Imaging Polarimetry using Taurus

[Instrument Description] [Instrument Setup] [Software Startup] [Checking System Operation] [Taking Data] [Calibrations] [Data Reduction]

Instrument Description

Taurus is used as an imaging polarimeter by placing a Wollaston prism in the collimated beam and using this in conjunction with a polarimeter module which enables a half-wave or quarter-wave plate to be rotated in the beam in front of the instrument. The polarimeter module was built by the University of Hertfordshire and originally intended for use with the RGO spectrograph.

The polarimetry modifications to Taurus (known as the Faint Object Polarimeter or FOP) have been available for some years, but the system has recently been upgraded by developing a mode of observation which uses CCD charge shuffling to perform the polarization modulation. It is this new system which is described here.

The polarimeter covers a rectangular field of view of about 26 arc sec by 100 arc sec (up to 160 arc sec with some vignetting). The size of the field is limited by the separation of the beams from the prism, and by the size of the half wave plate.

The charge shuffling mode allows relatively rapid cycling between four positions of the half-wave plate so that all of the information required for a linear polarization observation can be recorded on a single CCD frame. An alternative possibility is to use charge shuffling between two plate positions and two telescope positions. This allows a single Stokes parameters to be determined with rapid switching between the source and a sky position, thus giving very good sky subtraction even on extended sources which fill the field.

Instrument Setup

Most of the setup will normally be done by site staff or the support astronomer. The following things should be checked.

The Telescope

The F/15 cassegrain top end should be fitted. Taurus is also used at F/8, but the Wollaston prism is not large enough to accept the collimated beam size at F/8 so must use the F/15 focus.


The Wollaston prism must be mounted in the Taurus etalon wheel.

The Polarimetry Mask (a slot with width chosen to match the separation of the beams from the prism) should be mounted in the aperture wheel.

The Focal Plane filter wheel should contain whatever broad or narrow band filters are required for the observations.

The pupil filter wheel is not normally used during polarimetry and should be set to a clear position.

The file DISK$USER:[OBSERVER.ICL_LOAD]POL1.ICL is used to set the Taurus wheels. It should be edited if necessary to set the required positions for the Wollaston prism and polarimetry mask.

The Polarimeter Module

The polarimeter module mounts in the A&G unit on the spare guide probe arm (guide probe 1). The module is fitted with either a half-wave or quarter-wave plate. The half-wave plate is used for linear polarization observations, and the quarter-wave for circular polarization. The module is not easily accessible once installed so make sure the correct plate is installed for your run.

The polarimeter module is driven into the beam from the telescope control console. The telescope control software must be running. On the freestanding terminal next to the telescope console type GPROBE in response to the TASK? prompt. Then use the +Y and -Y buttons to move the probe to the required position. Y=586  puts it in the beam. Move Y to its lower limit (about 455) to move the module out of the beam for non-polarimetry observations. The X drive of guide probe 1 doesn't work but should be fixed at a value close to the required position (X=199).

Note that when the polarimeter is in the beam it restricts the range of motion of guide probe 2 (the actual autoguider). Thus it may be harder than usual to find guide stars when doing polarimetry.

Charge Shuffling Hardware Setup

The Charge Shuffling scheme uses the instrument sequencer microprocessor.  This micro uses the CCD "external sync" output pulse to determine when to move the polarimeter.  The instrument sequencer micro is located in the bottom of a control room electronics rack, the one which contains the XMEM.  You should ensure this is plugged in as follows.

Three connections are required to the instrument sequencer.  First, the instrument sequencer console RS232 port S1 must be connected to the AAOD2Q terminal server port 4, via the cable marked "ISconsole" at both ends. This port should have the logical name INSTSEQ_SIF. This connection is used to to command the instrument sequencer.

The second connection from from the Instrument Sequencer S3 serial line to the AAOD2A teminal server port 8, via the cable marked "ISapplic" at both ends.  This port should have the logical name IRISTEST_SIF. This is used by the instrument sequencer to send commands to the TELPOL_CONTROL program.

The third connection is for the CCD sync pulse.   This is a 55 way connection which normally plugs into the instrument sequencer I1 lemo connector and the S2 connector.  In the cass cage, the Lemo connector on the other end of this plugs in to the "sync out" port of the CCD controller to be used.

Starting Up the Software System

Running Taurus and Observer

This requires the following steps:

1.    Log in to AAT40A as OBSERVER

2.    Type the following commands (this assumes that we are using X-terminal aatxtk and CCD controller 1)

xon aatxtk
rvtaurus ccd_1

This will start up the Taurus SMS user interface and an OBSERVER window to control the CCD.

3.    On the Taurus SMS interface enter the startup menu and choose option NONE.

4.    Type the . key on the numeric keypad to get a command prompt. Enter the following command:

load disk$user:[observer.icl_load]pol1

This will position the Taurus wheels with the Wollaston prism in the beam and the polarimetry mask as the aperture, and set the focus.

