More about attached coordinate systems

The discussion elsewhere in this document tells you all that you need to know for using coordinate systems attached to image files in order to register them within CCDPACK. In fact for simple cases in which images are to be registered using, for instance, only object matching methods, it is not necessary to understand how the coordinate systems are handled by the registration programs. However, you can get a better understanding of coordinate systems and what you can do with them (including using them for image display) in the ``Using World Coordinate Systems'' section of the KAPPA document, SUN/95, and more detailed description of the underlying AST system in SUN/210.

In general CCDPACK conforms to the normal rules about World Coordinate System (WCS) image components and AST objects so other applications, for instance KAPPA's WCSTRAN and friends, can freely be mixed with CCDPACK applications. In fact some of the KAPPA applications provide similar facilities to those of the CCDPACK ones, and they can be used instead if for some reason you prefer them. There are a couple of additional conventions used within CCDPACK however:

• Some of the CCDPACK applications attach new coordinate systems to images, and these are normally given a domain (label) starting with the characters `CCD_'. Thus it is not a good idea to pick a domain which begins `CCD_' if you are assigning one yourself, unless you intend it to look like an automatically generated one. The special domains which are currently used by CCDPACK are as follows:

  CCD_REG

  The object-matching alignment coordinates found by the REGISTER program.

  CCD_WCSREG

  The global alignment coordinates found by the WCSREG program.

  CCD_SET

  The alignment coordinates inserted by the MAKESET program denoting a Set; this has special significance to most of the registration programs.

  CCD_OLDPIXEL

  A copy of the pre-transformation PIXEL coordinates stored by the TRANNDF task stored in the transformed image. This is not subsequently used by CCDPACK, but may be useful for comparing the old and new coordinate systems.

  CCD_GEN

  The coordinates in which images are generated by the CCDGENERATE program. This program is not documented but is used to generate the test data used in the demomstration scripts.

  CCD_REG1

  Used internally by CCDALIGN.

• Although there is nothing to prevent multiple coordinate systems in the same image having the same domain, this is likely to lead to confusion. CCDPACK will usually try to ensure that multiple coordinate systems with the same domain do not exist by deleting all but one. In this case a warning will be issued, but it is not considered a fatal error. For instance, the REGISTER program normally attaches a new coordinate system labelled `CCD_REG' to the images on which it operates; If any of them already has a coordinate system labelled CCD_REG, then the old one gets deleted, and you will be warned that this has happened.