AIPS NRAO AIPS HELP file for FARS in 31DEC18



As of Mon Jan 22 21:08:25 2018


FARS: Task to create Faraday rotation measure image

INPUTS

INNAME                             First image name
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
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 at the given set of lambda^2 is related
      with the brightness distribution as a function of Faraday
      rotation depth through the Fourier transform.
      The solve of the inverse problem: "Evaluation of the brightness
      distribution as a function of Faraday rotation depth using the
      measured brightness at the given set of lambda^2 "
      got the name "Faraday Rotation Synthesis".
      The task FARS carries out solution of this problem.

      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.
  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.
  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......Name of the user-supplied file to defining weights
              of the different frequencies data. The sequence of the
              weights must correspond to the sequence of frequencies
              at the input cubes. The weights has to be arranged
              in rows of 80 byte long. The weights should be separated
              any number of blanks (1 is preferable).
              Each weight must be represented by integer between 0 and 99.
              Although any integer is accepted.
  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 data 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 at half of the Fourier
                        transform output
                        The whole number is 2*APARM(1)+1
                        with zero at the center.
                        The value of APARM(1) should be chosen in accordance
                        of expected range of the Faraday rotation measures
                        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

               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
                            RE is recorded to the first output
                            IM is recorded to the second output
                        1=> the uncleaned Fourier transform is recorded
                            RE is recorded to the first output
                            IM is recorded to the second output
               APARM(5) 0=>original  data (RE and IM) are used at the
                           Fourier transform
                        1=> the shifted (to the center) data (RE, IM)
                            are used at the Fourier transform.
                            This option allows better discrimination
                            of different features at 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 convolve; So just the set of the clean
                             components is sent to the output files
                        No convolution is forced if:
                             uncleaned Fourier is sent out (APARM(4)=1)
               APARM(8) 0=> use the Gaussian as the convolve function
                        1=> use the Re of RMTF as the convolve function
               APARM(9) full width of the Gaussian convolve function,
                        at the 0.5 level, 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
                 found amplitude maximum and by GAIN.
                 The found function(production) is put by its median
                 on the position of found amplitude maximum, and is
                 subtracted from the having evaluated spectrum. The
                 process of the subtraction is terminated having
                 achieved number of iterations NITER or the flux 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 read the two input cube images which should have identical
      structure. The first axis of the cubes must be FREQ!!!!!!
      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 at RA-DEC plane) corresponded to the given Fraday
      rotation depth. The multi frequency observations are required to get
      this purpose.

      The task forms complex function of wavelength squares
      (the first input is real part, the second input is imaginary part)
      For each pixel at the image plane, FARS carries out the Fourier
      transform along the frequency (lambda square) 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 being cleaned. The real part of the
      output is recorded into
      the first output file and the imaginary part of this output is
      recorded in the second output file. The input data are determined
      at 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 one
      dimensional Fourier transform of the lambda^2 sampling function.
      RMTF is similar to dirty beam at the image 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 distribution of the lambda^2 is homogeneous the wider
      is area where imaginary part of RMTF is equal 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 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 (or better MQUBE). 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        0                    Last sequence # in set.
AIPS 1: IN3SEQ        0                    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

      The output cube image should have the following axes at 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 ofcets given at the FQ table created by MCUBE.

FARS requires another sequence of axes!!! So the AIPS task TRANS should
be applied to get it.
The example of TRANS inputs is following:

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 at 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