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ISIS imaging-polarimetry user guide

  1. Basic characteristics
  2. Afternoon settings and calibrations
  3. Configuring the telescope
  4. Acquiring objects and taking data
  5. Useful information
  6. Imaging-polarimetry observing commands

Basic characteristics

ISIS can be used as an imaging polarimeter following these steps:

  • replacing the grating by a flat mirror (performed by a support astronomer)

  • inserting a wide calcite analyser and a field mask instead of the spectrograph slit (performed by a support astronomer)

  • using permanently mounted waveplates (a half and a quarter waveplate), which are common for both ISIS imaging-polarimetry and spectropolarimetry modes
There are two flat mirrors available (for more information, see below), thus in principle one could use both arms of ISIS simultaneously. Unfortunately, in the imaging-polarimetry mode a dichroic causes an additional stellar image 11 arcsec away from the real stellar image and partly overlapping with the other real stellar image (we have two real stellar images due to the presence of a calcite analyser). Its intensity is less than 2 % of the intensity of the real stellar image, and therefore it is not seen for faint targets, for example. However, generally it is not recommended to use a dichroic for polarimetric observations for this reason.

It is possible to use 2 passband filters for the observations in both arms. For the red arm and a linear polarimetry, the unvignetted field of view is roughly 2 by 1.25 arcmin at centre and 2 by 1 arcmin at sides of a field. For the red arm and a circular polarimetry, it is roughly 48 x 44 arcsec at centre and 48 x 26 arcsec at sides of a field (see picture below). For the blue arm, the unvignetted field of view is the same as for the red arm for a circular polarimetry, and it is less vignetted at sides by about 15 arcsec for a linear polarimetry. See more details here.

The instrumental polarization measured from the zero-polarization standard star is p = 0.07 +/- 0.04 %.

fov

Fig. 1 - The unvignetted field of view and image characteristics for ISIS linear and circular imaging-polarimetry observations in the red arm. See more details here.

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Afternoon settings and calibrations

  1. Put the calcite analyser in the beam

    In order to have a correct information in the ISIS Mimic, Instrument Control Console and image headers, locate ISIS Observer tab of the Instrument Control Console, and in the Slit Unit section choose "Image polarimetry" option. This will move the multi-slit unit containing the calcite analyser to the light path, and the image-header keyword ISISLITU will show IMAGE_POL.

    In case you need to move back to a long slit, type:

    SYS@taurus> longslit

  2. Put the retarder plate in the beam

    Deploy the halfwave plate (for linear polarimetry) or the quarterwave plate (for circular polarimetry) in the light path:

    SYS@taurus> hwin (for linear) or qwin (for circular)

    To take halfwave or quaterwave plate out of the beam, type:

    SYS@taurus> hwout or qwout

  3. Insert a filter

    You can use two filters for the observations in both arms of ISIS. Filters are mounted in the following slides: RFILTA, RFILTB, BFILTA and BFILTB.

    To use a filter mounted in BFILTA (or BFILTB) slide type:

    SYS@taurus> bfilta 4 (or bfiltb 4)

    Similarly, to use a filter mounted in RFILTA (or RFILTB) slide type:

    SYS@taurus> rfilta 4 (or rfiltb 4)

    To remove a filter from a light path, type:

    SYS@taurus> bfilta 1 (or bfiltb 1, rfilta 1, rfiltb 1)

  4. Setup the detector

    Set the appropriate window and readout speed of the detector as usual.

    The windows below can be used as a reference. They cover the whole field of view and include an overscan region on the right side (see Fig. 3).

    For a linear polarimetry:

    SYS@taurus> window red 1 "[680:2148,1700:2470]"

    SYS@taurus> window blue 1 "[695:2148,1580:2440]"

    For a circular polarimetry:

    SYS@taurus> window red 1 "[770:2148,1820:2300]"

    SYS@taurus> window blue 1 "[800:2148,1750:2280]"

    flat

    Fig. 3 - This window includes the field and the overscan columns of the CCD (right end of the image). The double pattern of slits corresponds to the ordinary and extra-ordinary image of the comb mask as produced by the calcite analyser.


  5. Take calibrations

    Take usual set of calibrations described in ISIS cookbook, which includes biases and lamp or sky flat fields. Do not use internal tungsten lamps for flat fielding, as the stripes projected on the CCD are narrower than when using sky light.

    If you are taking dome or sky flat fields, then for linear (circular) imaging polarimetry observations you can use half-wave (quarter-wave) plate with continuous rotation of 1 Hz and integer integration time to depolarize dome flat-field or sky light.

    • Biases

      Before taking biases turn off all the dome lights and close the courtains in the control room.

