Keita, Austin, Betsy, Rahul
HXDS OM2 (Thermal SAMS) suspension has been installed in HAM6 chamber, replacing the old OM2 (i.e. Tip Tilt) - please see three pictures attached below.
This morning we started by recording the current position of the tip tilt suspension (old OM2) by placing a ruler in front of the suspension cage. Yesterday we had already confirmed the beam path using the aux laser set up by Keita :- beam from OM1 going to OM2, which is then directed to OM3 and into OMC. We placed some irises in their path to record the beam alignment. The Tip Tilt suspension was then removed from the chamber this morning. The new OM2 (T-SAMS) was then moved into the chamber and roughly positioned at first using the ruler as our guide (moving back 40mm in +y direction and then around 30mm in +X direction). The EQ stops were pulled back to freely suspend all three stages. Using the aux laser we fine tuned the position of OM2 until we roughly centered the beam on its optics and the reflected beam centered on the iris kept before OM3.
Once Keita was satisfied with the beam alignment, we attached 3 dog clamps for now (a couple more to be attached later on). . Then I connected the BOSEMS cable (using the tip tilt suspension cable) and heater cable. Given below are the Open Light Current (OLC), their offsets and gain,
| BOSEM (D060108-C) s/n | position | OLC | offset (-OLC/2) | gain (30000/OLC) |
| 389 | LR | 20600 | -10300 | 1.4563 |
| 317 | UL | 21750 | -10875 | 1.3793 |
| 552 | LL | 21450 | -10725 | 1.3986 |
| 672 | UR | 20390 | -10195 | 1.4713 |
After recording the OLC values , we pushed the BOSEMs to center it with the flag on the suspension.
On Monday, I will take top to top transfer function measurement of OM2 to rule out any rubbing and will also perform second round of heater thermistor test.
Also, this morning Betsy attached a beam dump behind the OM2 bottom mass - fastened to the suspension cage. Keita took some pictures which he will upload later on.
About Rahul's comment "moving back 40mm in +y direction and then around 30mm in +X direction".
What's ~40mm?
That's the difference between the bottom front edge of the old TT and that of the bottom adapter piece of the new HTDS.
Using a ruler (which is different from the ruler Rahul wrote about), looking at the TT mirror holder from the side opening of the cage, we decided that the mirror holder front surface is ~30mm recessed from the TT cage front edge.
OTOH, Don told Rahul that the bottom front edge of the HTDS adapter piece for T-SAMS is recessed from the mirror surface by ~10mm (i.e. the mirror surface is protruding from the adapter footprint).
Thus the HTDS adapter front edge needed to move by ~30+10=40mm from the bottom front edge of the old TT2 cage in +Y direction.
BTW any potential error in this procedure makes no real impact on the mode matching because the Rayleigh range of the OM1-OM2 beam as well as OM2-OMC beam are both ~70cm.
What's ~30mm?
That's a rough estimate of the difference between the +X edge of the TT bottom and the HTDS adapter bottom. We didn't measure it carefully or anything because it's just a lateral position of the cage.
What we did is to center the beam on the new OM2 horizontally.
Austin took pictures of the beam position on the viewer card while Rahul pressed a ruler against the front thermal shield (D2000272), the width of which was measured to be 178mm (turns out that it's 7"=177.8mm nominal). The mid point would be 89mm on the ruler. I estimated by using the viewer card that the distance between the incoming beam and reflected beam at the surface of the thermal shield was 4~5mm, so we shot for 86~87mm on the ruler for the incoming beam.
More pictures
1st and 2nd: Old OM2 (TT) position.
3rd, 4th, 5th: New OM2 (HTDS) position. You can see why we didn't use the cookie cutter. (No space.)
6th, 7th: Betsy's improvised beam dump solution because we have no space behind the new OM2 to mount the beam dump. It's hard to see but the beam dump is tilted down. We'll replace this when somebody invents a better and easy to implement solution.
8th: Rahul adjusts the position of HTDS.
OM2 heating test is going on.
I left H1:AWC-OM2_TSAMS_POWER_SET set to 4.6W, which results in ~21V across the heater element. This is the maximum hard-coded in the Beckhoff code to protect T-SAMS from overheating IN VACUUM (in the air the temperature doesn't go as high, so we're overly protected now).
Thermistor 2 is already stable at about 37.5C. Thermistor 1, which is closer to the mirror, is ~25.5C right now and is still going up.
I'll turn it off before I go to bed.
Upadte: After 7+ hours (~22:30 PDT), Thermistor 2 is about ~25.9C and it doesn't look like it's going up any more. Thermistor 1 is ~37.6C. I set the heating request to 0W.
New OMCR DCPD was connected in chamber and it's working.
See attached. A spike at around t=-7h is when I used a flashlight on the PD.
The PD is placed at a temporary location, and we haven't installed any DB9 cable bracket yet.
I have attached the image of the beam spot on the OM2 optic as mentioned in Keita's comment. As stated, we took the length of the thermal shield (178mm) to get the mid point on the ruler (89mm). In order to compensate for the distance between the incoming beam and reflected beam, we subtracted about 3mm from the mid point, giving our estimated spot position to be ~86mm, shown here.
OM2 (T-SAMS) Tranfer function results are posted in the following alog,
https://alog.ligo-wa.caltech.edu/aLOG/index.php?callRep=65028