5.    Log in to AAT40A again as OBSERVER in another window.

6.     Type the command:


where "n" is the running OBSERVER CCD number (1 or 2). This will initialise the instrument sequencer and provide a window with an ICL> prompt from which charge shuffling runs are started.


This program is run from the VAX NIGHT account to accept telescope and polarimeter commands (from the instrument sequencer).  Log into the night account on AAT40A and enter the following commands at the $ prompt.

(NOTE - currently the TELPOL_CONTROL program doesn't work from a terminal window connected using LAT. Make sure you specify the Telnet option (rather than LAT) when you start a terminal window on an X-terminal for this purpose.)

define instseq_sif IRISTEST_SIF:
The TELPOL_CONTROL program will display the string "Ready" when it has successfully started. If it does not appear, then see the section Debugging. The TELPOL_CONTROL program continues to run from this terminal.  To exit it at the end of the night, type EXIT.

This program also accepts direct commands from the terminal to move the polarimeter mechanisms or send commands to the telescope. The most important of these is the CAL command used to insert the polarization calibrators. The options here are:

CAL HN22  - selects a 100% linear polarizer
CAL CIRC    - selects the circular calibrator
CAL CLEAR  - selects clear position - (i.e. normal observing)

It is also possible to move the wave-plate. For example PLATE 45 will move the wave-plate to position 45 degrees.

Telescope or polarimeter commands sent by the instrument sequencer will  be reported on the terminal whilst the program runs. If the correct commands do not appear when doing a charge shuffling run you may need to reset the instrument sequencer (see  Debugging). This may be necessary after the Taurus charge shuffling mode has been used.

The Graphical User Interface

The graphical user interface is run from the solaris system aatssz. Log in on this machine as observer. Then type the command:


The graphical user interface will now come up. It has three tabbed panels which can be used to set up parameters for nod and shuffle runs as well as two types of polarimetry charge shuffling runs.

Checking System Operation

Linear Polarimetry

To verify correct operation of the polarimeter insert the calibration polarizer by typing the command CAL HN22 on the TELPOL_CONTROL terminal.

Then set up the instrument to observe the quartz lamp (On the lamp control unit insert the flap and turn on the quartz lamp. On the telescope console open the central dust cover, and select Main Focus). Take a charge shuffling linear polarization run as described below. The default parameters should be suitable.

The resulting image should look like the one here on the XMEM display. There are four pairs of images of the polarimeter slot, but the top pair has the lower image very bright and the upper one only just visible. The third pair has the bright and dark reversed, whereas the second and fourth pairs have the two images roughly equal. This is the modulation pattern expected for a 100% polarized source

Circular Polarimetry

For circular polarimetry select the circular calibrator (CAL CIRC) and take a run in Circular mode. The two pairs of images should show a large modulation similar to the first and third pairs in the above image.

Taking Data

The Charge Shuffling user interface used to define polarimetry runs

Polarimetry observations are normally taken using CCD Charge Shuffling:

1. Using the graphical interface set up the parameters for your run.

2. Click the Send to Vax button.

3. On the OBSERVER ICL terminal type the command SRUN.

This will start a charge shuffling run with the parameters set up in the GUI. The run takes some time to get going as it has to download information to the CCD micro and the instrument sequencer. Once the run starts you will see messages come out on the TELPOL_CONTROL terminal as the wave-plate is stepped.

If you want to repeat a run with the same parameters just type SRUN again.

Charge Shuffling Observation Parameters

The Polarimetry or Pol/Nod panels on the user interface are used for setting up polarimetry observations. The following parameters have to be set up:

Exposure time - This is the time in seconds spent exposing in each wave-plate position during each cycle.

Wait time - This is the time in seconds to wait for the polarimeter (or telescope in Pol/Nod mode) to settle after each move. A time of 3 seconds should be sufficient for straight polarimetry sequences. If Polarimetry plus Nodding is used the wait time must be large enough to allow the telescope to settle into position which may require a larger value.

Shift - The number of CCD pixels to shuffle by at each step. With the MITLL CCDs 400 pixels is a good number, shuffling far enough to keep the double images of the mask well separated.

Cycles - The number of charge shuffling cycles in the run.

Window - The CCD readout window to use. The MITLL_POLSHUFFLE window is a 1600 by 800 window large enough for four shuffles of the polarimeter field. It also has a 20 column bias region. There is also MITLL_CIRCSHUFFLE which is 800 by 800 large enough for two shuffles, as in circular polarimetry.

