Daniel, Jenne, Fil, TVo
In preparation for this weekend's work to try to do an OMC Scan measurement while varying SR3 RoC, I decided to try to make sure our new alignment of the IMC could make it down to HAM6. I was able to get light on AS_A_DC and AS_B_DC and close the centering loops but I noticed that AS_C had no light on it which is very odd.
All the electronics were turned back on from our grounding loop searching from April 26-27 however, the attached trend shows that something dramatic happened to that QPD segments around Apr 27 2018 15:53:41 PDT which caused the individual segments to get much noisier and the sum to drop dramatically. I'm pretty sure we were laser safe at the time so it wasn't like the a beam falling off the diode. So I thought this might be some sort of electronics problem like a disconnected cable in the midst of the madness, but everything in the rack to the chamber feed-through seemed to be connected properly, Fil and Daniel tested the in vacuum QPD chain from the outside with a multi-meter and confirmed that there is a connected diode in chamber.
So maybe we're just not seeing light? Again, this seems strange because we see light on both AS_A and AS_B.
In general, this won't deter our work this weekend but we might want to get this figured out before we open the doors early next week or put it on the task list for the chamber. One method we haven't tried is looking for AS_AIR and seeing if that comes out of the viewport. Also, there is a picomotor we can actuate to steer the beam onto AS_C, but I'm holding off on this unless we know what is going on.
Made a quick check that the downstream electronics works:
Thomas, Dan
We checked the AS air and OMC transmission beams coming out of the north HAM6 viewports. They were too dim to see with cards but visible with the IR viewer, both are clearing the viewports fine
After more looking in the viewport with Keita, Daniel and TVo (including turning the illuminator on/off, and sadly seeing no response on AS_C, although we did see a tiny response on the OMC QPDs which are inside the shroud), I worked on picomotoring the AS_C steering mirror, in a last-ditch effort to see if we could ever see a signal. The answer so far is no. Using the largest picomotor steps, I went +/- 6 steps in pitch and +/- 10 steps in yaw from the previous nominal. I thought I saw a beam briefly, but it turned out to be something upstream, since it affected all of the AS diodes (including the WFS, which aren't affected by this steering mirror). I did a rough raster scan of the picomotor actuation, mapping out a rectangle. When I was finished, I put the picomotor back to the place that I started, so even though it won't have gone precisely to the old pointing, it should be close.
AS_C seems to have a lot of 60 Hz + harmonics noise, more so than the OMC QPDs do, when the IMC is offline (MC2 misaligned). Daniel points out that this shouldn't affect our ability to see a DC response, and the rest of the electronics chain passed some basic checks, but it does seem like a concern.
Daniel pointed out to me that the picomotor to HAM6 likely wasn't connected since ISCT6 isn't really back in it's final position yet. And, he was right. So, today I connected the HAM6 picomotor controller using some 10' DB25 extension cables, and then re-did my raster of the AS_C pico. Step size of "magnum", 10 x 10 grid. I didn't see anything at all that looked promising. The picomotor is back nominally where I started, although it's of course not precise. I think we're going to have to pull the north door of HAM6 tomorrow, as we discussed was a possibility at this morning's meeting.
In case you find a problem with the QPD diode, the head, or the invac cable:
The cable currently installed for AS_C is D1101654 / S1202409 (40"). It has only one head (Head #29). The diode installed is Q3000 InGaAs QPD #18.
cf. LHO ALOG 12244
There is only one spare InGaAs QPD diode (#15) left as an LHO spare.
The spare 40" cable is D1101654 / S1301546. This cable has no head. Therefore, to use this cable, the head needs to be transplanted from S1202409 or D1000231 S1202412 (36"), which has two heads (#34 and #35) and two diodes (#55 and #76) attached.
See also: E1101174