This web page, and the site you found it in, is currently devoted primarily to debugging J. L. E. Dreyer's New General Catalogue and two Index Catalogues. Though compiled as carefully as possible, the three catalogues are unfortunately replete with hundreds of errors, large and small, almost all from the sources that Dreyer adopted. His own work was meticulous; very few errors in his catalogues are attributable to him. Rather, they are the result of either primitive methods of determining positions on the sky, or of hasty publication of "new" non-stellar objects.
Cleaning up the NGC/IC entries requires three things: 1) Access to the literature that Dreyer used to assemble the catalogues, 2) a source or sources of accurate positions for the objects included in the catalogues, and 3) a good photographic or digital sky atlas. For the first, I have over the years collected almost all of the original publications that Dreyer consulted. A few objects came in from private communications to Dreyer himself; these letters and lists may still be extant in his archives in England or Ireland, assuming they were kept after his death in 1932. For the second, I use CDS's VizieR service to consult nearly a dozen different catalogues of accurate positions for celestial objects of all kinds (a list is given below). The third requirement is fulfilled by HEASARC's SkyView; it provides easy access to the optical sky surveys (I occasionally use the non-optical surveys as well). The SDSS DR12 "Navigation Tool" is often useful, and the STScI Digitized Sky Survey page is occasionally helpful, too. I also use NED and SIMBAD for accessing modern data for individual objects.
This particular file primarily covers the second requirement -- accurate positions. It is split into six sections:
Files linked through this page are
|OC||open cluster (physical cluster)|
|OCR||open cluster remnant (physical cluster)|
|OptC||optical cluster (not a physical cluster, if this is known)|
|SA||stellar association, sometimes used interchangeably with SC|
|Ast||Asterism; usually from AH or TDM|
Here are some examples from the position tables with the format shown:
NGC/IC RA (2000.0) Dec (2000.0) R So n Comments, Notes, etc. (1) (2) (3) (4)(5)(6) (7) <------------------><------------><----------->|<--><> <--------------------------------------------------------------------------------------------------> N0015 00 09 02.5 +21 37 27 UZC N0015 00 09 02.472 +21 37 28.34 KHJ 1 = NPM1G +21.0004 N0015 00 09 02.48 +21 37 28.6 sUCA2 N0015 00 09 02.50 +21 37 26.9 CCA = UGC 00082. Nominal error = 1.4 N0015 00 09 02.54 +21 37 28.2 GSC 2 = 01184-01268 N2002.1 05 30 18 -66 53.5 oJH SC,LMC. "Place of double star ... the chief of a great cluster ..." in the LMC N2002.1 05 30 21 -66 51.7 cHCe SC,LMC N2002.2 05 30 20 -66 52.8 oJH GC,LMC. "vB, S, R" "nebula" in the LMC N2002.2 05 30 20 -66 53.0 ESOB GC,LMC = ESO 086-SC003 N2006n 05 31 19.9 -66 57 29 HCcd GC,LMC = [SL63] 538 N2006s 05 31 19 -66 58.3 ESOB GC,LMC = ESO 086-SC008 N2017 05 39 17.3 -17 50 42 c2MSP 6 6 sts; not a cluster. Mean position. N2017 05 39 18 -17 50.8 oJH N2017 e * 05 39 22.47 -17 51 18.9 c2MSP * N2017 m * 05 39 16.21 -17 50 58.0 c2MSP * N2017 n * 05 39 16.96 -17 49 43.0 c2MSP * N2017 ne * 05 39 19.40 -17 50 20.0 c2MSP * N2017 s * 05 39 20.48 -17 52 02.7 c2MSP * N2017 w * 05 39 08.06 -17 49 52.5 c2MSP * N2139=I2154 06 01 07.97 -23 40 23.7 cUB10 2 N2398 se comp 07 30 16.52 +24 29 12.4 UCAC N2398 nw comp 07 30 13.6 +24 29 26 WS N2399 07 29 49.9 -00 12 50 WS *** N2399 07 29 50.2 -00 12 51 HCm ***. GSC pos for 1, HCo for 2. N2399 07 29 50.3 -00 12 50 c2MSP 3 *** N2399 07 29 50.6 -00 12 44 oHS1 *** N2399 n * 07 29 50.26 -00 12 39.3 c2MSP * N2399 s * 07 29 50.49 -00 13 05.9 c2MSP * N2399 w * 07 29 49.96 -00 12 47.2 c2MSP * N2947=I0547=I2494 09 36 05.82 -12 26 11.7 s2MSP
These files are intended to provide a handy source for "sufficiently accurate" positions for all of the NGC/IC objects. Unlike the J2000.0 base files, they have only one entry per object, a "selected" position chosen by Brian Skiff or myself. (Earlier versions of these files also carried a unweighted mean positions calculated from the several acceptable positions in the base files when Brian or I had not yet selected an accurate position. All of these unweighted mean positions have been replaced with selected positions as of May 2016.)
The format of the files follows the base files closely with the same NGC/IC name I've used there in the first 20 columns. The selected RA follows in column 21 in HH MM SS.ddd format (with varying numbers of digits after the decimal depending on the precision of the data in the base files), the declination sDD MM SS.dd (same comment) in column 35. A one-letter code in column 51 indicates the source of the selected position: "s" for Skiff, "c" for Corwin, and "o" for original observer (currently used when the object may have a questionable identification or cannot be found; I have plans to include all of the original positions in later versions of the base files).
