This is the output of the MZ for the first couple hours after we set it up. Since we only watched the first few minutes on the scope, it appeared as though some transient from the slow influx of gas into the chamber was causing it to swing through fringes. This longer time series seems to paint a different picture. It shows something that looks like a damped oscillation that is also increasing in period.
An increase in period alone would be consistent with a slowing differential drift while the vacuum system reached its steady-state pressure, but the change in amplitude doesn't make sense (we have the output going onto a large-area PD using a lens, so even if we were just losing contrast the maximum power on the PD---bottom of the plot here---would stay roughly the same). Instead, this looks like something was actually causing it to oscillate between the maximum and minimum values of the output.
Not long after this, the large oscillation seemed to have gone away, leaving a more stochastic looking time series (see below). The abrupt jumps and subsequent slow drifts are probably coincident with the lab temperature control system turning on.
I chose a small chunk of time during which the time series looked relatively flat and near mid-range (~13:00:00 2/19/11) to take a spectrum. Since I now realize that we probably don't know the contrast for sure, I have to postpone the calibration until I can verify the full-swing amplitude tomorrow morning. Still, a few things can be learned by looking at how the spectrum compares qualitatively with the one I took in air. The main thing to notice is that the low-frequency hill is markedly shallower with respect to the higher-frequency mechanical resonances. There also appears to be a new family of resonances between 10-20 Hz, which might originate in the vacuum system itself.
We will have more information of how much better the low-frequency noise is once we've calibrated this.