Content-type: text/html Manpage of invert

invert

Section: User Commands (1)
Index Return to Main Contents
 

NAME

invert - Transform multi-pointing visibility data into a map  

PERSON RESPONSIBLE

rjs  

CATEGORIES

map making  

DESCRIPTION

INVERT is a MIRIAD task that forms images from visibilities. INVERT can form continuum images or spectral line cubes. It can generate images/cubes for several polarisations, as well as handling multi-frequency synthesis and mosaicing observations. INVERT can also form complex-valued images from non-Hermitian data (e.g. holography data). Appropriate point-spread functions can also be generated.  

PARAMETERS

vis
Input visibility data files. Several files can be given. No default.
map
Output map (image) file name. Each output file consists of a single polarization/Stokes parameter. If several different pols/Stokes images are being made, then several file names should be given. No default.
beam
Output beam (point-spread function) file name. The default is not to make a beam.
imsize
The size of the output dataset. The default is to image out to primary beam half power points. For options=mosaic, an image of this size is made for each pointing before a linear mosaic operation is performed.
cell
Image cell size, in arcsec. If two values are given, they give the RA and DEC cell sizes. If only one value is given, the cells are made square. The default is about one third of the resolution of the resultant images.
offset
When not mosaicing, this gives the sky position to shift to the center of the output images. The position is specified as an offset (in arcsec) from the observing center. The default is to perform no shifting.

When mosaicing, this gives the sky coordinate (RA and DEC) of the reference pixel in the imaging process. The value can be given in the form hh:mm:ss,dd:mm:ss, or as decimal hours and degrees. INVERT applies appropriate shifts to make this location fall on a pixel. The default is a central observing center.

fwhm
This determines a gaussian taper to apply to the visibility data. It specifies the FWHM of an image-domain gaussian -- tapering the visibility data is equivalent to convolving with this image-domain gaussian.

Either one or two values can be given, in arcsec, being the FWHM in the RA and DEC directions. If only one value is given, the taper is assumed to be symmetric. The default is no taper.

The signal-to-noise ratio will be optimised in the output image if this parameter is set to the FWHM of typical image features of interest.

If you are more accustomed to giving this parameter in the uv plane (as AIPS requires), then:

  fwhm(image plane) = 182 / fwhm(uv plane)
where the image plane fwhm is measured in arcseconds, and the uv plane fwhm is measured in kilowavelengths.
sup
Sidelobe suppression area, given in arcseconds. This parameter gives the area around a source where INVERT attempts to suppress sidelobes. Two values (for the RA and DEC directions respectively) can be given. If only one value is given, the suppression area is made square. The default is to suppress sidelobes in an area as large as the field being mapped.

The suppression area is essentially an alternate way of specifying the weighting scheme being used. Suppressing sidelobes in the entire field corresponds to uniform weighting (so the default corresponds to uniform weighting). Natural weighting gives the best signal to noise ratio, at the expense of no sidelobe suppression. Natural weighting corresponds to SUP=0. Values between these extremes give a tradeoff between signal to noise and sidelobe suppression, and roughly correspond to AIPS "super-uniform" weighting.

robust
Brigg's visibility weighting robustness parameter. This parameter can be used to down-weight excessive weight being given to visibilities in relatively sparsely filled regions of the $u-v$ plane. Most useful settings are in the range [-2,2], with values less than -2 corresponding to very little down- weighting, and values greater than +2 reducing the weighting to natural weighting.

Sidelobe levels and beam-shape degrade with increasing values of robustness, but the theoretical noise level will also decrease.

