VII/239A   History and Accurate Positions for the NGC/IC Objects  (Corwin, 2004)
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History and Accurate Positions for the NGC/IC Objects (Version July 2004)
     Harold G. Corwin Jr
    <The NGC/IC Project (2004)>
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ADC_Keywords: Nonstellar objects ; Galaxy catalogs ; Historical catalog

Description:
    The files icpos.dat  and ngcpos.dat  are intended  to provide  a handy
    source for fairly accurate positions for the NGC/IC objects. They have
    only one entry per object, giving an unweighted mean position followed
    by the number  of  positions  used  in  calculating  that  mean,   the
    calculated  standard deviations in RA and  Dec,  and the sources used.
    The precision to which the position is given reflects the precision of
    the most precise position in the full list.

    Note that precision does not necessarily reflect accuracy,  especially
    with unweighted means.  However,  obvious blunders have been rejected,
    and many objects  have enough  accurately-measured positions  now that
    overall accuracy  will not suffer much from  poor positions out on the
    wings of the  "bell curve".   Note also  that the  standard deviations
    reflect more accurately  the  uncertainties  in  the  unweighted  mean
    positions  rather than the mean errors.  Errors in published positions
    (see the "refs.dat" file) are almost always underestimated, so quoting
    the  standard deviation  increases the  qualitative perception  of the
    errors to about what they should be in an ideal world. Mean errors can
    be easily calculated by dividing the standard deviations by the square
    root of the number of positions used.

    Finally, these positions are on the FK5 system, and are not reduced to
    the International Celestial Reference Frame (ICRF).  However,  the FK5
    optical system  is consistent with ICRF to  within the known errors of
    the  older system (see  Ma et al,   1998AJ....116..516M and references
    therein for more information about ICRF).

    The individual positions are reported  in  the  files  icdata.dat  and
    ngcdata.dat.   The  files   icnotes.txt   and   ngcnotes.txt   contain
    discussions  about ambiguous,  non-existent or other mysterious NGC/IC
    objects.   A detailed introduction to the contents of these discussion
    files can be found in the "ninotes.txt" file.

File Summary:
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 FileName   Lrecl   Records  Explanations
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ReadMe         80        .   This file
explain.txt   104       56   General introduction (mostly included above)
nipos.txt     172      446   Introduction to the position files
                                  (*pos.dat and *data.dat files)
icpos.dat     134     6490   IC  positions (mean values)
icdata.dat    156    19154   IC  referenced positions and notes
ngcpos.dat    151     9027   NGC positions (mean values)
ngcdata.dat   178    36025   NGC referenced positions and notes
refs.dat       80     2365   References of positions
ninotes.txt    79      162   Introduction to files icnotes.txt and ngcnotes.txt
icnotes.txt    82    11259   Discussions of IC puzzles
ngcnotes.txt   80    15375   Discussions of NGC puzzles
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See also:
    http://www.ngcicproject.org/ : the NGC/IC Project Home Page
    VII/1 : Revised New General Catalogue (Sulentic+, 1973)
    VII/118 : NGC 2000.0 (Sky Publishing, ed. Sinnott 1988)

