This is an outline of our general working procedures for preparing
a "clean" version of the NGC (New General Catalogue) and IC (Index Catalogues), corrected as far as possible
in accord with the historical record. To pin the objects down once
and for all, we also include an atlas of images of all the NGC and
IC objects. The work is in two general parts: the catalogue and the
atlas. The catalogue itself is in several parts: data tables,
notes sections, and observing notes. This outline provides an
introduction to the project.
Here is an summary of what's to follow. First, we will answer the
simple questions "Why bother? What could we possibly gain by
expending a terrific amount of effort on what, on the surface,
appears to be relatively trivial questions?" Then, we will give a
brief history not only of the origin of the NGC and IC's
themselves, but of the various past attempts to revise it. Finally,
we shall lay out the working methods which we have adopted in our
work, giving a general overview of the finished catalogue as it is
now taking shape on the Web site -- keeping in mind that as time
goes on, we may well change our ideas, work methods, and the
presentation. An appendix gives a few examples of NGC corrections.
From now on, we'll refer to the NGC and the IC's collectively as
"the NGC" unless there is reason to discuss just one or the other.
We shall make it clear in the context when we are doing this.
At the present, there are at least a thousand known or potential
identification problems in the NGC itself, and certainly that many
again (perhaps more) in the IC's. This is an unfortunately large
percentage of the total number of objects in each catalogue (7840
in the NGC, 5386 in the two IC's). These problems include -- but
are not limited to -- poorly measured or simply wrong positions,
confusion of one nebula for another, poor or missing descriptions,
and so forth.
The major reason to make yet another attempt at removing the errors
from the NGC is that it is still used as a source of names. Most of
the brighter, larger, nearer, and therefore most interesting
non-stellar celestial objects are listed in the NGC. It has become
an astronomical tradition to refer to objects by their NGC numbers.
Therefore, it makes sense to be sure that the number used to refer
to an object does indeed belong to that object.
Aside from straightening out the confusion, and the simple fun of
doing so, there are compelling historical reasons to clean up the
NGC. Many of the people involved in the discovery and observation
of the nebulae were among the most famous astronomers of their day.
If the historical record is not correct, we run the risk of
misattributing discoveries, and even making mistakes about the
progress of astronomy itself. This evolves finally into a simple
matter of historical truth. The NGC is a window on the world of the
19th century astronomer -- but in its present form, it distorts our
vision of our predecessors and of their accomplishments.
In a far more general sense, the problem of proper attribution of
scientific discoveries and theories has been eloquently addressed
by Ralph Alpher and Robert Herman, two of the pioneers of the big
bang theory who saw their work generally ignored and little
appreciated for decades. Toward the end of a recent review of the
early work on the big bang (Alpher and Herman, 1990), they have
this to say:
One wonders about the forces that shape the activities of some
scientific authors. A number of questions have been raised in this paper
which beg to be looked at with proper objectivity by historians and
sociologists of science, particularly in terms of what such matters say
about scholarship and integrity in science, which is one of the most
important of human endeavors. We do not accept the argument of some that
correct attribution does not matter, but that only the furtherance of
science matters. This view does not reflect the ideals and realities of
the scientific enterprise. A correct history of science as a human
endeavor does matter, both for the present and for the future.
While knowing full well the relative importance of correct
attribution in the discovery of the nebulae and in the far more
vital history of cosmology, we could not agree more.
Building the NGC and IC -- J. L. E. Dreyer.
Dreyer was born in Denmark, but emigrated to Ireland in 1874 to
work at Lord Rosse's great observatory in Parsonstown. Though an
amateur, the third Earl of Rosse had built successively larger
speculum mirror reflecting telescopes through the late 1830's and
early 1840's. The series culminated in a massive 72-inch telescope,
the largest in the world from the date of its completion in 1845
until its dismantling just before the first World War. Rosse, his
son (the fourth earl), and his observers (Dreyer was one of these)
spent years examining and measuring the known nebulae in the
northern sky with the famous "Leviathan of Parsonstown," and
discovered many more fainter nebulae themselves.
During the observations, it became clear to Dreyer that it was time
to update Sir John Herschel's so-called GC (General Catalogue) of
nebulae and star clusters, published in 1864. Just a decade later,
there were simply too many new nebulae being discovered and too
many different lists to consult for previous discoveries. Preparing
observing lists or simply finding if a nebula had been previously
found by another observer had become a time-consuming chore. Thus,
Dreyer published a supplement to the GC of about 1000 new objects
in 1878, and -- having suggested yet another supplement in 1886 --
was instead asked by the Royal Astronomical Society to assemble a
"new general catalogue" of non-stellar objects. So, Dreyer added
the latest 1500 objects to the previous lists, combined them all in
Right Ascension (for 1860) order, and the "New General Catalogue of
Nebulae and Star Clusters" appeared as Volume 49, Number 1 of the
Memoirs of the RAS in 1888.
Assembling the NGC, Dreyer had to contend with conflicting claims
of position and description for what he often suspected to be the
same nebula. Some of these he could check himself; most -- because
of the overwhelming numbers of objects -- he simply had to accept
as published. Fortunately, he was an excellent transcriber -- very
few of the errors in the NGC can be traced to carelessness on his
part (his most common error, or that of his uncredited assistants,
was to apply precession with the wrong sign to declinations). Most
of the problems in the NGC are with the original positions and
descriptions, coming as they did from many different observers
using telescopes ranging in size from 2 inches to 72 inches, and
relying on auxiliary instrumentation that ranged from nonexistent
to state of the art.
