The Hipparcos Newly Reduced Astrometric Catalogue

A new reduction of the astrometric data as produced by the Hipparcos mission has been published by Floor van Leeuwen (Institute of Astronomy Cambridge), claiming accuracies for nearly all stars brighter than magnitude Hp=8 to be better by up to a factor 4, than in the original catalogue (further information). This new astrometric catalogue and several supplementary catalogues are accessible through the Virtual Observatory. Read on to learn how to find the catalogue and to query it.

Finding the Catalogue

Before querying the new Hipparcos Catalogue it must be found in VOExplorer. The following steps describe how to do this.

• Start the VO Desktop
• Open a VOExplorer window if one is not already open
• In the VOExplorer window, click New Smart List
• In the pane that appears enter a name for the new list and add the conditions as in the figure below (hint: clicking on the '+' icon adds another condition). When this is done click on Create.

• The new "New Hipparcos" smart list will contain a list of resources similar to the figure below. Note that this list is generated dynamically so the contents may vary if the services are modified in any way. If you select one resource in the list you get in the Information tab a description of it and some additional details. Every resource has a Curator associated with it and if there are problems with the service, this person should be contacted.

Querying the Catalogue - A Cone Search

A cone search is a Virtual Observatory Protocol allowing the retrieval of catalogue information on objects within a cone of given radius about a specified location. The section will describe how to perform a cone search of the new Hipparcos catalogue and to send the results to TopCat (see Helper Applications). Note that the cone search can only be performed on the main catalogue.

The guide will assume that the services have already been found, as per the instructions above, and the VOExplorer window is still open.

• Start the TopCat application
• Select the "New Hipparcos" smart list in VOExplorer to display matching resources
• Click to select the "Hipparcos catalogue" cone service, this will be represented by in the "Capabilities" column
• In the actions pane, select Query

• A new VOScope window will open
  • Enter a position or object in the At Position of Object Name box
    • e.g. enter Crab and hit your keyboard TAB button - after a short pause the object name will be replaced by its coordinates so for the Crab nebula you should see 83.633208,+22.014472
    • e.g. 145.4375,+54.4564
  • Enter a Search Radius, e.g. 180.0 in Degrees
  • Note that you can switch between Decimal Degrees and Sexagesimal by means of the radio buttons.
• Click the Search button - the search results will be visible in the main VOScope window
• Double-click the "Hipparcos catalogue" node to select it (the node will display the number of matching objects), it will change to yellow (see the figure below)

• In the actions pane, select Send tables to Topcat
  • Sending the file to TopCat might take some time depending on the number of results. Look for the activity indicator in VOScope during the transfer. Once the activity indicator ceases, it may take a while for TopCat to order the results internally, and display them.

• The table of results should now appear in TopCat (see the figure below)

• TopCat is a very powerful tool for viewing and analysing tables of data (see the TopCat help documentation for more information).
  • Clicking the (Spherical Polar Plot) icon to view the Newly Reduced Hipparcos object distribution over the sky (see the figure below, the colour of each data point has been configured to reflect the parallax of that object).

• And here a xy plot shows the RA, Dec distribution of the objects, colour coded aganst the number of observations per detection. This nicely shows the Hipparcos scanning law, a similar scanning strategy will be ued with ESA's GAIA mission in 2012.

Querying the Catalogue - An ADQL Query

ADQL or Astronomy Data Query Language is a standard language for querying astronomy databases. It is heavily based on SQL or Structured Query Language so if you are familiar with SQL you should find ADQL easy to understand. This section demonstrates how to perform an ADQL query of the new Hipparcos Catalogues.

The guide will assume that services have already been found, as per the instructions above, and the VOExplorer window is still open.

• Select the "New Hipparcos" smart list in VOExplorer to display matching resources
• Click to select the "Hipparcos catalogue" ADQL service, this will be represented by in the "Capabilities" column
• In the actions pane, select Build ADQL

• A new Task Runner window will appear, pre-configured for the ADQL query (see the figure below)

• Read Using Task Runner for more information on how to use the Task Runner window
• Enter an ADQL query such as one of the following examples:

SELECT TOP 100 * from maincat

Returns all columns (represented by the '*') of the top 100 rows in the 'maincat' catalogue which is the main astrometric catalogue.

SELECT m.HIP,m.Plx from maincat as m where m.Plx > 200.0

Returns the Hipparcos number and parallax columns from the main catalogue (which has been aliased as m) for objects with a parallax greater than 200 milliarcseconds.

SELECT Top 100 m.HIP, h.HD, m.Plx From maincat as m, hdtohip as h Where  (m.HIP=h.HIP )

This ADQL query creates a join between the 'HIP' column in the 'maincat' catalogue and the 'HIP' column in the 'hdtohip' which is a table relating Henry Draper numbers to HIP numbers. The query then returns the HIP and parallax columns from 'maincat' and the HD column from 'hdtohip' of the top 100 matching rows.

• Read General Help on Building Queries for more information on creating ADQL queries.
• After entering an ADQL query, click the Validate Edit button to check whether the entered ADQL query is valid - note that this is a syntax checker and it will not check whether any table or column names are correct.
• Clicking execute will send the query to the remote service, the status of the query will be visible in the "Execution" pane (see the figure below).

• When the query has completed, a link will appear in the "Execution" pane to the output file called Results.txt (see the figure below). Right-clicking this file will provide the option to view the file in your internet browser or save it to your disk.

• By default the format of Results.txt is VOTable which can be read into TopCat. The format of the output file can be changed to VOTable-binary, comma-separated or HTML format using the drop-down list in the "Input" pane (see the figure below).