Thomas Vo Scott(Apollo) Matt Evans


By offsetting the pitch and yaw test filters on the M0 stage we can see the optical lever beam move in pitch and yaw respectively.  For a sanity check, I also independently put an offset on the reaction mass test filters and this did not affect the optical lever beams so I'm sure that we are not seeing another ghost beam off the reaction mass.

BUT!!!

One thing I noticed is that there is another beam close to our currently aligned beam, approximately 3 cm away.  After looking up some specs for the test mass depth (~34cm) and the gap between the reaction mass (~5mm), Matt and I did some calculations and it looks like the beam that we are currently aligning to is the reflection off the back surface which will give you the same transfer function results as a front surface reflection.  Looking at the reflectivity/transmission data for the ETMy shows that the primary reflection off the front surface is comparable to the secondary reflection off the back surface so it is difficult to judge by eye which one is which.  These calculations will be needed in future installs for correct alignment. For optical levers with large lever arms such as ITMy (optical path ~ 60 meters), the wedge will play a significant role in determining the correct beam as well.

The solution is to go back and open up the enclosures to do yet another re-alignment to look for the correct beam off the front, that would require a soft close of the gate valve.  The reflection off the back surface will still be able to give pitch and yaw, unless the other beam encroaches on the photodetector.