
fabian.arellano - 8:51 Wednesday 11 August 2021 (17875) Print this report
Oplev alignment and QPD calibration.

The aim of the work yesterday was to set the length sensing QPD to a suitable position with respect to the lens. A more detailed entry will follow. The highlights are these:

  • I recovered the alignment of the oplev by moving the dirrerent stages of the suspension to suitable places and orientations.
  • I applied white noise actuation in the TM-P channel.
  • I found the suitable position of the length sensing QPD with respect to the lens by minimizing the amplitude at the pitch resonance at around 843 mHz.
  • Such position is 10.80 mm as read by the micrometer stage.
Comments to this report:
fabian.arellano - 20:24 Wednesday 11 August 2021 (17881) Print this report

I calibrated the QPD readout in terms of displacement.

  • I moved the IP close to the setpoint using the horizontal fishing rods.

  • I applied coil-magnet actuation to the GAS filters to bring them all to their new setpoints, which are zero all of them. Note that the values of the setpoints in the medm screen have not been changed to the new ones.

  • I acquired the data using the script provided by the oplev medm screen.

  • The data are in the directory: /opt/rtcds/userapps/release/vis/common/scripts/autocommissioning/oplev/data/

  • I'll analyze the data and post the result here.

fabian.arellano - 15:19 Thursday 12 August 2021 (17898) Print this report

Here the calibration data is. I'll add comments in a future entry.

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fabian.arellano - 10:35 Monday 16 August 2021 (17918) Print this report

I analyzed  the calibration data:

QPD parameter calibration factor (QPD-du/mm) Linear range per micrometer (mm) Tota range (µm)
LEN VER -7.84 [ 8.51  , 8.70 ] ±95
LEN HOR -8.43 [ 5.18  ,  5.33 ] ±75
TILT VER -1.39 [ 10.6   , 11.5 ] ±450
TILT HOR -1.33 [ 7.35  , 8.15 ] ±400

Observations:

  • The linear ranges for the Tilt QPD are larger than those for the length QPD.
  • The readout of the QPDs is not in units of counts. Counts refers to the DAC and ADC. I call the QPD readout units QPD displacement units or QPD-du.
  • I fitted the data in Excel and not with the script provided in the medm screen because I wanted to choose the data points considered to be within the linear range and report the size of the range.
fabian.arellano - 12:14 Monday 16 August 2021 (17920) Print this report

Notes can be found in the album SR3 remedying work after O3.

This entry provides more information on how I identified the suitble place for the length QPD with respect to the lens.

As pointed out in entry 17875, in the suitable position, the amplitude spectral density (ASD) of the LEN VER channel is expected to be minimum at the pitch resonance at around 843 mHz. The first plot attached shows the raw data in orange with normalized values in blue. The nomalization was done with TILT VER  values to take into account differences between measurements.

  • The amplitude at 10.80 mm is the lowest. As shown in the second screenshot (blue plot), the amplitude is actually buried in noise at 843 mHz in the ASD.
  • The amplitude at 10.60 mm is also buried in noise, but I still used 10.80 mm.
  • The files are at: /kagra/Dropbox/Subsystems/VIS/TypeBData/SR3/OPLEV/len_qpd_alignment/
  • The amplitude values at the resonance is included in the Excel file.
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fabian.arellano - 14:39 Friday 20 August 2021 (17971) Print this report

Pictures at SR3 remedying work after O3.

I constructed the oplev windshield with aluminium foil. It looks cozy and safe.

Terrence turned the control on just to make sure I had not disturbed the oplev alignment in the process. It worked well.

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