A2.0 CD-ROM organization
Last updated 22 Mar 2000
Version of this documentation for A1.0
Basic information about A2.0
Sites on the Internet where you can get
Ax.0 data
USNO site for 'Ax.0 and
derivatives'
C/C++ source code for extracting
Ax.0 data
Computing 'pseudo-V' magnitudes from Ax.0
R and B data
Compression of A2.0
Recalibrating Ax.0 magnitudes
A2.0 zones listed in declination order
A2.0, disk #1
A2.0, disk #2
A2.0, disk #3
A2.0, disk #4
A2.0, disk #5
A2.0, disk #6
A2.0, disk #7
A2.0, disk #8
A2.0, disk #9
A2.0, disk #10
A2.0, disk #11
SA2.0
A2.0 sizes after filtering faint stars
A2.0 zones, in declination order: The A2.0 (and A1.0) data are stored in zones, each 7.5 degrees high, numbered by distance from the south celestial pole ("SPD", "South Polar distance"). Thus, ZONE0000.CAT contains data from dec -90 to -82.5; ZONE0075 continues from -82.5 up to -75; and so on, up to ZONE1725 from +82.5 to +90.
This has two unfortunate consequences. First, the files near the celestial equator are enormous. Second, to fit the data on eleven CDs with minimal waste, any concept of orderly organization had to be sacrificed.
To add further confusion, the order for A2.0 is not the same as that for A1.0.
It might help to have a "reduced A2.0", a program that would take a .CAT file and produce a subset to a desired magnitude. I may get around to doing that someday (it's a fairly simple job.)
Filename Size A2.0 disk A1.0 disk dec range ZONE0000 CAT 23,910,768 Disk 1 1 -90 -82.5 ZONE0075 CAT 86,276,232 Disk 1 1 -82.5 -75 ZONE0150 CAT 246,296,364 Disk 9 6 -75 -67.5 ZONE0225 CAT 383,940,000 Disk 7 5 -67.5 -60 ZONE0300 CAT 457,993,524 Disk 5 3 -60 -52.5 ZONE0375 CAT 485,481,948 Disk 4 2 -52.5 -45 ZONE0450 CAT 489,880,236 Disk 3 1 -45 -37.5 ZONE0525 CAT 533,337,552 Disk 2 4 -37.5 -30 ZONE0600 CAT 537,821,028 Disk 1 6 -30 -22.5 ZONE0675 CAT 451,563,264 Disk 6 5 -22.5 -15 ZONE0750 CAT 259,313,868 Disk 7 7 -15 -07.5 ZONE0825 CAT 240,911,568 Disk 10 10 -07.5 0 ZONE0900 CAT 246,545,064 Disk 9 8 0 +07.5 ZONE0975 CAT 257,190,756 Disk 8 7 +07.5 +15 ZONE1050 CAT 250,388,400 Disk 8 8 +15 +22.5 ZONE1125 CAT 240,292,752 Disk 11 9 +22.5 +30 ZONE1200 CAT 241,716,960 Disk 10 9 +30 +37.5 ZONE1275 CAT 225,209,376 Disk 11 4 +37.5 +45 ZONE1350 CAT 227,169,384 Disk 6 10 +45 +52.5 ZONE1425 CAT 190,958,580 Disk 4 3 +52.5 +60 ZONE1500 CAT 123,258,960 Disk 2 2 +60 +67.5 ZONE1575 CAT 71,540,304 Disk 3 6 +67.5 +75 ZONE1650 CAT 34,832,736 Disk 3 2 +75 +82.5 ZONE1725 CAT 9,540,948 Disk 2 3 +82.5 +90 Total: 6,315,370,572
Dave Skillman provided the following helpful visualization of the A2.0 layout:
