Ibrahim, Oli, Thomas, Betsy
Before we flew the BBS in, we did a sanity-check experiment to compare slider values with a laser-pointer optical lever. This experiment follows Jeff's G1200698 math for an optical lever (picture also attached).
Setup:
A laser pointer was pointed at the BBS HR side such that the HR and AR reflections could be seen on a paper near the laser pointer. The initial position was recorded on a paper, and then slider values were driven to their maximum range in P and Y. The distance between the laser source and the HR surface was recorded. The experiment was done with BOSEMs and then with QOSEMs.
Method:
When the laser pointer is incident on the BBS, there are two reflected beams (AR and HR). The average distance of these was taken along with the average distance from the BBS. This was done by calculating the distance to the BBS and adding the half-thickness of 30mm. This gives us "one beam" and "one distance' to go off for P and Y. See the attachment below for an image of the paper used during the experiment. This was done once for BOSEMs and then again with QOSEMs.
A few notes:
- The setup was replicated and the distance was remeasured on a different day and this is why the distances are different.
- The QOSEM as-it-is is inversed in its actuator sign convention and you can see this in the attached diagram. One of our action items is to rectify this. So right now, +P = Pitching up instead of down with respect to the HR. In general, we will abide by T1200015, the SUS/Actuator Sign Convention.
Results and Analysis
Pitch is in purple and Yaw is in Orange. The equation used was obtained from "G1200698, the Oplev Infamous Factor of Two", which is essentially the same outline as our experiment, a diagram of this is attached.
This equation is: DØ= DS/2L Where,
Ø is the angle in radians (adjusted to urad below), S is the displacement in the beam spot, L is the distance from the laser source to the optic.
For these data, the total range of the Opticalign sliders was considered (from lowest negative to highest positive). Thus, a half the range would apply if the sliders were zeroed.
| BOSEM | QOSEM | |
| Distance from HR surface (in) | 130 | 118 |
| Distance from HR surface (mm) | 3302 | 2997.2 |
| Average distance from optic | 3332 | 3027.2 |
| Pitch Displacement Total - +-1500 cts | 5.5 | 5.75 |
| Yaw Displacement Total +-4000 cts | 46.75 | 40.75 |
| Pitch Angle Swept (urad) | 825.33 | 949.72 |
| Yaw Angle Swept (urad) | 7015.31 | 6730.64 |
| Total Pitch Count Sweep (+-1500) | 3000 | 3000 |
| Total Yaw Count Sweep (+-4000) | 8000 | 8000 |
| Pitch urad/Ct | 0.28 | 0.32 |
| Yaw urad/Ct | 0.88 | 0.84 |
| Pitch Ct/urad | 3.63 | 3.16 |
| Yaw Ct/urad | 1.14 | 1.19 |
Conclusion:
- For a QOSEM Drive:
- Pitch at M3 read +-475urad (1/2 the total sweep of 949.72 shown above), which is equivalent to 3.16 Opticalign Cts per urad.
- Yaw at M3 read +-3365urad (1/2 the total sweep of 6730.64 shown above), which is equivalent to 1.19 Opticalign Cts per urad.
- BOSEM and QOSEM ranges are comprable
- No cross coupling was observed. Remember that this was done on the test stand a few weeks ago, before we found P:Y and Y:P coupling in chamber.
Next Steps:
Next, we're going to compare these results with the M3 AOSEM readouts once these are reliable. We will also compare these results with the model (which is used to corroborate slider calibration). Ultimately, this will help with the calibration of our slider values (both within the actuator slider actuation vs. sensing and from counts to real units).
Once we get a working OpLev (it's not on the BBS right now), we'll be able to get a more fine measurement of this kind.
Again, this is just a gross alignment check for information about our pointing and the capacity of our actuation.
