How can you lock the PRMI without the REFL beams? c.f. this entry by Kiwamu
Which signals are you using for the locking?
I think the first priority is to find the fringes of the arms and lock them with POX/POY.
As for the POP, make sure the beam is not clipped because the in-vac steering mirrors
have been supposed to be too narrow to accommodate these two beams.
I was using AS55I for PRCL, and AS55Q for MICH. I snuck that into the last line of an unrelated elog, since I did both things at the same time: see elog 7551. Kiwamu's measurements (elog 6283) of the PRMI sensing matrix show that the PRCL and MICH signals are almost orthogonal in AS55 (although the optickle simulation doesn't agree with that...) He was able to lock PRMI with AS55 I&Q (elog 6293), so I thought we should be able to as well. Locking the PRMI was supposed to help tune the alignment of the PRM, not be the end goal of the night. Also, we only tried locking PRCL in the "middle right" configuration, not the "lower left" configuration, but we were seeing what looked like recycling flashes only in the "lower left" configuration.
I agree in principle that we should be working on the arms. However, since we can't use the old steer-the-beam-onto-the-cage trick to find the beam, I was hoping that we could steer the beam around and see some light leaking out of the ETM, onto the end table. However, with the 1% transmission of the ITMs and ~10ppm transmission of the ETMs, there's not a lot of light back there. I was hoping to align the PRMI so that I get flashes with a gain of 10 if I'm lucky, rather than just the 5% transmission of the PRM. With the PRMI aligned, I was expecting:
(1W through Faraday) * (10 PR gain) * (0.5 BS transmission) * (0.01 ITM transmission) * (10ppm ETM transmission) = 0.5uW on the ETM tables during PRCL flashes.
I was hoping that things would be well enough aligned that I could just go to the end table, and see the light with a viewer, although as I type this, I realize that if the beam was not on the end table (or even if it was...) any time I move the PZTs, I'd have to completely realign the PRMI in order to see the flashes. This seems untenable, unless there are no other options.
We then got sidetracked by trying to see the POP beam, and once we saw the POP beam we wanted to put something down so we could find it again. POP is also small, but not as small as expected at the end:
(1W through Faraday) * (10 PR gain) * (20ppm PR2 transmission) = 0.2mW on POP during PRCL flashes.
POP was very difficult to see, and we were only able to see it by putting the foil in the beam path, and using a viewer. I think that we once were able to see it by looking at a card with the viewer, but it's much easier with the foil. I'd like to find an iris that is shiny (the regular black iris wasn't helpful), to facilitate this alignment. Since we were just looking at the reflection off of the foil, I have no comment yet about clipping vs. not clipping. I do think however that the forward-going beam may have been easier to find....when the PRMI alignment drifted, we lost the beam, but I could still see the forward-going beam. Probably I should switch to that one, since that's the one that was lined up with the in-vac optics.
Ideas are welcome, for how to align the beam to the Yarm (and later to the Xarm), since our old techniques won't work. Aligning the PRMI was a distraction, although in hopes of getting flashes so we could see some light at the end tables. I'm going to go see if I can look through a viewport and see the edges of the black glass aperture, which will potentially be a replacement for the steering-on-the-cage technique, but if that doesn't work, I'm running out of ideas.