We have a plan for how we're going to measure the beam after PR3. Mike is going to write up a nifty program that will spit out the waist of the beam if you give it a bunch of razor blade measurement data.
Since the beam bounced off of the pitched ITMX is coming out of the chamber so high, it's kind of a pain to setup optics to steer the beam down the walkway next to the Yarm. So, I have a new vision.
I think that we can get the beam right after PR3 onto the PRM/BS oplev table using 3 clean mirrors (of which we have many spares, already clean). Once on the oplev table, we can put a 2" Y1 mirror to steer the beam down the walkway, after taking off the short east side of the table. Then we can use the little breadboard on the mobile blue pedestal for the razor blade / power meter setup.
The razor blade on a micrometer translation stage will be the first thing on that table that the beam sees. Then, a 2" lens to get the beam small enough to fit on the power meter. Then, obviously, the power meter. We can measure the distance between the oplev table and the razor blade using the laser range finder, which has pretty good accuracy (it's sub-centimeter, but I don't remember the exact number for the precision).
A lens is not okay if we're trying to get the beam directly onto the beam scanner, since it will distort the beam. However, as long as the razor blade is before the lens, and we're just using the lens to get the full intensity of the non-obscured part of the beam onto the power meter, I think using a lens should be fine. If we don't / can't use a lens, we're going to run into the same problem we have with the beam scanner, since the power meters all have a fairly small aperture. Even the big 30W power meter's aperture will be on the order of the size of the beam, so we won't be able to guarantee non-clippage.
The main problem I see with the technique as I have described it, is that the beam is going to hit 4 mirrors (3 in-vac, one outside) before going to the razor/lens/power meter. We have to make sure that we're not clipping on any of those mirrors. Also, this measurement version takes the beam after PRM, PR2 and PR3, but not after the BS and ITM. I don't think we're concerned with either of those 2 optics, (especially since this is refl off the front of the BS, so won't see any potential clipping on the BS cage), but just in case we are, this measurement isn't so useful, and we'd have to come up with a different way of placing the mirrors on the in-vac tables to get a beam bounced off of a yaw-ed ITMX.
Perhaps it would be easier to just go with the pitched ITMX version of the measurement, but I could use some ideas / advice on how to mount mirrors and lenses ~4 feet off the ground outside of the chambers, and not have them waving around on skinny sticks.
EDIT: Another idea is to instead use the beam transmitted through the BS, put a single clean steering mirror in the ITMY chamber, and get the beam out of the ITMY door. This could either be the beam before the ITM, or we could yaw the ITM a little and take the reflected beam.