The default is no down-weighting (robust=-infinity).

line
Standard "line" parameter, with the normal defaults. In particular, the default is to image all channels. See the help on "line" for more information. The "line" parameter consists of a string followed by up to four numbers, viz:

  linetype,nchan,start,width,step

where 'linetype' is one of "channel", "wide", "velocity" or "felocity".

ref
Line type of the reference channel, specified in a similar to the "line" parameter. Specifically, it is in the form:
  linetype,start,width
Before mapping, the visibility data are divided by the reference channel. The default is no reference channel.
select
This allows a subset of the uv data to be used in the mapping process. See the Users Manual for information on how to specify this parameter. The default is to use all data.
stokes
Standard polarisation/Stokes parameter selection. See the help on "stokes" for more information. Several polarisations can be given. The default is "ii" (i.e. Stokes-I, given the assumption that the source is unpolarised).
options
This gives extra processing options. Several options can be given (abbreviated to uniqueness), and separated by commas:
  nocal     Do not apply gains table calibration to the data.
  nopol     Do not apply polarisation leakage corrections.
  nopass    Do not apply bandpass table calibration to the data.
  double    Normally INVERT makes the beam patterns the same
            size as the output image.  This option causes the
            beam patterns to be twice as large.
  systemp   Weight each visibility in inverse proportion to the
            noise variance.  Normally visibilities are weighted
            in proportion to integration time.  Weighting based
            on the noise variance optimises the signal-to-noise
            ratio (provided the measures of the system
            temperature are reliable!).
  mfs       Perform multi-frequency synthesis.  The causes all
            the channel data to be used in forming a single map.
            The frequency dependence of the uv coordinate is
            thus used to give better uv coverage and/or avoid
            frequency smearing.  For this option to produce
            useful maps, the intensity change over the frequency
            band must be small.  Set the 'line' parameter to
            select the channels that you wish to grid.
  sdb       Generate the spectral dirty beam as well as the
            normal beam, when MFS processing.  The default is
            only to create the normal beam.  If the spectral
            dirty beam is created, this is saved as an extra
            plane in the beam dataset.
  mosaic    Process multiple pointings, and generate a linear
            mosaic of these pointings.
  imaginary Make imaginary image for non-Hermitian data
            (holography).
  amplitude Produce a image using the data amplitudes only.  The
            phases of the data are set to zero.
  phase     Produce an image using the data phase only.  The
            amplitudes of the data are set to 1.
  sin       Label the output map and beam as a SIN projection.
            Default is NCP unless non-east-west baselines are
            present or the field centre is within 3 deg of the
            celestial equator (because NCP blows up near the
            equator).  Note that this option simply changes
            ctype1 and ctype2 in the header, the translation
            only being correct to first order about the field
            centre.  A similar result could be obtained by
            running 'puthd' on the output map, e.g.
              puthd in=<map>/ctype1 value=RA---SIN
              puthd in=<map>/ctype2 value=DEC--SIN
            and likewise for the beam.
mode
This determines the algorithm to be used in imaging. Possible values are:
  fft    The conventional grid-and-FFT approach.  This is the
         default and by far the fastest.
  dft    Use a discrete Fourier transform.  This avoids aliasing
         but at a hugh time penalty.
  median This uses a median approach.  This is generally robust
         to bad data and sidelobes, has a even larger time
         penalty and produces images that cannot be deconvolved.
NOTE: Dft and median modes are not supported with options=mosaic.
slop
NOTE: This parameter should be used with caution! See the Users Guide for more information on its applicability.

When forming spectral cubes, INVERT normally insists that all channels in a given visibility spectrum must be good before accepting the spectrum for imaging. This keyword allows this rule to be relaxed. It consists of two parts: a tolerance and a method for replacing the bad channels.

The tolerance is a value between 0 and 1, giving the fraction of channels that INVERT will tolerate as being bad before the spectrum is totally discarded. The default is 0, indicating that INVERT will not tolerate any bad channels. A value of 1 indicates that INVERT will accept a spectrum as long as there is at least one good channel.

The replacement method is either the value `zero' or replaced with 0, or to be estimated by linear interpolation of two adjacent good channels. See the Users Guide for the merits and evils of the two approaches. The default is 'zero'.

vert.for,v 1.10 2011/06/27 14:46:08 pteuben Exp $


 

Index

NAME
PERSON RESPONSIBLE
CATEGORIES
DESCRIPTION
PARAMETERS

This document was created by man2html, using the manual pages.
Time: 18:35:38 GMT, July 05, 2011