feather

feather(highres, lowres, imagename='', sdfactor=1.0, effdishdiam=-1.0, lowpassfiltersd=False)[source]

Combine two images using their Fourier transforms

[Description] [Examples] [Development] [Details]

Parameters

  • 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

Description

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 normalization scale.

More information about feather can be found in the Image Combination section of the CASAdocs.

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-\(\omega\) t) where ‘\(\omega\) t’ is the Fourier transform of the ‘clean beam’ defined in the low-resolution image.

  5. 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.

../../_images/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 interferometric data.

Parameter descriptions

imagename

Name of output feathered image. Default is none; example: imagename=’orion_combined.im’.

highres

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.

lowres

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.

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 meters [m]. One can only reduce the dish effective dish diameter in feathering. Default is -1.0 which 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.

Examples

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.

feather(imagename='M100_Feather_CO.image',highres='M100_combine_CO_cube.image.subim',
        lowres='M100_TP_CO_cube.regrid.subim.depb')

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)
Development

No additional development details

Parameter 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
out.