A2.0 CD-ROMs and DVDs on a "pass-it-along" system
Last updated 18 May 2005
Reasons for doing this: I'm frequently asked about the possibility of laying hands on the USNO A2.0 catalogue in its eleven-CD form. USNO provided 500 copies of this catalog, free of charge, to interested persons. This came out of their not-very-healthy budget, and they're understandably not interested in making more copies for every citizen that asks for one.
Burning eleven CD-Rs is a large enough task that I am not very eager to do it often. However, Robert Burns has kindly made six copies of A2.0 available to me. I've burned a few more copies, and some people who have received copies have added still more to the list. We also now have some copies of A2.0 plus UCAC-2 distributed on two DVDs. These CDs and DVDs are now available "on loan" to anyone who wants them. The only "cost" is that you'll be asked to mail them on to the next person who wants them.
How this "pass-along" system works: Under this scheme, you will receive a set of A2.0 CDs or DVDs, at no charge, on loan. They will be sent to you by me or by a previous borrower. When you receive them, you and your friends can copy them to your hard drive and/or to CD-Rs or DVDs of their own.
Meanwhile, I'll be collecting names and shipping addresses of anyone else who would like to borrow the disks. Eventually, you'll get a message from me along the lines of: "When you're done with the A2.0 disks, would you please ship them on to...", followed by a name and address for the next borrower.
You do not send me, or anyone else, money. Your only expense should be that of shipping the disks on to the next person. This scheme has the great virtue that one set of disks can get the catalogue out to dozens of people, with minimal expense and effort on the part of any one person.
Be warned, though, that the "next person" may not be in your country. I'll attempt to arrange things so that if (for example) a copy goes to France, it's passed around among other French users, or at least European users. This should cut postage costs and shipping times. But there is some risk that you'll be asked to send it elsewhere. (Postage costs should be minimal, however; eleven CDs and some modest packaging are not very heavy. I am sending them inside the US via Priority Mail for $3.75, and overseas via Global Priority Mail for a mere $5.00.)
If you truly feel the need to "pay" for this in some way, then I would indeed appreciate it if, while burning a copy of the CD-Rs for yourself, if you also made yet another copy. I will then send you two "next person" addresses, and we'll have one more set of CDs for this scheme. A few such "extra" sets ought to cut down on the time required to get the disks out to everybody. Heck, if one person in ten does this, everybody on the planet would have a copy of A2.0 in a year or two... not that we'll really carry it to that extreme!
A warning about use of A2.0: If you're using A2.0 for astrometry, then I'd strongly urge you to consider using the UCAC-2 catalog instead, whenever possible. (UCAC-2 is included on the DVD version of A2.0.)
Why would I recommend this? UCAC-2 is not nearly as "dense" as A2.0, with about 80 million stars instead of over 500 million. But the positions for those stars are much more accurate than was the case for either GSC-ACT or A2.0, meaning you can do much more accurate astrometry.
If you have a very small field of view, it is possible that UCAC-2 won't provide enough reference stars for you. If you're going after extremely faint objects, it's possible that all the UCAC-2 stars will be saturated. UCAC-2 currently has almost no coverage north of declination +45; there aren't a lot of targets that far north of the ecliptic, but they do happen. Many astrometric programs may be a little slow in adapting to use of UCAC-2. For all of these reasons, it may still be a good idea to have a copy of A2.0 sitting on the shelf, "just in case".
For users of GSC in any of its forms, including GSC-ACT, only the +45 dec limit applies as an objection. The arrival of UCAC-2 effectively makes all forms of GSC obsolete.
How do I check that the disks were copied correctly? In general, I'd expect errors to be immediately obvious. When CDs go bad, you almost always get some variant of "Abort, Retry, Fail?" If that somehow failed to happen, the software used to read the A2.0 data would emit error messages if the data were bad; it's hard to come up with circumstances in which the data could be mangled, but still be legible to the software.
It's probably a good idea to make sure that you didn't miss a CD (it would be easy to copy ten CDs instead of eleven). To do this, make sure that you have 24 files of extension .cat and 24 of extension .acc. The 24 .cat files consume a total of 6,315,370,572 bytes. The 24 .acc files are 2880 bytes each.
I've written a page about the layout of the A2.0 catalog (which files are on which disks, their sizes, and the declination ranges covered... which can help when you need to know which disk(s) cover a particular range of declination.)
Better still might be a little utility to check each of the data files to (a) make sure they are present; (b) are of the right size; (c) perhaps check the first few bytes, the last few, and some in intermediate places. In truly dire situations, one might (d) do a check-sum to ensure the files are right, but this would involve re-reading all 6.3 GBytes and should be a last, almost never-necessary resort.
If you do find that there's some trouble with file(s) or CD(s), please let me know, and I'll send a replacement.
