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