Byte-by-byte Description of file: icpos.dat ngcpos.dat
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   Bytes Format Units    Label   Explanations
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       1  A1    ---      Note    [*] indicates a note in the file "ngcnotes.txt"
                                     or "icnotes.txt"
       2  A1    ---      Cat     [IN] Catalog: N=NGC, I=IC
   3-  6  I04   ---      NGC/IC  Number in the NGC or IC catalog
   7- 20  A14   ---    n_NGC/IC  Component or remarks (G1)
  22- 23  I2    h        RAh     Right Ascension J2000 (hours)
  25- 26  I2    min      RAm     Right Ascension J2000 (minutes)
  28- 34  F7.4  s        RAs     [0,60]? Right Ascension J2000 (seconds)
      36  A1    ---      DE-     Declination J2000 (sign)
  37- 38  I2    deg      DEd     Declination J2000 (degrees)
  40- 41  I2    arcmin   DEm     Declination J2000 (minutes)
  43- 48  F6.3  arcsec   DEs     [0,60]? Declination J2000 (seconds)
      49  A1    ---      qPos    [a-g] Precision on position (G2)
  51- 52  I2    ---      N       Number of positions used
  54- 59  F6.2  arcsec e_RAs     ? Standard deviation in RA
  61- 66  F6.2  arcsec e_DEs     ? Standard deviation in DE
  68-187  24A5  ---      Refs    References of positions detailed in "refs.dat"
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Byte-by-byte Description of file: icdata.dat ngcdata.dat
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   Bytes Format Units   Label     Explanations
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       1  A1    ---     Cat       [IN] Catalog: N=NGC, I=IC
   2-  5  I04   ---     NGC/IC    Number in the NGC or IC catalog
   6- 19  A14   ---   n_NGC/IC    Component or remarks (G1)
  21- 22  I2    h       RAh       Right Ascension B1950 (hours)
  24- 25  I2    min     RAm       Right Ascension B1950 (minutes)
  27- 33  F7.4  s       RAs       [0,60]? Right Ascension B1950 (seconds)
      35  A1    ---     DE-       Declination B1950 (sign)
  36- 37  I2    deg     DEd       Declination B1950 (degrees)
  39- 40  I2    arcmin  DEm       Declination B1950 (minutes)
  42- 47  F6.3  arcsec  DEs       [0,60]? Declination B1950 (seconds)
      48  A1    ---     qPos      [a-g] Precision on position (G2)
      49  A1    ---   f_Pos       [*o] rejected or original position (1)
  50- 54  A5    ---   r_Pos       Reference of position in "refs.dat"
  55- 56  I2    ---     N         ? Number of observations from original source
  58-178  A121  ---     Comm      Comments, Notes and Remarks (2)
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Note (1):
    Rejected positions are noted with an asterisk "*", and original
    positions from the source are noted with a lower case "o".

Note (2):
   Objects other than galaxies have their types noted as:
     GC = globular cluster
     OC = open cluster
     PN = planetary nebula
     BN = bipolar nebula
     DN = diffuse (bright) nebula
     EN = emission nebula
     RN = reflection nebula
    HII = HII region
    SNR = supernova remnant
     SA = stellar association
     SC = star cloud
      * = star, ** = double star, ... n* (n being a number) = asterism
   and a 'p' is appended if the NGC or IC number applies to part of an
   object only.

    This column contains also comments usually from the original sources,
    Harold Corwin's notes being enclosed in square brackets [...],
    cross-identifications with modern catalogues, etc. For more details,
    please refer to the "nipos.txt" file.
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Byte-by-byte Description of file: refs.dat
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   Bytes Format Units   Label     Explanations
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   1-  5  A5    ---     Ref       Reference code
       9  A1    ---   n_Text      [*=] * indicate a remark line, = the Bibcode
  10- 80  A71   ---     Text      Text of note.
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Global Notes:

Note (G1): the n_NGC/IC column may contain additional NGC/IC numbers,
     or the designation of a component of the NGC/IC number.
    Directional suffixes are generally used (e.g. nw=northwest,
    se=southeast, m=middle, etc.). See more details in the description
    file "nipos.txt".

Note (G2): the qPos column indicates the number of decimals in the
    columns containing the position (right ascension and declination).
    Accuracies better than 1mas have however been rounded to the nearest
    milli-arcsec (mas). The conventions are:
      a = 0.1ms  in RA, 1mas in Dec
      b = 1ms    in RA, 0.01arcsec in Dec
      c = 0.01s  in RA, 0.1arcsec  in Dec
      d = 0.1s   in RA, 1arcsec    in Dec
      e = 1s     in RA, 0.1arcmin  in Dec
      f = 0.1min in RA, 1arcmin    in Dec
      g = 1min   in RA, 0.1deg     in Dec

History:
  * 21-Jul-2004: From the NGC/IC Project at http://www.ngcic.com/corwin/
                 (version July 2004)
  * 10-Dec-2004: Version 'A' from author's version 2004-Nov-19
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(End)                                   Francois Ochsenbein [CDS]    10-Dec-2004