This the new name for the "SKYMAP Star Catalog" by Myers J.R.,Goddard Space Flight Center, Flight Dynamics Division. It as nothing to do whit the Sky Catalog 2000 from Sky Publishing.

Be careful when using this data for astrometric purpose, the proper motion error are sometime large.

This catalog is normally obsolete as its main purpose is Tycho data reduction. But from an amateur point of view it may be considered as a subset of the GSC up to magnitude 12. It remain very interesting for its compact size, homogenous limiting magnitude and color indication in some fields.

Magnitude band definition :

    band      emulsion/filter
    ----      ------------------------------------
     0        S  - IIIaJ + GG395
     1        N  - IIaD  + W12
     6        N  - IIaD  + GG495
     8        XE - 103aE + Red Plexiglass
    10*       XG - yellow objective + IIaD + GG495
    11        XB - blue objective +103aO
    12*       XB - blue objective +103aO
    13        XB - yellow objective + 103aG + GG495
    14*       XB - yellow objective + 103aG + GG495
    18        XN - IIIaJ + GG385
              * Calibrated with the GSC.

Object class definition :

cl object
-- ----------
0 star
1 galaxy
2 groupe of stars
3 non-stellar
5 possible artefact

Calibration catalog : 
A1.0 and SA1.0 : GSC
A2.0 and SA2.0 : ACT

North-South limit is -30° for A1.0 and -20°/-18° for A2.0.
Stars colour do not represente b-v as other catalogues but (mb-mr)*0.6

Performance : The catalogue structure do not allow fast access to the data. Effectively, the index file (.acc) define zone of 7.5° in declination by 15' in right ascension, this is a very large area for the catalogue scale. Each zone contain up to 3'000'000 stars ( 10 time the SAO !) and can only accept sequential access. So be patient and prefere the GSC for navigation purpose then switch to USNO-A when you are satisfied by all other parameters. 

Variability type : ( look at file cat\gcvs.txt for complete description)

        ACV       Alpha2 Canum Venaticorum
        BCEP      Beta Cephei
        BE        Be stars not GCAS
        BL        BL Lacertae
        BY        BY Draconis
        CEP       cepheid
        CST       constant
        CW        W Virginis
        DCEP      Delta Cephei
        DSCT      Delta Scuti
        E         eclipsing
        EA        Algol (Beta Per)
        EB        Beta Lyrae
        EW        W Ursae Majoris
        ELL       ellipsoidal
        GCAS      Gamma Cassiopeiae
        I         irregular
        IA        white irregular
        IN        irregular in a nebula
        INS       rapid irregular in a nebula
        INT       irregular of the T Tauri type
        IS        rapid irregular
        L         slow
        LB        slow red
        LBV       slow pulsating B stars
        M         Mira (Omicron Ceti) type
        N         nova
        NL        nova-like
        QSO       quasistellar object
        R         close binary with strong reflection
        RCB       R Coronae Borealis
        RR        RR Lyrae
        RRC       RR Lyrae, C subtype
        RV        RV Tauri
        S         rapid
        SDOR      S Doradus
        SR        semiregular
        SN        supernova
        UG        U Geminorum
        UV        UV Ceti
        UVN       UV Ceti in a nebula
        ZAND      Z Andromedae
        ZCAM      Z Camelopardalis
        ZZC       ZZ Ceti

  Notes :

      N: Notes found in the Notes table.
      O: Orbit. Where motion has exceeded 360 degrees, no
         values are listed in the position angle and separation
         columns. Orbits may be found in the "Fourth
         Catalog of Orbits of Visual Binary Stars" (Worley
         and Heintz, 1983), and in the Information Circulars
         published by Commission 26 of the IAU, as well as
        in the literature.
      a: Pair appears in an appendix list, not part of the
         discoverer's regular numbering system.
      r: Pair was listed, but in a "rejected" list, not part
         of the discoverer's regular numbering sequence.
      s: Pair has other discoverer's designations and numbers
         in the literature due to duplicate discovery.
         We have given credit to the earliest discovery
         observation (and correct identification) reported
         in the literature. The List of Synonyms follows the
         List of Additional Discoverers.
      p: Coordinates and proper motions from the ACRS, PPM,
         IRS, and FK5 catalogs.

