With Mark.

According to the plan to account for the bouyancy correction in GAS filters, ideally, in air we should set each keystone above its nominal zero position by the same amount it would sag in vacuum. In the case of SR3 these values are:

• BF: 460 µm.
• F1: 476 µm.
• F0: 195 µm.

The aim of the work reported in this entry to to check whether this goal is possible using the fishing rods and/or the coil-magnet actuators. The temperature was 24 ºC, which is what was requested.

Bottom filter

• The polarity of the stepper motor is such that a negative number of steps produces a positive displacement of the keystone per the LVDT.
• The position of the stepper before this excercise was -210,000 steps with LVDT readout 248 µm.
• The upper limit of the stepper motor is -275,000 steps. At the neighborhood of this position the number of steps may be lost but the motor does not jam.
• I moved the motor to -270,000 steps and got an LVDT readout of 340 µm.
• Then I applied magnetic actuation of 2500 counts to achieve the goal of 462 µm. This means we can compensate the bouyancy effect by using the coil magnet actuator and leaving the motor where it is now (upper end of range). 
• Finally, I put the keystone to zero for the alignmet team to use. I used -6450 counts for an LVDT readout of 14 µm.

Standard filter (F1)

• The fishing rod is not operational. It was not possible to identify the problem after several checks were done. See notebook for details.
• At the begining of the excercise the LVDT readout was -478 µm, whereas our goal is 476 µm.
• It was not possible to bring the keystone that high up with the coil-magnet actuator. With 15,000 counts we got 193 µm. But that is too much actuation already. So we decided not to pursue such strategy. We will just leave this filter alone for now.
• By the end of the excercise the position of the keystone had drifted to -532 µm.
• In vacuum ths keystone will sag ~ - 476 µm to reach about ( - 532 - 476 ) µm = ~ 1 mm. We will aim to compensate with F0 fishing rod.

Top filter (F0)

• At the begining of the excercise the keystone was at -1.4 mm. It had been set to -1.3 mm originally but it drifted down. Such a position was set according to the previous (and unsuitable) way to compensate for the bouyancy effect.
• Given the condition of F1, the top filter FR has to be used to compensate for the in-air position of F1 ( - 532 µm) and its sag in vacuum ( - 476 µm).
• Additionally, the FR should be used to compensate its own sag ( - 195 µm ).
• By visually inspecting the optic using Terada-san's reference I moved the F0 fishing rod up until it was at the correct height. This happend when F0 LVDT read -731 µm.
• The final LVDT reading is consistent with the expectation according to previous assesments. Originally, the optic was at the correct height when  BF LVDT = 0 µm, SF LVDT = 0 µm and F0 LVDT = -1.3 mm. The amount to lift required in the present configuration has to be 532 µm (F1 curent postion) plus 100 µm (F0 drift) which adds to 632 µm. What was actually moved up was 669 µm.
• It's worth pointing out that the keystone cannot be lifted all the way to zero with the coil-magnet actuator. (15,000 counts takes it to -530 µm only).

How to set the optic to the correct height in air (according to Terada-san's reference)

1. Set the BF LVDT to zero using the coil-magnet actuator. Use approximately -6450 counts.
2. The F1 keystone should be at -532 µm. In case it's not, applying a small amount of actuation with the coil-magnet actuator is expected to be enough. The fishing rod is not operational.
3. The F0 keystone should be at -731 µm. In case it's not, applying a small amount of actuation with the coil-magnet actuator is expected to be enough. The fishing rod could be used also but please don't use it without asking the VIS team.

This entry requires another section on how to set it correctly in vacuum. It will follow later.