Grid selected UV data and Fourier invert it to a (residual) dirty map EXAMPLE 1 --------- 0>invert Inverting map and beam Estimated beam: bmin=0.7503 mas, bmaj=1.8830 mas, bpa=-10.9 degrees 0> The two lines without prompts are the response from the invert command. The second of these is an estimate of the restoring beam size. EXAMPLE 2 --------- 0>mapsize 256,2 0>invert Inverting map and beam Your choice of large map pixels excluded 49.3% of the data. The x-axis pixel size should ideally be below 1.025 milli-arcsec The y-axis pixel size should ideally be below 3.466 milli-arcsec Estimated beam: bmin=9.189 mas, bmaj=23.87 mas, bpa=-11.54 degrees Estimated noise=6.17991 mJy/beam. 0> In this case 49% of the visibilities represented higher spatial frequencies than could be Nyquist sampled by the coarse pixel spacing of the chosen map grid. These visibilities were thus excluded from the map produced by invert. The third and fourth lines of output show the maximum pixel spacings that can be accomodated in RA and Dec without lossing data. One could satisfy both sizes by using the mapsize command to select a new square map pixel size of 1 arcsecond, or by using the mapsize command to select a rectangular pixel size of 1.0 x 3.0 arcseconds. CONTEXT ------- Invert is the command that takes UV data and transforms it to a dirty map and a dirty beam. Normally 'invert' is run for you by any commands that operate on the residual map, such as: clean - CLEAN a residual map. restore - Restore a residual map. mapplot - Display a residual map or beam. It is only run by these commands when the current dirty map and/or beam has been invalidated by modifications to the UV data. The beam often doesn't need to be recomputed under these circumstances. When it does need to be re-computed then 'invert' will also report a corresponding estimate of the new beam dimensions. RESIDUALS --------- If an established clean model exists then this will be subtracted from the observed visibilities while gridding. The result is to produce a dirty residual map approximately the same as the one left after clean, but devoid of any aliasing from the removed components. If a non-residual dirty map is desired, then discard the established clean model by typing 'clrmod true' and re-invert. RUNNING PARAMETERS ------------------ Although the command itself takes no arguments, the procedure that it runs takes a number of parameters. These are set and changed through a number of related difmap commands. Reasonable defaults are set for most of these, but you will be required to at least read a UV data file, using the 'observe' command and to specify a map grid, via the 'mapsize' command before first using invert. The rest of the commands offer a choice of visibility weighting and selection schemes which effect how the data are gridded on the UV plane. These commands are listed in the RELATED COMMANDS section below, and each command is discussed separately in its own help file. GRIDDING -------- Visibilities for each IF of an observation are combined in the UV plane during gridding. If each IF has a different frequency then this implements a crude form of multi-frequency-synthesis, however currently no provision is made to correct for spectral-index effects, so if you don't think that the source structure and flux are constant between the multiple frequencies then you are advised to split the IFs in AIPS and map them separately. Also note that if a range of spectral-line channels is selected via the difmap 'select' command, that a vector average of visibilities over those channels is formed before gridding. Thus different channels are not gridded separately and if the UV coordinates of visibilities change appreciably over the selected range them some degree of bandwidth smearing will be apparent. UV SAMPLING CONSTRAINTS ----------------------- In order to avoid aliasing and to avoid undersampling the image plane, the UV grid must be no more than half filled along either the U or the V axes. If the current map cell-size is too large to accomodate the range of U or V coordinates along the respective axes of the UV grid, then 'invert' will exclude visibilities that lie outside the Nyquist UV limits from the final map. The accompanying warning messages provide directions for remedying this situation with the mapsize command. Note that these are no more than warnings. There are times when using large pixels is the only practicle way to view a large enough field. If the Nyquist limits are exceeded along both the RA (x) axis and the Dec (y) axis, then the invert command will report corresponding limiting pixel sizes for the two axes separately. In such cases you could either take the smallest of the reported sizes and use the mapsize command to select square pixels, or you could use the mapsize command to select rectangular pixels to match both constraints. See 'help mapsize' for details. SIDE EFFECTS ------------ The latest CLEAN model will be added to the established model component list and model visibilities. If this is undesirable then use the 'clrmod' command to clear the latest CLEAN model before inverting. RELATED COMMANDS ---------------- mapsize - Specify the dimensions of subsequent maps. uvweight - Selects combinations of natural,uniform, uncertainty, and radial weighting. uvtaper - Apply a gaussian taper to the weights. uvrange - Constrain the range of UV radii within which to select visibilities for gridding. uvzero - Set a zero-UV-spacing flux.