Daniel, Keita, Rana, Aidan, Dale, Dick
Using the ITMY baffle PDs, we were able to get the green beam from EY to ITMY today! The aLIGO beam tubes are able to transmit light.
We spent hours today trying out various ideas about how the beam might be missing, but in the end we found it by moving the EY Transmon Suspension.
The MEDM screen for the TMS had a range of +/- 15000 cts. In fact, the TMS has an 18-bit DAC and so its full range is +/- 2^17 = +/- 131072 cts.
With the current ALS table alignment, the TMS biases which give the maximum voltage on each baffle PD channel are:
chan name |
P |
Y |
CS_CHAN_27 |
-10000 |
+2900 |
CS_CHAN_28 |
-69000 |
+1200 |
CS_CHAN_29 |
-48000 |
+1000 |
CS_CHAN_30 |
-46700 |
+17800 |
We used these numbers to "triangulate". Then we used the ITMY spool cam to further center the beam on the ITM. It is not at all visible on the ITM surface, but can barely be seen on the gate valve behind the ITM as well as a little glint on the edge of the hole in the baffle where the cavity axis is. Our random search pattern initially just gave flashes of ~50 cts, so the new digital filters helped after all. Eventually, the signal on each PD can separately be made to saturate at 32768 cts as we align the beam onto it.
The final TMS settings for "good alignment" to the ITM are P = -16000 and Y = +9250. Correction: there were some digital offsets in the 'test' filter bank. After zeroing those out, the new numbers are P = -42800 & Y = +59200.
Vincent has turned on the ETMY HEPI with low frequency boosts (although no integrators yet) so the pointing should be more immune to the HEPI hydraulic pressure fluctuations. Still need to fix up the ITMY HEPI. There's some seismic measurements going on overnight, but we can next align the ITMY to get the beam back to EY and then lock the green laser to the arm.
Angle calibration of TMS OSEM:
When tilting TMS without tilting the ALS beam on the table, the beam leaving the TMS telescope would tilt by almost the same amount (19/20 or 21/20) as the TMS itself. Here I just ignore 5% error and assume that the beam tilts as much as the TMS.
Using the baffle diode (https://dcc.ligo.org/cgi-bin/private/DocDB/ShowDocument?.submit=Number&docid=d0901376) we can calibrate the TMS OSEM. Note that the PD locations are not the same for H1 and H2. We're looking at H2 ITMY (left hole on page 3 of the above document).
There are three PDs horizontally aligned. The top one (PD3) is about 6.25 in away from the middle (PD2). The bottom one (PD4) is 11in lower than the middle. We know (from experience) that increasing P offset moves the beam down. And there's a fourth diode (PD1) that is at the same height as the middle one and horizontally off to the left by 11 in. PD1-PD4 naming convention is explained in E1100867, page 15.
PD3=CHAN_28
(P.Y)=(-69000, +1200)
PD1=CHAN_30,
(P,Y)=(-46700, +17800)
PD2=CHAN_29,
(P,Y)=(-48000, +1000)
PD4=CHAN_27,
(P,Y)=(-10000, +2900)
For PIT, PD3-PD2 is about 6.25 in for 21000 counts, so it's 6.25*25.4e-3/21000/4000 = 1.9 nrad/count.
Also for PIT, PD2-PD4 is about 11 in for 38000 counts, 11*25.4e-3/38000/4000 = 1.8 nrad/cout.
For YAW, PD1-PD2 is about 11 inches for 16800 counts, so 11*25.4e-3/16800/400 = 4.2 nrad/count.
There's no funny thing in the output matrix, PIT to coil matrix elements are just +-1, same for YAW, so you can multiply 131072 to get the range.
We can also get the initial alignment number using the offsets as of now (-42800 for PIT and +59200 for YAW)
1.9E-9 rad/count
Positive offset moves the beam down.
4.2E-9 rad/count
Positive offset moves the beam towards the inside of L.
Off by +81E-6 rad
The beam was pointing down.
Off by -250E-6 rad
The beam was pointing outside the L.
Note that the order of the channel numbers (27, 28, 29, 30) is reversed. Right now PD1, 2, 3 and 4 = Chan30, 29, 28 and 17.