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FAQ: Astronomical Image Processing System (AIPS)

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Archive-name: astronomy/aips-faq
Posting-frequency: monthly
Last-Modified: 1997/05/29
Version: 2.5

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The main source for this FAQ (list of Frequently Asked Questions) is AIPS
memo 87, "The NRAO AIPS Project --- A Summary" by Alan H. Bridle and Eric
W. Greisen.  What you are now reading expands in relatively minor ways the
contents of that memo.  A hypertext (WWW) version of this document is
available via the AIPS home page ("") and a
plain text version is also available via anonymous ftp to in
the /pub/aips/aips_faq.txt file.

This FAQ is posted to alt.sci.astro.aips, alt.answers, and news.answers
monthly, around the 15th.  Copies of the text version may be found on most
usenet archive sites under "astronomy/aips-faq".  Comments, suggestions
for improvement, etc. are welcome and should be addressed via e-mail to
aipsmail at  Unsolicited Commercial Endorsements are not
welcome, nor is junk e-mail.






     The NRAO Astronomical Image Processing System (AIPS) is a software
package for interactive (and, optionally, batch) calibration and editing
of radio interferometric data and for the calibration, construction,
display and analysis of astronomical images made from those data using
Fourier synthesis methods.  Design and development of the package began in
Charlottesville, Virginia in 1978.  It presently consists of over 4300
files containing 1.46 million lines of text.  These comprise over 400,000
lines of documentation and on-line help in over 1300 files, and almost a
million lines of text in over 2300 Fortran and C source files.  It
contains over 350 distinct applications "tasks," representing well over 60
person-years of effort since 1978.

     At its peak, The AIPS group in Charlottesville and Socorro had five
full-time scientist/programmers, and a few other computing and scientific
staff with partial responsibility to the AIPS effort.  The size of the
group has shrunk considerably since then.  The group is responsible for
the code design and maintenance, for documentation aimed at users and
programmers, and for making the code available to non-NRAO sites.  Since
its release under the Free Software Foundation's General Public License in
mid-1995, its availability via the internet has been made considerably
easier, and for that one release (15JUL95), about 150 sites downloaded the
software, and conservative estimates of the number of machines running
this one version of AIPS alone indicate about 450.  NRAO currently offers
AIPS installation kits (ready-to-run binaries) for most of the currently
available UNIX systems, with updates available semi-annually.  The number
of sites running some version of AIPS is probably in excess of 250,
possibly more than 300.

     In 1983, when AIPS was selected as the primary data reduction package
for the Very Long Baseline Array (VLBA), the scope of the AIPS effort was
expanded to embrace all stages of radio interferometric calibration, both
continuum and spectral line.  The AIPS package contains a full suite of
calibration and editing functions for both VLA and VLBI data, including
interactive and batch methods for editing visibility data.  In 1996/7,
considerable effort was expended to ensure that AIPS would be capable of
handling the data from Orbiting VLBI satellites such as VSOP.  For VLBI,
it reads data in MkII, MkIII and VLBA formats, performs global
fringe-fitting by two alternative methods, offers special
phase-referencing and polarization calibration, and performs geometric
corrections, in addition to the standard calibrations done for
connected-element interferometers.  The calibration methods for both
domains encourage the use of realistic models for the calibration sources
and iterated models using self-calibration for the program sources.


     AIPS has been the principal tool for display and analysis of both
two- and three-dimensional radio images (i.e., continuum "maps" and
spectral-line "cubes") from the NRAO's Very Large Array (VLA) since early
in 1981.  It has also provided the main route for self-calibration and
imaging of VLA continuum and spectral-line data.  It contains facilities
for display and editing of data in the aperture, or u-v, plane; for image
construction by Fourier inversion; for deconvolution of the point source
response by Clean and by maximum entropy methods; for image combination,
filtering, and parameter estimation; and for a wide variety of image and
graphical displays.  It records all user-generated operations and
parameters that affect the quality of the derived images, as "history"
files that are appended to the data sets and can be exported with them
from AIPS in the IAU-standard FITS (Flexible Image Transport System; see
newsgroup sci.astro.fits format.  AIPS implements a simple command
language which is used to run "tasks" (i.e., separate programs) and to
interact with text, graphics and image displays.  A batch mode is also
available.  The package contains over 7 Megabytes of "help" text that
provides on-line documentation for users.  There is also a suite of
printed manuals for users and for programmers wishing to code their own
applications "tasks" within AIPS.


