Circular polarimetry with FAPOL

Circular polarimetry is made with a 1/4 wave plate in the FAPOL unit and a calcite plate in the aperture wheel. The field vignetting is as for linear polarimetry limited by the size of the calcites (i.e. 140" in diameter). Verify with staff that the 1/4 wave plate is installed in FAPOL.

Observing steps

1. In the FAPOL window click on "Carriage is out". It will go in, then initialize and tell when it is ready. You can manually select retarder positions by clicking on the menu, or by running scripts to take one or several cycles at a time. For circular polarimetry the 1/4 wave plate should be rotated as a minimum in the 2 steps 0°, 90°, but better in the 4 steps 0°, 90°, 180° and 270°.
   
   
2. Imaging polarimetry:
   • Select Cal-90 in the aperture wheel.
     
     
   • Doing polarimetric imaging only, the simplest way to switch fast from normal to polarimetric mode, which involves substantial changes of the optical path, is to add +525 (foc-delta) units to the telescope focus value for the same filter in the beam. In this way you don't need to refocus if you already focused the telescope. Due to the substantial changes in the telescope focus, you will have to adjust the tv-focus to a value around 550. If you are starting up the night in imaging polarimetry mode, the default focus for circular imaging polarimetry is ALFOSC default 23230 + 525 , assuming the internal ALFOSC focus is set to the default value 1940). See also About focusing FAPOL .
     
     
   • Limit the CCD window size using e.g. alfosc.polwin, and take an image to check the position of your target. You will see two images of your target on the CCD, the ordinary component being the upper one and the extraordinary one about 15" below.
     
     
   • Start exposing. This can be done manually or using a script.
     A standard script is available:
     alfosc.cirpolexpose "number of angles" "myobject" "exposure time" "number of cycles"
     Options for number of angles are:
     2 (0°, 90°)
     4 (0°, 90°, 180°, 270°)
     
     
     
3. Spectropolarimetry:
   • You will use both the Cal-90 plate and the Calcite plate with a polarimetry slitlet mounted above it. These are both mounted in the aperture wheel. In order to have the slitlet in focus, the internal camera focus of ALFOSC must be set to 1070 (the default internal focus value being 1940). If you restart the ALFOSC obssystem you may have to reset the camera focus.
     
     
   • The telescope focus offset should then be about +275 from normal ALFOSC imaging mode (without filters). NB! Important! Have the lambda/4 retarder plate in the beam when you measure the focus!
     
     On Oct 29th 2011 the best focus-offset was found to be foc-del +275, obtained in 0.5" - 0.7" seeing adjusting the focus until the fwhm was about equal for the two ortogonal components.
     
     Due to the substantial changes in the telescope focus, you will have to adjust the tv-focus to a value around 520.
     
     
   • If you want to observe on the parallactic angle, preset to your target using the TCS numerical key [6]. If you want to align the slit along a preferred direction (e.g. along two stars), or the default field orientation is ok, then use TCS key [4] to preset, and use slitrot afterwards to find your preferred field orientation (see below).
     
     
   • For aquisition on the slit, put in the Cal-90 plate in the aperture wheel and window the readout using e.g. alfosc.polwin. Make sure you are autoguiding!
     
     If you want to align the slit along a preferred direction, then:
     • Take an image.
     • Type slitrot in the ALFOSC Sequencer window and follow the instructions. The principle is that you click on two points and get out a new value for the field rotation. This should be given into the TCS with the command: field-r xxx.
     
     When you are satisfied with the orientation, then place your target on the slit as follows:
     1. Take an acquisition image (exp t). You will see two images of your target, the ordinary component being the upper and the extraordinary one about 15" below.
     2. Type slitoff slit=xx in the sequencer window, xx being the number of your slit. Follow the instructions and put the cursor on the upper image of your target, press 'a' to determine the center, then 'q' to quit. The telescope will now move.
     
     Iterate once from step 1. Alternatively you can set the slit position yourself with slitoff xref=xxx yref=xxx.
     
     
   • Then put in the polarimetry slitlet and the grism of your choice.
     
     
   • Limit the window size (you can ask staff to write a small script that windows the readout according to your wishes). Two perpendicularly polarised spectra are produced per image.
     
     
   • Start exposing. This can be done manually or using a script.
     A standard script is available:
     alfosc.cirpolspec "number of angles" "myobject" "exposure time" "number of cycles"
     Options for number of angles are:
     2 (0°, 90°)
     4 (0°, 90°, 180°, 270°)
     
     
     

The 1/4 wave plate is a retarder which is used to convert elliptically or circularly polarised light into linearly polarised light, and the calcite plate produces the two orthogonally polarized beams. Both the ordinary and extraordinary components of a ray are shifted in phase by the quarter of a wavelength, i.e. the phase delay is 90°. The minimum requirement is to expose at 2 different angles of the 1/4 wave plate: 0° and 90° , but with 4 angles (stepped by 90°) the difference between the transmission of the orthogonally polarized spectra is eliminated in the reductions, since the polarisation modulation of the o- and e- rays have the inverse effect in the 3rd and 4th exposure compared with the 1st and 2nd.

Let O(i) and E(i) be the intensities of the ordinary and extraordinary images obtained through the calcite plate for each of the i=1,2 angles of the 1/4 wave plate. The percentage of circular polarisation (V) and its sign (left or right handed) is found as follows:

Q(i) = E(i)/O(i)
QM = Q(1) + Q(2)
V = 50.0*(Q(1)-Q(2))/QM

Intrinsic polarization across the field of the calcites is currently under investigation. At the moment we recommend observing your target at the same spot where you observe a Zero polarisation standard star.