Akutsu with remote Yokozawa

Summary

Checked several optical beam paths in the OMC chamber. Re-installed a beam dump that was detached the other day for the OMC stack modification (see 29804).

Details

Refer to Fig. 1 of 27252, which should be the latest drawing regarding the OMC ghost beams, perhaps. Yokozawa-san took care of bringing a stable main IR beam to OMC, and also OMC-ASC (OMMT2T centering + OMC DCQPD centering control). In this condition I performed the investigation for optical beams in the OMC chamber. Sometimes OMC locked but immediately lost during my work.

• AS_RF beam; This is a transmission beam from OSTM. As already reported in 30058, this beam missed the relevant steering mirror. Today I confirmed that this beam was about 3-cm off centered from this 2-inch steering mirror. It is quite strange situation, because on the upstream of this beam should pass through OFI, and should be within OMMT2 mirror aperture. What would make this kind of large off-centering????? I am afraid this would be a bad effect like a body blow. Anyway today I moved the relevant steering mirror to sufficiently catch this beam (Fig. 1); it was already centered in pitch from the first... this might mean there is no mis-centering happened in OFI??, so just horizontal shift about a few cm was enough. Then I tweaked the mirror so that the reflected beam illuminated the upper steering mirror of the relevant in-air periscope through a relevant viewport window, and also two ghost beams generated at the viewport window were caught be a relevant beam dump. Further in-air alignment should be done later by someone.
• Re-installing a beam dump: as reported in 29804, a beam dump was detached for the OMC modification work. I re-intalled it by referring the reference marker (Fig. 2). Then I detached the marker labels (Fig. 3). By the way, this beam dump is to dump a ghost beam generated when OMC locked; it was hard to confirm if this beam dump surely caught the relevant ghost beam or not, today.

Next step

• It was hard to see some ghost beams without increasing input power such as 10W. Maybe I need to put a tentative obstackle in the bow-tie OMC cavity path to prevent accidental resonance of such high power, then check some  ghost beams.
• Not yet checked all the beam dumps.
• As was written above, the re-installed one is the one of the most difficult beam dump to be confirmed if it is located properly. Can we make the OMC lock more stable...?????