Detector Watch 3

When watching the detectors, it helps to know what such things as DARM, AS, PO, WFS, REF, and GDS mean, with respect to the detection of gravitational waves.

You are right we should update the acronyms. For people who don't want to wade
through the PDF you point to here is a short explanation of terms:

DARM: (D)ifferential Arm, since the gravitational wave signal manifests as a difference in the length of the two arms we use this acronym a lot. The "DARM
loop" is the control loop that is designed to keep the interferometer operating
at the null point i.e. when both arms are the same length. So you'll see a lot
of reference to channels like

DARM_ERR: The error point of the loop. This is where any GW signal should
appear.

DARM_CTRL: The control signal for the loop

AS: Anti-Symmetric, this is the dark port of the interferometer and is read out
on ISCT4 (Interferometer Sensing Control Table 4). This acronym shows up a lot
in the log, for example the acquisition photo-diode is called AS5 or ASPD5.
When we operate we use ASPDn where n=1,2,3,4. You'll also see people talk
about AS_Q and AS_I. AS_Q is one phase of the AS port light which is closely
related to DARM_ERR and is also sensitive to changes in the differential arm length. AS_I is the orthogonal phase (i.e. rotated 90 degrees) and is sensitive to changes in the common length of the arms (i.e. Lx+Ly changes). One thing that
causes such changes is angular wiggling of the mirrors.

PO: Pick-Off, most of our optics have a wedged shape, this allows us to pick-off a small fraction of the light from the angled surface and use the information in the electric field of this light to control the interferometer. This shows up
in the log usually as POX (pick-off from Input Test Mass X (or ITMX)), POY (pick-off from ITMY) and POB (pick-off from the Beam-Splitter (BS)).

WFS: Wave Front Sensing, the WFS system is composed of a set of photodiodes
that are designed to sense the shape of the propogating wavefront of the light at various ports. This information is then fed back to the mirrors using the
ASC (Angular Sensing and Control) system, which as the name implies is designed to keep the mirrors from rotating about a vertical (yaw) or horizontal (pitch) axis.

REF: I think this should be REFL, which designates the REFLected port of the
interferometer also called the symmetric port. Since the dark-port is where
you have destructive interference, the bright port or symmetric port is where
you get constructive interference from the michelson.

GDS: Global Diagnostic Systems, this is the name given to a set of tools we
use to make measurements of the various channels. An important tool in this
system is DTT (Diagnostic Test Tools). This is what we use to make many of
the amplitude spectra, time-series etc.

Hope this helps. I'm impressed that people read our logs so carefully.

Brian