Mode -  There are four polarimetry modes, and three polarimetry/nodding modes. Linear takes four half-wave plate positions (0, 22.5, 45, 67.5) giving a full measurement of linear polarization. Circular takes two plate positions giving a circular polarization measurement (note that this requires the quarter-wave plate). The Q only and U only modes use two wave-plate positions measuring a single linear polarization Stokes parameter. In the polarimetry/nodding mode two wave-plate positions and two telescope positions are used giving four shuffle steps in all. There are thus Circular, Q only and U only modes but no Linear mode as this would require eight shuffle steps and there isn't room on the CCD for this.

RA Offset - For the offset mode the RA offset in arc seconds (for polarimetry/nodding modes only).

Dec Offset - For the offset mode the Dec offset in arc seconds (for polarimetry/nodding modes only).

Going back to normal CCD observation

The system returns automatically to the appropriate mode for doing normal CCD observations when the run is complete. Occasionally, due to an error, it may remaining in shuffle mode (indicating by the prompt). In this case, type "METHOD DEFAULT" in the Observer window or at the ICL> prompt.

Aborting Shuffling operations

To abort a shuffle, type ABORT in the Observer window and respond YES to the confirmation prompt. It may take a while for the abort to happen. When it does, you will have to type METHOD DEFAULT in either Observer or ICL window to return to doing normal CCD runs. The ICL sesssion should see the abort and tidy up itself. Beware that ABORTS are not handled well by the underlying CCD controller.   You may in some cases get a system failure and have to to a "RESET HARD" to recover, although recent reports indicate it does normally work.

Another way of taking Charge Shuffling Observations

The Graphical User Interface runs on the Unix machine aatssz whereas the rest of the software runs on the VAX. In the event of a failure of the network communications between these machines it wouldn't be possible to use this method. It is possible to take charge shuffling data using the VAX alone. (see using the NS command and nodshuffle program).


It is recommended that the following set of calibration  data should be taken.

1.    Flat field frames

2.    Observations of a star through the HN22 calibration polarizer giving a 100% polarized source.

3.    Observations of an unpolarized star.

4.    Observations of one or more polarized standard stars. There is a list of polarization standards in the AAO spectropolarimetry manual, a copy of which is kept in the control room. However, only the faintest ones can be observed with Taurus in broad band filters.

For circular polarization observations the CIRC calibrator should be used instead of the HN22. There are no real circular polarization standards, since few sources are circularly polarized and those that are are usually variable. However, there are a number of magnetic white dwarfs and AM Her binaries which have large circular polarizations and might be useful as a check on the operation of the system.

Data Reduction

Imaging polarimetry data can be reduced using the Starlink POLPACK package.

This is an example of a raw CCD image obtained in linear mode for the reflection nebula NGC 6729. POLPACK requires individual images containing the four wave-plate positions, so the charge shuffling data must be split up into its four individual shuffle positions. This can be done using the Figaro command ISUBSET.

The indivdual images should then be bias subtracted and flat-fielded. It is important that the same flat-field is used for the four wave-plate positions to avoid introducing any bias into the polarization.

To prepare data files for use with POLPACK the POLEXT command is used to set some items in an extension in the data files. The most important thing here is the WPLATE item which sets the wave-plate angle in degrees. For example:

polext file0 wplate=45 filter=V

would set the WPLATE item to 45 and the FILTER item to V in the data file file0.sdf.

A set of four or more such files covering the four plate positions can then be processed by the POLKA application. This performs extraction of the O and E images, alignment of the images, sky subtraction and generation of the Stokes parameters. POLKA is an interactive application with its own graphical user interface and a tutorial explaining its use.

The Stokes cube produced by POLKA can be converted to a catalogue of polarization vectors using the application POLVEC. This catalogue can be plotted as a polarization map by POLPLOT.

The application POLBIN can be used to bin the data in a polarization catalogue, and selections
from the catalogue can be made with the application CATSELECT from the CURSA package.

Below is an example of the reduced data obtained using POLPACK on the single CCD frame
shown above, for the NGC 6729 reflection nebula (illuminated by the pre-main-sequence star R CrA)

For more detail see the comprehensive POLPACK Users' Manual.

Taking Charge Shuffling Data using the NS command and nodshuffle program

You shouldn't normally need to use this method, as the graphical user interface is a much easier way of starting charge shuffling runs. However, it may occasionally be useful in the event of a breakdown in network communications between the VAX and the Unis systems, as this method can be run entirely from the VAX.

