[rgb]1,0,0IMAGER basically assumes everything fits into memory. This is in general quite fine for NOEMA data. However, for ALMA data, if you are working with a too small computer (such as my laptop, which is otherwise fine), you may be lacking physical memory (RAM, Random Access Memory), and [rgb]1,0,0IMAGER may become really inefficient by using Virtual Memory instead.
To avoid time losses, [rgb]1,0,0IMAGER prevents reading UV data whose size exceeds the available RAM, and warns the user if it exceeds half of the RAM. To treat these cases, [rgb]1,0,0IMAGER provides instead a number of tools that can work sequentially on the data set, instead of loading it in memory all at once.
Two commands have no equivalent using the [rgb]1,0,0IMAGER buffers, and work
only on files. Their magenta/FILE option is used to provide an
homogeneous syntax, but must be present for the command line
to be valid.
The magentaUV_SORT /FILE command also has a different behaviour than
its memory only version magentaUV_SORT. While the latter creates
a transposed version of the UV table for internal use (it cannot
be saved), the former keeps the normal
organisation with the visibility axis first.
An example of use of such facilities is the magenta@violet image-mosaic script
that splits the magentauv_map step into chunks that fit in the
computer memory available to the user.
By using the above commands, all operations can be done in a
quasi sequential way, avoiding to load in memory whole data sets.
the magentaREAD /RANGE command allows to select an ensemble of
channels from a UV data. If this ensemble is small enough, [rgb]1,0,0IMAGER
can work. At the end the magentaWRITE /APPEND and
magentaWRITE /REPLACE command will allow to put these channels at
their proper places in a deconvolved data cube.
Some operations on UV data, like time averaging, separation of line
from continuum emission, or self-calibration, and of course, spectral
resampling, are best done using all valid channels to avoid loosing
sensitivity.
To allow [rgb]1,0,0IMAGER to do them even for large data
files, most UV-related commands have a magenta/FILE option which
instructs the command to work from the corresponding data file,
instead of the UV buffers. This includes magentaUV_PREVIEW /FILE,
magentaUV_BASELINE /FILE, magentaUV_FILTER /FILE and especially
the magentaUV_SPLIT /FILE commands. Time averaging can be performed
by magentaUV_TIME /FILE, and prior sorting by time order can be
done by magentaUV_SORT /FILE. Spectral range extraction is possible by
magentaUV_EXTRACT /FILE, spectral resampling by magentaUV_COMPRESS /FILE,
magentaUV_RESAMPLE /FILE, and magentaUV_HANNING /FILE.
that allows to merge together an arbitrary number of UV tables,
in spectral line (with spectral resampling) or continuum (with
flux scaling according to a spectral index) modes.
that combines the capabilities of magentaUV_BASELINE and
magentaUV_FILTER in a single command, since both operations require
the same parameters and provide complementary informations.
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Gildas manager
2023-06-01