Summary
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This paper describes a merging of previously proposed FITS WCS conventions to fully support the multi-component distortion models derived from calibrations for HST detectors.  The application of this merged convention allows each science image to contain the full distortion model applicable to that specific image in an efficient and FITS compatible manner.  The use of this calibration in the DrizzlePac package has been demonstrated to correct science data to much better than 0.1 pixels across each image's field of view, with a typical RMS for aligning two ACS images on the order of 0.03 pixels in a suitably dense field of sources. This convention, despite making a few basic assumptions, retains each separate FITS convention's full functionality so that any software which understood, for example, the SIP standard will still work as before with the SIP keywords written out by the convention.  

All HST ACS and WFC3 images retrieved from the archive have been updated using this convention so that users will no longer need to retrieve the distortion calibration data separately. Anyone using HST images will now be able to use the STWCS and/or DrizzlePac package to perform coordinate transformations or image alignment based on this convention, while still being able to use external tools like DS9 to take advantage of the SIP conventions as well. This solution now provides the best possible solution for supporting these highly accurate, yet complex multi-component distortion models in the most efficient manner available to data written out in the FITS format. 