If you are still using older versions of the files which also contain unweighted mean positions, you will find the number of positions used to find the mean in columns 49 and 50. The calculated standard deviations in arcseconds in RA and Dec are in columns 53 to 58, and columns 59 to 64, respectively. The sources of the selected or mean positions are listed beginning in column 66. The precision to which a mean position is given reflects the precision of the most precise position for the object in the base file.
Note that precision does not necessarily reflect accuracy, especially with unweighted means. However, I've rejected obvious blunders, and most popular 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". And these have all been replaced with selected positions in current versions of the files, in any case.
Two sets of files are available, identical except for the equinoxes: J2000.0 and B1950.0. The equinox J2000.0 file gives the selected position (or in older versions, the unweighted mean of all the non-rejected positions) taken directly from the base file, while the equinox B1950.0 file has that selected (or unweighted mean) position precessed to B1950.0.
Please note that J2000 positions that I calculated between mid-2001 and
28 October 2005 may have errors of over an arcminute in them due to a bug in
GCC 2.96 (the GNU CC and Fortran compilers I was using at the time). Most of
the precessed positions are all right, but a few are not (e.g. NGC 1593 where
the RA seconds for the incorrect position was listed as 16.333; the correct RA
seconds should have been 06.133). Other positions in later batches were
affected by compiler errors which produced many precessed positions containing
"NaN" ("Not a Number"), indicating computation errors. The base B1950
positions were not affected by the compiler bug.
The latest (November 2010 and later) versions of the files processed with the GCC compiler that I used on workstations at IPAC, Caltech are correct; as are the positions processed with GCC 4.8.0 (that I adopted in February 2013) on a Macintosh using OS X 10.11.5.
My apologies for any problems arising from this.
If you are working with files from releases earlier than May 2016, you will find objects with unweighted mean positions still shown. For those, I've listed standard deviations rather than mean errors as these more accurately reflect the uncertainties in the unweighted mean positions. Errors in published positions (see the Sources 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. If I ever decide to calculate weighted means (unlikely) -- instead of or in addition to selecting "best" positions -- the mean errors will have more relevance. In the meantime, if you want them, mean errors can be easily calculated by dividing the standard deviations by the square root of the number of positions used. Again, this procedure only applies to older versions of the files.
As of March 2014, I am using J2000.0 for the base files rather than B1950. To find the J2000.0 positions, I simply precessed the old B1950.0 base files to J2000.0. Specifically, I did not reenter positions originally published at the equinox of J2000.0. Thus, any errors in the B1950.0 positions will be reflected in the J2000.0 positions that are now the basis for the positions I am currently using. The precession routine, discussed below, represents another potential source of error as well. I emphasize this because many of the older positions have as their native equinox B1950.0 tied to the FK4 list of fundamental astrometric standard stars. Many of the newly-adopted positions have equinox J2000.0 tied to either the FK5 or ICRS list of fundamental standards. The J2000.0 positions were precessed back to B1950.0 for use in the earlier base files. Precessing them forward again to the original equinox of J2000.0 will generally not recover the original positions as published -- approximations in the precession routines could lead to differences of as much as 2-3 tenths of an arcsecond. Simple rounding errors have occured as well. I will be checking all these carefully over the next few upgrade cycles, replacing positions that have been too strongly affected. So far, I have found very few that need fixing.
All of this raises the question of not only precession, but of conversion to the FK5 J2000 reference frame (not the more recently adopted ICRS, though for my purposes, the two are functionally identical; more on this below) which I have adopted here as a new standard. Let's talk about precession first.
The precession routine that I use now adopts the FK4 to FK5 reference frame corrections given in USNO Circular No. 163 (1981, ed. G. H. Kaplan. I gratefully acknowledge the invaluable help of Brent Archinal in working through these detailed accounts of precession, particularly the conversions from FK4 to FK5, and back; also see the discussion of precession in e.g. the Explanatory Supplement to the Astronomical Almanac, pp. 99-108 [University Science Books, 1992], and the Introduction to FK5). Note that I am specifically not using additional corrections taking into account the areal-dependent distortions between FK4 and FK5, nor have I adopted any magnitude terms, either. In practice, this version of precession agrees almost exactly with that used by the CDS services (VizieR and SIMBAD), and closely approximates the NED routine. Typical differences on test cases that I've used as checks are on the order of 0.1 arcseconds or less, though I have occasionally seen differences as large as 0.2-0.3 arcseconds.
Thus, the J2000.0 positions in the base files, derived from B1950.0 FK4 positions, are expected to be close to, if not exactly on, the FK5 system. Also note they have not been reduced to the more recently adopted International Celestial Reference System (ICRS) as reflected by the Hipparcos and Tycho-2 positions for bright stars. However, the FK5 optical system has been shown to be consistent with ICRS to within the known systematic errors (about 20 milliarcseconds) of the older system (see Ma et al, Astronomical Journal, Vol. 116, p. 516, 1998 and references therein; and the Introduction in Volume 1 of the Hipparcos and Tycho Catalogues, pp. 100-101, for more information about the relationship of FK5 and ICRS). Given the relatively large accidental errors (on the order of a few tens of milliarcseconds in the very best cases at J2000.0; more typical errors are on the order of a few tenths of an arcsecond for "astrometric" positions, and a few arcseconds for typical lists of galaxy positions) in the present data, you may also take the J2000.0 positions as being systematically consistent with -- again, if not precisely on -- the ICRS.
