GAIA astrometric calibration

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GAIA astrometric calibration

GAIA makes extensive use of the Starlink AST library, the most obvious of which is it's ability to display celestial coordinates sourced from FITS and NDF data files, but it is also capable of much more sophisticated astrometric functions. A quick list of these is:

  • Viewing the offsets between positions.
  • Plotting astrometric grid overlays.
  • Contouring images using astrometric alignments.
  • Changing to other celestial coordinate systems (FK5, FK4, Galactic, SuperGalactic, Ecliptic).
  • Interactively creating astrometric calibrations.

Much of this is referenced in the on-line help.

The following images show GAIA performing one of the possible methods that you could use to astrometrically calibrate an image. In this case it is assumed that you know the rough centre, orientation and image scale.

After displaying your image you should select the "Image-Analysis"->"Astrometry calibration"->"Type in known calibration" toolbox. In this you can enter the image centre, scale and orientation (in this case flipped top to bottom and left-right as shown by the signs of the increments, for North at the top and East to the left the signs would be reversed). Press "Test", "Accept" and your image now has a rough calibration.

The next stage is to get some reference positions from an on-line catalogue. Choose "Data-Servers"->"Catalogs"->"USNO at ESO", press "Search" and you should get a list of positions that are displayed as circles with interior crosses over your image. Ignoring the magenta circles it should look something like:

If you're good or lucky then you should be able to spot the pattern between the USNO positions and your image, if not go back and check the bootstrap data, get a new initial solution and then press "Plot" in the catalogue window to refresh the positions.

The next stage is to use these positions to fit a proper solution. To do this open the "Image-Analysis"->"Astrometry calibration"->"Fit to star positions" toolbox.

Press the "Grab" button and choose the USNO catalogue. This copies the reference positions. Now you need to place the magenta circles in the correction positions. In this example I just unticked "Move markers individually" and moved all the positions together (by dragging on a circle until in was in the right place). I then unticked this option, pressed "Clip" to remove any positions not on the image, had a quick scroll around to remove any clearly bad positions and pressed "Centroid" to position the circle accurately and finally "Fit/Test" to get the accurate solution. The final act was to save the image to disk with it's new astrometric calibration.

As I said above this is just one way to get a solution, you can also transfer reference positions from another image (probably DSS downloaded using "Data-Servers"->"Image Servers"), copy a solution from another image (another type of bootstrap), or refine an existing solution by adding in changes.

In the near future it should also be possible to achieve much of what has been described here "automatically" using a new tool "Autoastrom" (in fact a solution for this image has been produced in that way)...

Questions or comments to:

Copyright © 2005 Central Laboratory of the Research Councils
Copyright © 2006 Particle Physics and Astronomy Research Council
Copyright © 2008-2009 Science and Technlogy Facilities Council
Copyright © 2009-2013 Peter W. Draper
Last modified: 02-Jun-2016
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