      SYS@taurus> agcomp

      SYS@taurus> complamps off

      SYS@taurus> multbias red 11 & multbias blue 11 &

    • Dome flat fields

      In order to take dome flat fields, first ask the telescope operator (OSA) to point the telescope to a suitable position. Then you need to open mirror petals:

      SYS@taurus> petals open

      and direct the light from the dome to ISIS:

      SYS@taurus> agslit

      Then you will need to find a right illumination in order to have an integer integration time. There are in total 5 lamps mounted on the top-ring of the telescope: lamp number 1 = 9 W, number 2 = 25 W, number 3 = 150 W, number 4 = 500 W and number 5 = 500 W. With Sloan R filter in the red arm, lamp number 2 and exposure time of 10 s were used in a past. To turn on the lamp number 2, for example, type the following:

      SYS@taurus>fflamp 2 on

      or to turn on all the lamps:

      SYS@taurus>fflamp all on

      To rotate half-wave (quarter-wave) plate with continuous rotation of 1 Hz type:

      SYS@taurus>hwprot 1 (or qwprot 1)

      Now take some dome flat fields. To take N exposures, type:

      SYS@taurus> multflat red (or blue) <N> <int time> "dome flat"

      When done, switch off the continuous plate rotation:

      SYS@taurus>hwprot 0 (or qwprot 0)

      close the mirror petals and switch off all the lamps:

      SYS@taurus>petals close

      SYS@taurus>fflamp 2 off

      or

      SYS@taurus>fflamp all off

    • Sky flat fields

      In order to take sky flat fields, first ask the telescope operator (OSA) to point the telescope to a suitable blank field. The mirror petals will be opened by OSA. Turn on the continuous rotation of the half-wave (quarter-wave) plate:

      SYS@taurus>hwprot 1 (or qwprot 1)

      SYS@taurus> agslit

      Now take some sky flat fields:

      SYS@taurus> sky red (or blue) <int time> "sky flat"

      During twilight, the sky dims by a factor ~ 2 every 3 minutes. To avoid contamination by background stars, it is recommended to dither between successive exposures by a few arcsec:

      SYS@taurus>offset arc 10 10

      The coordinates given here are in arcsec, and are absolute, not relative.

      When done, switch off the continuous plate rotation:

      SYS@taurus>hwprot 0 (or qwprot 0)

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Configuring the telescope

  1. Set the telescope focus to a value about 0.45 mm lower than the nominal ISIS value (usually 97.85 mm), e.g.:

    SYS@taurus> focus 97.4

    This will enable you to take exposures and view the star roughly in focus on the CCD and on the slit-view mirror. To ensure that the star is also in focus on the direct-view mirror, set TVFOCUS 9500 in TVSCALE 5 at ICL.

  2. Determine the position of the centre of the field mask on the direct-view mirror:

    • Switch to agslit mode, point the telescope to a bright star and take a glance image with ISIS.

    • Identify the star in the image and ask the TO to apply offsets until the star appears centred on the central field mask slot.

    • Switch to direct view using agcomp and take an image with the direct-view camera.

    • Mark the position of the star on the direct-view camera DS9 display (take a note of the point coordinates in case the marks are erased). This position will be used to identify and roughly centre subsequent targets using the direct-view camera.

  3. Refocus the telescope accurately using the usual focus procedure. First, setup the spectrograph for observing on-sky by typing AGSLIT and ROT SKY 0. Then acquire a fairly bright star (about 11 magnitude will be fine) and put it in the centre of the central slot of the field mask found in the previous step. Take a test exposure to determine the optimal exposure time and use FOCUSRUN command in the usual way to determine the best telescope focus.
      
  4. Refocus the autoguider with the ICL command "AUTOFOCUS 600" (it will probably need reducing by >1000 from the usual Cassegrain focus because of the decreased telescope focus).

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Acquiring objects and taking data

  1. To acquire the objects you should use the direct-view camera:

    SYS@taurus> agcomp

    and position the target on the desired slot, as determined by the marks drawn on the TV screen in previous steps.

  2. Remove the direct-view mirror:

    SYS@taurus> agslit

    and check the acquisition by taking a short exposure (or glance) using ISIS. If necessary, do a final tuning by asking the TO to introduce offsets to centre the target in the field mask.

  3. Take data using imaging-polarimetry scripts in /home/whtobs:

    SYS@taurus> linimpolscript <camera> <int time> <title> [nloop] for linear polarimetry,

    SYS@taurus> cirimpolscript <camera> <int time> <title> [nloop] for circular polarimetry,

    where arguments are denoted by <> and nloop is a number of loops which equals to one if not specified.

    This is an example of scripts for a linear and a circular polarimetry which will take 4 and 2 images, respectively, at zero angles determined by your support astronomer. Zero angles are different for the red and the blue arm of ISIS, and also may vary with other spectrograph settings. Your support astronomer will modify the scripts to contain correct zero angles for your run. Fig. 4 is a linear-polarimetry example of an image of a 0 polarization standard.

    image

    Fig. 4 - Linear-polarimetry observation of a 0 polarization standard star.

