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ISIS spectropolarimetry SA notes

  1. Preparation before a run
  2. Setting up the spectrograph for spectropolarimetry
  3. Useful information

  1. Preparation before a run

    • Ask the optical engineer to change LIRIS halfwave plates for ISIS polarizers MF-POL-PAR and MF-POL-PER, if before the run they are not mounted in the main colour-filter slide in the WHT A&G box.

    • A standard configuration is to have the halfwave plate mounted on top of the quarterwave plate, so that the quarterwave plate is closer to the slit. The quarterwave and halfwave plates can be interchanged, to optimize a particular application. This has to be done when ISIS is off the telescope, hence you should find out whether your observer plans to use such a non-standard configuration and communicate this change to the optical engineers well in advance.

  2. Setting up the spectrograph for spectropolarimetry

  3. If the two ISIS arms are going to be used for observing, complete the spectropolarimetry setup for one arm and then proceed in the same way for the other arm.

    1. Mount ISIS polarizers

      Check whether ISIS polarizers MF-POL-PAR and MF-POL-PER are mounted in the main colour-filter slide in the WHT A&G box. If not (it can happen after a LIRIS run), proceed in a similar way as described in the LIRIS Setting-up web page, but other way around. MF-POL-PAR and MF-POL-PER have a serial number 1000 and 1001, respectively, in the ING filter database, and go in positions 2 and 3 of the A&G main colour-filter slide.

    2. Set up ISIS as usually

      Follow the standard procedure to set up ISIS (rotation, tip-tilt, and CCD focus) with the grating and central wavelength as requested by the PI.

      IMPORTANT! Remember that it is NOT recommended to use a dichroic, therefore do the ISIS set up without the dichroic:

      SYS@taurus>bfold 0, to observe in the red arm only

      SYS@taurus>bfold 1, to observe in the blue arm only

    3. Insert the polarization dekker

      After finishing the usual setup, you have to physically insert the polarization dekker in the dekker slide:

      SYS@taurus> dekker 1

      SYS@taurus> slit_door open

      Go to the WHT dome, open the slit door, pull out the 'Observing' dekker slide (labeled 'Obs') to take it out and put it back in its box. Take the 'Polarization' dekker slide (labelled 'Pol') and introduce it in the dekker unit. Be careful to arrive to the end of the rail where it should click into place. Close the slit door securely by snapping softly the locks. Finally, lock the slit door from the ICS:

      SYS@taurus> slit_door close

      Set the dekker name:

      SYS@taurus> setdekkerset polarisation

      Put the dekker in correct position (the default option is dekker in position 2):

      SYS@taurus> dekker 3

      Note that the dekker in 'position 2' is reached by the ICS command 'dekker 3'.
    4. Now click on the 'Update Filters' button in both the 'ISIS Observer' and 'ISIS Eng.' tabs of the Instrument Control Console so that the dekker mask drop-down menus are updated.

    5. Define the appropriate CCD window

      Move the Savart plate into beam:

      SYS@taurus> fcp calcite

      Move halfwave or quarterwave plate in:

      SYS@taurus> hwin, for linear polarimetry
      or
      SYS@taurus> qwin, for circular polarimetry

      SYS@taurus> agcomp

      SYS@taurus> complamps w

      SYS@taurus> flat red 0.5 "window" (adjust exposure time according to ISIS setup)

      Display the image in DS9 and define the appropriate window. In the case of the example here, dekker in position 2, you should see six stripes of light, the 'ordinary' and 'extraordinary' beams for each of the three dekker-mask apertures. Ideally, the window should cover the six stripes of light, and have a margin of about 50 pixels on either side of the most external stripes. See Fig. 1 as an example.

      Once you have defined the window set it as usual, for example:

      SYS@taurus> window red 1 "[890:1240,1:4200]"

      window
      Fig. 1 - An example of image with a window including a margin of about 50 pixels on either side of the most external stripes.
    6. Check the spectrograph focus

      Keep record of the initial ISIS collimator values (from step 2) and give them to observers. They will need this information in case they are planning to switch to normal ISIS observations during the run.

