\item[pointingcolumntouse] This parameter is NOT to be changed under normal circumstances. This is to be used by those who know what they are doing and want to try to use different columns in the POINTING table especially if they believe their dish direction is wrong. And if any of the {\tt \_OFFSET} columns is used do not expect to be able to use a different frame in the image setup in <link anchor="imager:imager.defineimage.function">defineimage</link>. Possible values are {\tt DIRECTION, TARGET, ENCODER, POINTING\_OFFSET, SOURCE\_OFFSET}
<casaxml xsi:schemaLocation="http://casa.nrao.edu/schema/casa.xsd file:///opt/casa/code/tools/xml/casa.xsd" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://casa.nrao.edu/schema/psetTypes.html">
<tool module="imager" name="imager">
<shortdescription>tool for synthesis imaging</shortdescription>
imager is an tool that accomplishes synthesis processing.
A {\tt imager} must be constructed for each MeasurementSet for which one
wishes to do processing. Multiple copies of {\tt imager} may
be made at any time (provide they are given different names).
<include>imager_forward.h</include>
<include>imager_private.h</include>
<method name="imager" type="constructor">
<shortdescription>Construct an imager tool</shortdescription>
This is used to construct {\tt imager} tools associated
with a MeasurementSet. The {\tt imager} tool may then be
used to generate various types of images. Note that
a new executable is started every time the constructor
This returns a Glish variable containing the tool functions of
imager in an alternate universe that you have to tunnel to with a wormhole
im.defineimage(nx=256, ny=256, cellx='0.7arcsec', celly='0.7arcsec')
im.image(type='corrected', image='3C273XC1.dirty')
<method name="advise" type="function">
<shortdescription>Advise (and optionally use) parameter values</shortdescription>
Advise on recommended values of certain parameters. Return these
values and optionally use them in Imager.
The calculations are performed as following:
\item[cell] The maximum uv distance in wavelength is found and then half of the
inverse is taken as the maximum cellsize allowed.
\item[pixels] The field of view is converted to a number of pixels
using the calculated cell size.
\item[facets] The number of facets on an axis is calculated in two
different ways. The first method simply requires that the peeling of
facets away from the celestial sphere should not cause an amplitude
drop of more than the argument {\tt amplitudeloss}. The positions may
be incorrect, but all the sources will be removed correctly. The
second method requires that the source positions be accurate to the
same fraction of the beam specified by {\tt amplitudeloss}. The
second calculates the second moment in w and in uv distance and
chooses the number of facets correspondingly. The first method does
the same but after fitting a plane to the sampling: $w = a u + b v$.
For an approximately coplanar array, the positions may be wrong but
the removal of sidelobes will be accurate. The number of facets
returned is the second, usually smaller, number. The formula used
N\_{facets} = N\_{pixels} \sqrt{{{\Delta \theta}\over{\sqrt{8 \delta A}}}{w\_{rms}}\over{uv\_{rms}}}
where $\Delta \theta$ is the cellsize in radians, and $\delta A$ is
the amplitude loss. This formula can be derived from (a) the peeling
of facets from the celestial sphere, and (b) a quadratic approximation
for the beam size both in the plane of the sky and along the $w$ axis.