INVERTF is derived from INVERT by the WSRT group to experiment with faster algorithms using no scratch space but more memory.
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.
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.
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.
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).
where ``linetype'' is one of "channel", "wide", "velocity" or "felocity".
linetype,start,widthBefore mapping, the visibility data are divided by the reference channel. The default is no reference channel.
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. You should 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 to only 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.
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.
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 `interpolate', indicating that the bad channels are either to be 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'.