Jason, Betsy, Rahul
Yesterday we re-started the alignment and balancing work on FC1 in HAM7 chamber after the lower wire loop was replaced and TF results were approved by Jeff Kissel (after a thorough investigation - see alog 60791). Jason measured all DoF (pitch, yaw, roll, length and height), while Betsy and I adjusted the weights and position of FC1 as directed, until we got it as per the requirement. A detailed report with numbers will be posted by Jason in another alog.
We started the day with adjusting the movable mass and screw (on the upper mass) to adjusted the pitch by several 100's of micro radians. However later on we changed our strategy and started adjusting the the add-on mass on the Intermediate mass (already existing weights = 284 gram in total, 149g front and 135g back). Later we had to remove 20g from the front and 20g from the back as we were more than 1.4mm lower than the requirement. After shedding 40g we were 0.9mm high but within the requirement of +-1mm. To adjust the pitch we put 7g on the front and then moved 2g forward (pitch changed by few 100s of micro radians). The final numbers on the add-on mass on the IM are as follows, total weight = 244g, front 138g, back 106g).
As we adjusted the add-on mass and movable mass, we also checked the position of the BOSEM flag with respect to the PD/LED. To re-center it we first adjusted the BOSEM itself, however when we ran out of range then we had to move the table cloth on the upper mass. During the day we moved the upper mass table cloth 'up' and 'side' ways to gives us more range for re-centering the flags. Betsy noticed that adjusting the add-on weights on the IM is better than sliding the movable mass (on the upper mass) for pitch adjustment as the BOSEM flags are very sensitive (gets very close the PD/LED very quickly) to the motion of the movable mass. Perhaps the blade springs on the upper mass (where the flags are attached) absorbs some of the pitch motion.
To adjust the yaw we pushed (clockwise or anticlockwise) the suspension with screw/clamp set up. The final fine tuning were done by torquing four clamps attached to FC1. The yaw is very sensitive to the applied torque on the clamps, hence this should not be changed unless required.
After every little adjustment we made on FC1, Betsy took several transfer function measurements for all DoF to check the health of the SUS and rule out any rubbing. We can confirm that FC1 is healthy and we manage to fulfill all the alignment requirements (see Jason's upcoming alog for details).
Picture of corner cube assembly clamped square against the FC1 structure front for distance check, attached. After Jason took the distance shot from the IAS monument area down by BSC3 to the corner cube, we used calipers to measure the distance from the cube plate (on center line of cube) to the face of the FC1 optic out near the 3oclock and 9oclock positions. (Used the holes on the plate that are spaced to probe the very edge of the optic.)
Final alignment numbers for LHO FC1. Coordinates are the site local coordinates w.r.t. WBSC2 0,0,0; yaw angle is reported as measured from the +Y axis, while yaw direction is reported assuming a top-down plan view of the optic (CW == clockwise, CCW == counterclockwise). We are using the same tolerances for position from the aLIGO install IAS work, but upon seeing the OSEM range available for pitch/yaw correction (off the top of my head it was something like 4mrad in pitch and 12mrad in yaw, but Betsy or Rahul can provide more accurate numbers) and the fact that the FC is much shorter than the arms, decided to widen the pitch/yaw tolerance to +/-150 µrad (150µrad causes an approximately 45mm displacement at FC2 ~300m away, so a little more than half-way between the center and edge of FC2 (~150mm diameter), and is well within the range of the FC1 OSEMs to correct). Our results:
Betsy used the pitch/yaw slider to test whether 100µrad on the slider corresponded to 100µrad on the autocollimator; the slider was driven such that FC1 pitched down by 100µrad, and this did indeed cause a 100µrad downward pitch as measured by the autocollimator (autocollimator read a pitch error of 30µrad up after the slider move, vs 130µrad up before). At the time we finished yesterday afternoon Betsy left this 100µrad pitch bias on FC1 (don't know if it has since been removed).
We also took a rough look at roll of FC1 around Center of Mass (COM), even though there is no spec that we knew of (as of yesterday afternoon). I measured this by sighting the center point of each triangular (to my view from the front of FC1) ear prism and recording the zenith angle reported by the total station (the total station reports the "up/down" angle as a zenith angle, with 0° being straight up). The difference between these 2 angles was then used to calculate the height difference between the 2 ear prisms (see attached exaggerated hand drawing); the roll around COM is then +/- half of this height difference. This estimation of course assumes the ear prisms are located directly across from each other and are both colinear with FC1 COM (I'm not sure how the prisms are placed, hence why this is an estimate). In this case, the right ear prism (ear A in the drawing) was directly in line with the center of FC1 (total station zenith angle of 90°9'0", or 90.15°; the total station sits on the FC optical axis, so looks "down" at FC1 from alignment monument LV37), with the left ear prism (ear B) slightly above. The calculated height difference between the ear centers is 2.1mm, giving a roll estimate of +/-1.05mm around FC1 COM.
Indeed, I am leaving the FC1 with Bias slider OPTICALIGN OFFSETs at this pointing in order to give us a clue that this is where we expect FC2 to see the beam.
Y = 0
P = 100
The as-built alignment numbers for FC1 posted above are incorrect. A 18mm error in the alignment solution calculation has been identified. Full details and corrected as-built coordinates for FC1 can be found at LHO alog 61485.