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Useful information

  • In the field mask of the calcite analyser (see Fig. 5) there are 7 slots (each 6 arcsec wide) separated by gaps 11 arcsec wide, producing in total 14 images on the CCD (due to the action of the calcite analyser).

  • calcite unit

    Fig. 5 -  The analyser unit for imaging polarimetry. The calcite is below the dekker field mask (in yellow).

  • Image characteristics as measured using blue arm observations, assuming a pixel size of 0.2 x 0.2 arcsec:

    • Each slot through the calcite projects on the CCD a stripe approximately 6.4 arcsec wide for the ordinary and 6.4 arcsec wide for the extraordinary beam, separated by a gap of 1.3 arcsec.

    • There is a projected gap on the detector of approx. 3.9 arcsec between adjacent slots.

    • The unvignetted field of view for a linear polarimetry is about 120 arcsec in the direction perpendicular to the beams, that is along the x-axis. Parallel to the beams, that is along the y-axis, the size is ~90 arcsec at the centre and ~75 arcsec at the left-most and right-most slots of the image.

    • The unvignetted field of view for a circular polarimetry is about 49 arcsec in the direction perpendicular to the beams, that is along the x-axis. Parallel to the beams, that is along the y-axis, the size is ~45 arcsec at the centre and ~27 arcsec at the left-most and right-most slots of the image.

  • Image characteristics (see Fig. 1) as measured using red arm observations, assuming a pixel size of 0.22 x 0.22 arcsec:

    • Each slot through the calcite projects on the CCD a stripe approximately 6.4 arcsec wide for the ordinary and 6.4 arcsec wide for the extraordinary beam, separated by a gap of 1.3 arcsec.

    • There is a projected gap on the detector of approx. 3.9 arcsec between adjacent slots.

    • The unvignetted field of view for a linear polarimetry is about 120 arcsec in the direction perpendicular to the beams, that is along the x-axis. Parallel to the beams, that is along the y-axis, the size is ~75 arcsec at the centre and ~60 arcsec at the left-most and right-most slots of the image.

    • The unvignetted field of view for a circular polarimetry is about 48 arcsec in the direction perpendicular to the beams, that is along the x-axis. Parallel to the beams, that is along the y-axis, the size is ~44 arcsec at the centre and ~26 arcsec at the left-most and right-most slots of the image.

  • Observers normally use the detector with 2x2 or 1x1 binning. The readout time for the usual CCD window is 6 s with 1x1 binning and 4 s with 2x2 binning.

  • Since 2016 there are two flat mirrors available for ISIS. The second mirror was used in a past as a spare mirror for NAOMI (adaptive optics), and therefore has a very good optical quality. As NAOMI was de-commissioned, ING decided to make another use of this mirror. In the red arm, the original flat mirror for ISIS (henceforth old mirror) and the new mirror have very comparable throughput. In the blue arm, however, old mirror is more efficient (for example, at 3870 (4430) Å the old mirror is by 30 (13) % more efficient than the new mirror). Therefore if both mirrors are to be used it is recommended to use the old mirror in the blue arm and the new mirror in the red arm.

  • The sky orientation for the red and blue arm is shown on the image below. If the rotator position angle on sky is zero, the slots of the dekker are aligned approximately east-west.

    The acquisition camera (after using a command SYS@taurus> agcomp) has the same orientation as blue arm CCD with sky PA = 0 (as shown on the image below) and also the same orientation as the slit viewing camera. The field of view of this acquisition camera is ~ 4 x 4 arcmin.

  • fov_dir

    Fig. 6 - The sky orientation for the red and blue arm with sky PA = 0.

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Imaging-polarimetry observing commands

Commands to move halfwave (quarterwave) plate

SYS@taurus> hwin (qwin), moves retarder plate in

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

SYS@taurus> hwp <angle> (qwp <angle>), moves retarder plate to requested angle (0-360 deg)

SYS@taurus> hwprot <rate> (qwprot <rate>), rotates retarder plate at requested rate (0 - 1 Hz)

SYS@taurus> hwstop (qwstop), stops the rotation of retarder plate

A continuous rotation for both quarter and halfwave plates has time-out of 12 hours in case an observer would forget to stop the rotation.


Other useful commands

SYS@taurus> longslit, moves the calcite analyser out of beam and moves to a long slit

SYS@taurus> mainfiltc MF-POL-PAR, selects polariser in the main colour-filter unit

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

In case retarder plates do not move, type:

SYS@taurus> inhw (inqw), which initializes halfwave (quarterwave) plate.



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Contact:  (ISIS Polarisation Specialist)
Last modified: 31 March 2017