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GAIA on the web

Note:This document was written for the Starlink Bulletin when GAIA first merged with Skycat and gained on-line catalogue access. It does not represent the current state.

The latest release of GAIA, the "Graphical Astronomy and Image Analysis Tool", has a range of new, astronomically interesting features. These include accessing and displaying catalogue, archive and image data obtained from the World Wide Web and displaying, creating and modifying image astrometry calibrations.

The new web browsing facilities are a product of changing GAIA to use the SkyCat tool from the European Southern Observatory (ESO), and the new astrometry features from converting to the Starlink astrometry library (AST). Existing users of GAIA shouldn't let these changes concern them, as it retains all its previous abilities for doing image display, colour table manipulations, aperture photometry, blink comparison, image patching and so on.

Taken all together, these new features make GAIA the ideal tool for investigating the objects and environment of your data (or target data), producing annotated images and determining sky coordinates.

On-Line Catalogues

The catalogues that are available at present are: ABELL, GSC, IRAS PSC, PPM, PPM1, QSO, RC3, USNO and ZCAT, all of which are made available through services at ESO and the Canadian Astronomy Data Centre (CADC). Successful queries to these servers result in the return of a list of positions (plus other data, if available), which are then identified on your image as markers of various shapes (assuming your data has a suitable astrometry calibration, of course). The result of two such queries is shown in Figure 1.

Figure 1: This image shows the results of making two queries, with identical parameters, on the Guide Star (GSC) and U.S. Naval Observatory (USNO) catalogues. The GSC positions are shown as squares and the USNO positions as circles. Not surprisingly, the catalogues are well correlated as the USNO positions are calibrated using the GSC. Such queries are useful to verify the astrometric calibration of your images. Contrary to the impression given in this picture, the USNO catalogue is just as useful as the GSC as the two can give different coverage.

In addition to these "normal" catalogues, you can also query the NASA/IPAC Extragalactic (NED) and SIMBAD databases about your field (these are also used to provide name resolution services). The result of a NED query on a field about the galaxy NGC1275 is shown in Figure 2. Some bibliographic details (mainly about where the object position originated) may also be available with links to the NASA Astrophysics Data System (ADS) where you can view the associated abstract.

Catalogues can be saved to local files, so you can re-plot the positions and manipulate them (once saved to a local file you can sort, add and remove entries from the list of positions).

Figure 2: GAIA is shown here displaying the result of a NED query on the region about NGC1275. Each of the boxes indicates the position of one of the objects listed in the query window. Further information about the object can be obtained using the "More Info" button, which starts up Netscape.

Data Archives

Using the SkyCat query facilities you can find out about any observations that have been taken by the HST and the ESO New Technology Telescope (again courtesy of CADC and ESO). The result of such a query on the field displayed in Figure 2 is shown in Figure 3. It is possible (when the data have been released to the general community) to see previews of HST observations. One such preview is shown in Figure 4. You can also find out more details about the observations by going on-line with Netscape (using the "More Info" button).

Figure 3: Results of a query on the HST archive at CADC for the NGC1275 field, shown in Figure 2. As well as the fields shown, the release date and the dataset name are available. The result of using the "Preview" option on one of the fields is shown in Figure 4.

Figure 4: This image shows one of the HST archive previews that are available for NGC1275. It also shows a grid overlay drawn by GAIA.

New Astrometry Facilities

The advantages gained by changing to the Starlink astrometry library (see Starlink User Notes 210 or 211, if you'd like to find out more about the details of this library) are that GAIA can now accept astrometry calibrations stored in most FITS "standards" (this includes the draft FIT-WCS) and plot very sophisticated celestial coordinate grids. It has also opened the door to a world were the creation and modification of astrometry calibrations is considerably easier. In GAIA you can see this flexibility in action in new toolboxes for changing the image celestial coordinate system and graphically fitting or modifying the astrometric calibration.

Just to show how good the graphics facilities are, two examples are shown in Figures 5 and 6. AST (and GAIA) can of course also draw more usual grids -- see the front page of this bulletin and Figure 4. Control is available over which parts of the plot are drawn, the colours, widths, scales, fonts, titles, labels and so on. It is also possible to change the celestial coordinate system (so that you can plot grids in FK5, FK4, Ecliptic, Galactic, SuperGalactic and Geocentric Apparent coordinates), or you can just plot a grid or set of axes in pixel coordinates. Naturally, this can all be printed to a postscript file for creating overheads.

Figure 5: This image shows GAIA displaying some results from the COBE satellite (obtained from SkyView). The overlay shows Galactic coordinates using a zenithal equal area projection.

Figure 6: This image shows a grid overlaid on a region of sky that includes the equatorial North Pole (FK5/J2000). The image is taken from the DSS.

You might say at this point, "that's really great, but how can I plot grids like that for my images"? The answer is to use the new toolboxes to astrometrically calibrate your image, after which plotting a grid is a couple of button presses away. There are four new toolboxes (in addition to the one that controls grid plotting), that allow you to:

• use reference stars to fit an astrometric calibration,
• copy calibrations from other images,
• use your knowledge of the image orientation, pixel scale and a reference point to define a calibration,
• transform a calibration to another celestial coordinate system,
• or finally allow you to "tweak" a calibration (that is add small corrections of offset, scale and rotation ) to make the fit "better".

The nice part of doing this type of work in GAIA is that it all happens graphically, so you can see where your reference positions really are, you can drag them around over your image and see them "jump" to their new positions when you make a fit etc.

Obtaining and identifying reference positions has also never been easier, as you can get these directly from on-line astrometry catalogues (i.e. GSC and USNO) and you can plot these over DSS images, before transferring the information to yours. Checking the quality of the calibration is also trivial, just look at any catalogue positions you have and/or read off some test positions.

There are many ways in which you can use the astrometry toolboxes to add an astrometric calibration, but the one that I've become used to while developing them, is to add a rough calibration (either by using a few reference positions, or by copying a calibration from a similar, say dithered, image, or as a last resort by trying out an image scale and reference position) and then to completely re-fit the image using all the positions I've got from one of the on-line astrometric catalogues...

Other changes

In this new release GAIA has also been changed in many other ways, not mentioned above, some of which are also the result of upgrading to more recent releases from ESO and some of which have been requested. A not very long or exhaustive list follows:

• GAIA can now be used from the IRAF CL command line (note this doesn't mean that it works with the cursor commands etc.).
• A new toolbox for getting image statistics has been added.
• The image slice can now be saved to a file.
• The main control panel can now be hidden at any time (good for large images and small displays).

To try out GAIA for yourself type the following command:

   gaia [image_file]


from the C-shell. Or alternatively:

   cl> gaia


if GAIA is installed on your system.

Other images...

Just for reference here's the image that was used on the bulletin front page.

Caption:
The picture shown above was produced by the latest version of GAIA, the "Graphical Astronomy and Image Analysis Tool".

The new release of GAIA also has a full suite of tools for graphically setting and modifying image astrometry and can browse catalogues and data archives using the WWW. See the GAIA article above for more. The grid overlay and annotations are drawn using the new Starlink Astrometry Library (AST), which is now generally available for use by C and Fortran Programmers.

The continuum subtracted H\alpha image of LMC was obtained by Dr. Mike Bessell from Mt. Stromlo Observatory (courtesy of Paul Crowther, UCL).

And one that didn't make in into the article.

This shows a region in orion and displays an offset ruler (this is activated using mouse button 3).