Miriad's CLEAN task - surprisingly called clean - implements the Högbom, Clark and SDI algorithms. Given an input dirty image and beam, it produces an output CLEAN component image which has flux density units of Jy/pixel. This CLEAN component image is CLEAN's best guess at what the source really looks like. Invariably its extrapolation at high spatial frequencies is very poor (and probably a reason why AIPS does not make it easy to view it). To reduce the undesirable effects of this extrapolation, and to add in any emission remaining in the residuals, you will need to use the task restor. This gives you what is normally thought of as the `CLEAN' or restored image.
The various input parameters to clean are:
modeparameter can be set to indicate the particular algorithm to use. Possible values are `hogbom', `clark', `steer' or `any' (the default). With `any', clean determines what it believes is the best algorithm for your particular image. In this case, clean can switch between different algorithms, as the nature of the residuals change. This is particularly useful when CLEANing a large image which contains some strong point sources and much lower brightness extended emission. In this case, clean may well switch from a Clark or Högbom algorithm to the SDI algorithm when it finds that the residuals are becoming very smooth.
regionparameter can be used to describe quite complex CLEANing regions. See Section 6.3 on how to specify this. Alternatively, you can use task cgcurs to describe the region interactively from a display of an image on a PGPLOT device. Task cgcurs (see Chapter 17.3) can produce a text file,
cgcurs.region, describing the region selected, which you can then input to clean (see Section 2.5). For example:
email@example.comThe default CLEAN region is the largest region (centred on the dirty image centre) that can be safely deconvolved.
cutoff. Alternatively, you can tell clean to stop when it encounters the first negative component, by using the options=negstop switch. Otherwise, CLEANing will proceed until niters CLEAN iterations have been performed. When CLEANing a cube,
nitersis the number of iterations per plane. For small and simple sources, a few hundred iterations are usually sufficient. For complicated and large sources, you can CLEAN forever.
You can set all three of cutoff, options=negstop and niters. Task clean will stop when any one of these stopping criteria are satisfied.
modelparameter to the name of the old output CLEAN component image, and setting the
outparameter to a new name. Actually the
modelimage need not have been produced by clean - it can be any image with units of Jy/pixel. Of course, it should be a representation of your source.
speedto some small negative number - typically -1 is good. The default is 0.
minpatchis the minimum full width (not half-width, as it is in AIPS) of the beam patch used in the minor cycle. The default is 51 pixels. However, for ATCA work, where the beam can often have large distant sidelobes, this value may be too small. Setting
minpatchto larger values will slow the algorithm, but may avoid CLEAN striping problems. The maximum value that clean will accept is 257 - larger values will be trimmed back to 257.
cliptimes the peak flux density are considered to represent true structure, and so are taken as components for that iteration. Typically the clip level is 0.9. The default that clean computes is image dependent, and will be a function of how many pixels there are across the beam. The default value is usually adequate.
Typical inputs are given below:
|out=vela.icmp||Output CLEAN component image|
|mode||Algorithm used - let CLEAN decide|
|region||Defaults to max area safely CLEANed.|
|phat||Unset means no Prussian helmet|
|cutoff=0||Terminate CLEAN at this residual level or|
|niters=500||Specify total number of CLEAN components|
|speed||Speedup factor; -1 for extended|
|sources, +1 for point sources|
|minpatch=127||Minimum beam size for minor cycles|
|clip||SDI clip level|
The total CLEANed flux density (i.e. the cumulative sum of the CLEAN components) should eventually settle down to a roughly constant number. This indicates that you are just picking up noise, and that there are no sidelobes left to remove. If the total CLEANed flux density starts to decrease again, this usually indicates that you have been a bit heavy handed, and CLEANed too much. You might also look at the result and see if you can see any sidelobes left over.
Having completed clean, you will almost certainly want to ``restore'' your image - see Section 14.6.