This directory contains the Hubble Guide Star catalog, version GSC-ACT. This
version eliminates most of the systematic errors found in the STScI GSC 1.1.
For more information see http://www.projectpluto.com/gsc_act.htm.

The files are in a special compressed format. The file structure mimics the
layout of the 2 CDROM set. The STScI format required 1.2GB, while this format
requires 160MB, a compression of 7.5:1. Like the CDROMs, the file names are
the GSC small region designation, but with a suffix of .ech.

The files are in a portable packed binary format but the header is one line of
plain ASCII. One way to view this line on UNIX is to use "head -1 <filename>".
The header line consists of the following fields. Because the absolute value of
the declination is stored one must also know that southern region ids are 4663
and greater.

	Version number
	Region ID
	Number of stars
	Western (smallest) RA, radians
	abs(Dec nearest 0), radians
	south (1 or 0)
	<newline>

The binary format then begins immediately following the newline and stores
each star in 8 bytes, as follows:

     2  delta ra, rads, 0 .. degrad(4) * cos(dec0), scaled 0 .. 0xffff
     2  delta dec, rads, 0 .. degrad(4), scaled 0 .. 0xffff
     2  small region id
     2  upper 2 bits are class, lower 14 are magnitude*900
     ---
     8  total bytes per entry

All values are stored in big-endian byte order. To achieve architecture
portability, the application must pull each record apart byte-by-byte and
construct aggregates at runtime.
