Recent changes to Masking

Masking modified by André Offringa

André Offringa — Fri, 01 Dec 2017 12:17:52 -0000

--- v6
+++ v7
@@ -7,7 +7,7 @@
 A basic example:

     :::bash
-    wsclean -fitsmask mymask.fits -niter 1000 -mgain 0.8 -size 1024 1024 \
+    wsclean -fits-mask mymask.fits -niter 1000 -mgain 0.8 -size 1024 1024 \
       -scale 10asec myobservation.ms

 A model image can be used as a mask file. In standard clean (i.e., not multi-scale clean) this can be used to limit the clean components to significant features. For example, one can first clean to 3 sigma (with the [`-auto-threshold`](BasicCleaning) option), then use the model as fitsmask and clean to e.g. 0.3 sigma. This does require two runs though, which is not necessary. Also, while this same method can be used for multi-scale cleaning, in general a multi-scale clean will make large parts of the model image non-zero, and cleaning will therefore not be very well constrained. To overcome both issues, WSClean implements a technique called auto-masking.
@@ -33,7 +33,7 @@
 In cases where the RMS varies strongly over the field of view, for example because of calibration artefacts, it might be useful to use [local RMS thresholding](LocalRMSThresholding), instead of thresholding relative to the global RMS.

 #### Limiting auto-masked deconvolution with a regular mask
-Auto-masking works in combination with normal masking, so that WSClean can be forced to to find components only in a certain area, and after reaching the auto-masking threshold it would continue with the automask. Since the automask will only contain components within the fitsmask, no components are found outside the fitsmask. Auto-masking and regular masking can be combined by specifying both `-fitsmask <file>` and `auto-mask <threshold>` on the command line.
+Auto-masking works in combination with normal masking, so that WSClean can be forced to to find components only in a certain area, and after reaching the auto-masking threshold it would continue with the automask. Since the automask will only contain components within the fitsmask, no components are found outside the fitsmask. Auto-masking and regular masking can be combined by specifying both `-fits-mask <file>` and `auto-mask <threshold>` on the command line.

 ### Availability

Masking modified by André Offringa

André Offringa — Thu, 22 Jun 2017 07:17:02 -0000

--- v5
+++ v6
@@ -38,3 +38,6 @@
 ### Availability

 Auto-masking is available in multi-scale since [WSClean version 2.1](Changelog-2.1), and is available in all modes since [WSClean 2.2](Changelog-2.2).
+
+### References
+The auto-masking algorithm is described and tested in [Offringa and Smirnov (2017)](http://arxiv.org/abs/1706.06786).

Masking modified by André Offringa

André Offringa — Wed, 05 Apr 2017 11:20:57 -0000

--- v4
+++ v5
@@ -32,6 +32,9 @@

 In cases where the RMS varies strongly over the field of view, for example because of calibration artefacts, it might be useful to use [local RMS thresholding](LocalRMSThresholding), instead of thresholding relative to the global RMS.

+#### Limiting auto-masked deconvolution with a regular mask
+Auto-masking works in combination with normal masking, so that WSClean can be forced to to find components only in a certain area, and after reaching the auto-masking threshold it would continue with the automask. Since the automask will only contain components within the fitsmask, no components are found outside the fitsmask. Auto-masking and regular masking can be combined by specifying both `-fitsmask <file>` and `auto-mask <threshold>` on the command line.
+
 ### Availability

 Auto-masking is available in multi-scale since [WSClean version 2.1](Changelog-2.1), and is available in all modes since [WSClean 2.2](Changelog-2.2).

Masking modified by André Offringa

André Offringa — Wed, 01 Mar 2017 14:07:41 -0000

--- v3
+++ v4
@@ -30,6 +30,8 @@

 The combination `-auto-mask 3 -auto-threshold 0.3` seems like a good general setting, which normally leaves almost no residuals behind.

+In cases where the RMS varies strongly over the field of view, for example because of calibration artefacts, it might be useful to use [local RMS thresholding](LocalRMSThresholding), instead of thresholding relative to the global RMS.
+
 ### Availability

 Auto-masking is available in multi-scale since [WSClean version 2.1](Changelog-2.1), and is available in all modes since [WSClean 2.2](Changelog-2.2).

