mem(algorithm, niter, sigma, targetflux, constrainflux, displayprogress, model, fixed, complist, prior, mask, image, residual, async)
Description
Makes a mem image using either the Cornwell-Evans maximum entropy or
maximum emptiness algorithms, using the single field or multi-field
contexts. The maximum entropy algorithm is the default. The mem is performed
on the residual image calculated from the visibility data currently
selected. Hence the first step performed in mem is to transform the
current model or models (optionally including a componentlist) to fill
in the MODEL_DATA column, and then inverse transform the residual
visibilities to get a residual image. This residual image is then
deconvolved using the corresponding point spread function. This means that
the initial model is used as the starting point for the
deconvolution. Thus if you want to restart a mem, simply set the
model to the model that was previously produced by clean.
Mask images are used to constrain the region that is to be
deconvolved. To make mask images, use either
boxmask (to define a mask via the
corner locations blc and trc) or mask (to
define a mask via thresholding an existing image). The default mask is
the inner quarter of the image.
The MEM deconvolution only operates on one Stokes parameter at a time.
Joint MEM deconvolution for multiple Stokes parameters will be
implemented in the future.
Some reference regarding MEM :
Cornwell and Evans,
Astronomy and Astrophysics (ISSN 0004-6361), vol. 143, no. 1, Feb. 1985,
p. 77-83.
Narayan and Nityananda,
Annual review of astronomy and astrophysics. Volume 24 (A87-26730
10-90). Palo Alto, CA, Annual Reviews, Inc., 1986, p. 127-170.
The mem algorithms possible are:
- Cornwell-Evans Maximum Entropy (entropy)
- The classic "vm" or "vtess"
deconvolution algorithm.
- Cornwell-Evans Maximum Emptiness (emptiness)
- The historic, but
largely undocumented, modification to the Cornwell-Evans algorithm
which seeks a model image which is consistent with the data and
simultaneously minimizes the number of pixels with no emission
(meaning "with pixel values below the noise level").
- Multi-field Maximum Entropy (mfentropy)
- Deconvolution is split
into minor and major cycles. For each field, the MEM analog of a Clark
Clean minor cycle is performed. In the major cycle, the emission thus
modelled is subtracted either from the original visibilities (for
multiple fields) or using a convolution (for only one field). The
latter is much faster.
- Multi-field Maximum Emptiness (mfemptiness)
- Just like mfentropy,
but with emptiness.
The multi-field mem (mfentropy or mfemptiness) should be
used if either of two conditions hold:
- 1.
- Multiple fields are to be deconvolved simultaneously OR
- 2.
- Primary beam correction is enabled. In this case, a
mosaiced mem is performed.
Note that for the single pointing algorithms, only a quarter of the
image may be deconvolved. If no mask is set, then the deconvolved
region defaults to the inner quarter. If a mask larger than a quarter
of the image is set, then only the quarter starting at the bottom left
corner is used. However, for the multi-field imaging, the entire
field may be imaged because the major cycles either do an exact
subtraction from the visibilities or because PSF extent is more than
twice the extent of the primary beam support.
Before mem can be run, you must run setdata and setimage.
Before mem can be run with a multi-field algorithm, you should run
setvp. You may want to run setmfcontrol before running mem
with a multi-field algorithm, though the default control values
may be acceptable.
Arguments
| algorithm |
in |
Algorithm to use |
|
|
Allowed: |
String:'entropy'|'emptiness'|'mfentropy'|'mfemptiness' |
|
|
Default: |
'entropy' |
| niter |
in |
Number of Iterations |
|
|
Allowed: |
Int |
|
|
Default: |
20 |
| sigma |
in |
Image sigma to try to achieve |
|
|
Allowed: |
Quantity |
|
|
Default: |
'0.001Jy' |
| targetflux |
in |
Target flux for final image |
|
|
Allowed: |
Quantity |
|
|
Default: |
'1.0Jy' |
| constrainflux |
in |
Constrain image to match target flux? else targetflux used only to initialize model |
|
|
Allowed: |
Bool |
|
|
Default: |
F |
| displayprogress |
in |
Display the progress of the cleaning? |
|
|
Allowed: |
Bool |
|
|
Default: |
F |
| model |
in |
Names of model images |
|
|
Allowed: |
Vector of strings |
| fixed |
in |
Keep model fixed |
|
|
Allowed: |
Vector of booleans |
|
|
Default: |
F |
| complist |
in |
Name of component list |
|
|
Allowed: |
String |
| prior |
in |
Names of mem prior images |
|
|
Allowed: |
Vector of strings |
| mask |
in |
Names of mask images (0=>no emission, 1=>emission permitted |
|
|
Allowed: |
Vector of strings |
| image |
in |
Names of restored images |
|
|
Allowed: |
Vector of strings |
| residual |
in |
Names of residual images |
|
|
Allowed: |
Vector of strings |
| async |
in |
Run asynchronously in the background? |
|
|
Allowed: |
Bool |
|
|
Default: |
!dowait |
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