By the way, you will occasionally see references to the ICRF2 (or just ICRF), the "International Celestial Reference Frame". This is the basic astrometric reference frame resting on about 3400 extragalactic radio sources observed with very long baseline interferometry (VLBI), with special emphasis on 295 "defining sources", including 3C 273B. (These are mostly quasars so have, in principle, no detectable proper motion. This ought to be a "frozen", non-rotating reference frame against which the motions of the stars can be measured for the foreseeable future.) The ICRS is the optical realization of the ICRF, essentially defined by the 100,000+- Hipparcos stars with well-known proper motions and not in multiple systems. See http://aa.usno.navy.mil/faq/docs/ICRS_doc.php and Fey et al., AJ 150, 58, 2015 for a more detailed discussion of the several astrometric reference systems, their developements, and interrelationships. (I note that preliminary work on ICRF3 has begun, but that is a reference frame for the future.)
All of this is about to be revised once again as Gaia scans the sky to something like 21st magnitude. The resulting catalogue of more than a billion stars will define the successor to ICRS, the optical astrometric system, to sub-milliarcsecond levels around the sky. Because it includes many of ICRF's QSOs as well, Gaia will be rigorously tied to the ICRF -- we'll soon have no excuse to not get it right. See the Gaia home page for more about this amazing project.
Finally, I note that the Gaia team has just (September 2016) published their
first data release. Details are available in
Occasionally, I give BVRc data for stars and galaxies in the notes for the positions (see e.g. NGC 1330). Once in a while, you may also see U and Ic numbers as well. I usually take these from the SDSS with the data converted with the following formulae from Jester et al (AJ 130, 873, 2005):
V = g - 0.59*(g-r) - 0.01 U-B = 0.78*(u-g) - 0.88 B-V = 0.98*(g-r) + 0.22 V-Rc = 1.09*(r-i) + 0.22 Rc-Ic = 1.00*(r-i) + 0.21
"u", "g", "r", and "i" are the SDSS magnitudes, and UBV are the standard Johnson/Cousins magnitudes, while Rc and Ic are the Cousins red and near-infrared magnitudes. These formulae are appropriate for all stars with Rc-Ic < 1.15, and will also work well for non-emission galaxies (also see the SDSS/UBVRI transformation page).
Note that the SDSS "magnitudes" are actually transformed from the measured fluxes with an inverse hyperbolic sine function. For objects brighter than about 22nd magnitude, these are numerically identical to standard Pogson magnitudes transformed using base 10 logarithms. For fainter objects, the "asinh" magnitudes more accurately reflect the fluxes at low signal-to-noise ratio, and even handle negative fluxes without crashing the transformations. Lupton et al (AJ 118, 1406, 1999) describes this magnitude system in detail. A useful introduction is given on the SDSS web site. Again, for "bright" objects, the SDSS magnitudes are functionally the same as the logarithmic magnitudes that we've been using for 1.5 centuries.
Much of astronomy from the time of William and John Herschel on through the end of the 19th century was devoted to the establishment of an accurate astrometric reference system. The star catalogues that the Herschels used often had systematic errors of several arcseconds, perhaps more. Crude proper motions could be found (and indeed were used by Sir William in 1805 to find the solar motion and apex), but parallaxes were first measured only in 1838, after the Herschel's sky surveys were done.
By the end of the 19th century when photography had supplanted visual observing as the primary means of determining positions of celestial objects, the reference system was well enough developed that it can be usefully accessed today for the derivation of proper motions of stars. The Carte du Ciel/Astrographic Catalogue project mapped the entire sky down to about 11th magnitude, providing an epoch now at least 70-80 years in the past for comparison with current epoch surveys.
Indeed, accurate proper motions were one of the goals of the astrometrists. By the early 20th century, they had a good idea of the large-scale motion of the nearby stars within the galaxy. Lacking, though, was an accepted theory of the nebulae and where they fit in with the stars. No one had found any verifiable proper motion for any of the nebulae in spite of years of trying. Large programs of observation of nebulae were mounted at several European observtories with the hope of determining proper motions for the objects.
But nebulae are tough to measure accurately. Many are fuzzy, ill-defined patches of pale light compared to the sharper star images. Setting a micrometer wire on the center of a nebular image is often a difficult task, though many of the brighter nebulae have sharp nuclei that help make the chore somewhat easier. The numbers of nebulae are small, too, at the brighter magnitudes. While there are something like a thousand nebulae -- most are galaxies, of course -- brighter than 12th magnitude, there are several million stars brighter than that same limit. A twelfth magnitude star is hard enough to measure with a visual micrometer, even with a relatively "large" telescope (say 30- to 50-cm); a nebula of the same brightness with most of its light spread out over an area several arcminutes in diameter is just that much more difficult.
So, by the turn of the 20th century, only a few thousand nebulae had been well-measured. Unfortunately, the resulting positions were typically published in observatory annals, essentially lost to the hundreds of researchers engaged in the still-new science of astrophysics. While stellar astrometry carried on, astronomers as a group were slow to start large-scale programs of measurement of nebular images simply to find accurate positions. By the 1920s, they knew that proper motions of the galaxies -- as they by then understood that most of the nebulae are extragalactic -- were essentially unmeasureable: Their distances were too great.
