QUAD 04 BUILD 03
Today the latest Build of QUAD 04 was completed by Jeff B. and Andres R. This work signifies the transition of "QUAD 04 BUILD 02" to "QUAD 04 BUILD 03". The following is an excerpt from an email sent by Jeff Bartlett on 11/10/2011 to members of the LHO SUS group explaining the mechanical adjustments made to QUAD 04:
"We removed shims from the Reaction chain to correct optical viewed
roll and fix optical viewed and ruler measured height. There are now
zero shims on the left blade and 3mm of shims on the right side. The
test masses for both chains are sitting at 21mm. This agrees with the
optical level. I did not think the effort was worth the reward for a
0.5mm height change. The optical lever shows a slight bit of pitch
differential between the chains. However, as I will discuss later this
measurement is suspect.
There is differential yaw between the masses at L2 and L3. The
measured yaw at L3 is almost zero in relationship to the frame. I trust
this measurement because the frame gap is set by the 5mm spacing plugs
and the frame is ridged at this level. The measured yaw for L2 is all
over the map. To tune this out will result in the yaw at L3 going out of
spec. Perhaps it was the wrong decision, but I though the gap at L3 was
more important than at L2.
There is still side shift between the two chains, which has been
reduced but is not gone. The side shift, of around 1mm, is the most
egregious at L2, but is still visible at all levels. We can tune it out,
but doing so induces out of spec yaw problems at L3 and L2. We have used
almost the entire swing range of the top stage blades to get it as close
as it is. I believe this same problem was observed at LLO, although I
don't know it's final resolution.
Above I alluded to a lack of trust in the optical lever (laser
reflection from mirrors attached to the centers of L3 masses)
measurements of pitch and yaw. This mistrust comes from the surfaces
machined into the faces of the dummy masses. The manufacturing
tolerances for these masses are not that tight and misalignment between
the dummy mass halves are known. Although these are small, they do not
lend themselves to accurate pointing of the target mirrors. This is not
as bad on the M0 dummy mass (D060355) because the center is drilled out
and the mirror sits on a small shelf. However, on the R0 dummy masses
(D060358 and D1002204), where the centers are solid, there is a bump
left over from machining, right in the center of the mass. This bump is
large enough to miss align the mirror by a considerable amount. On the
Quad-4 R0 dummy mass, we removed as much of this bump as we could and
used 0.010" shims as standoffs in an attempt to true the mirror. With
this, we were able to put the bubble within the circle of the level but
could not center it. As a result, yaw does not reflect true to the face
of the mass. Pitch looks OK but not is certain. "
-Jeff Bartlett on 11/10/2011
