R. Short, with guidance from K. Kawabe and J. Oberling

I've made some updates to the 'CLOSE_ISS' state in the IMC_LOCK Guardian, which handles the closing of the ISS second loop, that should make the closing of the second loop more consistent.

When the second loop is engaged, the DC working point of the ISS is determined by the output of the second loop's digital AC coupling (H1:PSL-ISS_SECONDLOOP_AC_COUPLING_DRIVE). This output is held when the second loop is closed, but it can occasionally be held far from the average of its oscillations before the second loop is engaged, which in the past has caused locklosses and was mitigated earlier this year (see alog 67347). While the second loop isn't causing locklosses anymore (that I can recall), we do still see the digital AC coupling output being held slightly off from the mean, causing the diffracted power to jump. I've expanded upon Georgia's logic by changing the way the output of the AC coupling drive is held to be more consistent. Instead of waiting to hold the output until it's near the mean over the past 10 seconds (a calculation that itself can take several seconds), the process is now as follows:

1. Take a 10-second average on the AC coupling drive output
2. Hold the output
3. Write the 10-second average value to the AC coupling drive's offset
4. Turn off the input and turn on the offset
5. Unhold then rehold the output (to grab the offset value)
6. Perform Georgia's "within 100 counts" check on the output
7. If successful, zero the offset and turn back on the input

I was able to test this logic during the maintenance period today, both with the IMC locked at 2W and 60W, with great success. We'll run with this for a while to see if over several lock acquisitions the second loop is being engaged with a more consistent digital AC coupling drive. The updated IMC_LOCK Guardian code is loaded and committed to SVN.