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Entry  Fri Sep 16 22:51:52 2011, Paul, Update, SUS, Calibration plan for the oplevs 
    Reply  Sun Sep 18 14:08:52 2011, rana, Update, SUS, Calibration plan for the oplevs 
Message ID: 5443     Entry time: Fri Sep 16 22:51:52 2011     Reply to this: 5448
Author: Paul 
Type: Update 
Category: SUS 
Subject: Calibration plan for the oplevs 

 In order to estimate the amount of noise that the oplevs are injecting into the GW channel, we first need to calibrate oplev signals in terms of angular change in the optic. I said in my previous post that there wasn't a calibration factor for OSEM values to radians, but I found that Kakeru had estimated this in 2009 - see entry 1413. However, Kakeru found that this was quite a rough estimate, and that it didn't agree with his calibrated oplev values well. He does quote the 2V/mm calibration factor for the OSEM readings though - does anyone know the provenance of this factor? I searched for OSEM calibration and found nothing.

Kiwamu and Suresh suggested a way to calibrate the oplevs without needing to calibrate the OSEMs in the way that Kakeru describes in entry 1413. This should give a calibration for the OSEMs _and_ the oplevs in fact. The method should be as follows:
1) Change the coil driver values in DC to give tip or tilt the optic. Measure the resulting change in spot position at a known distance from the optic, perhaps just using a ruler. Record the spot position and OSEM values for each coil driver value. This will definitely require a smaller spot size, so I'll implement the new telescopes first.
2) Knowing the length of the lever arm from the optic to the spot measurement position, we can calibrate the OSEM values to radians.
3) We can now put the beam onto the oplev QPD, and either change the coil driver values again in the same way (but over a smaller range), or excite the test mass in pitch or yaw, this time measuring both the OSEM values and the oplev QPD values. Since we can already convert from OSEM values to radians, we can now convert from oplev values to radians too.
4) I should be careful to consider the input sensing matrix for both the OSEMs and the oplevs in these measurements. Should I divide those out of the calibration to avoid that if they change the calibration factor changes too?
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