Declination zone mapping to A2 disk numbers
Dec 90.0 82.5 75.0 67.5 60.0 52.5 45.0 37.5 30.0
\ / \ / \ / \ / \ / \ / \ / \ /
Disk 2 3 3 2 4 6 11 10
Dec 30.0 22.5 15.0 07.5 00.0 -07.5 -15.0 -22.5 -30.0
\ / \ / \ / \ / \ / \ / \ / \ /
Disk 11 8 8 9 10 7 6 1
Dec -30.0 -37.5 -45.0 -52.5 -60.0 -67.5 -75.0 -82.5 -90.0
\ / \ / \ / \ / \ / \ / \ / \ /
Disk 2 3 4 5 7 9 1 1
Disk #1:
Volume in drive J is UA001
Volume Serial Number is B6CC-3F2C
Directory of J:\
DEMO TAR 51,200 09-24-98 10:15a DEMO.TAR
READAST V10 8,326 09-24-98 10:15a READAST.V10
README V10 12,727 09-24-98 10:15a README.V10
README V20 19,366 09-24-98 10:15a README.V20
READMESA V10 4,854 09-24-98 10:15a READMESA.V10
READMESA V20 1,199 09-24-98 10:15a READMESA.V20
READPHT V10 11,096 09-24-98 10:15a READPHT.V10
READPMM V10 29,134 09-24-98 10:15a READPMM.V10
READUSE V10 6,201 09-24-98 10:15a READUSE.V10
READUSE V20 6,381 09-24-98 10:15a READUSE.V20
ZONE0000 ACC 2,880 09-24-98 10:13a ZONE0000.ACC
ZONE0000 CAT 23,910,768 09-24-98 10:13a ZONE0000.CAT
ZONE0075 ACC 2,880 09-24-98 10:15a ZONE0075.ACC
ZONE0075 CAT 86,276,232 09-24-98 10:15a ZONE0075.CAT
ZONE0600 ACC 2,880 09-24-98 10:13a ZONE0600.ACC
ZONE0600 CAT 537,821,028 09-24-98 10:13a ZONE0600.CAT
16 file(s) 648,167,152 bytes
0 dir(s) 0 bytes free
Disk #2:
Volume in drive K is UA002
Volume Serial Number is C724-6122
Directory of K:\
ZONE0525 ACC 2,880 09-20-98 11:57a ZONE0525.ACC
ZONE0525 CAT 533,337,552 09-20-98 11:57a ZONE0525.CAT
ZONE1500 ACC 2,880 09-20-98 11:59a ZONE1500.ACC
ZONE1500 CAT 123,258,960 09-20-98 11:59a ZONE1500.CAT
ZONE1725 ACC 2,880 09-20-98 11:59a ZONE1725.ACC
ZONE1725 CAT 9,540,948 09-20-98 11:59a ZONE1725.CAT
6 file(s) 666,146,100 bytes
0 dir(s) 0 bytes free
Disk #3:
Volume in drive L is UA003
Volume Serial Number is 962B-7529
Directory of L:\
NEWBIN TAR 51,640,320 09-20-98 12:56p NEWBIN.TAR
SG1 TAR 26,511,360 09-20-98 12:55p SG1.TAR
ZONE0450 ACC 2,880 09-20-98 12:53p ZONE0450.ACC
ZONE0450 CAT 489,880,236 09-20-98 12:53p ZONE0450.CAT
ZONE1575 ACC 2,880 09-20-98 12:54p ZONE1575.ACC
ZONE1575 CAT 71,540,304 09-20-98 12:54p ZONE1575.CAT
ZONE1650 ACC 2,880 09-20-98 12:55p ZONE1650.ACC
ZONE1650 CAT 34,832,736 09-20-98 12:55p ZONE1650.CAT
8 file(s) 674,413,596 bytes
0 dir(s) 0 bytes free
Disk #4:
Volume in drive M is UA004
Volume Serial Number is A62F-7A25
Directory of M:\
ZONE0375 ACC 2,880 09-20-98 1:51p ZONE0375.ACC
ZONE0375 CAT 485,481,948 09-20-98 1:51p ZONE0375.CAT
ZONE1425 ACC 2,880 09-20-98 1:55p ZONE1425.ACC
ZONE1425 CAT 190,958,580 09-20-98 1:55p ZONE1425.CAT
4 file(s) 676,446,288 bytes
0 dir(s) 0 bytes free
Disk #5:
