Creating an accurate astrometric calibration

In some cases the approximate astrometric calibration derived in the previous stage will be adequate. However, it is possible to use additional astrometric reference stars to refine it. Assuming that you already have an approximate astrometric calibration proceed as follows.

1. Query the USNO at ESO catalogue to find the objects that overlay the JKT image, following the second part of the recipe in Section . Alternatively, you can load the example local catalogue ngc1275usno.tab.

2. Click on the Image-Analysis button on the menu-bar along the top of the main window. Choose the Astrometry calibration item and then Fit to star positions from the next menu. The Fit astrometry reference positions dialogue box (Figure ) should appear.

3. Import the catalogue of reference stars by clicking on the Grab button and choosing the appropriate catalogue.

4. Click on the Clip button to remove reference stars which fall outside the image.

5. Delete all the catalogue objects which correspond to galaxies, nebulae, blended double-star images etc, none of which make good astrometric reference objects. The procedure to delete an object is:
   • click on the symbol for the object in the main window, so that the object is highlighted in the catalogue window,

   • click on the Delete button and then confirm that the object is to be deleted.

6. When you are happy that you have removed all unsuitable objects, clear the catalogue markers: click on the Graphics menu in the main window's menu-bar and choose Clear.

7. Click on the Centroid button in the Fit astrometry reference positions dialogue box to refine the pixel positions for all the reference stars. New markers should be drawn over the JKT image showing the revised positions of the reference stars.

8. Click on the Fit/Test button and the revised calibration using all the reference stars will be created.

   If any unsuitable objects remain amongst the reference stars, or the fit is in some other way unsatisfactory, then repeat the steps above to remove the offending objects and repeat the fit.

   When the fit is acceptable click the Accept button.

9. Finally, save a version of the image with the revised calibration. Click on the File menu button in the main window, choose the Save as item and use the resulting dialogue box to save the image as a new file, perhaps called ngc1275jktast.sdf.