Assorted Astronomical Datasets

The following datasets are (or will be) available. Project Pluto has no association with these organizations (other than in using their datasets). However, Guide users sometimes ask about the availability of such data.

  • USNO-B1.0
  • Selected Astronomical Catalogs, Vol. 3
  • PGC-1996 (updated Principal Galaxy Catalog)
  • Hipparcos/Tycho datasets
  • Tycho-2 catalogue
  • Astrographic Catalog (AC 2000) CD-ROM
  • Astrographic/Tycho (ACT) CD-ROM
  • GSC-ACT Catalog
  • USNO A1.0 Catalog
  • USNO A2.0/SA2.0 Catalog
  • USNO SA1.0 Catalog
  • UCAC-3 (USNO CCD Astrograph Catalog)
  • UCAC-2 (USNO CCD Astrograph Catalog)
  • UCAC-1 (USNO CCD Astrograph Catalog)
  • USNO-B1.0

    USNO-B1.0 is the successor catalog to the USNO A2.0. Released in late 2002, it contains about a billion stars, twice as many as A2.0, going down to V=21. It has only one small defect, relative to B1.0: access. You could get A2.0 on a set of eleven CD-ROMs, but the only access to B1.0 is via Internet, and it's apt to stay that way. The catalog consumes 80 GBytes, due to the additional stars and the additional data about those stars. Distribution via DVD-R may be practical eventually... but not right now.

    Balanced against that, it has some tremendous advantages over A2.0. Instead of giving just an R and a B magnitude, you get "R1" and "R2" from two different plates, plus "B1" and "B2" from two more plates, and sometimes an I magnitude. You may get two to five of these, depending on the number of plates on which the star was found. I think the magnitudes may be better than those from A2.0, but wouldn't swear to it. ("Better" is a relative term; they still shouldn't be trusted for much.)

    The positions are somewhat better, mostly because B1.0 contains data from passably "current" plates, whereas A2.0 was based on 1950s-era plates, and stars moved a bit over those fifty years. Once the problems with access are worked around (they aren't insurmountable), B1.0 will become the catalog of choice for many astrometrists.

    Another advantage: A2.0 omitted some red stars, because they weren't bright enough to show up on the blue plate. B1.0 was able to use a looser "two out of five" scheme, and the red stars therefore need only show up on the two red plates.

    More details about B1.0 can be found here.

    Here's a comment on the B1.0 data format (not of much use to most of us; it shows the raw binary format of the original 80 GByte dataset, not the ASCII formats provided by the servers listed below.)

    B1.0 access via Internet: You can get B1.0 data for a given area from the Naval Observatory Flagstaff Station. Use within Guide 8 is simpler; you can just zoom in on the area of interest, then use the option to download and display B1.0 data. (Similar options are available to download GSC-2.2, A2.0, 2MASS, and/or DSS images covering the area Guide is showing on-screen.)

    Guide gets its data for displaying B1.0 (and 2MASS) via a VizieR query form.

    Selected Astronomical Catalogs, Vol. 3

    The Astronomical Data Center (ADC) has announced the availability of a third CD-ROM of assorted astronomical datasets (a list of the datasets is on this site). The previous two CDs were of extensive use in creating previous versions of Guide, and some of the datasets on this new CD will prove useful in Guide 6.0. As the document makes clear, many of the datasets are revised versions of datasets on the first two CDs.

    The following information is from the letter sent out by the ADC.

    The cost of each CD-ROM is US $10 (foreign orders add $5 for shipping and handling). Both ASCII and FITS versions are available, and can be purchased separately. Payment can be made by VISA, MasterCard, American Express, check or money order payable to Hughes STX Corporation in US DOLLARS on a US BANK. If you wish to pay by credit card, please FAX the NSSDC Request Coordination Office at (301) 286 1635, and ensure that your name and FAX number are clearly identified. If you wish to pay by check or money order, please mail your request to the Request Coordination Office, National Space Science Data Center, Code 633.4, NASA/Goddard Space Flight Center, Greenbelt, MD 20771. (Internet: or telephone (301) 286 6695.

    PGC-1996 (Principal Galaxy Catalog 1996)

    LEDA (Lyon-Meudon Extragalactic Database)

    The PGC-ROM 1996 gives access to the largest compilation of individual galaxies ever made.

