The JCMT Newsletter Number 10
 3-D Visualisation of JCMT Data
Heterodyne receivers on the JCMT generate data which are
naturally represented as `data cubes': three-dimensional grids of
points where two axes correspond to positions on the sky (or more
accurately positions on the celestial sphere projected on to a plane in
some fashion) and the third axis to frequency, usually displayed as velocity.
It is difficult to display data cubes using conventional techniques.
For example, SPECX can display data cubes as a collection of
small contour maps, laid out on a grid, with each contour map
corresponding to a different position on the velocity axis. However, even
with this technique it is difficult to gain an impression of the overall
structure in the cube.
In recent years a number of special-purpose `visualisation packages'
have been developed, some as commercial products, others in the public
domain. Displaying data cubes is one of the major functions of these
packages and they contain powerful and sophisticated facilities for
doing so. Data cubes can be shown as animations (or `movies') showing
a slice moving through the cube or alternatively as iso-surfaces (the
three-dimensional analogue of contours). The viewing direction,
magnification and colour table can all be manipulated. The
visualisation package recommended by Starlink is Data Explorer (DX),
a commercial package from IBM. A number of Starlink sites have bought
it, mainly for use in displaying numerical simulations.
Recently Starlink has done some work to facilitate visualising JCMT
data cubes with DX. A new application has been written
to convert JCMT data cubes into a format which can be imported
into DX. specx2ndf converts a SPECX map file into a standard
Starlink NDF (N-Dimensional Data Format) data cube.
It is quite a sophisticated
application which allows the dispersion axis to be expressed as either
a frequency or as a radial velocity about the rest frequency of the line
relative to one of a number of standards of rest. Once the data cubes
have been converted to NDF format they can be imported into DX, as well
as being accessible to various other Starlink packages, such as KAPPA
(see SUN/95). specx2ndf has been added to
CONVERT (see SUN/55), a package which contains applications for
converting between various different formats.
Additionally SC/2, The DX Cookbook, has been enhanced to include
complete recipes for converting, importing and displaying JCMT
data cubes. Simple visual programs (`networks' in the jargon of DX)
for visualising the cubes are available with the Cookbook.
Figure 1: JCMT CO spectral line map iso-surface of the S106 molecular
cloud. Note the two peaks separated in velocity.
Figure 1 shows a single iso-surface (or three-dimensional
contour) in a JCMT data cube. These data are an observation of
emission from the C18O J = 2 - 1 transition in
the molecular cloud S106. The iso-surface contour level corresponds to a
brightness temperature of 1K. The cube is viewed so that
velocity increases from left to right, Right Ascension increases from
bottom to top, and Declination increases `into' the page (the units of
the axes have all been normalised to run in a range from 0 to 1 for ease
of viewing, but any values can be chosen). When viewed from this `angle',
the cloud can be seen to contain two main peaks, separated in velocity
space, with the secondary smaller peak showing up at a higher velocity
than the main peak.
Figure 2: The same iso-surface as in Figure 1, but viewed from an angle
roughly `orthogonal' to that in Figure 1. The second peak is no longer
visible.
Figure 2 shows the same iso-surface from a different angle,
orthogonal to the first. Now Declination runs from left to right,
Right Ascension still runs from bottom to top, and velocity increases
into the page. This view is closer to the traditional two-dimensional
rendering of an image of a molecular cloud. The secondary peak is
obscured by the main peak and the velocity detail has been lost somewhat.
Figure 3: The same iso-surface as in Figure 1, but viewed from an angle
somewhat `above' that in Figure 1, and `zoomed' in. The second peak is
clearly visible.
Figure 3 shows the same iso-surface from a different angle
again; similar to that in Figure 1, but zoomed in to discern the
second peak more clearly, and seen from slightly `above' the viewing angle
of Figure 1. The second peak can be seen at a different velocity
from the major part of the cloud emission, and hence is probably at a
different distance to the main cloud.
This example is very simple, but is meant merely to illustrate how
DX can be used to inspect structure in JCMT spectral line maps.
These Figures are just examples
of the types of visualisations which can be produced with DX. It is
straightforward to write DX networks to produce visualisations tailored
to your own requirements.
Finding out more
DX is not available at all sites. The use of DX
on Starlink systems is documented in SUN/203 and SC/2. There is a
general introduction to scientific visualisation, not specific to any
particular package, in SG/8. The CONVERT package is described in
SUN/55. Starlink software is available via the World Wide Web,
and DX may be purchased direct from IBM.
Clive Davenhall, Starlink, Edinburgh (acd@roe.ac.uk),
Derek Ward-Thompson, Royal Observatory Edinburgh (dwt@roe.ac.uk).
References
SC/2.3: The DX Cookbook, A.C. Davenhall, 1 October 1997, Starlink.
SG/8.2: An Introduction to Visualisation Software for Astronomy,
A.C. Davenhall, 4 March 1997, Starlink.
SUN/17.6: SPECX - A Millimetre Wave Spectral
Reduction Package, 13 January 1995, R.M. Prestage, H. Meyerdierks
and J.F. Lightfoot, Starlink.
SUN/55.7: CONVERT - A Format-conversion Package,
1 August 1997, M.J.Currie,
G.J. Privett and A.J. Chipperfield, Starlink.
SUN/95.10: KAPPA - Kernel Application Package,
5 June 1997, M.J. Currie, Starlink.
SUN/203.3: DX - IBM Data Explorer for Data
Visualisation, 15 September 1997, D.S. Berry, G.J. Privett and
A.C. Davenhall, Starlink.
Last Modification Date 1998/03/2 - Last Modification Author: gdw
Graeme Watt (gdw)
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