AIPS NRAO AIPS HELP file for FARS in 31DEC18



As of Sun Jul 22 6:42:08 2018


FARS: Task to create Faraday rotation measure images

INPUTS

INNAME                             First image name Qpol
INCLASS                            First image class
INSEQ           0.0      9999.0    First image seq. #
INDISK          0.0         9.0    First image disk drive #
IN2NAME                            Second image name Upol
IN2CLASS                           Second image class
IN2SEQ          0.0      9999.0    Second image seq. #
IN2DISK         0.0         9.0    Second image disk drive #
OUTNAME                            First output image name
OUTSEQ         -1.0      9999.0    First output image seq. #
OUTDISK         0.0         9.0    First output image disk  #
INFILE                             Input file of weights
DOALIGN        -2.0         1.0    Should images be coincident?
                                   (See HELP.)
BLC             0.0      4096.0    Bottom left corner
TRC             0.0      4096.0    Top right corner
                                   See help for the 1st axis
                                   (must be frequencies!!!)
APARM                              Parameters for algorithm:
                                     1 number of pixels at
                                       half of the Fourier
                                       transform output
                                       The whole number is
                                         2*APARM(1)+1
                                     2 cell size in 1/m^2
                                       0 =>
                                       PI/(4*(Lmax^2-Lmin^2))
                                       Lmax,Lmin-max,min lambda
                                       at the data
                                     3 0 => regular output
                                       1 => output is RMTF
                                     4 0=> CLEANed Fourier
                                           transform
                                       1=> unCLEANed Fourier
                                           transform
                                     5 0=>original(shifted back)
                                          RE/IM are sent out
                                       1=>the shifted RE/IM are
                                           sent out
                                       2=>amplitude and phase of
                                          the data are sent out
                                     6 is not used
                                     7 0=> convolve the clean
                                       components
                                       1=> no convolve
                                     8 0=> use the Gaussian as
                                           the convolve function
                                       1=> use the Re of RMTF as
                                           the convolve function
                                     9 full width of Gaussian
                                       convolve function, at 0.5
                                       level, in 1/m^2
                                       0 => fit to real RMTF
                                    10 what send to output?
                                       0 => sum of CLEAN and
                                            residual
                                       1 => CLEAN result
                                       2 => residual
GAIN                               Gain in the CLEAN
NITER                              Maximum number of clean
                                   components
FLUX                               Minimum flux of clean
                                   component (Jy)
DOHIST         -3.0         1.0    -2 => copy 1st HI only
                                   -3 => copy no HI files
PRTLEV                             Number of Cleans to print

HELP SECTION

FARS
Type: Task

      The measured brightness of images at a set of lambda^2 is
      related to the brightness distribution as a function of Faraday
      rotation depth through the Fourier transform.  To solve the
      inverse problem: "Evaluation of the brightness distribution as a
      function of Faraday rotation depth using the measured brightness
      in the given set of lambda^2" has been called "Faraday Rotation
      Synthesis".  The task FARS performs this operation.

      FARS now writes 4 slice files in each output image.  They
      reresent the real part, imaginary part, amplitude, and phase of
      the RMTF or dirty beam of the synthesis.  TVSLICE and other
      slice functions may be used to examine these parts of the beam.
      The units displayed are based on the image and so they do not
      really apply.  The phase slice is in degrees and the others
      really in normalized beam units.

