IRAF Help page for: XOFFSET


xoffset (Jun01)                                     pipeline                                         xoffset (Jun01)

NAME
    xoffset  -  Performs  angular  cross-correlations  between  pairs of
    simultaneous velocity images (one being a "test"  image,  the  other
    being  a "reference" image). The output is an ASCII table giving the
    angular orientation of the test image,  under  the  assumption  that
    the orientation of the reference image is known.
   
    NOTE:  XOFFSET  was designed as one component of COPIPE, however, it
    can be used as a stand-alone routine.
   
   
USAGE
    xoffset input reference
   
   
PARAMETERS
   
    input
        List of test images having unknown camera offset angle
   
    reference
        List of reference images having (presumed) known  camera  offset
        angle
   
    geometry = "hgeom"
        Tells  the  code  which  geometry  information  it  should  use:
        fndlmb|hgeom
       
        If geometry =  fndlmb,  the  routine  will  read  the  following
        keywords from each image header:
       
           FNDLMBXC - the x-coordinate of the solar image center
           FNDLMBYC - the y-coordinate of the solar image center
           FNDLMBAN - the position angle (in degrees)
           FNDLMBMA - estimate for the semi-major axis in pixels
           FNDLMBMI - estimate for the semi-minor axis in pixels
           PIXLENX  - the pixel length along the x-direction
           PIXLENY  - the pixel length along the y-direction
       
        If  geometry  =  hgeom,  the routine will use the values of C_MA
        and C_MI  (as  determined  by  HGEOM)  for  the  semi-major  and
        semi-minor axes
   
    ref_offset = 90.
        The  camera  offset  angle  (degrees) of the reference images if
        not in header.
       
        The routine will attempt to read the value of the keyword OFFSET
        from  each  reference  image  header.   If this keyword does not
        exist,  the  angle  set  by  ref_offset  will  be  used  as  the
        default.   NOTE:  the  angle OFFSET is given in degrees measured
        clock-wise from the y-axis and represents our best  estimate  of
        the direction to Solar North.
   
    direction = "clock"
        Direction of positive rotation (clock|counter)
        i.e.,
       
            clock   => clock-wise
            counter => counter clock-wise
   
    nbins = 1024
        Number of angular bins (must be even)
       
        The  image  will  be  devided  into  nannuli  equal-area  annuli
        starting at R/Ro = rmin, out to R/Ro =  1.0.   Each  annulus  is
        then  divided  into  nbins  angular  bins,  or sectors.  Because
        FFT's are used to compute  the  cross-correlations  between  the
        annulus  data from each pair of input images, the number of bins
        must be even (a power of 2 is the most efficient).
       
   
    fint_fact = 1
        Fourier interpolate cross-correlation by 2**fint_fact
       
        One can obtain slightly higher precision  when  determining  the
        shift   of   the  cross-correlation  spectra  by  using  Fourier
        interpolation. This keyword allows the user to  set  the  amount
        of interpolation used.
   
    nannuli = 12
        Number of annuli to cross-correlate
   
    rmin = 0.7
        Fractional radius at inner edge of inner-most annulus
   
    width = 5.
        Width  of  the  zone  to  search for the correlation peak around
        lag=0 (degrees)
       
        Once the cross-correlation between two annuli from each pair  of
        images  is computed, we need to find the peak in the correlation
        spectrum.  To speed things up slightly, the  region  over  which
        the  code searches for the correlation peak is restricted to +/-
        width degrees around lag zero.
   
    updhdr = no
        Update input image headers with new offset angle
   
    output = yes
        Create output file?
   
    outname =
        Output data file name.
   
    compress = "gzip"
        Compression type |none|gzip|compress|
       
        Are the input images compressed, and what  type  of  compression
        was used?
   
    use_mrhiy = yes
        Use mrhiy file for initial guess?
       
        The  reference  images may not have the most recently determined
        offset values in their  headers.   By  using  the  latest  mrhiy
        file,  the  code  can  determine  compute the appropriate camera
        offsets without having to actually update  the  reference  image
        headers  (which  could  be  time  consuming  if  the  images are
        compressed).
   
    mrhiy_tbl = "mrhiy010730"
        Full path & name of mrhiy file
       
        Where to find the latest mrhiy file (full path and name)
   
    use_pm = no
        Use recent pm/drift scan data?
       
        This switch tells the  code  to  check  the  pm_info  file  (see
        below) for any recent PM events at the reference site.
   
    pm_info = "home$drift/drift.dat"
        Full path & name of file with recent drift scan results
   
    verbose = no
        Operate in verbose mode?



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