Volume in drive J is UA005
Volume Serial Number is BFCE-121F
Directory of J:\
ALPHA13 TAR 86,640,640 09-20-98 2:51p ALPHA13.TAR
BINARY TAR 81,326,080 09-20-98 2:52p BINARY.TAR
ZONE0300 ACC 2,880 09-20-98 2:49p ZONE0300.ACC
ZONE0300 CAT 457,993,524 09-20-98 2:49p ZONE0300.CAT
4 file(s) 625,963,124 bytes
0 dir(s) 0 bytes free
Disk #6:
Volume in drive K is UA006
Volume Serial Number is D337-8129
Directory of K:\
ZONE0675 ACC 2,880 09-20-98 3:43p ZONE0675.ACC
ZONE0675 CAT 451,563,264 09-20-98 3:43p ZONE0675.CAT
ZONE1350 ACC 2,880 09-20-98 3:47p ZONE1350.ACC
ZONE1350 CAT 227,169,384 09-20-98 3:47p ZONE1350.CAT
4 file(s) 678,738,408 bytes
0 dir(s) 0 bytes free
Disk #7:
Volume in drive L is UA007
Volume Serial Number is BEEB-402C
Directory of L:\
ZONE0225 ACC 2,880 09-20-98 4:41p ZONE0225.ACC
ZONE0225 CAT 383,940,000 09-20-98 4:41p ZONE0225.CAT
ZONE0750 ACC 2,880 09-20-98 4:45p ZONE0750.ACC
ZONE0750 CAT 259,313,868 09-20-98 4:45p ZONE0750.CAT
4 file(s) 643,259,628 bytes
0 dir(s) 0 bytes free
Disk #8:
Volume in drive M is UA008
Volume Serial Number is D5EA-3C2C
Directory of M:\
ZONE0975 ACC 2,880 09-20-98 5:34p ZONE0975.ACC
ZONE0975 CAT 257,190,756 09-20-98 5:34p ZONE0975.CAT
ZONE1050 ACC 2,880 09-20-98 5:38p ZONE1050.ACC
ZONE1050 CAT 250,388,400 09-20-98 5:38p ZONE1050.CAT
4 file(s) 507,584,916 bytes
0 dir(s) 0 bytes free
Disk #9:
Volume in drive J is UA009
Volume Serial Number is D7BD-1F2B
Directory of J:\
ZONE0150 ACC 2,880 09-20-98 6:22p ZONE0150.ACC
ZONE0150 CAT 246,296,364 09-20-98 6:22p ZONE0150.CAT
ZONE0900 ACC 2,880 09-20-98 6:18p ZONE0900.ACC
ZONE0900 CAT 246,545,064 09-20-98 6:18p ZONE0900.CAT
4 file(s) 492,847,188 bytes
0 dir(s) 0 bytes free
Disk #10:
Volume in drive K is UA010
Volume Serial Number is AE8E-0C21
Directory of K:\
ZONE0825 ACC 2,880 09-20-98 7:06p ZONE0825.ACC
ZONE0825 CAT 240,911,568 09-20-98 7:06p ZONE0825.CAT
ZONE1200 ACC 2,880 09-20-98 7:02p ZONE1200.ACC
ZONE1200 CAT 241,716,960 09-20-98 7:02p ZONE1200.CAT
4 file(s) 482,634,288 bytes
0 dir(s) 0 bytes free
Disk #11:
Volume in drive L is UA011
Volume Serial Number is CBA2-F42A
Directory of L:\
ZONE1125 ACC 2,880 09-20-98 7:44p ZONE1125.ACC
ZONE1125 CAT 240,292,752 09-20-98 7:44p ZONE1125.CAT
ZONE1275 ACC 2,880 09-20-98 7:48p ZONE1275.ACC
ZONE1275 CAT 225,209,376 09-20-98 7:48p ZONE1275.CAT
4 file(s) 465,507,888 bytes
0 dir(s) 0 bytes free
SA2.0:
Volume in drive K is USNOSA20
Volume Serial Number is 4B51-C695
Directory of K:\
READAST V10 8,326 09-24-98 11:05a READAST.V10
README V10 12,727 09-24-98 11:05a README.V10
README V20 19,366 09-24-98 11:05a README.V20
