Ed in this morning noticed that INPUT_ALIGN did not work. The cause is still UNKNOWN; It is NOT FIXED yet.

Symptom: the IR laser does not lock to the X arm in INPUT_ALIGN.

If you happen to do INPUT_ALIGN and come across the same issue, please consider doing the workaround written below.

A workaroud (temporary, manual and painful):

Request INPUT_ALIGN (although we assume that it is not going to lock).
Open the camera monitor of the AS port (or the one on the front TV screen in the control room),
Adjust the exposure time of the camera to something small, for example 36000 so that you don't miss a fast passage of fringes.
When the arm passes a resonance, the beam spot on the camera blinks. During this short blink, it shows a higher order mode (it may appear as a lateral shift of the beam spot depending on misalignment).
Using the information of the higher-order mode, move IM4 and PR2 in angle to improve the higher order modes.
- If the alignment is perfect,the beam just blinks without any higher-order-modes or lateral shift.
Once you get a satisfactory in the above process, check how high LSC-TR_X_NORM is. If it occasionally reaches 1, we are done. If not, keep improving the alignment.

What I tried (none of them worked):

I spent a couple of hours investigating the issue, but I could not identify or fix the issue. And I was never able to lock the laser the X arm. The below is a list of what I tried.

Different gain settings in LSC-XARM from 0.01 to 10. By the way, the nominal is 0.05.
- When the gain was at around 0.4, it consistently started catching fringes but for a very very short duration of about 0.1 sec seemingly with a gain peaking oscillation.
The opposite control sign.
Different combination of the whitening filters
- We normally use whitening filters 1 and 2. I tried a combination of 1 and 3, and also 2 and 3.
A different coil drive whitening.
- I tried state 2 in MC2 M2 stage which occasionally saturated due the lock transient.
Fine adjustment of dark offset in ASAIR_A
Different trigger settings, different filter-trigger settings and etc.