Update PBPS calibration guide
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## Operating principle
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Single shot diagnostic to measure relative pulse energy and beam position. Back-scattered X-rays from a thin target (typically 50 nm silicon nitride) are measured by four photodiodes positioned above, below, left and right of the X-ray beam.
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Position data maintains FEL pointing, while single-shot pulse energy measurements are used to normalise experiment data. There is one PBPS in the front-end and two further devices per end station.
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## How to use
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1. Open photon diagnostics [web app](https://sf-photodiag.psi.ch/app)
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2. Under Position/calibration tabs,s elect device from drop down menu 1
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3. Select target from drop down menu 2 - default target is 50nm Si3N4
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4. Press calibrate
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<img src="https://docs.google.com/drawings/d/e/2PACX-1vRGynTQ54ugugA_bM9pYc9eZDlet3Z9MSZWn17wJNwAEM2ORiJGoL8O61frpLBVRpFKlzJ7M05a5Mb1/pub?w=1245&h=1039">
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The calibration procedure normalizes the response of the 4 diodes, then scans the device horizontally and vertically over the FEL beam to calibrate the position response.
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Once completed (approx 1 minute) the plots are updated with the calibration response. Plotted on the x axis is the device motor position (horizontal - left plot and vertical - right plot) and on the y axis is plotted the normalized horizontal or vertical diode pair response. For a good setup the plot should be a straight line through the 3 data points.
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If you are happy with the calibration, press the "Push results/elog" button where the new calibration data is sent to the required data processing pipelines. After an automated restart (~30 seconds) both PV and BS position and intensity data will be calculated with the new calibration. An elog entry is made detailing the calibration data.
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## Error
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**Soft limits**
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Error message: `ERROR: Motor position outside soft limits`
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Problem is a soft limit on either the device Motor X or Y is blocking the scan motion
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1. Select all from drop down menu for motor X and motor Y of PBPS GUI (shown below)
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2. Change motor soft limits to 1 and -1 for both motors
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3. Retry calibration
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<img src="https://docs.google.com/drawings/d/e/2PACX-1vSga-Zluo4b0isNs92C5MXfawMueNVk3ideRRDA_ZGQmD05Jn4noTS_pOl0OHiOyjS31OhLTAelFkYn/pub?w=728&h=686">
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## Operating Principle
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The PBPS (Photon Beam Position Solid) is a **single-shot diagnostic** used to measure **relative pulse energy and beam position**. It functions by detecting **back-scattered X-rays** from a thin target, typically a **50 nm silicon nitride (Si3N4) film**. The scattered X-rays are captured by four **photodiodes** arranged around the beam:
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- **Top** (Up)
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- **Bottom** (Down)
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- **Left**
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- **Right**
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These measurements are critical for:
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- **FEL pointing stability** (by tracking beam position)
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- **Experimental data normalization** (by measuring pulse energy)
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SwissFEL features **one PBPS in the front-end** and **two PBPS devices per end station**.
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---
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## How to Use
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### **1. Open the Photon Diagnostics Web App**
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[Click here to open](https://sf-photodiag.psi.ch/app)
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### **2. Select the PBPS Device**
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- Navigate to the **Position** or **Calibration** tab.
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- In the **first dropdown menu**, select the PBPS device you want to calibrate.
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### **3. Choose the Target**
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- In the **second dropdown menu**, select a target.
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- **Default target:** **50nm Si3N4**.
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### **4. Start Calibration**
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- Press the **Calibrate** button.
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- The system will:
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- Normalize the response of the **four photodiodes**.
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- **Scan the PBPS device** horizontally and vertically across the FEL beam.
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- **Update the plots** with the calibration response (takes ~1 minute).
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### **5. Analyze the Results**
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- The updated plots show:
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- **X-axis** → Device motor position (**Horizontal: left plot**, **Vertical: right plot**)
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- **Y-axis** → Normalized **horizontal or vertical diode pair response**
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- **A good calibration** results in a **straight line** passing through the three scan points.
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### **6. Push the Calibration Results**
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- If the calibration looks good, press **"Push results/elog"**.
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- This updates the data processing pipeline and generates an **elog entry**.
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- After an **automated restart (~30s)**, the **PV and BS position & intensity data** will be updated with the new calibration.
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<img src="https://docs.google.com/drawings/d/e/2PACX-1vRGynTQ54ugugA_bM9pYc9eZDlet3Z9MSZWn17wJNwAEM2ORiJGoL8O61frpLBVRpFKlzJ7M05a5Mb1/pub?w=1245&h=1039">
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---
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The calibration procedure normalizes the response of the 4 diodes, then scans the device horizontally and vertically over the FEL beam to calibrate the position response.
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Once completed (approx 1 minute) the plots are updated with the calibration response. Plotted on the x axis is the device motor position (horizontal - left plot and vertical - right plot) and on the y axis is plotted the normalized horizontal or vertical diode pair response. For a good setup the plot should be a straight line through the 3 data points.
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If you are happy with the calibration, press the "Push results/elog" button where the new calibration data is sent to the required data processing pipelines. After an automated restart (~30 seconds) both PV and BS position and intensity data will be calculated with the new calibration. An elog entry is made detailing the calibration data.
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## Troubleshooting
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### **Soft Limits Error**
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#### **Error Message:**
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`ERROR: Motor position outside soft limits`
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#### **Cause:**
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- The PBPS **Motor X or Y** has hit a **soft limit**, preventing the scan.
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#### **Solution:**
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1. **Open the PBPS GUI** and locate the motor settings.
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2. **Modify the soft limits**:
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- Set **Motor X soft limits** to `-1` and `1`
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- Set **Motor Y soft limits** to `-1` and `1`
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3. **Retry Calibration**.
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<img src="https://docs.google.com/drawings/d/e/2PACX-1vSga-Zluo4b0isNs92C5MXfawMueNVk3ideRRDA_ZGQmD05Jn4noTS_pOl0OHiOyjS31OhLTAelFkYn/pub?w=728&h=686">
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### **Additional Debugging Tips**
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- Ensure the **PBPS device is correctly selected**.
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- Verify that the **target is installed properly**.
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- Try manually moving the motor **in small steps** to check functionality.
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- If the issue persists, check for **elog messages** indicating motor faults.
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---
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## FAQ
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**Q: How do I know if my calibration was successful?**
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A: If the diode response fits a straight line and the y-axis range is a few percent, your calibration is good!
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**Q: What happens if I push bad calibration results?**
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A: The **old calibration** will be overwritten, which may cause **incorrect beam position data**. Make sure to verify results before pushing.
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**Q: Why does calibration fail even after adjusting soft limits?**
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A: The device may be in a fault state. Try **resetting the motor controller** or checking the motor motion is completing by checking the motor GUI.
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---
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## 🎭 **Do Say / Don't Say**
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✅ **Do Say:**
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- *"I just calibrated PBPS like a ninja!"*
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- *"Let’s push these results to elog!"*
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❌ **Don't Say:**
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- *"What’s a soft limit?"*
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- *"Do I really need to calibrate this thing?"*
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---
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