# stub function definition file for docstring parsing
[docs]def feather(highres, lowres, imagename='', sdfactor=1.0, effdishdiam=-1.0, lowpassfiltersd=False):
Combine two images using their Fourier transforms
[`Description`_] [`Examples`_] [`Development`_] [`Details`_]
- imagename_ (string='') - Name of output feathered image
- highres_ (path) - Name of high resolution (interferometer) image
- lowres_ (path) - Name of low resolution (single dish) image
- sdfactor_ (double=1.0) - Scale factor to apply to Single Dish image
- effdishdiam_ (double=-1.0) - New effective SingleDish diameter to use in m
- lowpassfiltersd_ (bool=False) - Filter out the high spatial frequencies of the SD image
**feather** can be used as one method of combining single-dish and
interferometric images after they have been separately made.
The algorithm converts each image to the gridded visibility plane,
combines them, and reconverts them into a combined image. Each
image must include a well-defined beam shape (clean beam) in order
for feathering to work well; these could be a 'clean beam' for
interferometric images, and a 'primary-beam' for a single-dish
image. The two images must have the same flux density
More information about **feather** can be found in the `Image
section of the CASAdocs.
Feathering is a simple method for combining two images with
different spatial resolution. The processing steps are:
#. Regrid the low-resolution image to a temporary copy matching
the resolution of the high-resolution image.
#. Transform each image to the spatial-frequency plane (gridded).
#. Scale the low-resolution image (uv-grid) by the ratio of the
volumes of the two 'clean beams' (high-res/low-res).
#. Add to this, the uv-grid of the high-resolution image, scaled
by (1-:math:`\omega` t) where ':math:`\omega` t' is the
Fourier transform of the 'clean beam' defined in the
#. Transform back to the image plane.
One commonly used option for feather is sdfactor, which can be
used to adjust the flux scale of the SD image in the case that the
fluxes don't match where there is overlap between the
spatial-frequencies in the SD image and the interferometer image.
.. figure:: ../../tasks/_apimedia/featherimage.png
The results of feathering ALMA 12+7m and Total Power data for
M100 using three different sdfactors: 0.5 (right) 0.7 (middle)
1.3 (left). The higher sdfactor (1.3) recovers more of the faint
extended emission, as it is boosting the TP signal.
.. note:: **NOTE**: The **tclean** 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
.. rubric:: Parameter descriptions
Name of output feathered image. Default is none; example:
Name of high resolution (interferometer) image. Default is none;
example: *highres='orion_vla.im'*. This image is often a clean
image obtained from synthesis observations.
Name of low resolution (single dish) image. Default is 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.
Value by which to scale the Single Dish image. Default is 1.0.
Basically modifying the flux scale of the SD image.
New effective SingleDish diameter to use in meters [m]. One can
only reduce the dish effective dish diameter in feathering.
Default is -1.0 which means leave as is.
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.
Creating a image called 'M100_Feather_CO.image' from an ALMA
interferometric cube, 'M100_combine_CO_cube.image.subim', and a
single dish ALMA total power image,
'M100_TP_CO_cube.regrid.subim.depb'. The inputs have been
appropriately cleaned, regridded, and cropped beforehand.
Creating an image called 'feather.im' by combining the cleaned,
synthesis image, 'synth.im' and the SD image, 'single_dish.im'
while increasing the flux scale of the SD image by setting
sdfactor = 1.2.
feather(imagename ='feather.im', highres ='synth.im', lowres ='single_dish.im'sdfactor = 1.2)
No additional development details
Detailed descriptions of each function parameter
| ``imagename (string='')`` - Name of output feathered image
| Default: none
| Example: imagename='orion_combined.im'
| ``highres (path)`` - Name of high resolution (interferometer) image
| Default: none
| Example: imagename='orion_vla.im'
| ``lowres (path)`` - Name of low resolution (single dish) image
| Default: none
| Example: imagename='orion_gbt.im'
| ``sdfactor (double=1.0)`` - Value by which to scale the Single Dish image.
| Default: 1.0
| Basically modifying the flux scale of the SD image
| ``effdishdiam (double=-1.0)`` - New effective SingleDish diameter to use in m
| Default: -1.0 (leave as is)
| Obviously one can only reduce the dish
| effective dish diameter in feathering.
| ``lowpassfiltersd (bool=False)`` - Filter out the high spatial frequencies of the SD image
| Default: False
| 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