Object classification :
    Gx    Galaxy
    OC    Open star cluster
    Gb    Globular star cluster, usually in the Milky Way Galaxy
    Nb    Bright emission or reflection nebula
    Pl    Planetary nebula
    C+N   Cluster associated with nebulosity
    Ast   Asterism or group of a few stars
    Kt    Knot  or  nebulous  region  in  an  external galaxy
    ***   Triple star
    D*    Double star
    *     Single star
    ?     Uncertain type or may not exist
    blank Unidentified at the place given, or type unknown
    -     Object called nonexistent in the RNGC (Sulentic and Tifft 1973)
    PD    Photographic plate defect

Note on Desc: description of the object, as given by Dreyer or
    corrected by him, in a coded or abbreviated form. The abbreviations
    and their combination are fully described in the introduction
    to the published catalog.
    ab       about
    alm      almost
    am       among
    annul    annular or ring nebula
    att      attached
    b        brighter
    bet      between
    biN      binuclear
    bn       brightest to n side
    bs       brightest to s side
    bp       brightest to p side
    bf       brightest to f side
    B        bright
    c        considerably
    chev     chevelure
    co       coarse, coarsely
    com      cometic (cometary form)
    comp     companion
    conn     connected
    cont     in contact
    C        compressed
    Cl       cluster
    d        diameter
    def      defined
    dif      diffused
    diffic   difficult
    dist     distance, or distant
    D        double
    e        extremely, excessively
    ee       most extremely
    er       easily resolvable
    exc      excentric
    E        extended
    f        following (eastward)
    F        faint
    g        gradually
    glob.    globular
    gr       group
    i        irregular
    iF       irregular figure
    inv      involved, involving
    l        little (adv.); long (adj.)
    L        large
    m        much
    m        magnitude
    M        middle, or in the middle
    n        north
    neb      nebula
    nebs     nebulous
    neby     nebulosity
    nf       north following
    np       north preceding
    ns       north-south
    nr       near
    N        nucleus, or to a nucleus
    p        preceding (westward)
    pf       preceding-following
    p        pretty (adv., before F. B. L, S)
    pg       pretty gradually
    pm       pretty much
    ps       pretty suddenly
    plan     planetary nebula (same as PN)
    prob     probably
    P        poor (sparse) in stars
    PN       planetary nebula
    r        resolvable (mottled, not resolved)
    rr       partially resolved, some stars seen
    rrr      well resolved, clearly consisting of stars
    R        round
    RR       exactly round
    Ri       rich in stars
    s        suddenly (abruptly)
    s        south
    sf       south following
    sp       south preceding
    sc       scattered
    sev      several
    st       stars (pl.)
    st 9...  stars of 9th magnitude and fainter
    st 9..13 stars of mag. 9 to 13
    stell    stellar, pointlike
    susp     suspected
    S        small in angular size
    S*       small (faint) star
    trap     trapezium
    triangle triangle, forms a triangle with
    triN     trinuclear
    v        very
    vv       _very_
    var      variable
    *        a single star
    *10      a star of 10th magnitude
    *7-8     star of mag. 7 or 8
    **       double star (same as D*)
    ***      triple star
    !        remarkable
    !!       very much so
    !!!      a magnificent or otherwise interesting object

The following codes are used for Color:
'1' if brighter on blue Palomar plate;
'2' if equal on red and blue Palomar plates;
'3' if brighter on red Palomar plate;
'4' if visible only on the red Palomar plate.

The brightness scale varies from 
'1' (brightest) to
'6' (barely detectable).

Catalogues codes :

    1 NGC            2 IC              3 Berkeley          4 Czernik
    5 Dolidze        6 Collinder       7 Upgren            8 Tombaugh
    9 Ruprecht      10 King           11 Stock
   13 Trumpler      14 Markarian                          16 Haffner
   17 Hogg          18 Sher           19 Feinstein        20 Harvard
   21 Lynga         22 Westerlund     23 Basel            24 Blanco
   25 Baractova     26 Biurakan       27 Melotte          28 Pismis
                    30 Trapezium                          32 Pleiades
   33 Graff         34 Iskudarian     35 Stephenson       36 Roslund
   37 Hyades                          41 van den Bergh-Hagen
                    42 Bochum         43 Dolidze-Dzimselejsvili
   45 Antalova      46 Moffat         47 Havlen-Moffat    48 Frolov
                    50 van den Bergh  51 Mayer            52 Latysev
   53 Sigma Ori     54 Graham         55 Aveni-Hunter     56 Loden
   57 Grasdalen     58 Waterloo       59 Auner
   61 Schuster      62 Danks          63 Muzzio           64 =ref.378

Trumpler class :
 Trumpler concentration class [0 1 2 3 4]
 Trumpler Range class [0 1 2 3]
 Trumpler Richness class [m p r]
 Trumpler nebulosity [n u e] 

Hbeta magnitude = 10-2.5(10+log(FluxHbeta))

Morphology indicator :
   "<"  star-like appearance 
   ">"  fainter spherical envelope
   else blank