IMAGER is an interferometric imaging package in the GILDAS
software, tailored for usage simplicity and efficiency for
multi-spectral data sets.
Contrary to CASA, whose goal is to most accurately solve the (interferometer and single-dish)
measurement equation in its whole generality (wide field, ultra-wide bandwidths,
mosaicing, polarization, high dynamic range, sky curvature, etc...), at the expanse of complex
and thus relatively slow algorithms, the
IMAGER design was driven by the 4 main facts:
- “Simple” cases (unpolarized signal, single fields or relatively
small mosaics, relatively limited bandwidths, polarization for compact sources only)
constitute 99 % of the cases.
- there is no unique image that can be used to perform astrophysics from
a given interferometric data set.
- All deconvolution methods require some tuning of control parameters,
so speed is an important issue
- All images are ultimately limited by signal to noise considerations
Thus, speed is important (to allow trials), and numerical accuracy can be traded of
for speed provided the resulting “errors” remain smaller than the noise level.
The main goals of
IMAGER are
- to offer a proper implementation of imaging in case of
wide relative bandwidth, where the natural angular resolution
changes with frequency.
- to implement a simple and efficient scheme to process Mosaics,
including short spacings from single dish data
- to take advantage of improved capabilities of NOEMA and
ALMA, by offering new tools like self-calibration or wide bandwidth
analysis (and to some extent, polarization handling)
- to simplify user interfaces, by providing sensible defaults.
- to minimize image sizes
- to minimize processing time by using parallel programming as much
as possible and reducing Input/Output to the strict minimum.
IMAGER was developed and optimized to minimize Input/Output that
are the bottleneck of current computers.
Therefore,
IMAGER works mostly on internal buffers and avoids as
much as possible saving data to intermediate files. File saving is done
ultimately once the data analysis process is complete, which offers an
optimum use of the disk Input/Output bandwidth.
IMAGER also includes advanced display and image analysis tools, such as
simple overlaying of different data cubes, etc...
