Variations

The preceding recipe has described just one of the numerous different ways to apply an astrometric calibration to an image using GAIA. Many images already contain an approximate astrometric calibration and in such cases you can skip the first two stages of the recipe and proceed directly to the third to create an accurate astrometric calibration.

Comparison images retrieved from the SuperCOSMOS surveys rather than the DSS already have an object catalogue attached, making it un-necessary to retrieve a separate catalogue from the USNO PMM (but remember that the SuperCOSMOS surveys are currently only available south of Declination $+3^{\circ}$). Alternatively, you may not need to retrieve an object catalogue because you already know accurate celestial coordinates for a set of reference stars in your image; they might, for example, be listed in a scientific paper associated with the image. If you know an approximate astrometric calibration (typically, the orientation, plate scale and approximate central coordinates) for an image then you can simply type in the values (use the Astrometry calibration item from the Image-Analysis menu and choose the Type in known calibration... option).

If you have a series of similar images all overlapping the same area of sky, you could determine an astrometric calibration for the first using the method described. For the remaining images you can copy the WCS for the first image and then tweak it to fit the host image (use the Transfer button in the Fit astrometry reference positions dialogue box).

You can transfer a set of reference stars, with measured positions, from a DSS or SuperCOSMOS calibration image to the target image, then move the markers for the stars onto the corresponding objects in the target image (move the cursor to the appropriate reference star marker, hold down the left mouse button, move the cursor to the required position and then release the mouse button) and measure the positions in the target image (click on the Centroid button). A reference star can be deleted by positioning the cursor over the appropriate marker, holding down the Control key and clicking on the left mouse button. After confirmation the corresponding reference star is deleted.