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NRAO Home > CASA > CASA Task Reference Manual |
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0.1.24 feather
Requires:
Synopsis
Combine two images using their Fourier transforms
Description
The algorithm converts each image to the gridded visibility plane, combines
them, and reconverts them into an combined image. Each image must include
a well-defined beam shape (clean beam) in order for feathering to work well.
The two images must have the same flux density normalization scale.
Arguments
Inputs |
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imagename |
| Name of output feathered image
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| allowed: | string |
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| Default: |
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highres |
| Name of high resolution (interferometer) image
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| allowed: | string |
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| Default: |
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lowres |
| Name of low resolution (single dish) image
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| allowed: | string |
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| Default: |
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sdfactor |
| Scale factor to apply to Single Dish image
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| allowed: | double |
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| Default: | 1.0 |
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effdishdiam |
| New effective SingleDish diameter to use in m
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| allowed: | double |
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| Default: | -1.0 |
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lowpassfiltersd |
| Filter out the high spatial frequencies of the SD image
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| allowed: | bool |
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| Default: | False |
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Feathering is a simple method for combining two images with different
spatial resolution. The processing steps are:
1. Regrid the low-resolution image to a temporary copy matching the
resolution of the high-resolution image,
2. Transform each image to the spatial-frequency plane (gridded).
3. Scale the low-resolution image (uv-grid) by the ratio of the
volumes of the two ’clean beams’ (high-res/low-res).
4. Add to this, the uv-grid of the high-resolution image, scaled by
(1-wt) where ’wt’ is the Fourier transform of the ’clean beam’
defined in the low-resolution image.
5. Transform back to the image plane.
Both input images must have a well-defined beam shape for this task to work.
This could be a ’clean beam’ for interferometric images, and a ’primary-beam’
for a single-dish image.
The two images must also have the same flux density normalization scale.
Keyword arguments:
imagename -- Name of output feathered image
default: none; example: imagename=’orion_combined.im’
highres -- Name of high resolution (interferometer) image
default: none; example: highres=’orion_vla.im’
This image is often a clean image obtained from synthesis
observations.
lowres -- Name of low resolution (single dish) image
default: none; example: lowres=’orion_gbt.im’
This image is often a image from a single-dish observations
or a clean image obtained from lower resolution synthesis
observations.
sdfactor -- value by which to scale the Single Dish image. Default is 1.0
Basically modifying the flux scale of the SD image
effdishdiam -- New effective SingleDish diameter to use in m. Obviously one can only reduce the dish effective dish diameter in feathering. Default -1.0 means leave as is.
lowpassfiltersd -- If True the high spatial frequency in the SD image is rejected.
Any data outside the maximum uv distance that the SD
has illuminated is filtered out.
Comments:
This task can be used as one method of combining single-dish and
interferometric images after they have been separately made.
The clean task allows another method of combining single-dish and
interferometric data. The single-dish image can be used as a starting
model for the interferometric image-reconstruction. If there is
some overlap between the spatial-frequencies contained in the
single-dish image and the interferometer sampling function, then,
such a starting model will help constrain the solutions on the
short-baselines of the interferometric data.
More information about CASA may be found at the
CASA web page
Copyright © 2016 Associated Universities Inc., Washington, D.C.
This code is available under the terms of the GNU General Public Lincense
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