diff --git a/SOPHIE-Alignment.md b/SOPHIE-Alignment.md index 016aea0..58395cf 100644 --- a/SOPHIE-Alignment.md +++ b/SOPHIE-Alignment.md @@ -18,7 +18,7 @@ There are a number of strategies for dealing with energy drift: - **Post-fix**: Simply align the images during the data analysis. - **Rotate Endstation**: This essentially adjusts the X-ray beam axis. One can choose to either minimise the real drift, or to match the virtual drift (with opposite sense) so that they cancel each other out. Note that translating the endstation also results in a small rotation of the X-ray beam axis and so one should take care to reoptimise the endstation position after each rotation. The size and distance of the entrance apertures may limit the amount of endstation rotation that can be applied before the beam is clipped. - **Adjust Interferometer Mirrors**: This can be applied to either minimise the virtual drift, or to cancel the real drift. In practice, such adjustments can be very difficult and inconvenient due to the need to vent the system to access the manual adjustment screws and a limited number of adjustment screws included in the implementation of the interferometer system. One must also meet the constraints of having the interferometer beams sufficiently overlap at the detector. -- **Software Correction**: Drifts can be countered by applying a correction in the software of the instrument control system. A software correction can cancel a virtual drift completely by neutralising erroneous feedback systems and therefore eliminating fine stage movement limitations. However, using a software correction to minimise a real drift will increase feedback signals and cause issues with the fine stage limits. +- **Software Correction**: Drifts can be countered by applying a ZP-Z dependent correction to the transform to X/Y position that is calculated by Orocos. A software correction can cancel a virtual drift completely by neutralising erroneous feedback and the only negative consequence would be an extra (but typically insignificant) shift in the coordinate system when the interferometer is reset. However, using a software correction to minimise a real drift will increase feedback signals and can cause issues with the fine stage limits in extreme cases. ## The SOPHIE Endstation @@ -61,13 +61,13 @@ Sensor1.zCorrectMatrix[1] = 0.0 | ![start_panel_ZMI](uploads/dad9123d974015491a5cbf92559240be/start_panel_ZMI.png) | |:--:| -| Figure 4. EPICS panel for ZMI interferometer values and diagnostics. | - -If one is unable (or unwilling) to get the interferometer signal high enough to reliably lock after adjusting the reference mirrors, then the drift correction could instead be applied to the software zCorrectMatrix. +| Figure 4. EPICS panel for ZMI interferometer values and diagnostics. The "POS" values are the number of counted interference fringes, the "Status" values that end in a 5 indicate an OK status, and the "SSI" values are the signal strength indicator. An SSI value above 10k is good, but be sure to also check that blocking a beam causes a significant drop in SSI. | ### Fix the Virtual Drift -Apply the calculated screw turns to make the reference mirror parallel and then further adjust the interferometer components to compensate the beam alignment and optimise the signal strength. +Apply the calculated screw turns to make the reference mirror parallel to the ZP stage movement. and then further adjust the interferometer components to compensate the beam alignment and optimise the signal strength. + +If one is unable (or unwilling) to get the interferometer signal high enough to reliably lock after adjusting the reference mirrors, then the drift correction could instead be applied to the software zCorrectMatrix. ### Characterise and Fix the Real Drift (Requires X-rays) @@ -78,3 +78,7 @@ Apply the calculated screw turns to make the reference mirror parallel and then 5. Optimise the X-Y position of the Girder Mover for maximum X-ray signal. 6. Iterate steps 3-5 until the drift is minimised. + +### Apply a Software Drift Correction + +The zCorrectMatrix values [X,Y] are applied by Orocos in the J/K->X/Y coordinate transform. This means that all of the bad effects of a virtual drift are neutralised at the Orocos level. The only negative effect would be a (typically insignificant) shift in the coordinate system when the interferometer is reset at a significantly different photon energy from the previous reset. This occurs because the correction is calculated based on a reference point that is chosen as the position of the last "reset interferometer". Since an interferometer reset also defines a new coordinate system based on a reading of the position encoders on the coarse stages (with the fine piezo stage in a relaxed, 0 mV position) (and there will also be a coordinate shift due to previous positions being incorrect via accumulated errors in counting fringes), there is typically going to be a shift in the coordinate system anyway that will be more significant than the zCorrectMatrix shift.