Can I make copies of these disks for others?: Yes, with some very minor restrictions. The A2.0 data is copyrighted by the USNO and several other organizations that provided the plates from which the data was extracted. So you cannot make copies and sell them at a profit, and you cannot distribute subsets of the data. (You can distribute the data at cost for scientific and educational purposes, which is essentially what we're doing here.)
If you have a friend or friends who would like to borrow the disks to copy to their own hard drives and/or CD-Rs, this is perfectly okay. In fact, you will spare them the effort of joining in this "pass-it-along" scheme, saving some postage and some time/effort in organizing it all.
Do I really need all these files/disks?: Possibly not. The A2.0 layout page gives you some idea of what to expect here. Right away, I can tell you that the .tar files (which contain plate calibration data and the software used to create A2.0 in the first place) are probably going to be of no interest to most of us. You can also ignore some files that cover areas beyond your declination limit, though be warned that this won't cause a really drastic decrease in hard disk consumption.
Skipping entire disks is not apt to happen very much. Northern Hemispherians can usually skip disk 5, which only contains some archived software and the catalog data for declinations -52.5 to -60. The other disks all include some middle or northern-declination data.
A2.0 and UCAC-2 on two DVDs: Recently, I upgraded to a DVD writer. With this, one can put the entire 6.3 Gbyte bulk of the A2.0, plus the 2 GByte bulk of the UCAC-2, onto two 4.7 GByte DVDs. The first DVD contains the entire UCAC-2, plus the Northern Hemisphere part of A2.0. For some uses, that's all you really need: you'll want to use UCAC-2 in the area it covers, with the A2.0 as a backup plan for areas north of about declination +45.
The second DVD contains the A2.0 files for declination +7.5 degrees south. This allows for 7.5 degrees of overlap, which may help evade the need to swap DVDs in some situations.
The signup sheet for this list now includes a question as to whether you'd like to receive A2.0 on eleven CDs, or UCAC-2 plus A2.0 on two DVDs.
Software that makes use of the data: At least two freeware star charting programs can display A2.0: Hallo Northern Sky and Cartes du Ciel. Both programs have their fans, and both do a lot of nifty things aside from simply displaying A2.0 data.
Most commercial and freeware planetarium/star charting programs can also display A2.0. This includes my own Guide 8.0 software. Of course, as the author, I am strongly prejudiced in its favor! But rest assured: no matter what astronomy software you use, it probably supports A2.0 in one way or another.
Using the A2.0 disks with Guide: There are several different ways you can use these disks with Guide. The simplest is to zoom in on your area of interest, and use the Extras... Get Star Catalog Data... Get A2.0 from CD-ROM option. Guide will prompt you for the CD(s) you need to insert to get data for that region. (If you have the two DVD version of the data, just put in the first DVD if you're looking at the northern hemisphere and the second DVD if you're looking at the southern hemisphere. Both DVDs cover the region from dec +7.5 to the celestial equator.)
This does require a fair bit of swapping of disks. Nowadays, most people just copy all the A2.0 data to a folder on their hard drives. You can then use this method to persuade Guide to display A2.0 data from the hard drive.
Finally, DVD users may also want to be able to show the UCAC-2 data that is provided on the first DVD. Once this is copied to the hard drive, you can use this method to persuade Guide to display UCAC-2 data from the hard drive.
What about USNO-B1.0? Recently, USNO-B1.0, the "successor" catalog to A2.0, was released. (Click here for some details about B1.0.) The new catalog is, in almost all ways, an improvement over A2.0, and some have suggested we ought to be attempting to distribute this catalog instead of, or in addition to, A2.0.
The major obstacle is that B1.0 consumes 80 GBytes. USNO has distributed it on Linux-formatted hard drives to some of the major astronomical data centers, and the idea is that it's distributed to end users via Internet. You can, for example, get B1.0 data for a given area from the CdS VizieR server or from this server at the Naval Observatory Flagstaff Station. Distributing it by other means would be a hassle, in terms of time and money, which USNO has quite reasonably decided not to undertake.
We may someday have another "pass-it-along" system for B1.0, involving over 100 CD-Rs, or nine double-sided DVDs, or an external USB hard drive, or a combination thereof. But, for the nonce, A2.0 is somewhat easier to handle. I'll keep a list of persons asking about the A2.0 "pass-it-along" scheme, and will notify them if a similar scheme develops for other datasets.
Sign up to be on the list: Please send the following information to me, at pluto@projectpluto.com . I will reply with an e-mail telling you when you ought to see the A2.0 disks in the mail.
Name: Shipping address: E-mail address: Do you want A2.0 in its eleven-CD form, or in the two DVD form? For what purpose do you expect to use A2.0? (The above is purely a "curiosity" question; feel free to disregard it if you wish!)