S.L. Allen, D.A. Clarke
UCO/Lick Observatory
The Deep Extra-galactic Imaging Multi-Object Spectrograph (DEIMOS) under construction for Keck II will be among the first astronomical instruments to employ a detector comprised of a mosaic of 8 CCDs. Each of these CCDs may be read via 2 amplifiers. World coordinates are required to transform from FITS pixels to other systems including those of the amplifier, CCD, camera focal plane, slitmask, direct image of sky, and spectral image of sky. This amounts to dozens of different mappings which will be vital for quick-look interpretation and careful reduction of observations. Instrument simulators, slitmask design, and observation planning software require these mappings in reverse. Slitmask fabrication requires the mapping between sky and slitmask; in this case neither of the systems is represented as FITS pixels. We have designed a scheme which tolerates arbitrary numbers of transformations between any reference frames. The scheme uses a generalization of the proposed WCS keywords found in the current draft of ``Representations of celestial coordinates in FITS'' by Greisen and Calabretta. We avoid the problem of trying to document all these mappings within the 8-character namespace of FITS header keywords by storing the bulk of the WCS information in FITS tables associated with the FITS images. A simple tool replaces the ``principal'' WCS in the image header with any suitable row from the table, which permits an arbitrary FITS viewer to display using the desired coordinates. These WCS tables satisfy requirements for quick-look and pipeline data reduction, simulation and planning software, and archival documentation.