    The main astrophysical parameters are provided for more than 100,000 galaxies.

    Comprehensive sample programs in FORTRAN are also provided on the CD-ROM. They should work on any UNIX workstation. A DOS/Windows (TM) version is also available.

    To order:

    Complete and return attached form to the address overleaf. Orders can also be placed by fax. You will receive the package with the bill.








    ZIP code:

    Date and signature:

    For PGC-ROM 1996 with Fortran programs: 240 FF (US $48)

    Software for DOS/Windows: 100 FF (US $20)

    Delivery in Europe: 10 FF ($2)

    Delivery elsewhere: 20 FF ($5)

    FAX or MAIL orders to:

    Observatoire de Lyon
    F69561 Saint-Genis Laval CEDEX (FRANCE)

    FAX: (33) 78 86 83 86

    Hipparcos/Tycho catalogs

    Click here for information about ordering the raw data, in ASCII format, from the European Space Agency. (The ordering information also gives some specifications concerning the precision of the datasets that might be of interest to you.) Alternatively, if you've got a fast enough Internet connection, you can try downloading the data from the ADC ftp site.

    The Hipparcos and Tycho catalogs form the basis for Guide's display of stars for about a million of the brightest stars (down to about magnitude 11). They were gathered by the European Space Agency's Hipparcos satellite and released in June 1997, and represent an extreme leap forward over every previous catalog.

    The reason is that the satellite, being above the atmosphere, got astrometric (positional) data ten times more accurate than any previously available, and got far more precise magnitude data than ever seen before. Because of this positional precision, it was also able to get good parallax (distance) data for many of these stars; ground-based data has never been very accurate, except for the nearest stars.

    One point should be cleared up quickly. The Hipparcos catalog contains data for 118,218 stars, down to about mag 7 (with some fainter stars); it is at the highest level of precision. The Tycho catalog contains over 1 million stars, that were not so precisely measured in either position or in magnitude; it's complete to magnitude 10.5, with some fainter stars. Tycho is still far more precise than anything else out there, but it's not quite in the same league with Hipparcos. Tycho is now also known as "Tycho-1", since the release of an updated, expanded version in early 2000 that is known as Tycho-2.

    For some people, the fact that Guide uses Hipparcos/Tycho data is not very significant (it's unquestionably an advantage, but it may be "overkill" for your needs.) Click here for a discussion of the benefits of Hipparcos/Tycho data; it should allow you to decide whether Guide's use of Hipparcos/Tycho data really matters to you at all.

    When Guide is showing you a star chart, you can click on a star with your mouse and bring up full data on it (not only from Hipparcos, but from other catalogs as well). Click here to see an example of what Hipparcos will tell you about an example star, Eta Aquilae.

    One slight drawback to the Tycho data was that the proper motion data was not of wonderful quality. The satellite was only observing for a few years, not long enough to get a long baseline of observations. To repair this, the Tycho data was combined with older positions from the Astrographic Catalog (AC) to get excellent proper motions, in the Astrographic Catalog/Tycho (ACT) dataset. The current batch of Guide CDs use this catalog, not the proper motions from the original Tycho catalog. (Future batches will use Tycho-2, which uses similar methods and also has good proper motions.)

    Ordering the raw ASCII Hipparcos/Tycho data: You can get a set of six CD-ROMs containing full results of the Hipparcos mission, from the European Space Agency. No software is provided with the disks, but it's all in plain ASCII text and is quite well documented; if you're a programmer, making use of them is not at all difficult. (But you do have to write some software to make use of them!)

    The following details about these catalogs come from the order brochure mailed out by the European Space Agency.

