Task: moment Purpose: Calculate moments of a Miriad image. Categories: image analysis MOMENT calculates the nth moment of a Miriad image. The spectral axis, which is determined automatically, must be on axis 1, 2, or 3. Moments may be computed for other axis types by specifying the axis number, though the brightness units recorded in the output image header will likely not be correct. They can be fixed using PUTHD. Key: in The input image. No default. Key: region Region of the input image to be used, only the bounding box is supported. See documentation for region on how to specify this. Key: out The output image. No default. Key: mom -3: Velocity at peak intensity using a three-point quadratic fit around the peak (km/s). -2: Peak intensity using a three-point quadratic fit (units same as individual channels). -1: Average intensity (units same as individual channels). 0: 0th moment = sum(I) (integrated intensity, I x km/s). 1: 1st moment = sum(I*v)/sum(I), (velocity centroid, km/s), where v is the velocity. 2: 2nd moment = sqrt(sum(I*(v-M1)**2)/sum(I)), (velocity dispersion, km/s), where M1 is the first moment. If the line profile is Gaussian, this produces a map of Gaussian sigma, and FWHM = 2*sqrt(2*ln(2))*sigma = 2.355*sigma. The moments are calculated independently for each pixel using spectral channels with intensity satisfying the specified clip range. For frequency axes the radio velocity is computed from the line rest frequency recorded in the header. For velocity axes the axis scale is used directly. Key: axis Axis for which the moment is calculated. Moments may be computed for non-spectral axes though brightness units recorded in the output image header will usually be incorrect. Defaults to the spectral axis determined from the input image header. Key: clip Two values. For mom >= -1, exclude spectral channels with intensity in the range clip(1) to clip(2) inclusive. If only one value is given, then exclude -abs(clip) to +abs(clip). Default = 0 which excludes nothing. Key: span For mom >= -1, exclude channels further than the specified number of channels from the peak, hence using a total of 2*span + 1 channels to compute the moment. If the peak is too close to the first or last channel to allow the full span, then the pixel will be blanked in the output map. Please bear this in mind when specifying the channel range (via 'region'). Default = 0 which means no restriction. Key: rngmsk For mom > 0, mask pixels in the output map when the 1st moment lies outside the range of the spectral axis. This can happen because sum(I) can be arbitrarily small in emission-free channels but with sum(I*v) non-vanishing. Default: false. Key: pkmask Two values. For mom = -3, 1, and 2 (velocities) mask pixels in the output map for which the peak intensity lies within the range pkmask(1) to pkmask(2) exclusive. If only one value is given, then mask pixels with peak intensity less than pkmask. Default = 0. Revision: 1.17, 2011/10/25 09:43:44 UTC

Generated by miriad@atnf.csiro.au on 21 Jun 2016