J. Oberling, R. Savage

With the recent FSS RefCav TPD issues (see various alogs over the last few days), we went in to the enclosure today to address the issue.  All work was done with the ISS OFF.

We started by taking some power measurements at a couple points in the FSS path:

• After WP04: 151mW
• After PBS04: 119 mW

This was a big difference, so we tweaked WP04 to maximize the power after PBS04 at 150mW.  We then measured the power in front of the EOM (this is after the FSS AOM); the power here measured at 37mW, which is seriously low considering there was 150mW incident on the AOM (keep in mind, this is after we tweaked WP04 to increase the power incident onto the AOM, meaning the power being sent to the RefCav was even lower when we started).  This lead us to think something was not quite right with the FSS AOM alignment.  When preparing to tweak the AOM alignment, Rick noticed that the pitch adjustment screws (the AOM is mounted on a New Focus 5-axis mount) were very unequal, indicating that the AOM was not level; in fact, it was pitched up on the input side of the AOM.  Rick slowly walked the AOM alignment until it was more level, and this also improved the power sent to the RefCav; yaw was also tweaked.  In addition, a short iterative alignment between mirror M25 and the AOM was performed to ensure the return angle was optimized (M25 is the mirror that sends the beam back through the AOM, thereby creating the double pass).  All completed, we had ~89mW being sent to the RefCav.

We then worked to recover the RefCav alignment, which took the bulk of our time in the enclosure; during this alignment recovery we noticed that the beam exiting the EOM was fairly close to one edge of the output aperture, so the EOM was adjusted to better center the beam.  To be brief, to recover the RefCav alignment we used the RefCav reflected camera image and the iris in front of the RefCav as fiducials to rough align the beam; mirror M27 and the top periscope mirror were used to adjust the alignment.  A function generator was used to ramp the NPRO PZT and we looked at the TPD value on a scope; I manually adjusted the NPRO crystal temp to find the peak and keep it visible (it moves around, so requires constant adjustment) while Rick tweaked the alignment.  Finally, we tweaked the alignment into the RefCav RFPD.  That all done, we locked the RefCav without issue but the loop immediately began oscillating; the gains were reduced to make things more stable, and we had 5.2V on the RefCav TPD.

We took a TF of the FSS to set the gains (common and fast) properly with the new TPD value, see 1st attachment.  State of the FSS when we left the enclosure:

• RefCav TPD: 5.2V
• FSS UGF: 539 kHz
• Common Gain: 19 dB
• Fast Gain: 15 dB

For future reference, the 2nd attachment is the state of the FSS AOM alignment screws at the end of our adjustments.  The ISS was turned back on at ~19:05 UTC (12:05 PDT).