     An important aspect of AIPS is its portability.  It has been designed
to run, with minimal modifications, in a wide variety of computing
environments.  This has been accomplished by the use of generic FORTRAN
wherever possible and by the isolation of system-dependent code into
well-defined groups of routines.  AIPS tries to present as nearly the same
interface to the user as possible when implemented in different computer
architectures and under different operating systems.

     The NRAO has sought this level of hardware and operating system
independence in AIPS for two main reasons.  The first is to ensure a
growth path by allowing AIPS to exploit computer manufacturers' advances
in hardware and in compiler technology relatively quickly, without major
recoding.  (AIPS was developed in ModComp and Vax/VMS environments with
Floating Point Systems array processors, but was migrated to vector
pipeline machines in 1985.  Its portability allowed it to take prompt
advantage of the new generation of vector and vector/parallel optimizing
compilers offered in 1986 by manufacturers such as Convex and Alliant.  It
was extended in simple ways in 1992 to take full advantage of the current,
highly-networked workstation environment).  The second is to service the
needs of NRAO users in their home institutes, where available hardware and
operating systems may differ substantially from NRAO's.  By doing this,
the NRAO supports data reduction at its users' own locations, where they
can work without the deadlines and other constraints implicit in a brief
visit to an NRAO telescope site.

      The exportability of AIPS is now well exploited in the astronomical
community; the package is known to have been installed at some time on a
large number of different computers, and is currently in active use for
astronomical research at somewhere around 250 sites worldwide (Reference
** below indicated 140, but qualitatively the number is now guesstimated
to be considerably higher).  AIPS has been run on Cray and Fujitsu
supercomputers, on Convex and Alliant "mini-supercomputers," on the full
variety of Vaxen and MicroVaxen, and on a wide range of UNIX workstations
including Apollo, Data General, Hewlett Packard, IBM, MassComp, Nord,
Silicon Graphics, Stellar and SUN products.  It is available for use on
Intel-based personal computers under the freely available Linux operating
system (Linux, like AIPS, is covered by the GNU General Public License).
In late 1990**, the total computer power used for AIPS was the equivalent
of about 6.5 Cray X-MP processors running full-time.  It is now most
likely considerably more than that.

     AIPS is made available either as source code -- where a complete
compile/link cycle is needed -- or source plus binaries for a variety of
Unix systems.  Either form can be obtained via anonymous ftp, or by
submitting an order form for a tape to NRAO.  In addition, AIPS is now
available on CD-ROM from a third party, on the "Linux for Astronomy CDROM"
(see along with other freely
available Astronomical software such as IRAF, Midas, SAOImage, etc.  The
systems for which binaries are available include: Sparc (SunOS 4 and 5),
Dec Alpha (Digital Unix), IBM RS/6000 (AIX), Intel/Linux, Hewlett-Packard
9000/700, and Silicon Graphics.

     Similarly, but somewhat of mere historic reference now, a wide range
of digital TV devices and printer/plotters has been supported through
AIPS's "virtual device interfaces".  Support for such peripherals is
contained in well-isolated subroutines coded and distributed by the AIPS
group or by AIPS users elsewhere.  Interactive image display in now
provided directly on workstations using an AIPS "television" emulator and
X-Windows.  Hardware TV devices are now practically extinct, but those
used at AIPS sites in the past have included IIS Model 70 and 75, IVAS,
AED, Apollo, Aydin, Comtal, DeAnza, Graphica, Graphics Strategies,
Grinnell, Image Analytics, Jupiter, Lexidata, Ramtek, RCI Trapix, Sigma
ARGS, Vaxstation/GPX and Vicom.  With Printer/plotters, in the age prior
to PostScript becoming almost universally accepted as the language of
choice for these devices, AIPS support included Versatec, QMS/Talaris,
Apple, Benson, CalComp, Canon, Digital Equipment, Facom, Hewlett-Packard,
Imagen, C.Itoh, Printek, Printronix and Zeta products.  Generic and color
encapsulated PostScript is now produced by AIPS for a wide variety of
printers and film recorders.  The standard interactive graphics interface
in AIPS is the Tektronix 4012, now normally emulated on workstations using
an AIPS program and a terminal emulator such as xterm under X-Windows.