READMESA V10 4,854 09-24-98 11:05a READMESA.V10
READMESA V20 1,199 09-24-98 11:05a READMESA.V20
READPHT V10 11,096 09-24-98 11:05a READPHT.V10
READPMM V10 29,134 09-24-98 11:05a READPMM.V10
READUSE V10 6,201 09-24-98 11:05a READUSE.V10
READUSE V20 6,381 09-24-98 11:05a READUSE.V20
ZONE0000 ACC 2,880 09-24-98 11:05a ZONE0000.ACC
ZONE0000 CAT 3,052,152 09-24-98 11:05a ZONE0000.CAT
ZONE0075 ACC 2,880 09-24-98 11:05a ZONE0075.ACC
ZONE0075 CAT 9,083,196 09-24-98 11:06a ZONE0075.CAT
ZONE0150 ACC 2,880 09-24-98 11:05a ZONE0150.ACC
ZONE0150 CAT 14,968,068 09-24-98 11:06a ZONE0150.CAT
ZONE0225 ACC 2,880 09-24-98 11:05a ZONE0225.ACC
ZONE0225 CAT 20,472,024 09-24-98 11:06a ZONE0225.CAT
ZONE0300 ACC 2,880 09-24-98 11:05a ZONE0300.ACC
ZONE0300 CAT 25,635,408 09-24-98 11:07a ZONE0300.CAT
ZONE0375 ACC 2,880 09-24-98 11:05a ZONE0375.ACC
ZONE0375 CAT 30,337,008 09-24-98 11:07a ZONE0375.CAT
ZONE0450 ACC 2,880 09-24-98 11:05a ZONE0450.ACC
ZONE0450 CAT 34,453,488 09-24-98 11:08a ZONE0450.CAT
ZONE0525 ACC 2,880 09-24-98 11:05a ZONE0525.ACC
ZONE0525 CAT 37,932,072 09-24-98 11:09a ZONE0525.CAT
ZONE0600 ACC 2,880 09-24-98 11:05a ZONE0600.ACC
ZONE0600 CAT 40,948,416 09-24-98 11:10a ZONE0600.CAT
ZONE0675 ACC 2,880 09-24-98 11:05a ZONE0675.ACC
ZONE0675 CAT 41,942,196 09-24-98 11:11a ZONE0675.CAT
ZONE0750 ACC 2,880 09-24-98 11:05a ZONE0750.ACC
ZONE0750 CAT 40,867,848 09-24-98 11:11a ZONE0750.CAT
ZONE0825 ACC 2,880 09-24-98 11:05a ZONE0825.ACC
ZONE0825 CAT 41,095,488 09-24-98 11:12a ZONE0825.CAT
ZONE0900 ACC 2,880 09-24-98 11:05a ZONE0900.ACC
ZONE0900 CAT 40,869,072 09-24-98 11:13a ZONE0900.CAT
ZONE0975 ACC 2,880 09-24-98 11:05a ZONE0975.ACC
ZONE0975 CAT 40,153,776 09-24-98 11:14a ZONE0975.CAT
ZONE1050 ACC 2,880 09-24-98 11:05a ZONE1050.ACC
ZONE1050 CAT 39,405,480 09-24-98 11:15a ZONE1050.CAT
ZONE1125 ACC 2,880 09-24-98 11:05a ZONE1125.ACC
ZONE1125 CAT 37,838,388 09-24-98 11:15a ZONE1125.CAT
ZONE1200 ACC 2,880 09-24-98 11:05a ZONE1200.ACC
ZONE1200 CAT 35,579,316 09-24-98 11:16a ZONE1200.CAT
ZONE1275 ACC 2,880 09-24-98 11:05a ZONE1275.ACC
ZONE1275 CAT 32,420,964 09-24-98 11:17a ZONE1275.CAT
ZONE1350 ACC 2,880 09-24-98 11:05a ZONE1350.ACC
ZONE1350 CAT 28,552,992 09-24-98 11:17a ZONE1350.CAT
ZONE1425 ACC 2,880 09-24-98 11:05a ZONE1425.ACC
ZONE1425 CAT 24,210,864 09-24-98 11:18a ZONE1425.CAT
ZONE1500 ACC 2,880 09-24-98 11:05a ZONE1500.ACC
ZONE1500 CAT 19,156,344 09-24-98 11:18a ZONE1500.CAT
ZONE1575 ACC 2,880 09-24-98 11:05a ZONE1575.ACC
ZONE1575 CAT 13,976,316 09-24-98 11:19a ZONE1575.CAT
ZONE1650 ACC 2,880 09-24-98 11:05a ZONE1650.ACC
ZONE1650 CAT 8,609,532 09-24-98 11:19a ZONE1650.CAT
