Linking from Cartes du Ceil to the Virtual Observatory (and back again)

NEW 12th July 2015


Downloading to CdC from the VO and using VO applications to view and select the data


In my other page on using  Cartes du Ciel (CdC) with the Virtual Observatory(VO)  I described how you can download a position from a catalogue loaded from the VO to CdC via an application such as TopCat. This allows you to point your telescope at the position of the object in TopCat via CdC but does not draw the objects on the CdC sky chart.


Following a discussion with Michael Poxon on the CdC discussion group ( I discovered an alternative way that that enables the catalogue objects to be drawn on the CdC sky chart but also allows the VO to be used to select the particular object and to plot a whole sky chart of the particular catalogue.


In essence this method is the reverse of that on my other page, the high level steps are as follows:

  • Load the catalogue from the VO into CdC which can be done directly from CdC V3.10+

  • Set up CdC to draw the objects

  • Export the catalogue to TopCat using the VO Hub and SAMP communications protocol

  • Select items as required in TopCat

  • Send the position from TopCat to CdC using SAMP

  • CdC will then move the chart to the appropriate position

  • Your goto telescope will also slew to the position if it is connected to CdC

There are a couple of supplementary options:

  1. Plot the objects in TopCat all sky view so you can pick objects near each other or above your horizon

  2. Show the object position in Aladin so you can get a wide field or narrow field background image from the Digital Sky Survey (other images are available)

Loading the Catalogues into CdC from the VO


This function is covered in the CdC documentation here  you need to scroll down to the 'VO Catalog' section. The following is based round loading a catalogue of OB associations but there are currently 13,807 to chose from in  Vizier the VO web service CdC uses for catalogues.


1. Open CdC

2. Open the Setup - Catalog option

3. Click on the VO Catalog Tab

4. Click on the 'Add' button

5. In the Search box enter 'New List of OB associations' or what ever you are looking for

6. Catalog ID J/PaZh/21/13 should then be shown and highlighted

7. Click 'Select Catalog'

8. The Column headers will then be shown

9. You may want to change the colour by checking the force colour box and also select an object type not used such as Lozenge also increasing the size may help - unfortunately with this catalogue the symbol cannot be drawn using the object size in the catalogue.

10. Then click the 'Download Catalogue' button

11. The objects will then be loaded into CDC and should appear on the sky chart.

I had a bit of a problem here as I could not get CdC to search for an object even though it appears in the catalogue. As a result I am using the following work around.


Exporting the Catalogue to TopCat from CdC

CdC allows the broadcast of VO catalogues to other applications using SAMP. The following sequence of actions achieve this:

A. Open TopCat

B. Connect CdC to the SAMP hub ( under File - SAMP )

C. On the VO Catalog tab of CdC highlight the 'New List of OB

D. Rather than 'All SAMP Clients' Select 'TopCat'

E. Then Click 'Send Table to'

F. The table should then appear in TopCat

G. In the main TopCat window Activation Action select the 'Broadcast Row' box

H. Then click on Activation Action and a new window should appear

I. Click on the 'Transmit Coordinates' selection box and set the Target Application to 'Skychart'

J. The RA & Dec columns should be automatically selected

K. Click on Ok to close the window

L. In the TopCat main window Double click on the table name to open the table browser

M. If you now click on a row on the table to select the OB association you want TopCat will broadcast the Row and CdC should respond by moving the chart view to this position

N. The symbol you selected in 9 above should be shown, together with the label

O. As before if CdC is connected to your telescope it will slew to the selected object.

 TopCat Main Screen

This shows the TopCat main screen with the Catalogue of OB associations loaded as well as another VO catalogue of Supernovae. TopCat can have lots of catalogues open.

A couple of other possible actions

TopCat has an option to plot the table data ( received from CdC) in step F above using the Sky Plotting  facility. All you need to do is to select the button and select which fields in the catalogue represent RA & Dec. The table data points will then be shown on the surface of a sphere ( you are looking at the data from outside even if that is outside the Universe !). The sphere can be rotated by click and drag. If you click on one of the data points this will be selected in the TopCat table, broadcast to CdC and the CdC chart should move to the position.

A second option is to broadcast the row to Aladin as well. To do this start Aladin and in option 'I' above select 'All Clients' . NB you need to download Aladin rather than use the browser version.

TopCat Sky Plot

This image shows the TopCat sky plot with the OB associations plotted. Note this is in galactic co-ordinates and shows how the OB associations are in the plane of our home galaxy The Milky Way. The object selected in the plot is in row 72 of the table and is shown in the image below.

TopCat - Table Browser

CdC - OB Association SCO 2 D

Finally this screenshot from CdC shows that it has moved to row 72 the OB association SCO 2D.


John Murrell



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