    Hipparcos Catalogue

  • Number of entries 118 218
  • Limiting magnitude V ~12.4
  • Completeness Up to V=7.3 to 9.0
  • Median astrometric accuracy (J1991.25) ~1 milliarcsecond
  • Median photometric accuracy .0015 mag
  • Avg number of observations per star 110
  • Number periodic variables & light curves ~2700
  • Tycho Catalogue

  • Number of entries 1 058 332
  • Limiting magnitude V ~11.5
  • Completeness V ~10.5
  • Median astrometric precision 25 milliarcsecond
  • Median photometric precision (B, V, B-V) .07, .06, .10 mag
  • Mean number of observations per star 130
  • Invitation for Subscriptions

    Final results from the ESA Hipparcos space astrometry mission will be made widely available in June 1997. The results will be available in three formats:

    -- A 16-volume hard-bound printed catalogue, containing descriptions of the data reduction techniques, along with the Hipparcos catalogue and related annexes, plus an ASCII version of the Hipparcos and Tycho catalogues and annexes in a set of 6 CD-ROMs;

    -- A subset of the above consisting of Volume 1 (Introduction and Guide to the Data) and the ASCII CD-ROM set;

    -- Celestia 2000: a CD-ROM package containing the Hipparcos and Tycho catalogues and annexes along with interrogation software.

    Only a limited number of the hard-bound printed catalogue will be produced. To avoid future requests exceeding the available supply, you are invited to reserve your copy now, by returning the subscription form before 15 Jan 1997 to: Hipparcos Catalogue Subscriptions Astrophysics Division (SA) ESTEC PO Box 299 2200 AG Noordwijk (THE NETHERLANDS) Fax +31 71 565 4690 See also for a WWW order form


    Please reserve for me the following (prices include post & packing to any destination):

    ___ set(s) of the 16-volume printed catalogue (with ASCII CD-ROMs) @ 650 Dfl ($400) per set

    ___ set(s) of Introduction & Guide to the Data, with ASCII CD-ROM set @ 165 Dfl ($100) per set

    ___ set(s) of Celestia 2000 @ 80 Dfl ($50) per set





    The number of hard-bound volumes produced will be based on the response to this announcement. An invoice will be sent to all subscribers in early 1997. On receipt of payment your requested product(s) will be delivered to the address filled in above.

    Hipparcos Catalogue Subscriptions
    Astrophysics Division (SA)
    PO Box 299
    2200 AG Noordwijk
    The Netherlands

    FAX +31 71 565 4690

    Astrographic Catalog (AC) CD-ROM

    The AC 2000.2 catalog is available on free CD-ROMs from the USNO (US Naval Observatory). Send your requests for these disks to Norbert Zacharias (nz at usno dot navy dot mil). The catalogue is also available via FTP from the USNO ftp site... but it is a very large catalogs!

    The AC catalog contains data for about 4 million stars, from plates imaged about a century ago (the mean epoch is 1907). It therefore contains about four times as many stars as Tycho-1, 40% more stars than Tycho-2, and about a quarter as many as the Guide Star Catalog. For each star, a J2000 position, photographic magnitude, and the date of the exposure are given. The AC2000 catalog, the first digital version of this catalog, has since been superseded by AC2000.2.

    This catalog was a major endeavor early in this century; data for each star was quite painfully extracted. There's a very good article about it in the June 1998 Sky & Telescope ("The Astrographic Catalogue: A Century of Work Pays Off").

    By itself, this dataset would have to be considered of limited use. A century of proper motion has rendered it somewhat obsolete. However, it has been combined with Tycho data to produce the ACT and Tycho-2 catalogues; these catalogues are far from useless, as the following section describes.

    If you've acquired the AC2000 CD-ROM, then you can show those stars in Guide. Click here for information about AC2000 display in Guide.

    Astrographic/Tycho (ACT) CD-ROM

    With the arrival of Tycho-2, the ACT is now obsolete. It was once distributed on CD-ROMs by USNO, but is no longer available. Tycho-2 has more stars and more precise data, both for positions and proper motions. So the following data is now really of historical interest only.

    One slight problem with the original Tycho data ("Tycho-1") was that the proper motions were not very good. Proper motions from older catalogs such as the PPM and SAO were based on much longer time spans, which helped to offset the poorer measurement accuracy. The Hipparcos satellite was not able to run for several decades; if it had, we might get excellent proper motions from it.

    The result is that positions based on Tycho-1 data are excellent for times near 1991 (the midpoint of the satellite observations) and are slowly getting worse as time passes. One way to fix this problem is to improve proper motion accuracy; to do this, the USNO derived proper motions by combining data from the Astrographic Catalog with Tycho positions. This allows measuring proper motions over a time span of about a century, resulting in absolutely wonderful proper motion data. And this, in turn, can make Tycho data almost as precise today as it was in 1991.