      The RUN file named DOFARS defines a procedure that will do the
      initial transposes and find the weights text file, run FARS,
      delete the transposed input cubes, and transpose the output
      cubes to yield 4 output cubes in all.
Adverbs:
  INNAME......First image name. Q-polarization.  Standard defaults.
              In transposed order with frequency (or frequncy ID) as
              the first axis.
  INCLASS.....First image class.    Standard defaults.
  INSEQ.......First image seq. #.   0 => highest.
  INDISK......Disk drive # for the first image.  0 => any.
  IN2NAME.....Second image name. U-polarization. Standard defaults.
              In transposed order with frequency (or frequncy ID) as
              the first axis.
  IN2CLASS....Second image class.   Standard defaults.
  IN2SEQ......Second image seq. #.  0 => highest.
  IN2DISK.....Disk drive # for the second image.  0 => any.
  OUTNAME.....Output image name. Real part.  Standard defaults.
              Output image class for first image 'FARSre' or 'FARSam'
              Output image class for second image 'FARSim' or 'FARSph'
  OUTSEQ......Output image seq. #.  0 => highest unique.
  OUTDISK.....Output disk number. 0 => highest with space.
  INFILE......User-supplied text file defining weights for ALL
              spectral channels INCLUDING those < BLC(1).  The
              sequence of the weights must correspond to the sequence
              of frequencies in the input cubes. The weights are read
              with a free format and more than one may occur on each
              line of the input text file.  The weights are read as
              floating-point values and any value >= 0 is acceptable.
  DOALIGN.....Controls how the two input images are to be aligned
              True (>.1) means that the images must agree in
              their coordinates, though not necessarily in the reference
              pixel position.  Alignment is by coordinate values (if
              DOALIGN > -0.1) or by offsets from the reference pixel
              positions (if DOALIGN <= -0.1).  NOTE: all real axes (>1
              point) are aligned.  If DOALIGN = -2, the headers are
              ignored and the images are aligned at pixel (1,1,...).
              (see HELP DOALIGN).
  BLC.........Bottom left corner of the 1st input image. The other
              images are aligned by coordinates (see DOALIGN) on all
              axes having > 1 point.  The second image may have fewer
              real axes than the 1st.  The 2 windows must have the same
              dimension on the first 2 axes, but the task will select a
              smaller window than was specified if needed to overlap the
              2 images.
              The first axis (must be frequency. Use TRANS if not!!!)
              is controlled by BLC, TRC only for input data. The
              output size for the first axis is controlled by
              APARM(1,2,4)
  TRC.........Top right corner of input images. (See BLC.)
  APARM.......Parameters needed for algorithm:
               APARM(1) number of pixels for half of the Fourier
                        transform output.  The total number is
                        2*APARM(1)+1 with coordinate zero at the center.
                        The value of APARM(1) should be chosen in
                        accordance the expected range of Faraday
                        rotation measure and value of CELL (APARM(2))
               APARM(2) cell size of the outputs, in 1/m^2
                        The cell is recomended to be less or around
                        the default value
                            (pi)/4/(lambda^2max - lambda^2min)
               APARM(3) 0 => regular outputs
                        1 => outputs are RMTF ("1-D dirty beam")
                             Note that the RMTF is now written as
                             slice files so this option is normally
                             not needed.
               APARM(4) 0=> CLEANed Fourier transform, using inputs:
                            NITER, FLUX, GAIN. The CLEAN uses the
                            shifed (at lambda^2) data but the cleaned
                            components correspond to the original
                            lambda^2
                        1=> the uncleaned Fourier transform is recorded
                            RE/amp is recorded to the first output
                            IM/phase is recorded to the second output
               APARM(5) 0=>original data (RE and IM) are used in the
                            Fourier transform
                        1=> the shifted (to the center) data (RE, IM)
                            are used in the Fourier transform.
                            This option allows better discrimination
                            of different features in the Fourier
                            transform.
                        2=> amplitude and phase of the data are sent
                            out.
               APARM(6) is not used
               APARM(7) 0=> convolve the clean components
                        1 => no convolution; so just the set of the
                             clean components is sent to the output files
                        No convolution is forced if uncleaned Fourier
                        data are sent out (APARM(4)=1)
               APARM(8) 0=> use the Gaussian as the convolution
                            function
                        1=> use the Re of RMTF as the convolution
                            function
               APARM(9) full width at half maximumof the Gaussian
                        convolution function, in 1/m^2
                        0 -> fit the real part of the RMTF.
               APARM(10) What send to the output?
                         0 => sum of CLEAN and  residual
                         1 => CLEAN result
                         2 => residual
  GAIN........Gain in the CLEAN
  NITER.......Maximum number of clean components 0 => 1
  FLUX........Minimum Clean component (Jy)
                 The task can subtract the given number of complex
                 components. On each iteration, the maximum (and its
                 position) of the spectrum amplitude is determined.
                 The complex function RMTF is multiplied by the
                 complex value of the spectrum at the position of the
                 amplitude maximum and by GAIN.  This product function
                 is shifted to the position of the maximu and
                 subtracted from the current residual spectrum.  This
                 subtraction process is terminated when the number of
                 iterations reaches NITER or when the maximum found is
                 less than FLUX.
  DOHIST......Normally the HI file of input 1 is copied to the output
              history file and the HI file of the second input is
              appended.  The history of the Q and U cubes may be very
              similar, so to avoid having the file grow exponentially
              DOHIST = -2 => copy the first HI file only.
              DOHIST = -3 => copy no HI file, write FARS HI only.
  PRTLEV......> 0 => Number of Cleans to print.  <= 0 -> none.

