The GAIA tool developed by P.W. Draper at Durham is particularly well suited to preparing masks for use with TAURUS++ on the AAT. This is the demonstration we give below. However, note that the target astrometry has been prepared from an image which is astrometrically flat to high accuracy, in this case, the DSS survey. If you are preparing an astrometric catalogue from your own image direct from the telescope, you will no do doubt know that all, yes all, instruments have astrometric distortion. This document is of no use to you until you have removed gross radial distortions from the image. (GAIA, like all other graphics packages we are aware of, does not allow for radial distortion patterns, which is the most common form of distortion.) GAIA does allow for a moderate amount of distortion correction of the form described by orthogonal polynomials.
The AAT focal plane is flat to very high accuracy, which is why we make our mask with respect to a perfectly rectilinear frame. This is not the case for all telescope foci, e.g. most big telescopes today have curved focal planes so cave focus!
Through GAIA, we requested a target field 10' in diameter centred at RA=19h00m00s and dec=-31deg00'00''. Here are the commands you need to produce the result above. The actual mask manufactured with the procedure below can be inspected at the AAT.
** To open GAIA (from aaossj):
** To select an image of a particular region of sky: (gaia_dss.gif)
Data-Servers --> Image Servers --> Digitized Sky at ESO
RA and dec (and equinox of co-ordinates), and
also desired image width and height in arcminutes
=> DSS Image will be shown in GAIA window
Here is the actual field which will be returned as a FITS image. All operations are performed with respect to the FITS image.
** To overlay star positions: (gaia_usno.gif)
Data-Servers --> Catalogs --> USNO at ESO
RA and dec (and equinox), or select "Set From
Image" to automatically get co-ordinate centre of image.
and maximum radius (in arcminutes) of catalog
set magnitude limits on search by entering values
in `Brightest' and `Faintest' boxes
=> Search results will be shown in lower window, and
the positions will be overlaid onto image in GAIA
** To astrometrically calibrate an image: (gaia_fit.gif)
Image-Analysis --> Astrometry calibration --> Fit to star
positions from USNO catalog search by
clicking "Grab", then select the catalog list of interest
=> Object list will be displayed in the upper window
*If there are
reference objects lying outside the actual DSS image,
select "Clip" to remove these objects from the reference position
*Check co-ordinate and image parameters are satisfactory
=> RMS of fit, x and y pixel scales, and orientation
of image from the fit will be displayed in the lower
position list can be saved:
File --> Write positions to file...
to accept the astrometric fit
Note the astrometric calibration above.
** To overlay grid on DSS image: (gaia_img.gif)
Image-Analysis --> Overlay axes grid...
*Adjust colours, scaling, etc, of grid if desired
=> Grid lines are displayed over DSS image
** To save DSS image:
File --> Save as...
*Select directory and enter file name
=> Selected DSS image is saved as a FITS file,
with astrometric calibration information (if
** To output the catalogue to disk:
Note that the output ascii file has 5 columns:
ID, RA, dec, x pixel position, y pixel position
This file can be operated on with your own programmes and redisplayed over your image. However, note that you need to add four lines to the top of the file and use the file name extension .ASC rather than .dat or anything else.
# 1 ID
# 2 X_WORLD
# 3 Y_WORLD
# 4 X_IMAGE
# 5 Y_IMAGE
For the modified output file we obtained, see here. This is called the ASCII_HEAD format under GAIA which is much simpler than the catalogue toolbox formats of GAIA. For more serious catalogue work with GAIA, you need to embrace Tab Tables. Good luck!
** To reload the ascii file and superimpose on the image:
Data-Servers -> Local Catalogs -> Load from file...
To add overlay markers:
Options -> Set Plot Symbols...
which you will see in the catalogue window and then enter say "4.0" in
the "Size:" field, press "Add
Symbol" and "OK".
** All that remains now is to convert the .ASC file to a .dxf file which drives the mask cutter. Return to the previous web page to see how to do this.
Prepared by: JBH, S. Cianci, GFL (Sept 01); many thanks to P.W. Draper
for help with GAIA.