Detector Watch 4

You can search the logs timewise. I found the same stuff in say July's entries ( hit the 'Calendar button' )

No, I don't think so.

Yup, but as far as my reading has progressed, I think there's some subtleties here. Take this with a grain of salt:

The AS_Q is the anti-symmetric port signal - a photodetector voltage level derived from a photon flux via the physics of photodiodes. Whereas DARM_ERR is the correction signal applied via feedback to keep the Fabry-Perot resonance section locked. I think the difference between the two would be:

- a time lag.

- some transfer/amplification factor.

- a change of physical units.

You see the interferometer is a 'null' instrument. The idea is to not, as far as possible, actually allow the distance between the mirrors to change - once they are set at a resonance.

My analogy is that of driving a car down a straight road in a buffeting crosswind. For those of us who have done this, you know that to stay on a firm direction requires that a variable force has to be applied to the steering wheel to correct for movement induced by the wind hitting the side. It's an active process, is always slightly 'behind the play', but nonetheless suppresses side to side sliding across the road.

You could choose not to correct at the steering wheel, and simply use the lateral motion of the car as a proxy measurement of the sideways component of wind speed. You could even calibrate to known wind speeds, directions, tire and road characteristics etc.

But without any course correction, you would likely run out of road edge! However if you did correct the steering you could still measure/assess the wind: but by using the force applied to the steering wheel as a proxy. You would do something like this:

- note that the car has wandered by 'X'. ( ie. AS_Q )

- apply a force 'Y' to the wheel. ( ie. DARM_ERR )

- deduce a wind speed 'Z' ( ie. RMS strain aka. spacetime distortion ) that would have produced the 'X' thus stimulating the 'Y'.

This way you get to keep going down the road. Or in the case of the IFO's stay within the 'sweet spot' where the photon's keep circulating in such a manner to increase our effective path length and thus sensitivity. That resonance/path-folding trick is a key design feature which gives several orders of magnitude advantage.

I guess the analogy to going off-road would be when the IFO's lose lock!!

Cheers, Mike.

( edit ) Well done Chipper for nudging me back to this thread. I'd been somewhat distracted lately....

Also the 'hardware injections' I think are part of the procedure to calibrate matters. This would feed in a known amount of stimulus ( into DARM_ERR ? ) to then guage the response ( at AS_Q ? ) of that. We then build knowledge of such transfer characteristics to be applied later to unknown/native signals.