ZONE1725 ACC 2,880 09-24-98 11:05a ZONE1725.ACC
ZONE1725 CAT 2,858,460 09-24-98 11:19a ZONE1725.CAT
57 file(s) 664,587,272 bytes
0 dir(s) 0 bytes free
A2.0 sizes after filtering faint stars: The entire A2.0 tips the scales at 6.3 GBytes, a bit more than some of us want to put on our hard drives right now. One way to evade this is to filter out stars fainter than a certain magnitude.
This leads to two problems. The first is finding software to do the filtering. I didn't know of any until recently, but in February 2003, David Dean posted the following on the Astrometrica and Minor Planet Mailing List:
MPer's, I've written a perl script to filter the USNO A2 database at a given mag limit. (I posted on the Astrometrica group recently about this - sorry if this is redundant for you!). You're welcome to copy it from http://m57.org/contrib/DoubleD/usnoFilter.txt and give it a try. See the comments at the top of the script on how to use. You'll need perl on your local computer, running from a command prompt under Windows (or shell on any other platform perl runs on). I've found this handy to put a smaller database on my laptop to check pointing or get a quick fit at the scope. (I never have much luck with the SA subset). Use at your own risk, but I have done some sanity checks - filtering a zone at mag "100", for example, ouputs .acc and .cat files identical to the original A2 files. Feel free to write me off-group with any questions or comments. David Dean
The second problem is determining what the limit ought to be. Choose too faint a limit, and you may find (after carefully swapping in all eleven CD-ROMs) that the "subset" won't fit. Choose too bright a limit, and you'll find that you've plenty of space left over.
The following tables give the size, in megabytes, of each of the 24 A2.0 declination "zones" after filtering to a given magnitude, as well as the size of all 24 zones combined. Examples: if you filtered zone 0525 to R magnitude 17.5, the resulting data would consume 295 MBytes. Filtering the entire A2.0 to that level would consume 3318 MBytes. (The 'zone by zone' data lets you figure out, say, what size would result if you took everything north of dec -30 and brighter than 18.)
Three tables are given, for R magnitude, B magnitude, and V magnitude limits. The R and B data come straight from A2.0 itself. The V magnitudes are computed using a very rough approximation provided by T. Kato:
V = (5R + 3B) / 8
The results don't add up to the full size of A2.0, because some stars lack R and/or B magnitudes. A "megabyte" is defined to be 2^20 = 1,048,576 bytes.
