J. Kissel, K. Venkateswara

While trying to figure out the gain mismatch between the BRS and GND T240 at EX, we discovered that the calibration filter, "T240Cal" in FM2 of the GNDSTSINF A and B (but not C!) banks had been changed from the 254.3 [(nm/s)/ct] I'd installed in March (see LHO aLOG 10635), to 4.46 [(nm/s)/ct]. This gain calibration is different (reduced) by a factor of 57 -- which is the difference between high gain and low gain mode of the T240 interface chassis. This would therefore compensate for the gain in the chassis being switched from low (as it was in March) to high.

After a lot of aLOG trolling (thankfully there are only about three people on the planet who would have the knowledge to change the analog gain, and compensate for it in foton), I found that this is a result of Rich Mittleman (see LHO aLOG 11203). The filter file had not been committed to the userapps repo until I'd committed the file for other reasons (see LHO aLOG 13483), the aLOG did not mention the calibration factor, or the filter name, the filter bank name, or that he only changed 2 out of the three filter banks, rendering the aLOG quite difficult to find.

This does not explain the factor of 0.62 that we're missing, but it certainly threw us for a loop for a half-day. Please let this be a lesson to visiting commissioners and electronics engineers who make changes to hardware /software / electronics: add as much information as you can to your aLOGs to make them as search-able as possible, take the time to commit your work to repositories, and make sure you've done a complete job. Everything you do will eventually matter and confuse someone in the future. 

For all this, we still don't understand the factor of 0.62 between the T240 and the Rotation Sensor (see LHO aLOG 13544).