This is an entry that I was supposed to write some time ago but I never did..here it is, "Kissel style".

Problem : Keita  found  a couple of weeks ago that the mode as measured after the REFL WFS telescope in HAM1 was not what expected, and that mode, propagated through the REFL WFS sled, would give 35 degrees Gouy phase separation between the sensors, and even less between the tip-tilts actuators. Very bad. Sheila had to move the tip-tilts to compensate for that. We were puzzled because the mode as reaching HAM1 from HAM2 was not very far off from what expected (Paul's calculations, same entry), the telescope lengths were close to nominal, the sled was built according to the "nominal" design..everything was close to nominal, but somehow we ended up very far from the 90° Gouy phase separation we wanted.

Short version of the story: My best guess, based on the alamode model attached, is that all these "small" variations from nominal are indeed relatively small, but they all add up together to create the badness that we saw. My alamode file is attached.

Long version of the story

*All "nominal" parameters as designed:

Gouy separation between RM1 and RM2 is 66.5778
Gouy separation between WFS is 89.7558°

*Adding as built REFL WFS sled

Gouy separation between RM1 and RM2 is 66.5778°
Gouy separation between WFS is 68.0585°


* Adding as built REFL WFS telescope

Gouy separation between RM1 and RM2 is 56.7438°
Gouy separation between WFS is 66.3777°

*Adding as measured input beam parameters in HAM1, before the REFL telescope

- Horizontal

Gouy separation between RM1 and RM2 is 55.6327°
Gouy separation between WFS is 56.3534°

- Vertical

Gouy separation between RM1 and RM2 is 49.9092°
Gouy separation between WFS is 50.4782°

*This is still not quite as bad as Keita measured, but between the uncertainty on the mode measurement after the TTs and the tolerances on the ROC
of the curved mirrors (it looks like we are indeed sensitive to variations from nominal of the +1.7m mirrors), we might be quite close to explain what we saw.


The message : No major problem, small things add together to make big things, need to measure the beam before and after the telescope to (hope to) get this right. 
 
Additional message : The current REFL WFS telescope gives us 90° Gouy phase separation by having one of the WFS at the waist. I don't think this is really what we want: the beam size on the WFSs is different by a factor of 3, and having one of the WFS at the waist makes us more sensitive to errors in our measurements/calculations. Not a problem in the near future, but we might want to keep this in mind for when HAM1 is open again. I will see if I can find another solution similar to the "design" one.