    Tycho-2 sidestepped the whole issue by combining the AC catalog (and several others) right from the beginning.

    Guide 8.0 uses Tycho-2 data, and therefore has no ACT-based issues. Tycho-2 wasn't available for earlier versions, so the situation there is more complex.

    Guide 7.0 uses the ACT proper motion data, in place of Tycho data. Most people will not see the difference, but there are two groups of people who will care deeply about this. One is people doing astrometry with the Charon software. The other group will be people examining asteroid occultations of stars; Guide can show the paths of these events (click here to read about occultation/eclipse path display in Guide) , but use of ACT data has made such predictions somewhat better. (See the February 1998 issue of Sky & Telescope, page 86, for details about this.)

    Guide 6.0 used the original Tycho data, but one can get it to use ACT data by going through a few extra hoops. Click here for details on how the ACT can be displayed and used in Guide 6.0.

    USNO A1.0 Catalog

    The USNO A1.0 catalog was the recordholder for the World's Largest Star Catalog, with nearly 500 million stars. It was rendered obsolete by the subsequent A2.0 version of the catalog, and still more obsolete by the arrival of the B1.0 catalog. Like these later catalogs, A1.0 covers the entire sky, and was created by scanning in Palomar and UK-SERC plates. Both red and blue plates were scanned, and objects appearing on only one plate were thrown out. This helped to evade the GSC problem of spurious objects, and also meant that color data was available through comparing the red and blue magnitudes.

    The precision of the positions was already somewhat better than that in the GSC. The authors then recalibrated the data using the Astrographic Catalog/Tycho data, resulting in still better precision. This version, the A2.0 catalog, is now available.

    The A1.0 was distributed on a set of ten CD-ROMs, but is available only to researchers with a demonstrated need for the data. In any case, one would now want the A2.0 catalog, on eleven CDs; this is available, under the A2.0 "pass-it-along" scheme.

    Though the USNO A1.0 CDs are not widely available, you can still download small portions from Lowell Observatory, which has set up a means to request data for a given region in RA/dec. You can get the data as ASCII text or in binary form; the latter can then be displayed in Guide or used for astrometry with Charon.

    USNO SA1.0 and SA2.0 Catalogs

    The USNO A1.0 catalog consumes ten CDs. USNO A2.0 contains a few more stars, and consumes eleven CDs. Neither is very widely available, since USNO distributes them at no charge and can't afford to make millions of sets. To combat this problem, the USNO also provided a single-CD version of each catalog, called "SA1.0" and "SA2.0" respectively. These contain 10% of the A1.0 or A2.0 stars (about 50 million each), in the same format as A1.0 or A2.0. They are not the brightest 10%, however; they are the 10% best suited for use in astrometry, because of their well- determined positional accuracy and because they provide a very even coverage of the sky (the density of stars is practically constant everywhere). Both CDs were freely available from USNO, but they have since run out. The SA2.0 data can be downloaded, however, using the directions found at

    SA1.0 and 2.0 are nearly useless for making star charts, because 90% of the stars are omitted; no recognizable patterns remain when you do that. For similar reasons, it doesn't work very well with the Charon astrometry software, which expects to match star patterns in a catalog to star patterns in a CCD image. However, it can be used with Charon, if some care is taken; click here for details.

    SA1.0 can be displayed in Guide 7.0, without qualification. SA2.0 can be displayed in Guide 7.0, if you download current software and a small index file specially created for SA2.0; click here for details.

    USNO A2.0 Catalog

    As is discussed on the USNO PMM page, the A1.0 has some significant astrometric and photometric problems, mostly inherited from the Guide Star Catalog. A new version, the A2.0 catalog, has been calibrated using the Astrographic Catalog/Tycho (ACT) data, leading to considerable improvement in this regard. It is now available over the Internet, and on a set of eleven CDs (A1.0 only consumed ten disks, but A2.0 has a few extra stars in it.) The full set of CDs used to be available from USNO, but they ran out of copies. You can still get A2.0 disks at no charge, provided that you are willing to copy them, then pass the originals on to someone else; click here for details of the A2.0 "pass-it-along" scheme.

    Be warned that the photometry in A2.0 still isn't very good. Dave Monet stated that "...the photometric calibration of USNO-A1.0 is about as poor as one can have and still claim that the magnitudes mean something." Things are a little better in places with A2.0, but not by much.

