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Allen, M. G., Bonnarel, F., Boch, T., Fernique, P., & Louys, M. 2003, in ASP Conf. Ser., Vol. 314 Astronomical Data
Analysis Software and Systems XIII, eds. F. Ochsenbein, M. Allen, & D. Egret (San Francisco: ASP), 261
Organization of Data Sets in Virtual Observatories
Mark G. Allen, François Bonnarel, Thomas Boch,
Pierre Fernique
Centre de Données astronomiques de Strasbourg, France
Mireille Louys
Laboratoire des Sciences de l'Image, de l'Informatique
et de la Télédetection, France
Abstract:
Browsing and accessing local and distributed datasets
is an important aspect enabling Virtual Observatories. We present
an example implementation of a data tree in the Astrophysical
Virtual Observatory prototype tool. This is a dynamically built
data tree containing information on image datasets, based on
the IDHA data model. We show how the
meta-data representation of the GOODS dataset in the AVO demo allows
efficient data browsing and selection, and how the tree may be
used to access local and distributed data. This capability is being
developed in the framework of the CDS Aladin image browser,
and AVO prototypes.
Browsing and visualization of image datasets will be an
important part of Virtual Observatory operations. Such
datasets may range from a small set of images stored on
a local disk, to the tera-byte collections of modern
surveys. Standardized and scalable descriptions of image
metadata will be required to enable dataset browsing,
selection and visualization.
The data tree that was developed for the Astrophysical
Virtual Observatory (AVO)
1st year demonstration represents a prototype implementation of a
scalable, hierarchical metadata description
(in
VOTable/XML)
for image datasets. The data tree mechanism allows
any image data available via URL to be described and
accessed via an hierarchical tree.
Here we provide a very brief outline of the image
metadata description that forms the basis of this data access
scheme, and show how the data tree is used in the AVO
prototype to enable ``smart browsing'' and selection of data.
Figure 1:
The Data Tree in the AVO Prototype. The lower right
Server Selector window shows a request sent to the Aladin image
server for data available in the CDFS, and the resulting list
of images. This tree is presented in the Treeview window
along with the results of subsequent requests. The fields of
view of images in the tree are interactively shown in the
top right visualization window, where the images can be selected
and loaded into the image and catalog plane stack.
 |
The information requirements for the VOTable description
of the data tree have been designed to be flexible
enough to allow for a simple listing of a set of images,
or a rich description based on the
IDHA
data model. Details on the required, and optional fields, including
an annotated example are described on the
Euro-VO web page.
The IDHA data model seeks to provide a generic description
of astronomical data. Using the model as a basis for image
metadata description is beneficial because data described in a
tree that conforms to the IDHA model schema can be organized into
nodes corresponding to objects of the model. Also uniform
description of the image metadata (such as 'coordinates' and 'observed
wavelength') allows software interfaces to make
use of this information for data browsing and selection.
Such capabilities have been built into the AVO prototype.
Figure 2:
Field of view display of the GOODS ACS image tiles. The
original image tiles are shown, along with the currently selected tile.
The small square shows the outline of the image cut-out centred on the
cursor position, that can be generated by the Aladin image server on request.
 |
Figure 1 shows the data tree interface in the AVO prototype.
The hierarchical tree displays information on the image data
available in a given region of the sky. In this example
the tree was dynamically generated by the Aladin image
server as a result of a request for data within a 
0.5
radius of the Chandra Deep Field South (CDFS). The results
show the GOODS data (Giavalisco et al. 2004) in the Aladin image
server, including WFI, ISAAC, HST-ACS and Chandra images.
The images are organized by instrument, bandpass and epoch
and the interface provides a mechanism to interactively re-sort
the nodes based on the metadata.
The metadata stored in the tree is utilised
to provide an efficient means for ``smart browsing'' and selection
of data. Browsing the cursor over the tree causes the field
of view outlines for each tree node to be displayed. An example
is shown in figure 1 where image tiles of a single epoch
of the CDFS ACS images are overlaid on a WFI image.
Conversely, browsing the cursor over the image highlights nodes in the
tree when data are available at the cursor coordinate. This
interaction between the tree and the image display
provides an efficient means to simultaneously select all the
data available at a given point in the sky.
Image servers that provide image cut-outs are also
supported by the full IDHA implementation of the
data tree. Figure 2 shows the
fields of view of the original HST ACS image tiles,
plus the outline of the image section that would be
generated if the data request was submitted to the
image server.
In addition to data stored in the Aladin image server,
images available via the Simple Image Access (SIA)
protocol, and indeed any images available via a direct
URL including images on a local disk, may be accessed
and viewed in the tree with their field of view outlines
shown in the display. Interfaces to the SIA servers for
SkyView,
and the
NOAO Science Archive
have been implemented in the prototype. For example, the result
of a query to SkyView for the CDFS is shown in the tree in
figure 1. Data trees for images available via direct URLs
may be constructed and loaded into the prototype. Such
trees have been demonstrated for WFPC2 association images
served from ESO, and for VLA data served from Jodrell Bank.
Data trees for local images may be automatically generated
by specifying the top level directory at the load interface.
The data tree functions developed for the AVO prototype
are also fully enabled in the Aladin 2.0 release (Bonnarel,
Fernique & Boch 2004).
References
Bonnarel, F., Fernique, P., Boch. T. 2004, this volume,
221
Giavalisco, M. & the GOODS team, 2003, ApJ in press
© Copyright 2004 Astronomical Society of the Pacific, 390 Ashton Avenue, San Francisco, California 94112, USA
Next: MMDSL: a Multi-Mission Data Server at LAEFF
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Previous: DRACO and the Italian Participation in Virtual Observatory Activities
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