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Astronomical Data Analysis Software and Systems VII
ASP Conference Series, Vol. 145, 1998
R. Albrecht, R. N. Hook and H. A. Bushouse, e
Ö Copyright 1998 Astronomical Society of the Pacific. All rights reserved.
ds.
Recent Developments in Experimental AIPS
Eric W. Greisen
National Radio Astronomy Observatory 1
Abstract. An experimental version of AIPS is being developed by the
author with the intent of trying new ideas in several areas. Copies of this
version are made available periodically. For interferometric data, there
have been significant developments in spectral­line calibration, data edit­
ing, wide­field imaging, self­calibration, and analysis of spectral cubes.
For single­dish data, there have been improvements in the imaging of
``on­the­fly'' spectral­line data and new programs to analyze and image
continuum, beam­switched data. Image display techniques which retain
their state information to any other computer running this version have
been developed. Results from several of these areas are presented.
1. Introduction
The Astronomical Image Processing System (``AIPS'', Greisen 1990; van Moorsel
et al. 1996) was developed at the NRAO beginning in the late seventies. Since
then, we frequently encountered the conflict between the desire to develop new
ideas and capabilities and the desire to provide to the users a stable, reliable
software package. Some new capabilities are simply new programs added with
little or no e#ect on current AIPS usage. However, since AIPS is in many areas
a tightly connected system, some new ideas are potentially disruptive since they
a#ect, for example, how all interferometer data are calibrated or how image
models are computed. The conflict between stability and the desire to continue
to develop new ideas led the author, beginning in April 1996, to develop a
separate, openly experimental version of the AIPS package called AIPS CVX.
This version contains a variety of new and improved capabilities developed by
the author as well as the full functionality of the version that is sent out by
NRAO. Periodically, a copy of CVX is frozen in tar files and made available
for down­loading over the Internet. The current state of CVX is described on
the author's home page 3 which includes instructions, comments, and links for
installing the most recent copy of CVX. The remainder of this paper is used to
describe some of the highlights of the new code.
1 The National Radio Astronomy Observatory is a facility of the National Science Foundation
operated under cooperative agreement by Associated Universities, Inc.
3 http://www.cv.nrao.edu/”egreisen
204

Recent Developments in Experimental AIPS 205
2. Changes in infrastructure
The ability to allocate memory at run time for use by fortran routines was
added to AIPS just before the initiation of CVX. The memory is actually allo­
cated by a C­language procedure with malloc which passes the start address of
the memory back as a subscript into an array whose address was provided by the
fortran calling routine. A top­level ``Z'' routine acts as an interface between
the programmer and the C procedure and manages the allocation/deallocation
in an attempt to avoid memory leaks. Dynamic memory is now used where
appropriate in CVX in order to scale memory to the size of the problem and to
avoid, for example, burdening continuum users with large spectral­line arrays.
The AIPS ``TV'' image display has always been ``stateful.'' A program
could query the system to determine which portions of the display are visible
and what the coordinates and other header parameters are for the image(s) dis­
played. This was implemented through disk files containing device descriptions
and other files containing the display catalog. This technique tied the displays
to a particular local­area­network, a particular computer byte ordering, and a
hand­maintained set of link files for the cooperating workstations. In CVX, for
the now ubiquitous X­windows form of TV display, I have eliminated both sets
of disk files, placing the device and catalog information inside the software (XAS)
which implements the display. I have simplified the machine­dependent routines
to remove all knowledge of the meaning of the bytes being passed. Instead, the
packing and unpacking of data to/from the TV (including conversion to/from
network­standard integers and floats) is done by the routines which must know
about the TV, namely XAS itself and the XAS version of the TV virtual device
interface (the ``Y'' routines). Among numerous benefits, this simplifies the addi­
tion of new capabilities and allows passing of data words of any length and type.
To implement the remote catalog, machine­independent routines convert the
AIPS header structure to/from standard FITS binary forms. XAS was enhanced
to do initialization and vector and character generation internally, reducing the
amount of data which has to be sent from the calling programs.
XAS communicates with programs running on the same and di#erent com­
puters through an assigned Internet socket. It was found that, when the socket
is busy for one program, a second program's request for the socket is apparently
honored even when XAS is coded not to queue requests. The second program
then hangs waiting for the socket to respond to its opening transmission. In
the past, this undesirable delay was avoided by doing file locking on the image
device file (a shaky proposition over networks) which will respond immediately
when the file is already locked. To avoid this hanging in CVX in which there
are no disk files for locking, a new TV lock server was written. The Internet
socket to it may still hang, but the lock server does so little that the hanging
time is negligible. In this way, any computer running CVX may display images
on any workstation anywhere in the world running CVX's XAS server. Collisions
are noted immediately so that calling programs need not wait for busy displays.
Pixel values, coordinates, and other parameters of the displayed images are then
available to the compute servers. Thus, a scientist in Sonthofen might reduce
data in a large compute server in Socorro while interacting with displays run on
the local workstation. The only limitations are security restrictions on Internet
sockets (if any) and available data rates.