    You can also grab A2.0 data through Lowell Observatory. Right now, you can get A1.0 data through the Strasbourg Data Centre (CDS) in France , or through the ESO (European Southern Observatory) site in Germany. It seems to me a safe bet that these two sites will also be switching to A2.0 as soon as they can.

    Using A2.0 and/or SA2.0 in Guide: You can, without question, use A2.0 downloaded from the Internet in Guide 6.0 or 7.0. This required no modification whatsoever to the software, because the format did not change.

    Guide 8.0 has a nifty feature wherein you can zoom in on a desired area, then ask Guide to get A2.0 data via Internet for that part of the sky; click here for details. The same feature can be used in Guide 7.0, if one gets the last update to the Guide 7.0 software. This update will also allow you to use Guide 7.0 with the A2.0 CD-ROMs; click here for details.

    Tycho-2 catalogue: (8 Feb 2000) This dataset has been released! You can download the actual data, and pieces of information about the catalogue, from Erik Høg's site, or from the VizieR site at CDS.

    You can get quite a bit of data about the details of how Tycho-2 was constructed from this Web page provided by the authors of this new catalogue (E. Høg, C. Fabricius, and V.V Makarov of Copenhagen University Observatory; U. Bastian and P. Schwekendiek of the Astronomisches Rechen-Institut in Heidelberg, Germany; and A. Wicenek of the ESO.) They provide a paragraph on the Web site, plus a link to a PostScript version of their paper describing Tycho-2. For those who can't print out PostScript, here are the high points:

    The Tycho-2 catalog does not involve any additional observations by the Hipparcos satellite; instead, the original data gathered between 1989 and 1993 is to be reprocessed using better techniques and faster hardware that were not available for the "original" Tycho catalog, also known as Tycho-1. The resulting catalog contains "good results" for about 2.5 million stars brighter than about Vt=12 (as opposed to Tycho-1, which got data for a little over 1 million stars to about magnitude Vt=11.5.) Not only does the new catalog contain 2.5 times as many stars; the photometric and astrometric data generated is more precise (smaller error bars) than that in Tycho-1. Also, many double stars not resolved in Tycho-1 are resolved in Tycho-2.

    I expect that this dataset will be of tremendous use in astrometry. At present, I know of only one Charon user (Gordon Garradd) using Tycho/ACT to do astrometry. He has a large enough field of view so that, with some planning, he can get four or five Tycho-1 stars in his field of view. The resulting data is of wonderful quality, with small astrometric residuals suitable for last-minute astrometry to improve occultation predictions and for use in computing the orbit of 433 Eros for the NEAR mission. But very few people have big enough CCDs to use Tycho for routine astrometry. Tycho-2 should make this far easier. It should also evade another bane of Tycho/ACT use: Gordon usually finds that to get good data on faint asteroids, the exposure has to be long enough to saturate the Tycho/ACT stars. Having Tycho run "deeper" by half a magnitude will help here.

    How Tycho-2 was created: (You need know none of this to make use of Tycho-2, but it is interesting.) Tycho-1 worked by examining the data and considering a "detection" of a star to occur if a certain signal/noise ratio was reached (1.5 or 1.8). At the time, a better approach was considered, in which data from two consecutive measurements would be combined. The result would be a better S/N ratio, and the limiting magnitude would be improved by about 0.4 magnitudes. The authors of the paper state that "this was beyond the available capabilities with respect to software development and computing facilities when discussed in 1991".

    For Tycho-2, a still better solution was used. The data from all measurements of a given star are used. It sounds a bit like a more sophisticated version of the process used in CCD imaging, where you may not detect an object in four images, but averaging the four improves the signal/noise enough to make the object detectable. The paper gives full details on this, and was reasonably understandable to this non-expert (unusual; these things usually seem written to make the subject as opaque as possible.)

    UCAC-3 catalogue: UCAC-3 is a continuation of the UCAC-2 project. It provides high-quality astrometry and decent photometry for 100 million stars, covering the entire sky. It is available on a double-sided DVD, and will probably be the standard for people doing asteroid photometry. It essentially renders GSC in all forms, including GSC-ACT, completely obsolete.

