This discussion is somewhat short, and only valid for the original Elixir-RAW style format/mode.

    The format for a catalog region file is as follows:

    --
    Header
    --
    Averages
    --
    Measurements
    --

The Header follows the format of a FITS header but always stored with 3 blocks to speed up read time: 2880 x 3 bytes. Two important keywords in the header: NSTARS & NMEAS

All average values are stored consecutively in a single block. They are written as an array with NSTARS elements of the Average structure defined below. The individual measurements follow the averages as a block. To find a measurement for a given star, you need to get the value of the offset for the star. this is the element number for the first measurement of this star, and are followed by next measurements for this star, until the value Nm is reached. Here are the structures being used for the average and measurement values:

    typedef struct {
      float R;                    /* RA  in decimal degrees */
      float D;                    /* DEC in decimal degrees */
      short int M;                /* thousandths of mag (-32.000 to 32.000 valid range) */
      unsigned short int Nm;      /* number of measurements */
      unsigned short int Xp, Xm;  /* chisq values in tenths */
      unsigned int offset;        /* offset to first Measure-ment */
    } Average;                    /* = 20 bytes / average */

    typedef struct {
      char dR, dD;                /* tenths of arcsec (-12.7 to +12.7 valid range) */
      short int M;                /* thousandths of mag (-32.000 to 32.000 valid range) */
      unsigned char dM;           /* thousandths of mag (0.000 -- 0.255 valid range) */
      float t;                    /* time in seconds (what is reference?) */
      unsigned int average;       /* reference to corresponding Average entry,
    				 upper byte of value contains flags.
    				 limit of 16,777,215 stars (Naverage)
    				 in a file (=0xFFFFFF).
    				 flags = average & 0x1000000 */
    } Measure;                    /* = 13 bytes / measure */

The above two structures define the entries in the photometry database. The database consists of a large number of files representing a small patch on the sky (roughly 1.5 degree$^2$ in most places). These files are organized into directories representing bands of Declination. A reference file determines the coordinate boundaries for each of the files so that a given point on the sky can unambiguously be associated with a specific file in a specific directory. The sky coordinates for each file is the same as those used by the HST Guide Star catalog, except for the region around the North celestial pole, for which all stars are included in a single file.

Within a given file, the data are stored in a binary format, with an ASCII FITS-like header. The header is examply in the format of a normal FITS header, but with the exception that all files have a fixed number of header blocks (for now 3 blocks = 8640 bytes). This is done to speed loading the header and finding the beginning of the binary data. The number of 3 blocks seems quite generous, as currently only a few FITS keywords have been defined for each file, basically keywords to define the number of stars and the total number of measurements stored in the file, as well as values to define the RA and DEC range of the file.

The first section of data following the header blocks consists of average measurements for each uniquely observed star. Each star occupies 20 bytes, the size of the Average structure defined above. The Average structure contains the average Ra, Dec, and Magnitude for the star, as well as the number of measurements, and \chisq\ values for the magnitude and position. Finally, there is a 32 bit integer which defines the offset to the first measurement for this star. This offset is defined as the number of Measure records from the start of the Measure structure.

The second section of data, following the Average data contains all measurements for each star listed in Average. Each measurement occupies 13 bytes, the size of the Measure structure. This structure contains the difference of this position from the average RA and DEC, and the instrumental magnitude of this measurement (in the units defined by the fstat program, which give m = -2.5*log(cts) + Mo, where Mo is currently 24.5 [10/15]). There is also the magnitude error for this measurement, the time of the measurement (in seconds relative to a to-be-determined zero point), and a reference to the entry in the Average structure so we can relate a given measurement with a given star. This last entry also includes a byte of flags, of which only 4 have currently been defined. This means the Average offset can only be as large as 16,777,215 (0xffffff), limiting the possible number of stars allowed in a given file. This does not seem like a long term problem, though: aside from the fact that this number is very large and we only expect in the vicinity of 20,000 stars per file, the file can easily be divided into pieces at a later date if needed. this last step is trivial, consisting of splitting the data up into smaller RA,DEC regions, and updating the reference catalog. With the above definitions for the Average and Measure structures, we will typically expect 20,000 * 15 measurements per year and 20,000 average entries in a given file. This implies a file size of about 4.3 MB at the end of a year. It is also possible that we will choose to split the files up in the future if the number of measurements makes their size unweildy. 4.3 MB is sufficiently small that this is not a problem, but after only 5 years, the files will be over 21 MB each, getting to be fairly significant.