In this mode a CSX file is generated using the program nodshuffle. See the "Generating CSX files"  section below.  When you do this there are two values output which you need to record as you will need them later. The items are

To run a nod and shuffle operation, you use the "NS" command at the ICL> prompt. This will prompt you for various values, being

TRUE/FALSE Is the run to be a bias RUN.  Defaults to FALSE.
CSX Filename
Here you specify the name of your charge shuffling description file.   See "Generating CSX files"  below.  The default location is  DISK$USER:[OBSERVER.CS_FILES] and the default file type is "CSX".
Timer Resolution
Used to determine the resoultion of the exposure and other timers in the CCD controller.  This is required as the range of the timers are much more limited then the range you may desire.  A common value here is "3", which indicates a resolution of 1 milli-second but limits you to an exposure plus an offset time of about 65 seconds. See "Generating CSX files" below.
External Delay
This is the amount of time the system must wait  for the external device (telescope to offset).  This value is given in terms of the timer resolution.  See "Generating CSX files" below
Cycle count
The number of observation cycles.  1 to 65535.
Use Axes
This should always be False for polarimetry runs.
RA Offset
        The RA offset - you have to specify this and dec offset even if the run does not involve nodding
Dec Offset
The Dec offset - One of the offsets must be non-zero or it will be rejected.
The window to use for reading out the detector. The system will save the existing window before specifying this one. IT will then restore the original window when the run is complete.
Finally, the system will output a summary and prompt you for confirmation. If you confirm, the run is started.  Note, it can take a while to start a charge shuffling run, but you should see messages every few seconds during this time.

Generating CSX files

CSX files can be generated from the graphical user interface by using the Save As... option.

It is also possible to generate CSX files using a program which runs on the VAX.   This program is run from the OBSERVER account using the NODSHUFFLE command.

Command options are

If specified, we are using the axes.
        Specifies a linear polarization run with four wave plate positions (0, 22.5, 45, 67.5)
        Specifies a polarization run with two wave plate positions (0, 45)
        Specifies a polarization run with two wave plate positions (22.5, 67.5)
        Specifies a circular polarization run with two wave plate positions
        Specifies a polarization/nodding run with two plate positions (0, 45) and two telescope positions
        Specifies a polarization/nodding run with two plate positions (22.5, 67.5) and two telescope positions
        Specifies a circular polarization/nodding run with two plate positions and two telescope positions
-offtime n
Specifies the offset time in seconds (real number)
-exptime n
Specifies the base exposure time in seconds (real number)
-offexptime n
Offset exposure time in seconds (real number)
Shutter time in seconds (real number)
-shift n
Rows to shift, (integer)
-output file
Specify output file. The default is standard output



Only one of the mode flags (axes,lpol,qpol,upol,cpol,qnod,unod,cnod) should be specified. If no mode is
specified offset mode is used.

The program writes to STDOUT the charge shuffling file.  It writes to STDERR the timer resolution index value and the external device delay value. For example, assume an axes mode operation, exposure time of 10 seconds in both positions, offset time of 2 seconds and shift of 300 rows.  The command is as follows

nodshuffle -axes -exptime 10 -offtime 2 -shift 300
This produces
The timer resolution value is "3"
The external device delay is "2000"
PR 0,65535,10000,12100,1,300,0,0,1
PR 0,65535,10000,12100,65535,300,0,0,2
PE 0,65535,1,2100,0,65535,0,0,1
PE 0,65535,1,100,0,65535,0,0,0
To catch the charge shuffling file in a disk file, you need to redirect STDOUT.  E.g
define/user_mode sys$output myfile.csx
nodshuffle -axes -exptime 10 -offtime 2 -shift 300
The timer resolution value is "3"
The external device delay is "2000"
And the file myfile.csx  which contains
PR 0,65535,10000,12100,1,300,0,0,1
PR 0,65535,10000,12100,65535,300,0,0,2
PE 0,65535,1,2100,0,65535,0,0,1
PE 0,65535,1,100,0,65535,0,0,0


The OBSERVER CCD software works as normal, and it is suggested that you first do a normal CCD GLANCE run to check the CCD system is working.

If the instrument sequencer task is not powered up or not communicating with the VAX, then you should expect the following just after entering the TDFNOD command

INSTSEQ initialising
!! No error to report (improper use of EMS)
!! OBEYW error from task INSTSEQ - action INITIALISE TAURUS_CCD
ADAMERR   %SYSTEM, device timeout
In Procedure: NS_LOAD
If you get this error, physically check the instrument sequencer micro. Ensure there are three lights down the left hand side (+5V, +12V and -12V). Ensure is is cabled correctly (see "Hardware Prepration" section). Hit the reset button.

Exit ICL and then start it again (you need only type ICL this time, although doing the full "TDFNOD 1" command does no harm)
If the system still does not start up then you may need to reset the terminal server (AAOD2Q) being used by the instrument sequencer before trying again. (Wait 20 seconds after power cycling the terminal server before trying again) In this case, you will also need to EXIT and restart the TELPOL_CONTROL program since it uses the same terminal server.


If the "Ready" string does not appear from TELPOL_CONTROL, then reset the terminal server (AAOD2Q) and go through the sequence described in the previous section for checking and restarting the instrument sequencer (since it uses the same terminal server) before restarting TELPOL_CONTROL

Jeremy Bailey