Back to the NIKA2 main page

To start polarimetric observations with NIKA2

  1. Put rotating half-wave plate (HWP) in the optical path on NIKA2
  2. Done by the operator using a pneumatic system
  3. The rotation of the HWP is managed through the acquisition software
  4. If the acquisition was running before, there will be a noticeable change on the screen when the HWP is put in front of the NIKA2 window
  5. Start the server for MPPSYNC, needed for the HWP operation
  6. Open a terminal at nika2@nika2-a
  7. Go to the appli folder: ~/NIKA/Processing/NIKA_lib_V3_IRAM/Acquisition/appli
  8. Call ./mppsync.sh
  9. Call ./server
  10. The mppsync server can be left running after the polarimetric observations. It should not affect the total power observations.
  11. Restart the acquisition software (LAUNCH ACQUI_V3_IRAM) on the nika2-a machine with the correct ini file
  12. For ease of use, three ini files are created at the beginning of each pool: runXX.ini, runXX_tp.ini and runXX_pol.ini
  13. The runXX_pol.ini file is modified accordingly for polarimetric observations with NIKA2 (div_kid=20 instead of 40 and _enable=1 instead of 0)
  14. The acquisition software is reading always the file runXX.ini, so we should copy runXX_pol.ini to runXX.ini before we start (similarly copy runXX_tp.ini to runXX.ini to go back to total power observations)
  15. Check the MPPSYNC value in the acquisition window
  16. Expected to be around -400 to -500 us
  17. If not, use the buttons "start/stop" and/or "header/reconnect" or restart the acquisition altogether
  18. Start the resonances monitor (LAUNCH MON_V3_IRAM), as usual
  19. Launch HWP operation window
  20. Select "U MPPSYNC" from the drop-down menu of the "trace" window
  21. Click buttons "power" and "start"
  22. If the HWP is off, the traces look as expected for total power observations (i.e. relatively flat and noise-like)
  23. If the HWP is on, there is a clear sinusoidal wave in the traces, caused by the HWP rotation

  24. Rotation sense checked on Feb 2, 2022 by visual inspection within the receiver cabin. The HWP rotates correctly, in the sense marked on its frame, when the "move_dir" button is not clicked.

  25. Set the phase of the HWP
  26. Click on "setPhase" button
  27. The "phase" entry in the mppsync terminal decreases or increases until it locks at around 50% with small oscillations around this value
  28. When the phase is locked, the yellow arrow in the MPPSYNC window points upward
  29. After the HWP tuning, the spectra of the traces show clear peaks at around 3 Hz, 6 Hz etc.
  30. Start observations and real-tile data reduction. At the moment, the polarization data are reduced with the IDL pipeline
  31. It's best to use the alias "idl_polar" on nika2-a instead of "idl". This alias ensures that IDL does not try to overload some CPUs needed by the acquisition and that are more critical when running at 47Hz.
  32. Other than that, the reduction is done the same way as total power for pointing, focus, skydips, calibration scans, beam maps and science observations.

  33. Sometimes is needed to open multiple IDL instances to accelerate the reduction process (e.g. for focus sequences). This can be done by calling multiple "xterm &" instances from a single terminal of observer@nika2-a and then calling "idl_polar" in each one of them. This way only one IDL license is used.

  34. The imbfits files are not used by IDL but created because they will be used eventually by PIIC when this mode is available

To go back to total power observations

1. Stop the HWP rotation and turn it off

2. Close the acquisition software as usual

3. Restart the acquisition software (LAUNCH ACQUI_V3_IRAM) on the nika2-a machine with the correct ini file

4. Start the resonances monitor (LAUNCH MON_V3_IRAM) and proceed with the tuning as usual