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Polarimetry with NIKA2

The following text gives information to observers and 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.

Information for observers

Since the summer semester 2024, we invite proposals in open time for NIKA2 1-mm polarimetry. The polarimetry mode has been commissioned and the PIIC reduction software has been updated to create I, Q, U Stokes maps. The following quick overview of the polarimetry capabilities is based on the summary of the NIKA2 Pol commissioning and the updated PIIC tutorial.

NIKA2 polarimetry makes use of a half wave plate (HWP) rotating with 2.98 Hz in front of the entrance window of the cryostat thereby modulating the incoming signal. In addition, a wire-grid polarizer inside the cryostat splits the two 1mm polarizations. The sensitivity in polarization is better than in total power, with values close to 20 mJy sqrt(s) in Q and U. The NEFD in total intensity in polarimetry is comparable to the total power sensitivity reported in Perotto et al. 2020. Polarization angles are stable with a dispersion of less than about 5 deg.

• In general, NIKA2-Pol observations should be carried out under stable atmospheric conditions (see Atmospheric stability issues section at NIKA2 Observing Session) and low wind conditions speed to reduce tracking deviations. In addition, HPBWs should be monitored (see Pointing section at NIKA2 Observing Session). Beam stability during day time observations is much improved after re-painting the primary in summer 2024.

• To avoid beam smearing, scanning speeds of 35-40 arcsec/s shall not be exceeded. These maximum speeds are still consistent with a Nyquist sampling of the 1 mm beam at each phase of the HPW. For more details please read chapter 6 of the latest PIIC tutorial.

• The correction for leakage of intensity to Stokes Q and U, i.e. for the instrumental polarization (IP), is not easy (cf. commissioning report). The IP can be up to 3% for compact sources. For more extended sources, the IP smears out and is less of a problem. PIIC allows for correction of the IP.
• Different methods can be employed to characterize the IP, depending on the availability of IP calibration sources in the sky or not. Generally speaking, these sources shall be observed at similar elevation and focus as the science target. Sources to be observed to determine the IP correction:

  • Uranus shall be used when available as it is pointlike, non-polarized and stable.

  • Neptune rises few hours before Uranus.

  • Mars can become large filling the main beam when in Opposition and close to Earth. This makes measuring the leakage pattern more difficult. On 20/2/2025 it is 11.8" Current angular diameter

  • An alternative method may be to observe strong QSOs as 3C286, 3C454 or 3C84 (included in the catalog polar_qso.sou in the NIKA2 pool account). However, quasars are variable and polarized and this method has not yet been successfully tested. The B-FUN project is trying out.

  • Observing the science source over a range of parallactic angles (transit-method), can allow to disentangle the IP, which is fixed in Az/El, and the source intrinsic polarization, which is fixed in RA/Dec. Weather conditions need to be really stable during a transit. The B-FUN project is trying this method.

  • In the future, we may use the database of observed Uranus scans.
  • Consult the AoD to define the best strategy and sources for your specific science case and observations.
• Observing procedure, when using a point-like source to measure the IP for a given NIKA2-Pol session (set of science scans):

  • perform a @calib_1scan.pako script (4 min. duration) before a new focus correction and with elevation conditions as similar to those of the science target as possible (see, for instance, Section 6.8 of the latest version of the PIIC tutorial).

  • These calibrations need to be performed approximately every hour.
  • after the calib_1scan, check the focus with a quick pointing scan and re-focus if needed. If the focus changes, it is preferable to perform another calib_1scan before moving back to the science target.

• For NIKA2-Pol beam maps, always use the script @beammap1scan95sub_polar_hwp3hz_29min.pako provided in the NIKA2 pool account.

For Operators and AoDs: Going from total power to polarimetry observations and vice-versa

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.

   • Check that no server process is running and, if so, kill it

   • 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 .ini files are currently stored in the folder: nika2@nika2-a:~/NIKA/Processing/NIKA_lib_V3_IRAM/Acquisition/experiments/

   • 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 tutorial.

   • 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
   • Press the "stop" button run the MPPSYNC window
   • Press the "power off" button
   • Press the "stop" button again

   • 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 in the nika2@nika2-a:~/NIKA/Processing/NIKA_lib_V3_IRAM/Acquisition/experiments/ folder before restart

   • The mppsync server terminal can be left open (it is convenient leaving it running during the NIKA2 pool if possible). 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

Created: 2022.FEB.04, I. Myserlis

Last update: 2025.FEB.13, C. Kramer & S. Berta