The CGPS Archive at CADC
by Daniel Durand, CADC, HIA, NRC
The CGPS project in short
The Dominion Radio Astrophysical Observatory ( DRAO ) at NRC’s Herzberg Institute of Astrophysics, in collaboration with an international consortium of astronomers, is carrying out a high-resolution spectral survey of the atomic hydrogen and radio continuum emission from our Milky Way galaxy. O bservations of 373 fields with the DRAO Synthesis Telescope are presented in sixty-eight 5 x 5 degrees mosaics, covering Galactic longitudes from L = 63 to 175 degrees and latitudes from B = -3.5 to +5.5 degrees and a latitude extension to B = +17.5 degrees, between L = 100 to 116.5 degrees, with a resolution of about 1’ at 1420 MHz. For each mosaic (1024 x 1024 pixels), atomic hydrogen observations are presented as a data "cube" with 272 spectral channels having a velocity resolution of 1.3 km/s. Stokes I continuum images at 1420 MHz and 408 MHz are produced. Complementary images in the four IRAS infrared bands and data cubes of CO (J=1-0) emission have also been created as part of the s urvey. In addition to the 5 x 5 degrees mosaics, a series of nine 15 x 15 degrees mosaics of 408 MHz emission, and possibly other lower radio frequencies, will be created.
The CGPS archive has been developed by the HIA ’s CADC ( NRC ) in collaboration with the staff at the DRAO , NRC . Mosaics are archived as they are processed and released to the consortium. Archived data are in FITS format, except for informational text files and are stored within the CADC’s storage system on hard drive for fast access.
The new CGPS interface
One of the biggest challenges related to the development of the CGPS archive is dealing with large data cubes which makes images quite difficult to view and manipulate. The new CGPS archive interface which has been released to the community this last May is now offering ways to navigate through the data cube in a very user friendly way. In addition to the regular browse and retrieve functions, users are now capable of navigating through any data sets, overlaying any published catalogues from the Vizier interface thanks to the new Aladin display interface which was developed by the staff at the Centre des Données de Strasbourg. Let’s pass through the basic features.
The basic navigation
a) Regular browse and fetch
We are using the regular web interface to catalogue data to browse through the CGPS collection. Just a few fields need to be specified (like L,B or RA,DEC) to reach the desired field.
b) Data exploration
There are two ways to navigate through the data. The “Data Visualisation” button allows the user to display a specific velocity plane using a selection panel if the data is 3D data cube, or a direct display if the data is only 2D continuum images.
But the most interesting interface is the “Mosaic Navigation”. When clicked, the Aladin interface appears as well as the list of data files for this specific area of the sky. The user can then select the data file of interest and it will be displayed. If the data is 3D, a panel will allow the user to select the desired velocity plane. More of these velocity planes could be displayed with Aladin allowing fast comparison.
Examples:
Display of the metadata information, allowing user to display, preview and retrieve a given file.
This panel appears when selecting a data cube. Please note the channel/velocity plane selector at the bottom.
In this demo, after displaying a selected velocity channel, we used the load command from the ALADIN interface to overlay NVSS sources (in red) and radio sources from another catalogue (in blue). When selected on the image display, more information about these sources is displayed at the bottom of the Aladin window.
c) Aladin specifics
Aladin is a very rich and complex tool. Prospective users are advised to consult the guide document available on the Aladin web site (see at the end of this article for references). Aladin can display images and overlay known catalogues from the Vizier collections as well as other surveys. The user could load local catalogues and images as well. Multiple images could also be displayed side by side, up to 16 at a time, using the multiview button. For example, CGPS users could compare IRAS images with CO very easily.
Other capabilities
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Fetch the data from your command line
On most UNIX platforms, there are a few commands available to save into a file any information pointed to by an URL. These standard commands are curl and wget , and should be available on most UNIX-like systems. They can be used to obtain CGPS data from CADC directly from your command line. The examples below use curl:
How to get an image directly from CADC :
curl -o mev1_h1.fits “ http://www.cadc-ccda.hia-iha.nrc-cnrc.gc.ca/anonProxy/getData?file_id=CGPS_MEV1_HI_line_image&archive=CGPS&wcs=original ”
Just change the image name to get a different one.
How to get an image slice directly from CADC :
curl -o mev1_hi_116.fits “ http://www.cadc-ccda.hia.iha.nrc-cnrc.gc.ca/anonProxy/getData?file_id=CGPS_MEV1_HI_line_image&archive=CGPS&cutout=[ *,*,116:116,*]”
Just change the image name (look in the catalogue on the CADC interface) and the plane number to get a different one. In fact, we are using the IRAF syntax: cutout=[1:100,1:100,100:110,*] will extract channels 100 to 110 for an image section of 100 X 100.
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Cookbook
The CADC is in the process of writing an extensive cookbook for CGPS users. It should be ready by the end of the summer. Please check the main CGPS web page for more information.
The future
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Virtual Observatory access
The CADC is now very involved in designing, developing and implementing the Virtual Observatory ( VO ). It is our plan to implement access to the CGPS through the various VO interfaces. We are waiting for the implementation of the necessary standards suitable for the specific nature of radio data. These standards should be available in late fall. Then the interfaces will be able to be modified accordingly.
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VPGS
On a more rapid timeline is the extension of the CGPS done at the VLA ( VGPS ). This should appear at CADC in early fall.
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SGPS
CADC is also planning to integrate more Galactic plane surveys and information under the same tools like the SGPS and the H-alpha surveys.
Acknowledgements and references
The CADC would like to thanks Lewis Knee and Roland Kothes for the fruitful discussions and reviews which led to the release of the new interfaces. We also thank François Bonnarel and Pierre Fernique at CDS for having implemented the necessary modifications to their Aladin tool to allow the navigation of 3D radio data.
CADC: http://www.cadc-ccda.hia-iha.nrc-cnrc.gc.ca /
Aladin: http://aladin.u-strasbg.fr
Vizier: http://vizier.hia.nrc.ca /
CGPS: http://www.cadc-ccda.hia-iha.nrc-cnrc.gc.ca/cgps/