aLIGO LHO Logbook

H1 SUS

betsy.weaver@LIGO.ORG - posted 17:05, Wednesday 20 March 2013 - last comment - 19:52, Wednesday 20 March 2013(5852)

PR3 and PR2 bias slider "calibration" to alignment beam

While we were at it, I measured how much slider range it took to move the beams on various optics.

Using the PR3 pitch bias slider, we moved the IAS beam 81mm at the PR2 optic. The PR2 and PR3 are separated by 16.16m (so says T0900043, optical layout numbers for aLIGO). Since this is an optical lever type measurement, 2*theta = 81mm/16.16m = 5mRad, theta = 2.5mRad. The slider amount used to move the beam the 81mm was 2000uRad (units as advertized on the medm) = 2.0mRad. So, comparable numbers.

PR3 yaw: 2*theta = 54mm/16.16m = 3.3mRad, theta = 1.65mRad, while the slider says that we had moved 1500uRad = 1.5mRad.

Similarly for PR2, looking at the beam down on PRM:

PITCH

2*theta = 60mm/16.61m = 3.61mRad, theta = 1.8mRad

Slider says we moved 1550uRad = 1.550mRad

YAW

2*theta = 150mm/16.61m = 9mRad, theta = 4.5mRad

Slider says we moved 4000uRad = 4.0mRad

Again, the slider was roughly in agreement with what we saw. However, we did encounter 2 other oddities:

1) It seems strange that the IAS direct alignment of PR3 advertised that we had aligned the PR3 in pitch to within +/- 50 uRad. We had achieved this, yet as recorded above, we used 1.5mRad of slider bias to center on PR2. Why??

2) We found the bias sliders quit actuating on the PR3 optic in PITCH at ~4500 uRad, and in YAW at ~2000 uRad. The slider full range is +/-12,300uRad, so I'm not sure what is physically stopping the slew of the optic at ~4000. We'll investigate tomorrow, via repeating with others. Mark B. and Arnaud are also chewing on the original uRad calibration. The MC optics have alignment biases in some cases near 3000 and 4000 uRad, so I wonder if they have seen this...

Comments related to this report

christopher.wipf@LIGO.ORG - 19:52, Wednesday 20 March 2013 (5853)

Link

The SUS alignment bias screen can be thoroughly misleading.  We saw this while aligning TMSY.  The range of the calibrated slider has no particular relation to what the actuators are capable of doing.  And the "DAC counts" shown on that screen are later rescaled by a matrix -- so it leaves you with no clue as to what's really coming out of the DAC.

Taking a glance at PR3, the 4 mrad pitch bias appears to use up more than 90% of the available actuation force (123000 counts on the DAC outputs to the T2/T3 coils; the limits are +/- 131072).  On MC1 and MC3 the pitch biases are modest, but there are 4 mrad yaw biases, which take up about 50% of the DAC range.  (On MC2 the biases are quite small.)