So, twentieth century galaxy cataloguers essentially started over with positions estimated with accuracies of a couple of arcminutes, often from the NGC or ICs. Who needed better? That accuracy was good enough to find the galaxies, and that was pretty much all an astrophysicist needed to point a telescope with a spectrograph or camera attached. Adjustments at the eyepiece could easily center the object for further study.
Use of the galaxy images as a reference frame for astrometry was slow to come, too. While galaxies in principle could define a precise and -- for all practical purposes -- unmoving reference frame, finding the nuclei in the larger images proved to be as difficult as always; on photographic plates, the nuclei are sometimes lost in the overexposed central regions of the galaxies. Plates, however, have a huge dynamic range, and many galaxy nuclei have essentially stellar profiles, so several projects were done using galaxies as reference objects.
However, galaxies are found in large numbers only away from the Galactic plane. The Milky Way is precisely where many of the most interesting classes of stars are located, so the astrometric reference system there had to depend on the stars themselves. If one is forced to use stars over a third of the sky, why not simply do it over the other two thirds as well? That, in fact, is usually what happened until the extragalactic quasi-stellar radio sources were recognized in the 1960s as a potential reference system.
By then, it became obvious that we needed good optical positions for the quasars to compare with the radio positions (see the discussion of the ICRF and the ICRS in the previous section). With the dawning realization that when we observe quasars (or their radio-quiet cousins, QSOs) we are looking at the nuclei of very distant galaxies, interest in determining accurate positions for galaxies once again bloomed.
Now, of course, we use the positions from the modern sky surveys, scanned directly from the sky, or from "second generation" survey plates taken by the big Schmidt telescopes. These positions are strictly on the same system as the stars, so -- assuming that the scanning machines can accurately find the galaxies' nuclei -- we have our accurate positions in hand.
Finally, we need to relate the positions from the surveys to the NGC and IC objects. Automated scanners don't have the NGC in their software as they go over the sky or a plate, so we have to take the time to make the association of the NGC/IC number with the accurate position.
That is what I am doing here.
The NGC -- and to a lesser extent, the two ICs -- has been used as a source for deep sky objects since it was published. Most of the large, bright optical objects are there. So, it has been subject to extensive work since its publication to determine its accuracy.
The unfortunate summary is "Not very accurate." Dreyer recognized this and published long lists of corrections as appendices to both ICs, as well as in his edition of WH's complete papers. Other early 20th century efforts at correcting the catalogues were made by (for example) Father Hagen at the Vatican Observatory, Karl Reinmuth at Heidelberg, and several Harvard astronomers led by Harlow Shapley. Other lists of corrections came from Lick and Mt. Wilson Observatories in California, Helwan Observatory in Egypt, and several of the European observatories.
Guillaume Bigourdan at the Paris Observatory probably made the biggest contribution, observing about 6600 of the NGC and IC objects from the early 1880s to 1911, some several times over, measuring their positions as accurately as possible with respect to nearby stars. This enormous set of micrometric positions uncovered and corrected hundreds of the NGC's errors, and is still valuable today for helping to identify the objects that the 19th century visual observers saw.
The mid-20th century saw the husband-and-wife team of Gerard and Antoinette de Vaucouleurs carry on the vast chore of galaxy cataloguing as a necessary first step toward astrophysical studies of the galaxies. They and their colleagues eventually published the three Reference Catalogues of Bright Galaxies where the problem of designation and position errors had to be dealt with for not just the NGC/IC objects, but for other galaxies as well.
Other specialized catalogues had an easier time of it -- there are far fewer open and globular clusters; planetary, emission, and reflection nebulae across the sky than galaxies, so the recent catalogues of these objects are -- at least compared to the galaxy catalogues -- relatively error-free.
Two explicit revisions of the NGC were published before 1990: "The Revised New General Catalogue of Nonstellar Astronomical Objects" by Jack Sulentic and William Tifft; and "NGC 2000.0", edited by Roger W. Sinnott (this includes the two ICs as well). Neither was particularly successful at removing errors from the old catalogues, but that was not a major goal in either case.
For that, amateur astronomers have played a major role in recent decades. The NGC/IC Project (of which my particular effort was a part) attempted to collect the most prominent of these into a single, coherent web site, and largely succeeded in that. Unfortunately, the Project itself, while still available online, has become largely static since its latest revisions in 2007. Lately (April 2015), there have been expressions of interest in updating the Project's web pages. Check there from time to time for new files, new data and images, etc.
The southern sky was relatively better covered well before the north thanks to the efforts of one man: Andris Lauberts. His ESO/Uppsala Survey of the southern sky is a monumental work and was originally the single most valuable source of positions for deep sky objects south of -17.5 degrees. Among the southern objects for which he specifically searched were all the NGC and IC objects.
My own work with the de Vaucouleurs (in the Southern Galaxy Catalogue, SGC), and with Brian Skiff (the south-equatorial extension to SGC, SEGC) supplemented Andris's work, and led to generally correct identifications for the NGC objects -- particularly galaxies -- south of +3 degrees, as well as for the larger IC galaxies, and for many other southern deep sky objects.
