A big issue behind this alignment activity was that the PR2 and PR3 mirrors had moved vertically in the past weeks due to the change in the lab temperature.
Even though such vertical movement changes the spot positions on the PR3 and BS significantly, this doesn't immediately make the oplevs useless.
[Size of misalignment]
Since PRs are strongly curved, this must have introduced misalignment in pitch through the curved surfaces. According to a simple ABCD matrix computation (shown at the bottom of this entry), a 3 mm vertical movement (upward) on all the PRs would introduce the following misalignment at the PR3 mirror.
- Shift in the beam position = -24 mm (i.e., the beam is lower than the mirror center at PR3)
- Angle change = 23 urad (pointing downward as the beam propagates to the End stations)
Not surprisingly, the angle of the main beam to the end stations does not change so much because the PR2-PR3 optical system is desgined to be a telescope (-ish). In other words, the telescope amplifies the beam translation rather than the beam angle.
Since the opleves are trustable at the level of 10s of urad based on my experience, this angle change by the vertical movement doesn't seem fatal from the point of view of the daily alignment routine.
However, we should note that the spot position must have changed a lot after the PR3.
[Some math]
Building ABCD matrix for the PR telescopes, one can find
(PR telescope) = [[1, 0], [-2/R3, 1] ] * [[1, d], [0, 1]] * [[1, 0], [-2/R2, 1]]
= [[8.19, 11], [-7.6e-3, 0.11]]
where
R2 = -3.0764 m (RoC of PR2)
R3 = 24.9165 m (RoC of PR3)
d = 11.0661 m (telescope separation)
All the parameters are taken from the MIF wiki page. As for the PRM, it had been locked at the standard filter (SF). Therefore I assumed that the PRM didn't move.