Astrometric Calibration

This recipe describes how to apply an astrometric calibration to an image. In this context an `astrometric calibration' is a mathematical transformation relating the $x,y$ positions of pixels in the image array to celestial coordinates on the sky. Applying an astrometric calibration is just another term for creating a World Coordinate System (WCS; see Section ) for the image, with the World Coordinates being celestial coordinates.

GAIA provides numerous options for astrometric calibration, only one of which will be used here (though Section , below, gives a few hints about what else is available). In most of the techniques stars occurring in the image are used as fiducial marks and a transformation is defined between their $x,y$ pixel positions measured in the image and their celestial coordinates obtained from an astrometric reference catalogue. The most interactive (and problematic) part of the procedure is identifying a given object in the image with the corresponding entry in an astrometric reference catalogue.

The principal information which you need to know about an image before attempting astrometric calibration is the approximate position on the sky corresponding to the centre of the image and the size of the field of view. The former can usually be obtained from examining the auxiliary information included with the image (see Section ). The latter may also be found in the auxiliary information or from the documentation for the instrument or telescope. If you have no prior information whatsoever about the region of sky observed then astrometric calibration will usually be impossible.

In this recipe an astrometric calibration will be created for the V band CCD image of NGC 1275 obtained with the JKT which was used in the recipe in Section . The field of view of this image is about six minutes of arc in each axis.

The process of creating the astrometric calibration divides naturally into three stages:

1. finding suitable astrometric reference stars,

2. creating a preliminary astrometric calibration,

3. creating a final accurate astrometric calibration.

Each stage is described separately in individual sub-recipes below.