Andris went through the Magellanic Clouds, too. In addition, the Clouds have been covered by several observers including Jenni Kay, Mati Morel, and Brian Skiff; and particularly Brent Archinal in his recent book "Star Clusters" with Steven Hynes (the objects in the Clouds are discussed further below). I have not yet had time to reflect all of this work here, though I have made a start. I have, however, done a thorough re-evaluation of the NGC and IC objects in both Magellanic Clouds with the original observations, particularly John Herschel's, at hand.
The northern sky was not as fortunate with respect to the NGC and ICs until 1997 (more below). Many of the objects are too faint to be included in the professional mid-20th-century surveys of brighter galaxies (the CGCG, MCG, and UGC), and -- as I noted above -- the simple existence of an NGC or IC number is not in itself sufficient to excite the interest of a professional astronomer intent on building a career "pushing back the frontiers of science." The same is true of the many so-called "non-existent" star clusters (in Sulentic and Tifft's "Revised New General Catalogue of 1973) discussed by Brent Archinal in his 1993 Webb Society monograph; they are just no longer interesting enough to professional astronomers to merit much study.
The NGC/IC position situation drastically improved in 1997 when Wolfgang Steinicke published his "Revised New General Catalogue and Index Catalogue". A second edition appeared in mid-1998 and has been followed by more or less yearly updatings since. The version that I've adopted for the position files is the one that Wolfgang posted in 2001. His latest version (as of this writing in May 2016) is dated February 2016, and I encourage readers to look to that (or its successors) for a collection of data (magnitudes, diameters, and so forth) beyond the positions and object types that I list so far.
Using the available literature, or the digitized sky surveys (available in the 1990s as RealSky and the Digitized Sky Survey), Wolfgang collected or measured accurate positions for thousands of NGC and IC objects not previously having them. These include the nearly 1900 positions that Wolfgang took from the Lick NPM1G catalogue. These (and about 48,000 fainter) galaxies served as the reference frame for the Lick Northern Proper Motion survey, so their positions are very accurately known: standard deviations are less than 0.3 arcsec, though there is a 0.2 to 0.3 arcsec declination offset with respect to the ICRS. Wolfgang himself has measured thousands more positions using RealSky or RealSky South. There is also a systematic offset of about two arcseconds to the south in the declinations measured using RealSkyView; these are due to a bug in the RealSkyView software (the same bug affects positions derived from DSS with any software originating at Software Bisque, e.g. "The Sky", "CCDSoft", etc.). I have folded all of these positions, without systematic corrections, into my accurate position files -- they are accurate enough to identify the objects they are meant to.
Note, too, that Wolfgang published his PhD thesis on the discoverers of the NGC objects in 2010 as a book: "Observing and Cataloguing Nebulae and Star Clusters"; it is available from the Cambridge University Press. This book gives information about nearly all the early observers, their telescopes, and their observing techniques. It is a valuable complement to the actual lists of deep sky objects which they discovered.
The publication late in 2003 of "Star Clusters" by Brent Archinal and Steven Hynes has at last given us a nearly-definitive list of star clusters in the Milky Way Galaxy, the LMC, SMC, M31, and the Fornax Dwarf Spheroidal Galaxy. I am typing in the positions for the NGC and IC clusters from this marvelous book (the Magellanic Cloud clusters are currently in). Here, the positions come primarily from Brent Archinal himself, or from a massive paper in A&AS, 84, 527, 1990 by Kontizas et al for the LMC. While I had called most of these LMC objects "open clusters" -- with only those with a "compact" note being called "globular" -- it's clear, simply looking at the clusters, that many are indeed actually globulars, at least morphologically. Many others are stellar associations ("star clouds"). I've since reclassified most of the Magellanic Cloud clusters based on their appearance.
I have also folded in other large lists of optical positions for galaxies and planetaries, including those from Brian Skiff at Lowell Observatory, Steve Gottlieb in Albany, California, and the extragalactic groups at Bologna, Italy (Vettolani et al), NRAO (Condon et al), and Lyons, France (Paturel et al).
This still left thousands of NGC/IC galaxies, nebulae, and clusters with either no accurate position or only a single position (subject to various kinds of errors including simple typos). For all of these objects, I have measured positions on the POSS prints, the SERC films, or on DSS images on HEASARC's SkyView web site; have extracted positions from one of the astrometric catalogues (primarily GSC until about 2006 or 2007; 2MASS, SDSS, UCAC, URAT1, CMC, or USNO-B1.0 after); or have adopted positions measured by others in the NGC/IC Project as noted above.
Finally, extending the work I've done to revise the list of 195 bright, large, and/or nearby galaxies in the Astronomical Almanac (work now being carried on by Ronald Buta at the University of Alabama), I have made an effort to measure very accurate positions for the few hundred brightest galaxies using the 2-Micron All-Sky Survey (2MASS) images available through IPAC's Infrared Science Archive web site. Though these images are at longer wavelengths (roughly 2 microns, as the survey name indicates) than the optical images I've used in the past, almost all the positions derived from them are for the galaxies' bright nuclei lost to overexposure on optical sky survey images. These nuclei are almost all coincident with the optical or radio nuclei. The exceptions tend to be "interesting" and/or unusual peculiar systems like NGC 520, or very inclined galaxies like NGC 4945 where the optical nuclei are lost behind dust in the galaxies' disks.
