***** File IMAGFORM.TXT Image Data Descriptions The International Halley Watch agreed early in the project that all data would be submitted from the individual Disciplines to the Lead Center using the FITS format (Wells et al., 1981). When the decision was made to distribute this information on CD-ROM, it was determined that the data had to have even broader accessibility. For this reason the original FITS files, with contiguous headers and data, were split into separate files distinguishable by their filename extensions (.HDR for headers). The file sizes were preserved as multiples of 2880 bytes, allowing the original FITS byte stream to be recovered by concatenating the appropriate header and datafile. PDS labels were constructed to allow definition of the datafiles for the Planetary Data System. For each datafile there must always be an associated FITS header. The convention for naming files on the IHW CD-ROMs was proposed by the Lead Center and NASA/Goddard Space Flight Center (GSFC) personnel to include a unique data qualifier for the data. Specifically, a set of subnet codes was established to enable identification of the IHW Discipline/subdiscipline from the filename itself. A CD-ROM running number and file extension complete the filename (example: LSPN0059.IBG). A short list of this convention by Discipline and subnet (or experiment) is given below for image data: PDS Object FITS Discipline Subnet Extensions (description) NAXIS = Code ______________________________________________________________________ image 2 IR Studies IRIM .img image (browse) 2 " LSPN .ibg image 2 Near Nucleus NNSN .img image 2 Radio Studies RSCN .img image (multiple) 3 " RSOC .img image (spectrum) 2 Spectroscopy SPEC .img ______________________________________________________________________ A table linking the subnet codes above and the subdiscipline names is given in Section 5 ('Filenaming Conventions') of the file HALGUIDE.TXT. Concerning the numeric portion of filenames, calibration files for IRIM, LSPN, and SPEC begin at 4001, whereas the Halley data themselves for all disciplines and subdisciplines start at 0001. The file extensions follow suggestions by the Planetary Data System (SPIDS v1.1; Martin et al., 1988) for tabular and image data. In addition, for IHW FITS, the original headers and data were split into separate files, with filename extensions as listed below. .HDR - FITS header records .IBG - data records for subsampled browse image .IMG - image data records .LBL - detached PDS stream format These PDS labels are metadata (as headers describing data submitted to the archive). There has been no effort to duplicate the documentation contained in the full FITS headers because the PDS and FITS headers for a given data file differ only in the filename extension. Instead we have attempted to use the power of the PDS label syntax to fully describe the data structures and thus gain access to software by that group. Standards for the Preparation and Interchange of Data Sets, Martin, T. Z., et al, Document D-4683, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA document version 1.1 was the primary reference to the Object Description Language (ODL) necessary to create the PDS labels. (We acknowledge R. Borgen and M. Martin, PDS-CN, JPL, for assisting the IHW through version 2.0 of the ODL implementation for SPECTRUM.) The basic PDS descriptors such as SFDU_LABEL, RECORD_TYPE, RECORD_BYTES, and FILE_RECORDS are explained in the SPIDS document. The RECORD_TYPE for all data files is FIXED_LENGTH. The PDS labels have been formed as fixed length (78 byte) plus an embedded CR and LF. In the case of images, we have included a new keyword describing the byte ordering of the data (MSB_INTEGER) required by FITS. In PDS, images (.IMG, .IBG) are in terms of LINES (FITS keyword NAXIS2) and SAMPLES (FITS keyword NAXIS1), given knowledge of the SAMPLE_BITS (FITS keyword BITPIX), and are easy for the split files. The final form of the label for compressed images is still under active discussion. Unlike previous PDS efforts with compressed images, we chose not to compress the header (or label) and thus have included a keyword to describe the type of compression (ENCODING_TYPE = "PREVIOUS_PIXEL") used. The label for compressed images also contains information to permit software to skip over the data if the decoding algorithm is unknown (ITEMS, ITEM_TYPE, and ITEM_BITS). We use ODL to indicate various subclass structures for the data objects. An example of this is the DIFFERENCE modifier applied to IMAGE, yielding the keyword DIFFERENCE_IMAGE, which indicates that a processing step was applied to the original image. REFERENCES Martin, T.Z., Martin, M.D., Davis, R.L., Mehlman, R., Braun, M., Johnson M.: October 3, 1988, Standards for the Preparation and Interchange of Data Sets, Version 1.1, JPL D-4683. Wells, D.C., Greisen, E.W., and Harten, R.H.: 1981, Astron. Astrophys. Suppl. Ser. 44, 363.