Masking modified by André Offringa

André Offringa — Fri, 20 Jan 2017 11:00:10 -0000

--- v2
+++ v3
@@ -19,7 +19,7 @@
  * Only one run of clean is required;
  * It maintains scale-dependent masks, which improves multi-scale cleaning.

-Currently (as of [WSClean 2.1](Changelog-2.1)), auto-masking only works in multiscale mode. The general syntax is as follows:
+Auto-masking works in all modes since [WSClean 2.2](Changelog-2.2). The general syntax is as follows:

     :::bash
     wsclean -multiscale -auto-mask 3 -auto-threshold 0.3 -niter/-mgain/-size/-scale/... obs.ms
@@ -32,4 +32,4 @@

 ### Availability

-Auto-masking is available since [WSClean version 2.1](Changelog-2.1).
+Auto-masking is available in multi-scale since [WSClean version 2.1](Changelog-2.1), and is available in all modes since [WSClean 2.2](Changelog-2.2).

Masking modified by André Offringa

André Offringa — Tue, 20 Dec 2016 08:48:34 -0000

--- v1
+++ v2
@@ -6,6 +6,7 @@

 A basic example:

+    :::bash
     wsclean -fitsmask mymask.fits -niter 1000 -mgain 0.8 -size 1024 1024 \
       -scale 10asec myobservation.ms

@@ -20,6 +21,7 @@

 Currently (as of [WSClean 2.1](Changelog-2.1)), auto-masking only works in multiscale mode. The general syntax is as follows:

+    :::bash
     wsclean -multiscale -auto-mask 3 -auto-threshold 0.3 -niter/-mgain/-size/-scale/... obs.ms

 This will start multi-scale cleaning first towards a threshold of 3 sigma. During multi-scale cleaning, the positions and scale of each component is recorded and put in a scale-dependent mask. Only the center pixel of the kernel is placed in the mask. Once the 3 sigma level is reached, cleaning will continue towards the final threshold of 0.3 sigma. During that stage, the scale-dependent masks are used to constrain the cleaning.

Masking modified by André Offringa

André Offringa — Tue, 20 Dec 2016 08:47:02 -0000

Masks are often used to be able to clean up to/below the noise level. Deeper cleaning leaves fewer undeconvolved residuals behind, hence the image quality is higher, and it decreases the self-cal bias when used inside a self-cal loop.

Providing a mask

WSClean accepts masks in CASA format and in FITS file format. A mask is a normal, single frequency, single polarization image file, where all zero values are interpreted as being not masked, and all non-zero values are interpreted as masked.

A basic example:

wsclean -fitsmask mymask.fits -niter 1000 -mgain 0.8 -size 1024 1024 \
  -scale 10asec myobservation.ms

A model image can be used as a mask file. In standard clean (i.e., not multi-scale clean) this can be used to limit the clean components to significant features. For example, one can first clean to 3 sigma (with the -auto-threshold option), then use the model as fitsmask and clean to e.g. 0.3 sigma. This does require two runs though, which is not necessary. Also, while this same method can be used for multi-scale cleaning, in general a multi-scale clean will make large parts of the model image non-zero, and cleaning will therefore not be very well constrained. To overcome both issues, WSClean implements a technique called auto-masking.

Auto-masking

Auto-masking allows automated deep cleaning and solves the two problems mentioned above:

Only one run of clean is required;
It maintains scale-dependent masks, which improves multi-scale cleaning.

Currently (as of WSClean 2.1), auto-masking only works in multiscale mode. The general syntax is as follows:

wsclean -multiscale -auto-mask 3 -auto-threshold 0.3 -niter/-mgain/-size/-scale/... obs.ms

This will start multi-scale cleaning first towards a threshold of 3 sigma. During multi-scale cleaning, the positions and scale of each component is recorded and put in a scale-dependent mask. Only the center pixel of the kernel is placed in the mask. Once the 3 sigma level is reached, cleaning will continue towards the final threshold of 0.3 sigma. During that stage, the scale-dependent masks are used to constrain the cleaning.

A scale-dependent mask makes sure that when a certain scale-kernel size was cleaned, only that same scale is allowed at that position when the mask is used.

The combination -auto-mask 3 -auto-threshold 0.3 seems like a good general setting, which normally leaves almost no residuals behind.

Availability

Auto-masking is available since WSClean version 2.1.