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Basic Information on immerge
Purpose: Linear merging of images.
Categories: map combination
IMMERGE is a Miriad task to merge linearly two images with
different resolutions. The two images must be of the same field
and use the same coordinate system.
In combining the data, it is assumed that the low resolution
image better represents the short spacing data, whereas the
high resolution best represents the fine structure. Commonly,
the low resolution image will be a single-dish observation,
and the high resolution will be a mosaiced interferometric
This gives the two images to be merged. The first input must be
the high resolution image, and the second the low. There is
no default. The two images must be on the same coordinate grid.
If necessary, use REGRID to achieve this.
The output image. No default.
The flux calibration factor. Ideally the two inputs will have
correctly calibrated flux scales, in units like Jy/beam. In this
case, the flux calibration factor would be 1. In practise, the
calibration may not be perfect, and an extra calibration factor
will be required to align the flux scales. "factor" gives the
factor to scale the low resolution image to put it on the same
flux scale as the high resolution image. Note that this factor
is in addition to the scaling needed to convert Jy/beam at a low
resolution to Jy/beam at the high resolution.
If no value is given for "factor", IMMERGE determines this by
comparing the values in the Fourier plane, of data within an
particular annulus (after accounting for the differing
resolutions of the two inputs). IMMERGE finds the scale factor
that minimises differences between the data of the two (in a
robust/L1 sense) in the annulus.
This specifies an annulus, in the Fourier domain, where the
high and low resolution images should agree (after allowing
for resolution differences). This is the annulus of data used to
deduce the flux calibration factor, and with options=feather.
Two or three values can be given. The first two give the inner
and outer radius of the annulus in the Fourier domain. The
third argument gives the units. Possible units are "klambda"
(kilo-wavelengths), "meters", "feet" and "nanoseconds". The
default is "klambda".
Values for "uvrange" must be given either if "options=feather"
is used or if the flux calibration factor is being deduced.
Region-of-interest parameter. See the help on region
for more information. NOTE: This parameter is ONLY used for
determining the flux calibration factor. Only plane selection
(e.g. via the "image" command) is allowed. Typically you
would want to select a range of planes which contains
significant signal in the overlap region.
PGPLOT device for a plot. When determining the flux calibration
factor, IMMERGE can produce a plot showing the correspondence
between the high and low resolution data points in the annulus
(after correcting for resolution effects and the deduced flux
calibration factor). Ideally it will show a line with "y=x".
The default is not to produce a plot. It also plots the
difference from this "y=x" line as a function of spatial
Before Fourier transforming, the images are padded with a guard
band. "guard" gives one or two values, being the minimum width
of this guard band in pixels, in the x and y directions. The
actual guard band used is such that the size of the image plus
guard band is a power of 2.
Task enrichment parameters. Several can be given, separated by
commas. Minimum match is used.
normalize Rather than the output being the merged images, the
output is the low resolution image corrected by
the flux calibration factor.
zero By default, IMMERGE pads the guard band with data
that minimizes FFT edge effects. If the input
images are really zero beyond the edges of the two
input images, then padding with zeros might be
preferable. This is particularly so if IMMERGE is
deducing the flux calibration scale factor.
feather This merges the two images in a fashion similar to
AIPS IMERG. This method is generally less
desirable than the default scheme used by IMMERGE.
shift Determine the optimum shift to apply to the low
resolution image to make it align with the high
notaper Normally the low-resolution image is tapered to
match any residual primary beam response in the
high-resolution image. This option causes this
step to be skipped.
Revision: 1.6, 2011/10/06 07:18:56 UTC
Generated by email@example.com on 06 Oct 2011