EXPLAIN SECTION

FARS: Task to solve Faraday Rotation Synthesys problem
DOCUMENTOR: L. Kogan
RELATED PROGRAMS:

      The main ideas of the algorithm are  described by
      B.J. Burn at:
      MNRAS, 133, 67, 1966 and

      M.A.Brentjens
      and A.G.de Bruyn at:
      Astronomy & Astrophysics, 441, 1217-1228(2005)

      FARS reads the two input cube images which should have identical
      structure.  The first axis of the cubes must be FREQ or FQID.
      If not, the task 'TRANS' should be used to achieve this order.
      The first cube should correspond to the Q-linear polarization
      component.  The second cube should correspond to the U-linear
      polarization component.  The purpose of the task is to get the
      brightness distribution (for each pixel in the RA-DEC plane)
      corresponding to the given Fraday rotation depth.
      Multi-frequency observations are required to achieve this.

      The task forms complex function of wavelength squared (the first
      input image is the real part, the second input is the imaginary
      part).  For each pixel of the RA-DEC plane, FARS carries out the
      Fourier transform along the wavelength squared axis.  This
      Fourier transform converts the observed complex brightness as a
      function of the lambda^2 to the output complex function of the
      Faraday rotation depth. The Fourier transform can be directed to
      the outputs directly or it can be deconvolved with a complex
      Clean algorithm. The real part or amplitude of the output is
      recorded into the first output file and the imaginary part or
      phase of this output is recorded in the second output file. The
      input data are determined from the positive set of lambda^2. The
      output of the Fourier transform (complex function) is the
      convolution of the actual brightness (as a function of the
      Faraday depth) and the so called Rotation Measure Transfer
      Function (RMTF). The RMTF is a one dimensional Fourier transform
      of the lambda^2 sampling function and is recorded in slice files
      attached to the output images.  The RMTF is similar to the dirty
      beam in aperture synthesis.  The dirty beam is real function
      because the brightness distribution is a real function.  RMTF is
      a complex function, but it's imaginary part can be zeroed at the
      vicinity of zero Faraday depth if we shift the origin of the
      lambda^2 set to the median.  The more homogeneous the
      distribution of the lambda^2 the wider the area where the
      imaginary part of RMTF is near zero.  FARS carries out all
      calculation with the centered data, and then rotates the outputs
      by the phase corresponded to the central lambda^2. The output
      (RE, IM) can be shifted back to the original lambda^2
      distribution or not (or amplitude and phase can be sent to the
      output) under control of APARM(5).

      It is supposed the images for each of the frequencies are available
      The example of the typical image header is following:

AIPS 1: ----------------------------------------------------------------
AIPS 1: Type    Pixels   Coord value     at Pixel     Coord incr   Rotat
AIPS 1: RA---SIN  2048    12 28 16.418    1024.00      -0.100000    0.00
AIPS 1: DEC--SIN  1024    12 39 58.294     513.00       0.100000    0.00
AIPS 1: FREQ         1   8.0851000E+09       1.00  5.0000000E+07    0.00
AIPS 1: STOKES       1   2.0000000E+00       1.00  1.0000000E+00    0.00

