Table of contents

1. Visualisation of Catalogs
2. Manipulation of Catalogs
3. Cross-Matching of Catalogs

Visualisation of Catalogs

1. VOplot visualisation of Vizier catalogs

Select catalogs from the Vizier Catalog service choosing "HTML table" as the output format, and selecting decimal degrees for the coordinates.

VOPlot may then be launched from the bottom of the results page, and provides a simple plotting tool as an applet in a web page. [Does not work correctly when sexagesimal units are chosen]

2. VOPlot standalone application

VOPlot is designed for plotting data in VOTable format. A related application conVOT is available at the same location, can be used for converting between various table formats.

VOPlot is a simple 2-d plotter with advanced statistical functions for rank correlations and significance tests. Additional columns, and subsets of points can be defined.

A 3-d version VOPlot3D, and a version for handling large (106) numbers of rows is also available.

3. Topcat

Topcat is a download-able application for handling tables in virtually all formats used in Astronomy. The underlying table processing libraries are also available to download and use from a command line.

It includes plotting facilities in 2- and 3-d, as-well as histograms, density and spherical polar plots. It is very flexible for converting tables between formats, and quickly visualising table and catalog data. It is 'astronomy aware' in terms of having built in functions for converting coordinate units. It provides spreadsheet-style presentation of tables and their associated meta-data, and allows for sub-setting and rearrangement of columns.

4. Aladin catalog visualisation

Aladin is a tool for catalog and image data, as-well as general access to VO and other on-line resources. It allows display of catalog points overlaid on any image with sky coordinates, and can display different astronomical coordinate projections. It provides means for filtering catalogs to change the way points are displayed, for example points varying in size proportional to another column in the catalog. Help, manuals and tutorials on its use are available at the web site.

Manipulation of Catalogs

VOPlot, Topcat and Aladin all allow manipulation of catalog data including definition of new columns, and making subsets of the rows based on mathematical expressions. VOplot and Aladin allow relatively simple expressions, and Topcat includes more sophisticated expressions by utilising math libraries. Aladin allows a more interactive visualisation of catalog manipulation of objects projected on sky images. Topcat includes a interactive tool for selecting regions of any shape in 2- and 3-d parameter space plots.

The overall capabilites for manipulating catalogs in these interactive tools are however limited compared to a programming environment, but the flexible input and output (especially Topcat) make it convenient to use these tools alongside more robust programming or scripting tools.

Cross-Matching of Catalogs

Cross matching of catalogs is an important capability in the VO where one of the main scientific drivers is to combine data and information from different sources.

Cross-matching tools

Cross-matching capabilities of various levels are available in the Aladin and Topcat tools.

Aladin - positional cross matcher based on sky-distance, with ability to specify closest matching, all matching, or non-matching sources in 2 input catalogs.

Topcat - multiple table positional cross match (coordinates need to be decimal degrees)

Discrimination based on positional uncertainties is not done automatically in these tools, although it is usually possible to propagate the required uncertainties via column manipulations albeit cumbersome.

Cross-matching services

There are on-line services which can perform cross matches of tables that are made available as "skynodes", for example the NVO OpenSkyQuery interface (http://openskyquery.net/Sky/skysite/). These include some of the very large Astronomical catalogs such as SDSS, USNO etc. Currently this service is however limited to matching 5000 sources.