Following on from Patrick's entry regarding the reference cavity transmission falling below 0.9 V ...

Attached are two plots of the reference cavity transmission and the Laser Room temperature as measured
by a sensor on the table, along with an image of the various spots from the pre-modecleaner and reference
cavity.  Since I didn't save any previous camera image, I will simply have to go on memory.  The
pre-modecleaner reflected spot looks like it's drifted to the right in yaw (as you look at the image).
The reference cavity reflected spot has moved up in pitch.  Both alignments were tweaked during last
Tuesday's maintenance period.

Also attached is a plot of the U100 mirror mount drift due to temperature and similar data from the
Newport website for their Suprema series mirror mount.  The plots suggest that pitch suffers with
any temperature variation, although it's hard to quantify given the ~0.5 degC temperature change.  The
plot for the U100 mount, suggests that the pitch adjustment does not recover.

The degradation in the transmission signal seems to coincide with colder temperatures of late.  Prior
to last week the last time the alignment was tweaked was mid-July.

Another possible reason might be that the reference cavity is slipping in its suspension, for whatever
reason.  Given the fact that the transmission can almost always be corrected for by a pitch alignment
I don't think this is a likely reason.  The reference cavity is the same as that used for the Initial
LIGO H1 PSL and I do not recall any mention of frequent alignment tweaks to preserve the transmission
signal.