The registration process

Registering a set of images requires identifying a single coordinate system which can apply to all of them, so that each pixel in each of the images has a definite position in the overall picture as well as within its own grid. This single coordinate system within which all the images can be embedded may be an actual sky coordinate system with RA and Dec coordinates, or an arbitrary one which coincides with the pixel coordinates of one of the images, or some other kind. Information on how to embed the images in the same coordinate system may come from one or more of a variety of sources, for instance:

Telescope pointing information:

The telescope system may insert FITS headers recording the position in the sky from which the observation was taken. Alternatively, this could simply be encoded in the filename or known to you in a non-machine-readable form.

Mosaic camera geometry:

If the instrument is a mosaic camera in which several CCD chips are adjacently positioned on the focal plane, then one observation will comprise a set of related image files, whose relative positions may be known.

Multiple exposures:

If multiple exposures are taken without moving the telescope or otherwise altering the observational setup, then trivially the images in question will share the same coordinate system.

Object matching:

When a number of images overlap and the same features appear in more than one of them, the relative positioning of the images can be determined by matching up the features.

For a given set of data, some of these sources may be more accurate or reliable than others.

It is possible to attach any number of these coordinate systems to an image. Once all the images that you are interested in have the same coordinate system attached to them, then they can all be resampled so they match pixel for pixel, ready for combination or comparison. There are two ways of keeping track of which coordinate systems are attached to an image: firstly, each coordinate system has a label (sometimes called its domain) which can be used to identify it. Some of these labels have special meaning, for instance a coordinate system labelled `SKY' indicates positions on the sky itself and coordinates are usually reported in RA and Dec (the other special labels are GRID, PIXEL and AXIS). Secondly, an image always has a Current coordinate system, which is the one in which positions are normally reported. These attached coordinate systems are understood by other Starlink applications so that for instance using KAPPA's DISPLAY command:

% display myimage axes

will display myimage with axes showing coordinates from the Current system attached to the image.

CCDPACK provides four main categories of facility for adding coordinate systems to sets of images, which are described later in this section as follows:

Object matching methods:

FINDOBJ, FINDOFF, PAIRNDF, CCDALIGN and REGISTER are described here. These are suitable if you have a set of mutually overlapping images which can be aligned by identifying the same objects in different images.

Direct manipulation of coordinate systems:

WCSEDIT is decribed here and can be used to examine coordinate systems attached to an image, add them, remove them, or change which one is Current for an image when it's necessary to do this manually.

Dealing with externally stored coordinate systems:

ASTIMP and ASTEXP are described here. These allow coordinate system information to be saved in and restored from external files, and can be used to align multiple sets of images in the same way relative to each other -- this can for instance be used for observations from mosaic cameras. The MAKESET command can also be used to read coordinate information from an external files -- see the section on dealing with Sets ().

Combining information from existing coordinate systems:

WCSREG is described here, and can be used to produce a unified coordinate system out of several present in a set of images when this is possible.

Resampling and mosaicing registered images is discussed in sections §, and , and some examples of putting it all together are given in §.