1) The coordinate system uses red, Naki and SPD (South Polar Distance)
2) The coordinates are converted into an integer, and the method is as follows.
If the original red class (indicated in hours) is RA, Natelush (indication degree), actually stored in the Star Table
Ra = ra * 15 * 3600 * 100
DEC = (DEC 90) * 3600 * 100
3) The whole sky is divided into 24 regions in accordance with SPD, each spanning 7.5 degrees.
4) Each day, the entries of the stars are arranged in the order in which the red criterion is incremented.
5) Each of 24 regions corresponds to two files, the name of the file is named in the following:
Zonexxxx.YYY
among them
XXXX is 10 times the SPD (0,75,150 until 1725)
YYY is
ACC represents ASCII speed files
Cat Represents binary directory file
6) In a directory file (.cat), each (stellar data) records 3 32-bit integers. The storage method is like the two-dimensional array INT [3, Length] in the C language, as shown below:
| RA (1) | DEC (1) | MAG (1) | RA (2) | DEC (2) | MAG (2) | ... where RA, DEC each occupies a 32-bit integer, the third integer contains There are still other information in red brightness, and blue brightness. This integer is constituted: this integer value (10) is: sqffbbbrrr where S: If 1 indicates that the record is related to an ACT star, 0 indicates that it is not related. Q: If 1 indicates that the brightness may be wrong, it is 0 无 无 FFF: The target's observation area BBB: 10 times the blue star, etc. RRR: 10 times red star, etc.) byte order BIG_ENDIAN, but Intel PC Using the little_endian therefore need to do byte sequential conversion when extracting data from a directory file.
(According to the CPU design, when the data width exceeds one byte (byte), the high byte is preceded in front, and the high byte is called Big-endian. The opposite manner is called Little-Endia. For example, long finished 0x12345678 at Little-Endian When the system uses two groups of two bytes (Words) to store characters, the memory arranging within the memory should be 0x78563412, in the Big-Endian system 0x12345678.hp, IBM, Motorola 68k series uses BIG-Endian design. Intel x86, DEC VAX use Little-endian design. PowerPC supports two formats, called Bi-endian.) 8) Since the star table file is relatively long, USNO-A2.0 provides a quick index positioning file for each star table file (.acc) .
