Diff for "Continuum/PolarimetricObservations"

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
   • Done by the operator using a pneumatic system
   • The rotation of the HWP is managed through the acquisition software
   • 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
2. Start the server for MPPSYNC, needed for the HWP operation
   • Open a terminal at nika2@nika2-a
   • Go to the appli folder: ~/NIKA/Processing/NIKA_lib_V3_IRAM/Acquisition/appli
   • Call ./mppsync.sh
   • Call ./server
   • The mppsync server can be left running after the polarimetric observations. It should not affect the total power observations.
3. Restart the acquisition software (LAUNCH ACQUI_V3_IRAM) on the nika2-a machine with the correct ini file
   • For ease of use, three ini files are created at the beginning of each pool: runXX.ini, runXX_tp.ini and runXX_pol.ini
   • 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)
   • 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, see below)
4. Check the MPPSYNC value in the acquisition window
   • Expected to be around -400 to -500 us
   • If not, use the buttons "start/stop" and/or "header/reconnect" or restart the acquisition altogether
5. Start the resonances monitor (LAUNCH MON_V3_IRAM), as usual
6. Launch HWP operation window
   • Select "U MPPSYNC" from the drop-down menu of the "trace" window
   • Click buttons "power" and "start"
   • If the HWP is off, the traces look as expected for total power observations (i.e. relatively flat and noise-like)
   • If the HWP is on, there is a clear sinusoidal wave in the traces, caused by the HWP rotation

   • 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.

7. Set the phase of the HWP
   • Click on "setPhase" button
   • The "phase" entry in the mppsync terminal decreases or increases until it locks at around 50% with small oscillations around this value
   • When the phase is locked, the yellow arrow in the MPPSYNC window points upward
   • After the HWP tuning, the spectra of the traces show clear peaks at around 3 Hz, 6 Hz etc.
8. Start observations and real-time data reduction. At the moment, the polarization data are reduced with the IDL pipeline
   • 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.
   • Other than that, the reduction is done the same way as total power for: pointing, focus, skydips, calibration scans, beam maps and science observations.

   • 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.

   • 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
   • Click the "stop" button run the MPPSYNC window
   • Unclick the "power" button
2. Close the acquisition software as usual
   • Call ./reset_nika.pl in the appli folder (nika2@nika2-a:~/NIKA/Processing/NIKA_lib_V3_IRAM/Acquisition/appli/)
3. Restart the acquisition software (LAUNCH ACQUI_V3_IRAM) on the nika2-a machine with the correct ini file
   • Copy runXX_tp.ini to runXX.ini before restart
   • The mppsync server window can be left open. It shouldn't affect the total power observations
4. Start the resonances monitor (LAUNCH MON_V3_IRAM) and proceed with the tuning as usual
   • When the correct ini file is selected for total power observations. The "trace" window doesn't even have the "U MPPSYNC" option in its drop-down menu