     The principal users of AIPS are VLA, VLBA, and VLBI Network
observers.  A survey of AIPS sites carried out in late 1990** showed that
61% of all AIPS data processing worldwide was devoted to VLA data
reduction.  Outside the NRAO, AIPS is extensively used for other
astronomical imaging applications, however.  56% of all AIPS processing
done outside the U.S. involved data from instruments other than the VLA.
The astronomical applications of AIPS that do not involve radio
interferometry include the display and analysis of line and continuum data
from large single-dish radio surveys, and the processing of image data at
infrared, visible, ultraviolet and X-ray wavelengths.  About 7% of all
AIPS processing involved astronomical data at these shorter wavelengths,
with 7% of the computers in the survey using AIPS more for such work than
for radio and another 7% of the computers using AIPS exclusively for
non-radio work.

     Some AIPS use occurs outside observational astronomy, e.g., in
visualization of numerical simulations of fluid processes, and in medical
imaging.  The distinctive features of AIPS that have attracted users from
outside the community of radio interferometrists are its ability to handle
many relevant coordinate geometries precisely, its emphasis on display and
analysis of the data in complementary Fourier domains, the NRAO's support
for exporting the package to different computer architectures, and its
extensive documentation.  


As well as producing user- and programmer-oriented manuals for AIPS, the
group publishes a newsletter that is sent to over 775 AIPS users outside
the NRAO soon after each semi-annual "release" of new AIPS code.  There is
also a mechanism whereby users can report software bugs or suggestions to
the AIPS programmers and receive written or email responses to them; this
has in the past provided a formal route for user feedback to the AIPS
programmers and for the programmers to document difficult points directly
to individual users.  With the 15JAN96 release, the old "gripe" system
will now have the ability to automatically submit problem reports directly
via email to the AIPS group within NRAO.  This is expected to complement
the existing informal approach involving support via electronic mail.  In
addition, a registration mechanism has been instituted that will enable
NRAO to keep track of the distribution of AIPS, as serious sites are
expected to register in order to obtain user support.

     Much of the AIPS documentation is now available to the World-Wide Web
so that it may be examined over the Internet (start with the AIPS home
page at "").  Also, this information is
available via anonymous ftp on the machine (
The NRAO knows of over 230 AIPS "tasks," or programs, that have been coded
within the package outside, and not distributed by, the observatory.

     There is a closed, moderated mailing list called "bananas" that
serves as a conduit for important announcements pertaining to AIPS, as
well as an occasional forum for questions and discussion about the
software.  To subscribe to this list, send an email to:

and put this in the BODY of the message:

	subscribe bananas

Once your request has been approved by the list owner, you will receive an
introductory message.  This mailing list also has a moderated two-way
mirror to the alt.sci.astro.aips usenet newsgroup.


     The AIPS group has developed a package of benchmarking and
certification tests that process standard data sets through the dozen most
critical stages of interferometric data reduction, and compare the results
with those obtained on the NRAO's own computers.  This "DDT" (Dirty Dozen
Tasks) package is used to verify the correctness of the results produced
by AIPS installations at new user sites or on new types of computer, as
well as to obtain comparative timing information for different computer
architectures and configurations.  It has been extensively used as a
benchmarking package to guide computer procurements at the NRAO and
elsewhere.  Two other packages, "VLAC" and "VLAL", are less widely used to
verify the continued correctness of continuum and spectral-line

     The "AIPSMark(93)" is often used as a measure of the performance that
a given machine will produce.  It, and other aspects of the DDT package,
are described in AIPS Memo 85.  The original baseline for the AIPSMark was
a Sparcstation IPX, set by definition at 1.0.  Since then, the details of
the test (i.e. the application programs) have changes slightly.  In
general, high-end Intel Pentium Pro 200MHz systems may give an AIPSMark of
around 3.3, high-end Sparc Ultra systems around 8 or 9, and top of the
line Digital Alpha and SGI processors may produce 12-14.  It is important
to note that the benchmark measures total system performance as it is
based on total elapsed time.  Thus, factors such as disk latency and
transfer rates, memory, swapping, and others are perhaps almost as
important as raw floating point performance.