      With the calcite in the light beam, the spectrograph focus should be about 9200 microns higher than the one obtained in the standard ISIS set up (step 2). The reason is that there is now an extra optical element (calcite plate) between the slit and the collimator, and the spectrograph focal length will change. In order to stay in anastigmatic region of the spectrograph, we will need to add about 9200 microns to the previously determined best spectrograph focus. So, for example, optimal collimator value for the red arm with GG495 and no dichroic should be rcoll = 10100 + 9200:

      SYS@taurus> rcoll 19300

      Similarly, optimal collimator value for the red arm without GG495 and no dichroic should be rcoll = 9300 + 9200 = 18500, and optimal collimator value for the blue arm should be bcoll = 5100 + 9200 = 14300.

      SYS@taurus> slitarc 0.7 (use a fairly small slit width)

      SYS@taurus> complamps cune+cuar (check that no ND filter is deployed in the calibration-lamps unit)

      SYS@taurus> arc red 2 "focus"

      Note that longer exposure times should now be used due to the presence of the wave plate and the calcite slab. Display the arc image in DS9 and check the FWHM of several isolated arc lines close to the centre of the detector for the central ordinary (o) and extraordinary (e) beams (for example, arc lines marked in green on Fig. 2). Then move the collimator value in steps of about 500 microns in both directions, take an arc in each step and measure FWHM of the same isolated arc lines as before. Repeat until the minimum FWHM in both rays is obtained, where also a difference of FWHM between o and e rays should be minimal. This is the final collimator value which should be used for observations.
      arc
      Fig. 2 - Arc image where green labels mark example arc lines to measure the best spectrograph focus.
    7. Find the zero angle of the setup for the required retarder plate
    8. Zero angle is the angle of the retarder plate for which the contrast between the ordinary (o) and extraordinary (e) beams is maximized. Feed the spectrograph with linearly polarized light by inserting the MF-POL-PAR polarizer in the beam:

      SYS@taurus> mainfiltc MF-POL-PAR

      Because we are feeding the spectrograph with linearly polarized light, the theoretical zero angles should be around 45 deg for circular polarimetry and 0 deg for linear polarimetry. However, the respective zero angles in ISIS are around 50 deg and 8 deg instead. Besides, these values can vary by up to 4 deg depending on the grating and central wavelength in use. Table 1 in the section Useful information lists some zero angles determined for different set-ups (note that +45 deg is added to the measured zero angle for circular polarimetry - see the following text about circular polarization).

      Now you should take W-flats around the expected zero angle:

      SYS@taurus> complamps w

      SYS@taurus> hwp <angle> or qwp <angle>, to move the required plate to a requested angle between 0-360 deg

      SYS@taurus> flat red 2 "angle"

      To measure the average brightness use the central o and e rays. You can use the iraf imstat command on two rectangular regions, which must have the same size (see Fig. 3). For example:

      cl> imstat r123456.fit[1][200:210,1800:2500] for the o ray

      cl> imstat r123456.fit[1][240:250,1800:2500] for the e ray

      Always use a fairly long region for good statistics on both rays. Take a note of the average count values and compute the difference. Do the same at remaining angles and record the angle of the maximum difference between both rays which will be the zero angle.
      rectangular regions
      Fig. 3 - A zoomed region of a flat field taken with the quarterwave plate at 39 deg. The ordinary (o) and extraordinary (e) rays are labeled. The blue rectangular regions are used to compute average brightness values.
      Linear polarization (using the hwp plate)
      Theoretically, for linear-polarization observations, the target should be observed at the halfwave-plate angles 0, 45, 22.5, and 67.5 degrees. Hence, you should add +0, +45, +22.5, and +67.5 degrees to the zero angle you have found and use these angles for science observations by introducing them in the linear-polarimetry script at /home/whtobs/isis_scripts/linpolscript.

      It is a good practice to check that other plate angles (+22.5, +45, and +67.5) make sense. With the half-wave plate at 22.5 and 67.5 deg away from the zero angle the difference in the intensity between the ordinary and extraordinary beams should be minimal, at 45 deg maximal (see Fig. 4).

