Polarimetry

Point of contact: Miles Lucas <mileslucas@arizona.edu>

Warning

WIP: This document is a work in progress. I recommend emailing me if you plan to do polarimetry.

Introduction

Devices and Stages

Polarizing beamsplitter (PBS)

The polarizing beamsplitter cube (PBS) splits incident light into orthogonal polarization states for each science camera. The orientation of the PBS is such that camsci1 measures horizontally polarized light while camsci2 measures vertically polarized light. Polarimetric observables are obtained from differencing the two cameras’ images or fluxes

X = I_cam2 - I_cam1

to normalize the observable, divide by the single sum

I = I_cam2 + I_cam1
x = X / I

To use the PBS, it must be inserted in stagescibs (‘pol’). This can be done through INDI or from either camsciCtrl GUI. In data analysis, the beamsplitter option can be confirmed in the STAGESCIBS PRESET NAME FITS header (should be ‘pol’).

Important

Don’t forget to use the same filter for camsci1 and camsci2! We’re doing PDI, not SDI.

Half-wave plate (HWP)

Because the PBS has a fixed orientation, each camera can only measure the horizontal and vertical polarized light (Stokes Q). Therefore, to measure +45° and -45° polarized light (Stokes U) a half-wave plate (HWP) modulates input polarized light such that incident Stokes U light can be measured as horizontal and vertically polarized light.

Additionally, the HWP is a critical component for cancelling instrumental polarization effects. In the same way it modulates incident Stokes U light into horizontal/vertical, it can modulate incident Stokes Q light into the -Q orientation. By making two measurements with different HWP orientations, one for Stokes +Q and one for Stokes -Q, we can differentially cancel instrumental effects:

I_HWP0 = +Q + inst
I_HWP45 = -Q + inst
0.5 * (I_HWP0 - I_HWP45) = 0.5 * (+Q + inst - -Q - inst) = Q

In order to use the HWP, it must be inserted into the beam using stagepollin. Once it is in, the key MagAO-X application to use is hwptrack. To move the HWP to a given angle, set hwptrack.hwp_position.target. The standard sequence for linear Stokes polarimetry steps through 0°, 45°, 22.5°, and 67.5°. These correspond to measuring Stokes +Q, -Q, +U, and -U in sequence. These names are shown in the hwptrack.hwp_position_name.value and can be used for grouping frames for data analysis.

The last feature is the ADI synchronization tracking mode for the HWP, set by hwptrack.tracking.toggle. This synchronizes the HWP with the image rotator, adding an offset on top of the desired HWP angle (shown with hwptrack.hwp_tracking_offset.value). This keeps the eigenpolarizations of the instrument coincident with the sky even while the field is rotating due to pupil tracking. This also simplifies the data analysis–when each frame is derotated to orient north up and east left, the additional HWP offset will perfectly align the Stokes Q and U axes to the sky axes.

For data analysis, the following FITS headers are provided–

HWPTRACK SET ANGLE / [deg] The polarization angle of the HWP, typically 0, 45, 22.5, and 67.5
HWPTRACK ACTUAL ANGLE / [deg] The actual orientation of the HWP, only differs from the SET ANGLE when tracking is on
HWPTRACK POS NAME / The named HWP position, one of 'Qplus', 'Qminus', 'Uplus', 'Uminus', or 'unknown'
HWPTRACK TRACKING / [int] 0 if synchro_adi tracking is off, 1 if tracking is on

Dual rotating quarter-wave plate compensator (DQWP)

Operation

  1. Make sure the PBS is selected in stagescibs

  2. Make sure both science cameras are in the same filter, r, i, or z.

  3. (Optional) Insert the DQWP

    1. Make sure AO loops are open since we will occult the beam

    2. stageqwplin to preset in

    3. (TODO; code doesn’t exist yet) qwptrack start tracking

  4. Ensure the science cameras are synchronized via the synchro property

  5. Insert the HWP

    1. Make sure stagezaber power is on and stagepollin USB is on in the ninja tab of pwrGui

    2. Move stagepollin to preset in

    3. Turn stagezaber power off and stagepollin USB off

  6. Prepare the HWP

    1. Make sure stagepolrot power and USB are on in pwrGui

At this point, all optics are ready for polarimetry. Polarimetry is run via the hwpSequencer app or inside the hwpSequencerGUI.