R Magnitude sizes:
band 0000 0075 0150 0225 0300 0375 0450 0525 0600 0675 0750 0825
---- ---- ---- ---- ---- ---- ---- ---- ---- ---- ---- ---- ----
13.0: 0 1 3 6 8 8 8 8 7 7 5 4
13.5: 0 2 5 9 12 11 11 11 11 10 8 7
14.0: 0 2 7 13 17 16 16 16 15 15 12 11
14.5: 1 3 10 20 25 23 23 23 22 22 18 16
15.0: 1 5 14 29 36 33 32 34 32 32 27 24
15.5: 2 7 21 42 53 48 47 50 48 47 39 35
16.0: 3 11 32 63 78 71 69 75 70 69 55 50
16.5: 4 16 48 97 117 107 104 113 107 100 78 69
17.0: 6 24 75 147 176 162 156 174 165 145 104 93
17.5: 10 40 127 239 284 270 257 295 276 215 135 120
18.0: 21 80 223 356 418 451 443 490 493 370 171 153
18.5: 22 82 234 365 436 462 466 505 510 417 209 189
19.0: 22 82 234 365 436 462 466 505 510 426 237 217
19.5: 22 82 234 365 436 462 466 505 510 429 246 228
band 0900 0975 1050 1125 1200 1275 1350 1425 1500 1575 1650 1725 total
---- ---- ---- ---- ---- ---- ---- ---- ---- ---- ---- ---- ---- -----
13.0: 5 5 5 5 5 4 4 3 2 1 0 0 116
13.5: 8 8 7 7 7 6 7 5 3 2 1 0 170
14.0: 12 12 11 11 11 10 10 8 5 3 1 0 247
14.5: 17 18 17 17 17 15 15 12 7 4 2 0 358
15.0: 26 26 24 24 25 21 22 18 11 6 3 1 520
15.5: 37 38 36 35 35 31 32 26 16 9 5 1 752
16.0: 53 54 50 49 49 43 46 36 22 13 7 2 1082
16.5: 73 75 70 68 68 60 64 51 32 18 9 2 1561
17.0: 99 102 95 92 92 81 86 70 43 24 12 3 2239
17.5: 127 132 124 120 120 106 111 91 57 32 16 4 3318
18.0: 159 167 157 151 151 135 140 117 73 41 20 5 4994
18.5: 195 204 195 186 186 170 175 148 94 52 25 6 5545
19.0: 224 234 226 216 217 201 204 172 110 63 30 8 5877
19.5: 234 244 238 228 229 213 215 181 117 67 33 9 6004
B Magnitude sizes:
band 0000 0075 0150 0225 0300 0375 0450 0525 0600 0675 0750 0825
---- ---- ---- ---- ---- ---- ---- ---- ---- ---- ---- ---- ----
13.0: 0 0 1 2 4 5 5 5 5 5 5 3
13.5: 0 0 1 3 6 8 7 7 7 7 7 5
14.0: 0 0 2 5 9 11 11 11 10 10 10 8
14.5: 0 0 2 7 13 17 16 15 15 15 15 12
15.0: 0 1 4 10 20 25 23 22 23 22 23 19
15.5: 0 1 5 15 30 36 34 33 33 33 33 28
16.0: 0 2 8 23 44 54 50 48 49 48 49 40
16.5: 0 3 12 35 67 80 74 71 74 71 72 57
17.0: 0 5 17 53 100 119 111 105 110 107 104 80
17.5: 0 7 25 81 149 176 165 156 166 160 150 107
18.0: 0 10 37 126 219 260 246 232 252 241 215 141
18.5: 0 16 62 194 311 367 380 359 397 385 321 184
19.0: 0 22 81 231 361 430 458 459 498 502 415 226
19.5: 0 22 82 234 364 435 462 465 504 509 428 245
band 0900 0975 1050 1125 1200 1275 1350 1425 1500 1575 1650 1725 total
---- ---- ---- ---- ---- ---- ---- ---- ---- ---- ---- ---- ---- -----
13.0: 3 3 3 3 3 3 3 3 2 1 1 0 75
13.5: 5 5 5 5 5 5 4 4 3 2 1 0 113