     In 1992, the NRAO joined a consortium of institutions seeking to
replace all of the functionality of AIPS using modern coding techniques
and languages.  The aips++ project is expected to provide the main
software platform supporting radio-astronomical data processing sometime
around the turn of the century.  Future development of the original
("Classic") AIPS will therefore be somewhat limited, mostly to calibration
of VLBI data, general code maintenance with moderate enhancements, and
improvements in the user documentation.

     AIPS++ has now had its first limited Beta release.  More details can
be found on the AIPS++ web page at:

or, for a non-framed version:

In addition, an experimental version of AIPS called "CVX" was cloned in
1996 for the purposes of adding more support for single-dish Radio
Astronomy instruments (e.g. through programs for analyzing "on-the-fly"
mapping data), and new bandpass calibration and deconvolution methods.
Details of this system may be found via the main AIPS web page under the
title "Related Software".


    Since the 15JUL95 release of AIPS, the software is Copyright (C)
1995-1997 by Associated Universities, Inc., and is protected by the Free
Software Foundation's General Public License (GPL).  It is freely
available under the terms of this License on our web and anonymous ftp
servers.  However, there is a registration mechanism, and only those AIPS
installations that have registered with NRAO will be eligible to receive
any form of support.

    Prior to this release, AIPS was proprietary software issued to various
people under what is now an obsolete user agreement.  For academic or
educational or research oriented users, there was no charge for the
agreement, but there was a fee for commercial users.  This is no longer
the case.

    Why was this so?  Why did NRAO/AUI try to control distribution?  The
answer to both of these is twofold.  First, it was labelled as proprietary
code to prevent third parties from taking the code (for free), slightly
changing it, slapping a copyright on it and sueing NRAO to cease and
desist from distributing the original AIPS.  While this may sound
unlikely, this sort of thing has happened to others.  The GNU General
Public License now protects NRAO and our users from this sort of scenario
in a less restrictive way.

    Second, it is really important to us to have a clear picture of how
many users of AIPS there are out there.  Not only does this give us a
certain amount of leverage with hardware and software vendors, but it
helps to justify allocation of resources (people, computers) specifically
for continued support of AIPS.  That is why we have retained some flavour
of the old system in the new "register for support" scheme.  This has
already helped to provide a picture of both the extent of AIPS use, and
the type of hardware on which it is being installed, to the AIPS group.

    The AIPS group encourages all AIPS "customers" who intend to use the
software for Astronomical Research (especially those working in Radio
Astronomy) to register with NRAO when they receive and install a copy of
AIPS.  The installation procedure now does this for you automatically if
you approve, and will even e-mail in the registration form to us.
Eventually we hope to have a forms-based interface for registering on our
web pages (when we find that elusive spare time).

     Further information on AIPS can be obtained by writing by electronic
mail to aipsmail at or by paper mail to the AIPS Group, National
Radio Astronomy Observatory, Edgemont Road, Charlottesville, VA
22903-2475, U.S.A.  Use this address to request copies of AIPS or
ancillary documentation also.  Do not use it for UCE, SPAM, junk e-mail or
other net.abuse.


    The best source for information about AIPS Installation is the "AIPS
Installation Summary".  This document is updated for every release,
usually after extensive install-testing on many diverse architectures.  It
can be found online via the main AIPS web page, under the section covering
the latest release.  For those few people who still order tapes from NRAO,
a hardcopy version is provided.

    There is a separate, but unmaintained, document called the "AIPS Unix
Porting Reference".  However, this has not been updated since the July
1994 release, and significant portions of it are now either largely
irrelevant or have been incorporated into the Installation Summary
described above.  It might be somewhat useful for ports to new Unix or
non-Unix systems.

    In the future, it is hoped to make this available on-line via regular
web pages as opposed to a TeX/PostScript document.  Also, a section on the
most frequent problems will be added; this may eventually evolve into a
separate FAQ.


    The closest thing to such a document is probably the AIPS Cookbook.
It may be found online at <> in the
form of a table of contents and individual PostScript files for each

[**] The 1990 AIPS Site Survey", AIPS Memo
     No. 70, (Warning!  WordPerfect binary file!)  Alan Bridle and Joanne
     Nance, April 1991 

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