Systematically, the 2MASS coordinate system is defined by ICRS. The positions measured on the 2MASS images also have very small standard deviations -- on the order of 0.2 arcseconds in both coordinates. The positions in the 2MASS extended source catalog (XSC; online at http://irsa.ipac.caltech.edu/applications/Gator/) and the 2MASS large galaxy atlas (T. Jarrett et al, AJ 125, 525, 2003) typically have errors on the order of an arcsecond. Positions in the XSC are occasionally off for large objects near the edge of a 2MASS scan; superposed stars or companion galaxies can also confuse the position. The large galaxy atlas was created specifically to address this and other shortcomings of the automated processing that created the extended and point source catalogues.
More recently (2005 to the present) the Sloan Digital Sky Survey (SDSS) has provided positions and photometry (diameters as well as magnitudes) for half a billion objects, most in the northern sky. The positions are again very accurate (standard deviations on the order of 0.15 arcsec), but only for single, isolated objects with well-defined luminosity profiles. Large, multiple, bright, or otherwise complicated objects have often been broken up by the image-finding algorithm that SDSS uses, so data for the bright objects in the NGC/IC must be approached with considerable caution. For the smaller, fainter objects here, however, the SDSS offers data of very high accuracy at optical wavelengths.
Other surveys that I regularly turn to include the US Naval Observatory surveys (UCAC, USNO-B1.0, and URAT1 in particular), the Hipparcos and Tycho catalogues, the Guide Star Catalog (particularly GSC 2.2, GSC 2.3 and GSC-ACT), the Carlsberg Meridian Catalogue, and PPM/PPMXL. Each has its strengths and weaknesses, but the data from them that I've adopted is not just highly accurate, but appropriate for the object at hand.
Since December 2003, there have been no gaps in the position lists. All of the NGC and IC objects have "accurate" positions, and all have now (June 2016) been looked at by a "modern" observer with the history of the object in mind. In particular, Wolfgang Steinicke has collected data for the NGC and IC objects, as well as usually considering the observational circumstances of its discovery; Steve Gottlieb is well along with his visual observations of all the NGC objects; Malcolm Thomson has extensive notes on many of the IC objects; Courtney Seligman is building a "Sky Altas" around images and detailed descriptive notes of each of its objects, including all of the NGC and IC objects; and I have notes for over 5000 of the NGC and IC objects (the "notes" files mentioned above), as well as Brian's and my collection of selected accurate positions.
Still to be done is a reconciliation of these several lists. Most of the objects are labeled correctly in all the lists, but some differences remain. There are also differences of opinion among us as to which object(s) (if any!) a few of the numbers apply.
I also still have in progress a careful historical check of the positions that I have taken from other sources: are the objects that the modern positions point at really the ones that were found a century or two ago? While we have made a lot of progress toward answering that question for almost all of the NGC and IC objects, there is still some that needs to be done. Regard these position files, along with the notes files, as a major step in that direction.
As I mentioned above, the positions have been updated by Brian Skiff and myself to include a single, selected, "best" position for each object. These selected positions are always very good, with errors on the order of 0.2-0.3 arcsec for stellar objects, or for objects with identifiable nuclei. We have simply looked at a list of the accurate positions reported by VizieR (I am currently -- beginning in 2014 -- using 2MASS PSC and XSC, UCAC, URAT1, SDSS, GSC-A, GSC 2.3, the Carlsberg Meridian Catalog, USNO-B1.0, and the Tycho-2 Catalogue) and have chosen a position that is close to the mean of all the values available. I prefer positions from the surveys that scanned the skies directly -- 2MASS, UCAC, URAT1, SDSS, CMC, Tycho-2 -- rather than those from photographic plates. Similarly, I prefer any of these "objective" positions to those estimated by me (or any other observer) from a photograph or on-screen image. Obvious outliers are almost always rejected, though in cases where the positions are more discordant than the usual errors suggest, I have listed them all and have selected one that more or less represents the center of the object. These are fortunately rare.
For the nebulae clearly associated with stars, such as planetary nebulae, reflection nebulae, and some HII regions, I almost always give the positions of the stars themselves as representative of the object. Occasionally, lobes or clumps of nebulosity within the larger object will warrant positions of their own; I list these as separate objects with unique names (based on the NGC number) and positions.
The exceedingly rich globular clusters, as well as the relatively compact clusters in the Magellanic Clouds, often have two or more positions listed in the objective catalogues, probably for individual stars or close clumps of stars, as the result of incipient resolution. I've taken simple means of the several positions where this leads to a center that seems representative of the object. Lately, researchers needing kinematic or dynamical centers of globular clusters have adopted more sophisticated schemes based on isopleths or isophotes; see, for example, the papers by Goldsbury et al (AJ 140, 1830, 2010) and Noyola and Gebhardt (AJ 132, 447, 2006). I've adopted these positions when they represent the visual star distribution well enough.
For open clusters, I've been guided by the original observer's description and sizes (if given). These come primarily from the Herschels, with John Herschel's positions almost always quoted in the GC and NGC. I've listed these positions in all the cases that I've looked at, and will eventually have them all included in the position list for easy comparison with more modern positions. For the positions that I've determined myself using Goddard's SkyView service, I look, first, at 30 arcminute square DSS2 fields using the red plate images, or if those are unsatisfactory in some way -- e.g. two or more fields poorly stitched together, or a field that is too small -- I switch to another color plate or a larger field, usually one degree across. Once I have the entire cluster in the image, I estimate the lengths of the major and minor axes and put the cluster's center at the intersection of the two axes.