14.0: 7 8 8 8 7 8 7 7 5 3 2 1 168
14.5: 11 12 13 12 11 12 10 11 8 5 3 1 249
15.0: 17 18 19 18 17 18 16 16 12 7 4 2 365
15.5: 25 27 28 26 25 26 23 24 18 11 7 4 534
16.0: 36 38 40 37 36 37 33 34 27 16 10 5 774
16.5: 51 54 56 52 50 51 46 48 38 22 13 7 1115
17.0: 70 74 77 72 69 71 63 66 52 31 18 10 1596
17.5: 95 100 104 97 93 95 85 88 70 42 24 13 2257
18.0: 125 130 137 127 122 123 110 115 93 56 32 16 3178
18.5: 162 169 177 165 158 158 143 148 122 75 42 21 4528
19.0: 204 210 219 209 198 199 183 188 157 99 56 27 5644
19.5: 226 233 243 236 225 226 211 213 179 115 66 32 5965
V Magnitude sizes:
band 0000 0075 0150 0225 0300 0375 0450 0525 0600 0675 0750 0825
---- ---- ---- ---- ---- ---- ---- ---- ---- ---- ---- ---- ----
13.0: 0 0 1 2 3 3 3 3 3 3 1 1
13.5: 0 0 2 3 4 4 4 4 4 4 3 2
14.0: 0 1 3 5 6 6 6 6 6 6 4 4
14.5: 0 1 4 8 9 9 9 8 9 9 7 6
15.0: 0 2 6 12 14 13 13 13 13 14 11 10
15.5: 1 3 9 18 21 20 19 19 20 21 17 15
16.0: 1 5 14 27 31 29 28 28 29 31 25 22
16.5: 2 7 22 41 47 44 42 43 44 45 37 32
17.0: 3 11 34 61 70 66 63 64 65 66 52 46
17.5: 4 16 51 91 103 99 93 95 96 96 72 63
18.0: 7 23 76 130 150 144 135 138 139 135 96 84
18.5: 9 32 107 179 206 200 188 193 192 189 129 110
19.0: 11 42 143 231 270 264 249 260 257 251 174 149
19.5: 14 54 179 277 329 332 316 333 328 313 224 199
band 0900 0975 1050 1125 1200 1275 1350 1425 1500 1575 1650 1725 total
---- ---- ---- ---- ---- ---- ---- ---- ---- ---- ---- ---- ---- -----
13.0: 1 2 1 1 1 1 1 1 0 0 0 0 43
13.5: 3 3 3 2 3 2 2 2 1 0 0 0 65
14.0: 4 5 4 4 4 4 4 3 2 1 0 0 98
14.5: 7 7 7 7 7 6 6 4 3 2 1 0 147
15.0: 11 11 11 10 10 10 9 7 4 3 1 0 219
15.5: 16 17 16 15 16 15 15 11 6 4 2 0 326
16.0: 24 25 24 22 23 22 22 16 9 6 3 1 479
16.5: 34 36 34 32 33 31 31 23 13 8 5 1 701
17.0: 48 51 48 45 47 43 45 33 19 12 7 2 1010
17.5: 66 70 65 61 64 59 61 46 26 16 9 2 1439
18.0: 88 93 87 82 86 79 82 62 35 21 12 3 2000
18.5: 116 123 113 107 111 101 105 82 46 28 15 4 2696
19.0: 155 163 150 141 145 133 137 110 63 37 20 5 3571
19.5: 205 212 198 186 189 175 178 148 89 52 26 7 4574
Compression of A2.0: A2.0 is distributed in a binary form, and standard compressors such as PKZip or GZip won't reduce the size by much. (They also make use of the data a pain; you have to decompress everything before you can access any data, and you can't "reach in" and decompress a chunk of data in the middle.) But it does not necessarily follow that A2.0 can't be compressed much. There is really a lot of redundancy in A2.0; that's the basic requirement for compressibility.