206 Greisen
3. Processing of interferometric data
A major area of experimentation in CVX has been the calibration of interfer­
ometric data, especially multi­spectral­channel data. Because the software did
not handle time dependence properly, users were required to handle the antenna­
based, channel­dependent complex gains (the ``bandpass'') as if they were con­
stant or only slowly varying in time. I changed the bandpass solutions to use
weighting of antennas in solving for and averaging bandpasses and corrected the
bandpass application code to apply time­smoothed calibrations properly. These
two corrections have allowed overlapped spectra to be ``seamlessly'' stitched to­
gether for the first time (J. M. Uson and D. E. Hogg, private communications).
It has been long believed that VLA spectral­line images should, but do not,
have noise which is independent of channel. Using a variety of new techniques
to record and report the failure of calibrator data to contain only antenna­based
e#ects, I have found that these closure errors are a function of spectral channel
that closely matches the noise spectra found in the final image cubes. Since
some configurations of the VLA correlator have very much worse closure failure
than others we hope to find and correct the hardware cause of the problem.
Better calibration is only useful if bad data are properly excluded. With
instruments as large as the VLA and VLBA, it is very time consuming to exam­
ine all the data and bad data are not always readily apparent (i.e., unusually
small amplitudes tend to be invisible). Using object­oriented programming in
fortran (Cotton 1992), I have developed an editor class which allows the user
to interact with the visibility data, or various tables associated with those data,
to prepare tables of deletion commands. It has been found that editing based
on the antenna system temperatures is not particularly time consuming and
detects a very large fraction of the data which should be deleted. The editor
class uses the AIPS TV display as a multi­color graphics device to display a
function menu, the data to be edited, and data from other antennas/baselines
for reference. The editor class is implemented in normal AIPS CVX tasks to edit
uv data using the system temperature and the uv data themselves. Since it is a
class, I inserted it as a menu choice in the iterative imaging and self­calibration
task SCMAP by adding only about 100 lines of code some of which pass the new
user parameters needed by the editing. An example screen from SCMAP may be
found in the author's home WWW page previously cited.
AIPS has long supported the imaging and deconvolution of # 16 fields
surrounding the direction toward which the telescopes were pointed. Among
other things, this allows for removal of sidelobes due to distant sources from
the field of primary interest. Previously, the phases of the data were rotated to
the center of each field before imaging, but no other geometric corrections were
made. This makes each field parallel to the tangent plane at the antenna pointing
direction. Such planes separate from the celestial sphere very much more rapidly
with angle from their center than would a tangent plane. Therefore, I have
changed CVX to o#er the option of re­projecting the data sample coordinates
for each field so that all fields are tangent planes capable of a much larger
undistorted field of view. (In direct comparisons, the re­projection surprisingly
adds virtually no cpu time to the job.) This option requires each field to have
its own synthesized beam image and to be Cleaned in a somewhat separated
manner. In CVX, up to 64 fields may be used; source model components may

Recent Developments in Experimental AIPS 207
GREY: 3C336 IPOL 4835.100 MHZ 3d Model.ICLN.2
Grey scale flux range= ­40.0 400.0 MilliJY/BEAM
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RIGHT ASCENSION (B1950)
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GREY: 3C336 IPOL 4835.100 MHZ 3d Model.ICLN.1
Grey scale flux range= ­40.0 400.0 MilliJY/BEAM
0 2 4 6
DECLINATION
(B1950)
RIGHT ASCENSION (B1950)
16 31 08.5 08.0 07.5 07.0 06.5 06.0 05.5 05.0
24 41 20
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Figure 1. Model image with (left) and without 3D correction.
be restored to all fields in which they occur, rather than just the one in which
they were found; and a Clean search method may be used to minimize the
instability that arises if the same direction is searched in more than one field.
The results of using the ``3D'' option are illustrated in Fig. 1 which shows the
Cleaned image of a model source separated from the pointing direction by 2 #
(18575 synthesized beams) and separated from the field center by 14.9 arcsec (36
synthesized beams). The non­signal rms of the uncorrected image is 35 times
that of the corrected one. The source model found with 3D­Cleaning may be
used in all other AIPS tasks in CVX, in particular for self­calibration. Since
sidelobes of distant sources are frequency dependent, the ability to make and
subtract accurate models of such sources greatly increases the spectral dynamic
range and the field of view free of systematic errors in multi­channel images.
References
Cotton, W. D., 1992, ``Object­Oriented Programming in AIPS fortran,'' Aips
Memo No. 78, 4 NRAO.
Greisen, E. W., 1990 ``The Astronomical Image Processing System,'' in Acqui­
sition, Processing and Archiving of Astronomical Images, eds. G. Longo
and G. Sedmak, (Osservatorio Astronomico di Capodimonte and Centro
di Formazione e studi per il Mezzogiorno), Naples, Italy. Also appeared
as AIPS Memo No. 61, 1988, 3 NRAO. See also Aips Memo No. 87
van Moorsel, G., Kemball, A., & Greisen, E. W. 1996, ``AIPS developments in
the Nineties,'' in ASP Conf. Ser., Vol. 101, Astronomical Data Analysis
Software and Systems V, ed. George H. Jacoby & Jeannette Barnes (San
Francisco: ASP), 37
4 html://www.cv.nrao.edu/aips/aipsmemo.html