      Circular polarization (using the qwp plate)
      We were feeding the spectrograph with linearly polarized light when we measured a zero angle (we used the MF-POL-PAR), and therefore a final zero angle is obtained by adding 45 degrees to the measured angle. That is, if 50 degrees was the zero angle with the MF-POL-PAR, then the angle in the script for circular polarimetry should be 50+45=95 degrees. We will use this angle and +90 degrees in the circular-polarimetry script at /home/whtobs/isis_scripts/cirpolscript. Note that instead of 95 and 185 degrees one can use also 5 and 95 degrees for the observations.

      When you are finished, remember to remove the MF-POL-PAR filter out of the beam:

      SYS@taurus> mainfiltc out
      Polarization dekker set
      Fig. 4 - Image taken using the halfwave plate, showing the expected difference between the zero angle (top) and the zero angle + 22.5 degrees (bottom).

      The table below summarizes the expected ISIS wave plate angles when doing linear and circular spectropolarimetry observations.

      Polarimetry mode
      Retarder
      Angles (deg)
      Linear
      Halfwave plate
      8, 53, 30.5, 75.5
      Circular
      Quarterwave plate
      95, 185

    9. Useful information

    10. For a spectropolarimetry user guide, click here.

      Acquisition

      The insertion of the halfwave plate after the acquisition (without the halfwave plate) does not move the star image relative to the slit, within the accuracy of acquisition of ~ 0.1 arcsec.

      After a spectropolarimetry run

      If you want to configure ISIS from a spectropolarimetry into a longslit mode, proceed as following:

      • Physically change dekkers: take the polarization dekker out and insert the observing dekker. Then change the dekker name:

        SYS@taurus> setdekkerset observing

      • Remove polarization optics from a light path:

        SYS@taurus> hwout (qwout), moves halfwave (quarterwave) plate out

        SYS@taurus> fcp_out, moves the Savart plate out of beam

        SYS@taurus> mainfiltc 1, removes polarizer in the main colour-filter unit

      • Set usual collimator values:

        SYS@taurus> rcoll 10960 for observations in the red arm with a dichroic and GG495

        SYS@taurus> bcoll 5100 for observations in the blue arm.

      • Set the appropriate window, e.g.:

        SYS@taurus> window red 1 "[555:1520,1:4200]"

        SYS@taurus> window blue 1 "[585:1550,1:4200]"

      Dust on the calcite block

      Each bit of dust on the calcite block generates two dark streaks on the image, in the ordinary and extraordinary ray, which at first look like remanence after a dekker exposure. An example of the dust on the calcite block can be seen in the image below.

      If you encounter dust on the calcite block, please let the IS and the optical engineer know. Otherwise, the calcite block should be serviced at regular intervals, every 2 to 3 years, due to the optical gel drying out.
      dust on calcite
      Fig. 5 - Tungsten-lamp flat taken with a calcite block and without a dekker in the beam. There is a light loss of 10 % in the left-hand dip, and few % in the right-hand dip, due to the dust on the calcite block.
      Zero angles measured for different set-ups

      Zero angles determined for different set-ups are listed in the table below.

      Tab. 1 - Zero angles with ISIS.
      ISIS arm
      Grating
      Central wavelength (Å)
      Spectropol. mode
      Zero angle (deg)
      red
      R316R
      6000
      linear
      8
      red
      R600R
      6000
      linear
      8
      red
      R158R
      6400
      linear
      9
      red
      R158R
      6700
      linear
      10
      red
      R158R
      7500
      linear
      9.5
      red
      R1200R
      8650
      linear
      11.5
      red
      R158R
      6400
      circular
      99
      red
      R158R
      7500
      circular
      94
      blue
      R1200B
      4100
      linear
      4
      blue
      R300B
      4400
      linear
      5
      blue
      R158B
      4500
      linear
      4
      blue
      R300B
      4500
      linear
      4
      blue
      R600B
      4700
      linear
      6
      blue
      R1200B
      4100
      circular
      94
      blue
      R300B
      4400
      circular
      96.5
      blue
      R600B
      4440
      circular
      95
      blue
      R300B
      4500
      circular
      94.5
      blue
      R1200B
      4500
      circular
      95


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Contact:  (ISIS Polarisation Specialist)
Last modified: 13 December 2016