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\paperID{P4-27}
\title{The AIPS++ Project}

\author{J.P. McMullin, K. Golap, S.T. Myers}
\affil{National Radio Astronomy Observatory}

\contact{J.P. McMullin}
\email{jmcmulli@nrao.edu}

\paindex{McMullin, J. P.}
\aindex{Golap, K.}     % Remove this line if there is only one author
\aindex{Myers, S. T.}

\authormark{McMullin, Golap, \& Myers}

\keywords{computing, AIPS++, astronomy: radio, ALMA}

\begin{abstract}          % Leave intact
The AIPS++ (Astronomical Information Processing System) Project has developed 
a codebase of libraries, tookits, and applications for the analysis of radio 
astronomical data. We discuss the overall architecture and core components of
this package along with the current technologies in use. The existing package 
features many applications which are required for the next generation 
telescopes under development (e.g., multi-field, multi-scale, and wide field
imaging algorithms, full primary beam Stokes I,Q,U,V imaging, automated and 
interactive statistical data editing, flexible calibration and self-calibration
based on the telescope measurement equation; we highlight some of this 
functionality and discuss the future directions for the applications and 
underlying technology. 
\end{abstract}

\section{Introduction}

AIPS++ (Astronomical Information Processing System) was developed by an 
international consortium of observatories (ASTRON, ATNF, JBO, NCSA, NRAO)
as a toolkit for the analysis/reduction of radio astronomical data. As of
April 2003, the consortium was disbanded; however, development continues based 
on project/instrument needs. In particular, AIPS++ is the baseline reduction
package for the ALMA (Atacama Large Millimeter Array).

AIPS++ was designed as a large-scale, distributed system. Its architecture
is based upon distributed components, making it easy to add, replace, or
upgrade functionality. This also facilitated the distributed development
within the consortium, enabling many workers to participate in the development
(i.e. scalable effort). It was also developed with modern languages and
modern computer science techniques (e.g. object-oriented programming). This
makes the package easily expandable and parallelizable at different levels
(from the core C++ libraries to the scripting level).

The underlying mathematical framework for the analysis within AIPS++ is the 
Measurement Equation (Hamaker, Bregman, Sault, 1996) which provides a general 
basis for modeling the instrumental errors in the calibration and imaging
system.
algorithms, and further, it makes no hardware specific assumptions, i.e., 
no assumptions regarding connected-element arrays or uniformity of antennas.

\begin{figure}
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\plotone{P4-27_f1.eps}
\caption{The AIPS++ MeasurementSet.} \label{P4.27-fig-1}
\end{figure}

AIPS++ employs a well defined data model, the MeasurementSet. It was 
designed to meet the following requirements:
\begin{itemize}
\item the format must accomodate synthesis and single-dish data from a variety
of telescopes, both current and future, in as broad a framework as possible
(i.e., it must be able to adapt to handle unforseen telescope operations, 
hardware devices, calibration plans, observing modes and data reduction 
processing).
\item it must be compatible with the requirements of the measurement equation
formalism.
\item it must build upon the knowledge and experience associated with previous
data formats such as the AIPS and MIRIAD internal formats and the FITS
format for interchange.
\end{itemize}

\section{Toolkit}

The AIPS++ code base has extensive functionality. In particular, the
core radio astronomy applications are already in place. 

\begin{enumerate}
\item data fillers - available for many instruments and several archive
	and interchange data formats
\item editing and visualization - available for both visibility and image
	data; rasters, contours, vector, and 3-D slice displays.
\item calibration - flexible calibration of all corrupting terms 
	\begin{itemize}
	\item based on underlying measurement process
	\item arbitrary parameterization and polarization basis for effects
	\item visibility-plane components supported
	\begin{itemize}
	\item P - parallactic angle correction (pre-computed)
	\item C - polarization configuration (pre-computed)
	\item G - electronic gain; solvable
	\item T - atmospheric correction; solvable
	\item D - instrumental polarization response; solvable
	\item B - bandpass response; solvable
	\item F - ionospheric correction; pre-computed from global, empirical
		model (PIM)
	\end{itemize}
	\item pre-computed or solved using chi-square based on the measurement
	equation
	\end{itemize}
\item imaging - complete suite of imaging and deconvolution tools 
	\begin{itemize}
	\item supports polarimetry, spectral-line, multiple fields, mosaicing,
	non-coplanar baselines (simultaneously)
	\item single dish OTF, holography
	\item clean algorithms: Hogbom, Clark, Schwab-Cotton, Multi-scale
	\item incremental multi-field deconvolution
	\item Non-Negative Least Squares and Maximum Entropy deconvolution
	\item flexible in image size (2$^n$ not needed)
	\item supports wide range of coordinate systems
	\item tracks moving objects
	\end{itemize} 
\item tools for unlimited data access/exploration (e.g., TaQL, LEL)
\item good scripting, pipeline capabilities
\item infrastructure necessary for high level applications development is 
	already in place (e.g., quanta, measures, coordinates, lattices, 
	tables, etc.)
\item provides for development and prototyping at different levels (low level
	access to higher level applications)
\end{enumerate} 

\section{Framework}

The existing framework of AIPS++ is built upon proprietary technology (e.g., 
Glish) and as such represents a fairly closed system. It is unable to take
advantage of new technologies wich have become readily available since 
the original design of AIPS++ over a decade ago 
(e.g., Python, Java, CORBA, etc). However, replacing Glish as the underlying
software bus and task manager implies essentially a new system, providing
an opportunity to open the architecture of the package while maintaining 
and enhancing the science data processing functionality (i.e., the existing
C++ application libraries are maintained). 

\begin{figure}
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\plotone{P4-27_f2.eps}
\caption{System Framework. The grey boxes indicate the existing framework;
the white boxes indicate the additions possible through changes in the
software bus.} \label{P4.27-fig-2}
\end{figure}


Figure 2 illustrates the existing and future framework for the AIPS++ codebase.
Replacing Glish as the software bus with a version of CORBA provides interfaces
to other existing languages and tools (e.g., JAVA, Python, etc) which are
both well-tested and well-known in the community. Since the original design of
AIPS++ used the object-oriented component concept, little work is required to
preserve the existing applications/libraries and make them available in such
a new framework. As a technology exploration, a prototype pipeline for ALMA
has been constructed using AIPS++ applications/libraries called through the
ACS environment (ALMA Common Software - a CORBA-based common software 
infrastructure).

For more information on AIPS++, see: http://projectoffice.aips2.nrao.edu

\begin{references}
\reference Hamaker, J.\ P.\, Bregman, J.\ D.\, \& Sault, R.\ J.\ 1996, 
   \aap 117, 137
\end{references}


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