The problem with an approach like this for the poor, scattered clusters -- found particularly by John Herschel -- is not being able to properly visualize his view from his description, especially when he does not give apparent diameters. This may have led to some rather fanciful "clusters" on the sky; NGC 6773, 6774, and 6775 are three such objects (see my notes on these). I doubt very much that these are physical clusters of stars, but the detailed studies needed to determine that have not been done, or I've simply not taken the time to dig them out of the literature. I welcome pointers to papers discussing objects such as these. Of course, information on many of these clusters, published before 2003, will be found in the remarkably complete compendium "Star Clusters" by Brent Archinal and Steven Hynes. I am grateful, too, for extensive correspondence with Brent; that has clarified many objects that I would otherwise still be puzzling over.
Other diffuse objects like bright nebulae have their centers estimated in the same way, that is by assuming they are simple ellipses. They, of course, almost always have irregular shapes, so the approximation as an ellipse can sometimes lead to misleading positions. I've adjusted these by eye accordingly, usually on the DSS2 red images.
For late-type galaxies without obvious nuclei, I have chosen the center of the bar (if there is one) or of the bulge or disk as the position for the galaxy. All these positions, while carrying the precision warranted, are nevertheless more than accurate enough for unambiguous identification of the intended NGC or IC object. They may not, however, be adequate for physical studies of the object -- caution is advised!
For the missing NGC/IC objects, I simply present the original observer's nominal position from the published monograph or paper. Occasionally, I have access to original observing logs or documents (e.g. the Herschel Archive from the Royal Astronomical Society, or sketches by the Leander-McCormick observers) with more information than eventually appeared in print. The position from NGC or IC is given only if it is relevant to the correct identification of the object, or if it figures in a discussion of the adopted position.
Finally, I'd better say something about NED, the NASA/IPAC Extragalactic Database, of which I was a team member from early 1991 until my retirement in mid-2011. NED is a database of literature and data about galaxies, aimed at the professional astronomer. Thus, it reflects the professional literature. However, as I note below, I corrected all of the NGC/IC identifications in NED in 2006, though some objects with recent reevaluations are not yet updated. Many of NED's NGC/IC identifications have been wrong in the past. But as of March 2006, almost all are correct in that they represented my latest considerations of the objects' identifications.
Nevertheless, if you find a difference, trust my most recent files on this web site, not NED. (If you find a difference that is really annoying, send me a note so that I may ask my colleagues in Pasadena to fix NED. Or simply send NED a note directly.) This effort grew out of requests from NED users for the NGC numbers on objects in the Magellanic Clouds. Most of these are star clusters, and had not been previously included in NED. This work has also led to the resolution of many more NGC puzzles, primarily in the south where Lauberts, Steinicke, or I had not made positive identifications.
Three special notes of thanks are due:
First, to Bob Erdmann for his dozen years of work on the NGC/IC Project's Web site. It is through that work that the Project got its start on the World Wide Web, and that it has received the recognition that it has. Were it not for Bob's dedicated effort, our individual work would still sit in our separate file systems, or in scattered notes in the deep-sky journals known only to a few diehard fans of the historical sky.
Second, to Christopher Watson who picked up the Web site when Bob had to step down as webmaster in early 2009. Chris was able to take over the day-to-day chores of running the site with enviable ease. All of us who have data on the site, and who use it frequently, are grateful for Chris's database and Web experience that made the transition possible at all. Chris had to give up the webmaster's role in 2011 due to the pressure of work and family committments. Currently, the Web site is being maintained by Mark Wagner and Steve Gottlieb, with financial support by myself.
Third, as mentioned above, in March of 2008 my long-time colleague and collaborator Brian Skiff at Lowell Observatory began collecting and assessing positions for NGC objects from the "new" astrometric catalogues like 2MASS, UCAC, USNO B-1.0, GSC 2.3, and others. This is fulfilling one of my already-mentioned long-term goals for this work -- letting it be an astrometric reference as well as a guide to the history of 19th century nebular astronomy.
I take great pleasure in thanking several people specifically for their help in this "informal" quest for perfection. First, my colleagues in the NGC/IC Project who have contributed data and copies of the old literature, furnished positions, and suggested identifications, specifically (in no particular order) Brian Skiff, Malcolm Thomson, Wolfgang Steinicke, Bob Erdmann, Steve Gottlieb, Brent Archinal, Chris Watson, Steve Coe, Jenni Kay, Ron Buta, Dave Riddle, and the late Glen Deen. Most of these folks have also spent considerable time writing articles or preparing speeches about the project -- getting the word out and helping others to discover the rewards of historical deep-sky work.
Others who have done special digging in libraries and observatory archives, or who have made contributions of positions, time, or materials include Andris Lauberts, Brian Cuthbertson, Brian Marsden, Daniel Green, Tom DeMary, Harvey MacGillivray, Mati Morel, Jason Adamik, Jim Caplan, Sue French, Don Osterbrock, Leos Ondra, Yann Pothier, Marion Schmitz, Paul Brown, James Bryan, Tony Flanders, Guiseppe Longo, Glen Cozens, Jeffery Corder, Steve Waldee, Courtney Seligman, Jeff Kanipe, Perry Remaklus, Kathrine Haramundanis, Ashraf Shaker, Steven Dick, Brenda Corbin, Angus McDonald, and probably many others whom I must have forgotten -- my apologies to all who have fallen victim to my leaky memory. Please remind me; you deserve credit here, too.