Technically, this sort of compression is quite possible. But legally and practically, there are problems. Ax.0 must be distributed in its "raw", uncompressed form. And while the following compression method would be "old hat" for most professional programmers, it would be understandably scary to your average Basic or Fortran programmer.
The following assumes we want to keep things relatively simple (as compression algorithms go), and that we want lossless compression (no bits are sacrificed in the creation of this data, and you could decompress it and re-create the original byte for byte). For this, we can achieve compression of 25% or 33% almost trivially. Somewhat more complex algorithms could get us to the 50% level with a little effort.
Each A2.0 (or A1.0) star is currently stored as three long integers (12 bytes). The first contains the RA. The second contains the dec (actually, dec + 90 degrees, "south polar distance", or "SPD".) The third contains magnitude data and the identity of the plate ("field", in A2.0-speak) used. To compress this 25% would mean storing each star in nine bytes instead of twelve; 33% would require eight bytes/star.
Both the RA and dec are stored in "units" of .01 arcsecond (360000 units/degree). Stars are stored in 24 files, each covering a declination band of 7.5 degrees.
In any given declination band, the dec will obviously only cover a range of 7.5 degrees, or 2.7 million units. That can be stored in 22 bits. Let's round that up to a full three bytes. We've lopped off one byte already.
The RA for a given zone does run through a full 360 degrees, also stored in the .01 arcsecond units. Let's divide each zone into 2160 pieces in RA. At the equator, each "piece" would measure 10' east-west, a.k.a. 60000 units.
Therefore, the RA can be stored in sixteen bits, a.k.a. two bytes. We're down to nine bytes a star. That's the 25% compression format.
There is a small cost for this: we need to know where each "piece" begins and ends. That requires an "index" of 2160 long integers per zone (much like the 96 long integers currently given in .ACC files). With 24 zones, we'll need to invest (24 zones) * (2160 pieces/zone) * (4 index bytes/piece) = 207360 bytes for our index. But that's a small price to pay, considering we just cut A2.0 down by 25%, or about 1.6 GByte.
We could re-define our "piece" so that, instead of covering 60000 "units" in RA, it covers only 256. Doing that means we can store an RA value in only one byte (saves 500 MBytes), but our index grows by about 250-fold (consumes 50 MBytes, about 2 MBytes/zone). Still, doing this boosts the compression from 25% up to 33% (eight bytes/star instead of nine).
Further compression does get a little harder to do. The declination values will vary almost randomly across those 7.5 degrees within a zone, and by definition, you can't compress random data. (You could get a little from the fact that stars will cluster a little here and there, and you could store the dec in 22 bits instead of 24 bits; this last eliminates two bits/star, or one byte for every four stars. With a half billion stars in A2.0, we'd be talking about saving 125 MBytes. But it would a fair amount of pain for only modest gain.)
I've been keeping the number of bytes/star constant for the sake of discussion. This is an unusual restriction; most compression methods store "common" values in fewer bits than they do "rare" values. Abandon that restriction, and all sorts of wonderful possibilities open up. The difference in RA between one star and its neighbor, for example, is highly correlated (because each zone is sorted in RA) and would compress nicely. The "field" (plate) identifier also could be compressed fairly well, and cross-indexing to GSC, PPM, Tycho-2, and other catalogs would be straightforward if you abandon the constant size requirement.
I've also kept all the data (lossless compression). The last two or three bits of the RA and of the dec are basically "noise", and could be dropped painlessly, saving 250 or 375 MBytes. If it's a dire emergency, I suppose we could do that... but the ability to reconstruct the original A2.0 from its compressed version could be valuable. In fact, Dave Monet might well insist on it from a legal perspective.
33% is not, by usual standards, impressive compression. But A2.0 consumes about 6.3 GBytes in its 'raw' form. 33% compression brings us down to 4.7 GBytes, 50% to 3.2 GBytes... either of which would fit nicely on a 5.2 GByte DVD, with some room for other catalogs.