aLIGO LHO Logbook

H2 SUS

keita.kawabe@LIGO.ORG - posted 12:34, Thursday 06 September 2012 - last comment - 13:08, Monday 24 September 2012(4112)

ETM and ITM upper stage SUS BOSEM and Oplev sign question

From initial alignment data, we know the following:

Positive offset in PIT (H2:SUS-ETMY_M0_OFFSET_P and H2:SUS-ITMY_M0_OFFSET_P) will tilt the mirros such that the reflected beam off of the mirrors will go down.

Positive offset in YAW will tilt the mirrors such that the reflected beam off of the mirrors will go toward the inside of L.

That is, the upper stage of ITM looks like the mirror image of the ETM. Why is this the case? I thought that they are identical.

Also, I think oplev sign is somehow wrong. It's not consistent with initial alignment data.

FYI, the sign of the things in initial alignment was figured out by:

First using baffle diodes to figure out the sign of the TMS to figure out the TMS sign, and make the first beam hit the center of the ITM.

Then using ETMY cage and CCD camera, make the reflected beam from the ITM hit the cage bars to figure out the sign of ITM.

Then move offset of ETMY so that the beam comes back to the table, then move TMS and repeat, to see if ETMY sign is the same as TMS (it is).

As you can see, there is not much ambiguity there.

Comments related to this report

keita.kawabe@LIGO.ORG - 12:59, Thursday 06 September 2012 (4113)

Link

Attached is the oplev and upper stage offset. (Jumps not caused by the offset are from HEPI.)

For positive SUS offset, the following is true for Oplev:

	Positive PIT offset	Positive YAW offset
ETMY	Oplev goes negative	Oplev goes positive
ITMY	Oplev goes positive	Oplev goes positive

From this, oplev seems to think that positive PIT offset moves ETMY down but ITMY up, and positive YAW offset rotates both ETM and ITM in the same direction.

Images attached to this comment

mark.barton@LIGO.ORG - 16:51, Friday 07 September 2012 (4130)

Link

Mark Barton

I did some followup on this issue and it looks as if the F2 and F3 OSEMs may be swapped on ITMy. See attached plots which have Keita's channels (divided up into separate plots for ETMy and ITMy), plus additional ones of interest, including the M0F1, M0F2, M0F3, L1UL and L1LR sensors, the estimated P and Y from the OSEM2EUL blocks at M0 and L1, and the requested drives to the M0F1, M0F2 and M0F3 coils before magnet sign correction. I also zoomed in on a 3 hour period from 12-09-06-02-00 to better show the events of interest.

With ETMy, everything is as expected. The pitch OL reads negative for positive pitch offset but this is as designed - the OL is trying to be a measure of beam height and positive SUS pitch is down. (Yaw is left=positive viewing the QPD from the optic, which is the same convention as for SUS.)

With ITMy, everything internal to SUS to do with pitch is as expected, but the OL does not have the expected opposite sign. In yaw, the M0 and L1 Y channels have opposite sign and the yaw OL agrees with M0 yaw.

This would be consistent with the F2 and F3 OSEMs on the ITMy being swapped. A further data point in favour of this is that the signs in the ITMy COILOUTF block are the opposite of expected from E1000617 (F2 should be opposite F1 and F3, and is for ETMy, but it's F3 that's opposite for ITMy). This was earlier put down to a magnet swap, but the comparison with the L1 level suggests it's actually the OSEMs that are swapped. This wouldn't be a hard mistake to make because the convention in E1000617 is a bit confusing: both M0 and R0 face OSEMs are labelled

F1
F2 F3

as viewed from the _back_ (i.e. the reaction chain side), so the M0 OSEMs are

F1
F3 F2

from the side you would work on them from.

As far as OL's are concerned, things are consistent with both ITMy OL channels being flipped, as if the QPD were upside down.

Images attached to this comment

thomas.vo@LIGO.ORG - 13:08, Monday 24 September 2012 (4279)

Link

Mark B. Thomas V.

We buzzed out the the QPD with a laser pointer on ITMy and found that the QPD is upside down from what the MEDM screen on the SUS quadrants are indicating.

The segments of the QPD are laid out as such:
+-------+
| 2 | 4 | ^
|---+---| | This way up
| 3 | 1 | |
+---+---+

I believe the error came from a miscommunication in the exchange of information between SUS and OptLevs. I had originally mapped out the quadrants on 07/24/2012 according to ALOG 3573 using the MEDM screens. I wasn't aware that the top level ITMY SUS QUAD model had been re-ordering the signals as such (as described in Jeff K's ALOG 3613):

Analog Signal ADC Channel SEG#
1 1_0 SEG2
2 1_1 SEG1
3 1_2 SEG4
4 1_3 SEG3

According to ALOG 3613, Jeff had re-ordered ADC Channel and SEG# to 1:1 as it makes the most sense to be that way! I think this sequence of events led to us being confused on why the signals look like they're upside down since the diagonals of the signals are switched. This fix explains why Keita's original entry shows that the OptLevs look "backwards" in some sense.

For future reference, I'll try to be more clear on what I'm measuring when mapping out the orientation of the optical lever QPD, as well as run tests with the suspension offsets in pitch and yaw to make sure they coincide with each other. This will be added to the Optical Lever Installation Procedure (E1200063).