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
cl> gaiadisp [image_file]
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).
| 