A few other specific acknowledgments are necessary:
Laurent Cambresy at CDS first suggested to me that this work might find a place in their holdings (VII/239), and Francois Oschenbein has prepared these lists for their inclusion in the CDS collection. Francois has also helped track down missing and obscure references that I had let go by the wayside over the years.
Gerard and Antoinette de Vaucouleurs got me started on this quest through their support and encouragement beginning in 1965, and ending only with Antoinette's death in 1987 and Gerard's in 1995. Their Reference Catalogues of Bright Galaxies led me directly into the historical byways.
I would also be remiss if I did not acknowledge the extraordinary patience of my wife Kathleen who sacrificed evenings and weekends to the never-ending quest for an accurate representation of the historical deep sky. Morris, Thumper, Angelica, Crabcake, Bunty, Dante, and Nicco have also provided much-needed grounding at various times over the past quarter century, pulling me back to their solid feline world of food and affection, clearly the real objects of earthly delight. Galaxies can sometimes, rightly, become mere phantoms in the sky.
I must also thank all those many musicians, and electronic and audio engineers, who make the great music of the past thousand-plus years available to us at any time of day or night. We live in a special place and time ("a glorious accident"), and we are far more fortunate than we usually acknowledge or even realize. I'm happy now to explicitly recognize their work, and to paraphrase, "A day without music -- or galaxies! -- is a day without life."
Finally, here are some formal acknowledgments for the several data services without which almost none of this work would have happened. The people behind these services made this project possible, and continue to make it easy.
The Sky Surveys, DSS, and STScI:
The Digitized Sky Surveys were produced at the Space Telescope Science Institute under U.S. Government grant NAG W-2166. The images of these surveys are based on photographic data obtained using the Oschin Schmidt Telescope on Palomar Mountain and the UK Schmidt Telescope at Siding Spring. The plates were processed into the present compressed digital form with the permission of these institutions. [Go to http://archive.stsci.edu/dss/index.html]
The National Geographic Society - Palomar Observatory Sky Atlas (POSS-I) was made by the California Institute of Technology with grants from the National Geographic Society.
The Second Palomar Observatory Sky Survey (POSS-II) was made by the California Institute of Technology with funds from the National Science Foundation, the National Geographic Society, the Sloan Foundation, the Samuel Oschin Foundation, and the Eastman Kodak Corporation.
The Oschin Schmidt Telescope is operated by the California Institute of Technology and Palomar Observatory.
The UK Schmidt Telescope was operated by the Royal Observatory Edinburgh, with funding from the UK Science and Engineering Research Council (later the UK Particle Physics and Astronomy Research Council), until 1988 June, and thereafter by the Anglo-Australian Observatory. The blue plates of the southern Sky Atlas and its Equatorial Extension (together known as the SERC-J), as well as the Equatorial Red (ER), and the Second Epoch [red] Survey (SES) were all taken with the UK Schmidt.
We acknowledge the use of NASA's SkyView facility ( http://skyview.gsfc.nasa.gov) located at NASA Goddard Space Flight Center.
This research has made use of IPAC's Skyview Image Display and Analysis Program, developed with support from the National Aeronautics and Space Administration. [Go to http://www.ipac.caltech.edu/Skyview/].
This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. [Go to http://ned.ipac.caltech.edu].
IRSA (2MASS access):
This research has made use of the NASA/IPAC Infrared Science Archive (IRSA) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. [Go to http://irsa.ipac.caltech.edu].
This research has made use of the SIMBAD database, operated at CDS, Strasbourg, France. [Go to http://simbad.u-strasbg.fr/].
This research has made use of the VizieR catalogue access tool, CDS, Strasbourg, France. The original description of the VizieR service was published in A&AS 143, 23, 2000. [Go to http://vizier.u-strasbg.fr/viz-bin/VizieR].
We have made use of the LEDA and HyperLEDA databases (http://leda.univ-lyon1.fr).
This research has made use of NASA's Astrophysics Data System Bibliographic Services. [Go to http://adsabs.harvard.edu/ and http://adsabs.harvard.edu/historical.html].
Funding for SDSS-III has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Science Foundation, and the U.S. Department of Energy Office of Science. The SDSS-III web site is http://www.sdss3.org/. SDSS-III is managed by the Astrophysical Research Consortium for the Participating Institutions of the SDSS-III Collaboration including the University of Arizona, the Brazilian Participation Group, Brookhaven National Laboratory, Carnegie Mellon University, University of Florida, the French Participation Group, the German Participation Group, Harvard University, the Instituto de Astrofisica de Canarias, the Michigan State/Notre Dame/JINA Participation Group, Johns Hopkins University, Lawrence Berkeley National Laboratory, Max Planck Institute for Astrophysics, Max Planck Institute for Extraterrestrial Physics, New Mexico State University, New York University, Ohio State University, Pennsylvania State University, University of Portsmouth, Princeton University, the Spanish Participation Group, University of Tokyo, University of Utah, Vanderbilt University, University of Virginia, University of Washington, and Yale University.
JPL's Solar System Dynamics:
Finally, I am pleased to acknowledge JPL's SSD Small-Body Identification tool, and their HORIZONS System ephemeris generator for help with comets and asteroids mentioned in the old observations.