    UCAC-3 is available from the US Naval Observatory on a double-sided DVD at no cost. Click here for more info on UCAC-3, including on how to get the data. If USNO runs out of DVDs, which seems quite likely, check this page; I'll probably attempt to arrange a system wherein the disks can be passed around and copied.

    I've posted C/C++ source code for accessing UCAC-3 data.

    UCAC-2 catalogue: UCAC-2 is a continuation of the UCAC-1 project. UCAC-1 provided high-quality astrometry for 27.4 million stars, most below declination -15. It's a very useful catalog for southern hemisphere astrometry, but hasn't been quite so useful for the rest of us.

    As is discussed on the UCAC home page, however, the second release of the catalog has been released (August 2003). This version basically continues on up to declination +45 or so, and should be tremendously useful for astrometrists. It's distributed on three CD-ROMs. I will be e-mailing persons who ordered the GSC-ACT CD-ROMs from me, suggesting that they get UCAC-2 and use their GSC-ACT disks for drink coasters, or Frisbees, or whatever.

    I expect UCAC-2 to become the standard for asteroid astrometry, with the exception of persons in the following situations:

  • Imaging objects north of the +45 (or so) limit of UCAC-2. Such persons would ideally then use USNO-B1.0, but a more practical (and likely) choice would be that catalogue's predecessor, USNO-A2.0.
  • Imaging objects so faint that the UCAC-2 stars get saturated. Not very likely, since the catalog goes to about Rmag=16. But some people do chase faint enough objects for this to be a concern.
  • Extremely small fields of view. The catalog will be much denser than GSC, but not as dense as A2.0. You'd have to have a very narrow field of view for UCAC-2 to prove unusable, but it could happen.
  • You can display UCAC2 data in Guide, and you can use UCAC2 as an astrometric reference catalog in Charon. I think most other astrometry software will support UCAC2, too.

    UCAC-1 catalogue: (Be aware that this catalogue has been superseded by UCAC-2.) The UCAC (USNO CCD Astrograph Catalog) is an ongoing project; click here to see the home page for UCAC.

    The above site also gives information about the availability of a data CD-ROM. To request a copy of the CD-ROM, you should send an e-mail to nz at pisces dot usno dot navy dot mil.

    My copy of UCAC arrived on 6 April 2000. The disk contains positions and proper motions for 27.4 million stars, most below declination -15 (the northern limit varies from -5.5 to -21). The data was gathered by the USNO with a CCD astrograph at Cerro Tololo; the plan is to move the hardware to the Northern Hemisphere and do the same thing for the other half of the sky.

    This looks to be an incredible dataset. It gives us roughly the astrometric accuracy of Tycho-2, but with about four times the density of GSC and a slightly greater density than SA. Where possible, I expect this to be the astrometric catalog of choice in the future.

    There are only two serious problems. First, as the README file puts it: "No attempt has been made to provide good photometric data. In fact, observing progresses even during nights with cirrus clouds." * Second, the area covered by UCAC1 doesn't include much of the ecliptic, and few asteroids happen to be conveniently south enough to be covered yet.

    Charon can now use this dataset in astrometric reductions; click here for details as to how to do this. Also, you can now display UCAC1 in Guide as a user-added dataset.

    The CD is a little awkward to use. The data is provided as a 600+ MByte GZipped TAR file, so you can't run directly from the CD. UnZIPping it results in a 1.8 GByte file, which must then be unTARred to produce a slew of files; you'll need 3.6 GBytes temporarily when unpacking everything, and 1.8 GBytes to store the results. Fortunately, multi-GByte drives are common these days.

    If placed in a binary form, the data could consume about 540 MBytes and be directly readable. I may go that route, rather than let the ASCII data consume 1.8 GBytes. You would need either 2.3 GBytes or 3.6 GBytes while unpacking everything. (There's no way to use the CD "as is", the way you can use Tycho-2 or Ax.0. The data has to be unGZipped and unTARred on your hard drive.)

    * The README file may be overly pessimistic about the quality of the photometry, as suggested by the following comment:

    ======================================================== (Arne Henden) wrote:-
     UCAC magnitudes are all unfiltered CCD, and as such
        make a consistent magnitude system and one that
        matches most CCDs better than the GSC/USNO photographic
        values.  They quote ~0.1mag accuracy; it should
        be better locally.