      The images for different frequencies should be combined in the cube
      using the AIPS task MCUBE. The recommended inputs for MCUBE are
      following:
AIPS 1: MCUBE:  Task to collect a set of n-dim maps into a (n+1)-dim map
AIPS 1: Adverbs     Values                 Comments
AIPS 1: ----------------------------------------------------------------
AIPS 1: INNAME     'M87X'                  Input name(name).
AIPS 1: INCLASS    'Q-COS'                 Input name(class).
AIPS 1: INSEQ         1                    Input name(seq. #). 0=>high
AIPS 1:                                    First sequence # in the set
AIPS 1: INDISK        3                    Input disk drive #. 0=>any
AIPS 1: IN2SEQ        4                    Last sequence # in set.
AIPS 1: IN3SEQ        1                    Sequence # increment.
AIPS 1: OUTNAME    'M87XCUBE'              Output name(name).
AIPS 1: OUTCLASS   'Q-COS'                 Output name(class).
AIPS 1: OUTSEQ        0                    Output name(seq. #).
AIPS 1:                                      0 => highest unique
AIPS 1: OUTDISK       3                    Output image disk drive #
AIPS 1:                                      0 => highest with space
AIPS 1: DOALIGN       0                    Alignment control parm
AIPS 1: AXREF         1                    n+1 axis pixel of map INSEQ.
AIPS 1:                                    0 => 1
AIPS 1: AX2REF        0                    n+1 axis pixel of map IN2SEQ
AIPS 1:                                    0 => opposite end from INSEQ
AIPS 1: NPOINTS       4                    Number of pixels on axis n+1
AIPS 1: DOCONCAT      2                    > 1 => make a SEQ.NUM or
AIPS 1:
      NPOINTS is the number of combined images, which should be prepared
      under the same NAME and CLASS but with different INSEQ=1,2,3,4
      Use task FQUBE is the images are not distributed linearly in
      frequency.  This task adds an FQ table to the output image
      listing the actual frequency of each plane in its cube.

      The output cube image should have the following axes in the image
      header:

AIPS 1: ----------------------------------------------------------------
AIPS 1: Type    Pixels   Coord value     at Pixel     Coord incr   Rotat
AIPS 1: RA---SIN  2048    12 28 16.418    1024.00      -0.100000    0.00
AIPS 1: DEC--SIN  1024    12 39 58.294     513.00       0.100000    0.00
AIPS 1: FQID         4   1.0000000E+00       1.00  1.0000000E+00    0.00
AIPS 1: STOKES       1   2.0000000E+00       1.00  1.0000000E+00    0.00
AIPS 1: FREQ         1   8.0851000E+09       1.00  5.0000000E+07    0.00
AIPS 1: ----------------------------------------------------------------

The axis 'FQID' reports the number of frequencies at the cube.
The axis 'FREQ' reports the reference frequency.
The actual frequencies are the sum of the reference frequency and
the offsets given in the FQ table created by MCUBE or FQUBE.

FARS requires another order of the axes.  So the AIPS task TRANS should
be applied to get it.  An example of TRANS inputs is:

AIPS 1: TRANS:  Task to transpose a subimage of an up to 7-dim. image
AIPS 1: Adverbs     Values                 Comments
AIPS 1: ----------------------------------------------------------------
AIPS 1: INNAME     'M87XCUBE'              Input name(name).
AIPS 1: INCLASS    'Q-COS'                 Input name(class).
AIPS 1: INSEQ         1                    Input name(seq. #). 0=>high
AIPS 1: INDISK        3                    Input disk drive #. 0=>any
AIPS 1: OUTNAME    'M87XTRANS'             Output name(name).
AIPS 1: OUTCLASS   'Q-COS'                 Output name(class).
AIPS 1: OUTSEQ        0                    Output name(seq. #).
AIPS 1:                                      0 => highest unique
AIPS 1: OUTDISK       3                    Output image disk drive #
AIPS 1:                                      0 => highest with room
AIPS 1: BLC        *all 0                  Bottom left corner of image
AIPS 1:                                      0 => entire image
AIPS 1: TRC        *all 0                  Top right corner of image
AIPS 1:                                      0 => entire image
AIPS 1: TRANSCOD   '31245'                 New axis order in terms of
AIPS 1:                                    input axis numbers
AIPS 1: BADDISK    *all 0                  Disks to avoid for scratch

So the final image cube should have the following axes in the image
      header:

AIPS 1: ----------------------------------------------------------------
AIPS 1: Type    Pixels   Coord value     at Pixel     Coord incr   Rotat
AIPS 1: FQID         4   1.0000000E+00       1.00  1.0000000E+00    0.00
AIPS 1: RA---SIN  2048    12 28 16.418    1024.00      -0.100000    0.00
AIPS 1: DEC--SIN  1024    12 39 58.294     513.00       0.100000    0.00
AIPS 1: STOKES       1   2.0000000E+00       1.00  1.0000000E+00    0.00
AIPS 1: FREQ         1   8.0851000E+09       1.00  5.0000000E+07    0.00
AIPS 1: ----------------------------------------------------------------

The same set of steps should be carried out to get the second linear
polarization input image for FARS.

AIPS