Structure of UCAC1 compressed files

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The files are organized in zones, each zone being 0.5° wide, starting with zone 001 (file z001.bin) at the South Pole.

The first 80bytes of each of these files contain a text treated as a magic number - some keys which identify the file. Here are two examples:

file z001.bin: #UCAC1-18#12-1234(80chunks)#1998#750: Z001 00000001-00000908
file z123.bin: #UCAC1-17#12-1234(80chunks)#1998#750: Z123 21128972-21409220

The numbers at the end describe the zone number, and the range in the UCAC1 numbers contained in the file; the preceding numbers indicate the lowest values used for the magnitudes (7.50) and the Epoch (1998). The 17 or 18 number represents the length of the coded records - as 18 bytes near the Poles, or generally 17 bytes.

This first line is followed by 80 binary numbers of 4 bytes each (i.e. a total of 320 bytes) which represent the seek position, in the file, of chunks of data records, one chunk being approximately 18.5minutes wide in RA -- the exact range of RA is 2²⁴mas = 16777.216arcsec = 1118.4811sec.

Each chunk of data records starts with a chunk header which specifies the origins used for each parameter, the number of data records, etc. The data, made of fixed-length records of 17 or 18 bytes, follow this header.

The various structures are described in the following table:

file header (400 bytes)

80bytes ``magic'' header

80*i4 320bytes seek positions of the 78 data chunks

Chunk#0 Header (32+... bytes)

i4 4bytes total length of chunk in bytes, rounded to a multiple of 4 bytes

i4 4bytes UCAC number of first star in the chunk

i2 2bytes Length of this header (32+...) bytes

i1 1bytes Number of this chunk = leftmost byte value of RA

i1 1bytes N(m4) = Number of out-of-range proper motion values requiring a storage on 4 bytes

i1 1bytes N(mu) = Number of out-of-range proper motion values requiring a storage on 2 bytes

i1 1bytes N(Vm) = Number of out-of-range magnitude values

16*i1 16bytes out-of-range Nobs values

i2 2bytes (unused)

N(m4)*i4 N(m4)*4bytes Out-of-range proper motions requiring 4-byte storage

N(mu)*i2 N(mu)*2bytes Out-of-range proper motions requiring 2-byte storage

N(Vm)*i2 N(Vm)*2bytes Out-of-range magnitudes

Chunk#0 Body

RA 24bits Right ascension in mas (offset in chunk) - is 32 bits near the poles

SPD 21bits South Polar Distance in mas (offset in chunk, value between 0 and 1800000)

Epoch 11bits Epoch of Position, in myr (origin 1998.000)

sRA 8bits Sigma on RA (mas)

sDE 8bits Sigma on DE (mas)

UCmag 10bits Magnitude in cmag: values between 0 and 950 are cmag with origin at 750, values from 951 on are out-of-range values reported in the chunk header

Nobs 5bits Number of observations offset 2; values above 16 are out-of-range values reported in the header

Ncat 5bits Number of catalogues offset 2

pmx 13bits Proper motion in 0.1mas/yr, offset -2048; values larger than 4096 (i.e. outside the limits [-409.6 +409.6[mas/yr are out-of-range values (2-byte values if smaller than 6143, 4-byte proper motions for values larger than 6144)

spmx 9bits Sigma on pmx in 0.1mas/yr

pmy 13bits Proper motion in 0.1mas/yr, same conventions as pmx

spmy 9bits Sigma on pmy in 0.1mas/yr

file header (400 bytes)
80bytes		``magic'' header
80*i4	320bytes	seek positions of the 78 data chunks
Chunk#0 Header (32+... bytes)
i4	4bytes	total length of chunk in bytes, rounded to a multiple of 4 bytes
i4	4bytes	UCAC number of first star in the chunk
i2	2bytes	Length of this header (32+...) bytes
i1	1bytes	Number of this chunk = leftmost byte value of RA
i1	1bytes	N(m4) = Number of out-of-range proper motion values requiring a storage on 4 bytes
i1	1bytes	N(mu) = Number of out-of-range proper motion values requiring a storage on 2 bytes
i1	1bytes	N(Vm) = Number of out-of-range magnitude values
16*i1	16bytes	out-of-range Nobs values
i2	2bytes	(unused)
N(m4)*i4	N(m4)*4bytes	Out-of-range proper motions requiring 4-byte storage
N(mu)*i2	N(mu)*2bytes	Out-of-range proper motions requiring 2-byte storage
N(Vm)*i2	N(Vm)*2bytes	Out-of-range magnitudes
Chunk#0 Body
RA	24bits	Right ascension in mas (offset in chunk) - is 32 bits near the poles
SPD	21bits	South Polar Distance in mas (offset in chunk, value between 0 and 1800000)
Epoch	11bits	Epoch of Position, in myr (origin 1998.000)
sRA	8bits	Sigma on RA (mas)
sDE	8bits	Sigma on DE (mas)
UCmag	10bits	Magnitude in cmag: values between 0 and 950 are cmag with origin at 750, values from 951 on are out-of-range values reported in the chunk header
Nobs	5bits	Number of observations offset 2; values above 16 are out-of-range values reported in the header
Ncat	5bits	Number of catalogues offset 2
pmx	13bits	Proper motion in 0.1mas/yr, offset -2048; values larger than 4096 (i.e. outside the limits [-409.6 +409.6[mas/yr are out-of-range values (2-byte values if smaller than 6143, 4-byte proper motions for values larger than 6144)
spmx	9bits	Sigma on pmx in 0.1mas/yr
pmy	13bits	Proper motion in 0.1mas/yr, same conventions as pmx
spmy	9bits	Sigma on pmy in 0.1mas/yr