Continuum/PolarimetricObservations

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The following text describes the procedure to change the NIKA2 system from total power to polarimetric observations mode. The procedure is described in a sequential form of the steps that need to be followed. Also, a reverse procedure is given below to change back from polarimetric observations to total power mode. For more information about the polarimetric characteristics and capabilities of NIKA2, we point the reader to the summary of the polarization commissioning report or also Chapter 6 of the PIIC manual.

To start polarimetric observations with NIKA2

1. Put the 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 traces when the HWP is put in front of the NIKA2 window

2. Start the server for MPPSYNC (hereafter mppsync terminal), needed for the HWP operation

   • Open a Xterm_V3_IRAM terminal in the nika2-a:1 VNC, if necessary. It must be logged to nika2@nika2-a

   • Go to the appli folder: ~/NIKA/Processing/NIKA_lib_V3_IRAM/Acquisition/appli

   • Call ./mppsync.sh

   • Call ./server

   • The mppsync server (running from archeops@mppsync1:~/NIKA_lib_AB_OB_gui/Acquisition/instrument/mppsync/) can be left running after the polarimetric observations. It should not affect the total power observations. Running status in the mppsync terminal brings useful information for monitoring the system (i.e. phase, power, direction, movement, speed; see below)

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 in box "U")

   • 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 left side of the acquisition (acqui) window

   • Expected to be around -400 to -500 us; the tab background should be in green colour
   • If not, use the buttons "start/stop" and/or "header/reconnect" and/or restart the acquisition altogether. Sometimes, several tries are necessary until the previous condition is met
5. Start the resonances monitor (LAUNCH MON_V3_IRAM), as usual, and get traces for (e.g.) array 2
6. Launch HWP operation window
   • Select "U MPPSYNC" from the drop-down menu of the "trace" window
   • Activate the checkmark on "power" and click "start" button

   • If the HWP is off (check the power status in the mppsync terminal), 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 (if NIKA2 is illuminated, of course), caused by the HWP rotation

   • Rotation sense last checked on Feb 20, 2024¹ by visual inspection within the receiver cabin. The HWP rotates correctly (CW), 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 stabilises 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.
   • The PIIC monitor processes the polarimetry data as total power, i.e. does not produce Stokes I-Q-U cubes.
9. Polarimetric data reduction.
   • NIKA2 polarization data can be reduced with two independent pipelines: PIIC-Pol and IDL.

   • For PIIC-Pol, Stokes I-Q-U cubes can be created offline. For a detailed description, please read chapter 6 of the latest PIIC manual.

   • For the IDL pipeline here are some instructions:

     • 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 (calib_1scan), 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 used by the PIIC-Pol pipeline but not by the IDL one.

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

   • The message "power OFF" should appear at the top of the MPPSYNC window. Check the power status in the mppsync terminal

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 terminal 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

Some recommendations for polarimetric observations with NIKA2:

• It is not suggested to reduce polarization beammap scans during an observing session because it will take a long time and delay the observations flow, especially if a pointing or focus scan is needed in the (near) future.

• Scanning speed for science observations should not exceed 35"/s - 40"/s . For more details please read chapter 6 of the latest PIIC manual.

To Do List:

Created: 2022.FEB.04, I. Myserlis

Last update: 2024.FEB.26, I. Myserlis