At one extreme were the careful observations of the astrometrists
working at professional observatories. These observers were
actively involved not only in nebular astronomy -- indeed, with one
or two exceptions, the nebulae consumed very little of their time
and attention -- but in setting up the fundamental coordinate
reference system which we still use today in a form little
different than it was a century ago. These observations, made with
precision optical micrometers, tie the positions of the nebulae
into the fundamental system with random errors on the order of a
few tenths of an arc-second. Only in the past two decades have
better positions for a significant number of galaxies been
determined from plates taken with astrographic telescopes.
On the other hand, there were many amateur and semiprofessional
astronomers actively making discoveries. Some of these observers
were well-equipped with large, solidly-mounted telescopes fitted
out with modern instrumentation. Some, like Isaac Roberts, were
even at the forefront of astronomical photography. Others were
neither as fortunate in their observing gear, nor as careful in
their observing habits. Many scanned the skies with nothing more
than an eyepiece and setting circles at their disposal. Yet, they
published their discoveries along side those from the professional
observatories. Dreyer had to handle them all.
Thus, the NGC is a compendium of the good and the not so good.
Through long experience and comparison of lists of positions from
various observers, Dreyer had a good idea whose observations were
trustworthy, and whose were not. He also realized the importance
of not letting this information be lost (as well as simply giving
credit where it's due!), so he listed the discoverers of each
nebula and star cluster, and gave fairly complete references to
the discovery publications. These references are our key today to
unraveling the problems in the catalogues.
Previous Attempts At Cleaning The NGC
Curiously, most of the previous attempts at cleaning out the errors
in the NGC ignored this gold mine of information in the references.
The modern astronomers involved simply looked at the NGC position
-- if there was a nebula there, great! this was obviously what the
NGC was referring to.
Unfortunately, this works in only a few of the problem cases. The
reason is simply the heterogeneity of the original discovery
observations. More detective work is needed to decipher the often
meager clues left us by the discoverers of the nebulae.
It's still instructive, however, to look at the previous major
attempts to debug the NGC. Here, more or less in chronological
order, are comments about these.
- Dreyer published, in the Notes to the IC's, improved positions
and other comments relevant to the identifications of several
hundred NGC objects. He collected these from the scientific
literature of the late 19th and early 20th centuries; and from
many private communications from the observers themselves. He
also published another shorter list of corrections and
comments in a Monthly Notices article in 1912; this resulted
from his editing for publication in that same year a
collection of all of William Herschel's scientific papers,
including Sir William's three lists of nebulae and clusters
which Dreyer heavily annotated in the Collected Papers.
Finally, he exploited the new method of photographic
astrometry, publishing in the Journal of the Royal Dublin
Society positions of many nebulae from photographs.
(Photography, by the way, stopped cold all but a few diehard
visual micrometric observers from continuing their work. The
obvious advantages of having images frozen on a photographic
plate made the arduous visual work obsolete by the turn of the
century.) He also asked the Royal Greenwich Observatory to
examine a particularly confused region of the northern sky
where a large systematic error crept into Sir William's
discovery observations of more than a dozen nebulae. This
paper appeared in 1911.
- One of the last observers to cling to the visual micrometer
was Guillaume Bigourdan. The major work of this astonishing
observer is today little known and even less appreciated:
micrometric observations of nearly every NGC object (and many
IC objects) visible to him in Paris, altogether about 20,000
measurements of more than 6,600 nebulae and clusters. Since
one of his prime goals was to discover whether the nebulae had
measurable proper motions, he attempted to measure each of
these objects not just once, but twice or more. He began this
mammoth project in 1884 and continued his observations for
more than a quarter of a century. He eventually received the
Royal Astronomical Society's Gold Medal for it in 1919 after
its final publication (delayed by World War I) in 5 massive
The value of Bigourdan's observations to us is that he
published the details of what he observed, right down to the
transparency and seeing. The really valuable part of the
observations are the details of the micrometric observations,
presented not only in their raw form of position angles and
distances from the comparison stars, but also as RA and Dec
offsets. This means that the exact location which Bigourdan
measured can be pinpointed on the sky today. While he was not
overly concerned with debugging the NGC, or with discovering
new nebulae, he did a bit of both in the course of his work.
His descriptions of the objects are detailed and often contain
measurements of nearby stars other than the comparison star;
these are useful for verifying identifications. Absolute
positions (RA and Dec) can be derived from his offsets since
he lists the comparison stars and equinox 1900 positions for
them. Assuming that the proper motions of the comparison stars
are small, modern positions for them (from the GSC, or if they
are brighter, from Hipparcos/Tycho or any other modern catalogue)
will allow positions accurate to about two arcseconds for the
nebulae to be found from Bigourdan's observations, at least
for the brighter nebulae which he could see well.
Many other, though less extensive, lists of micrometric
observations are available in the 19th and early 20th century
literature. These include (but are not limited to)
- more than a dozen lists of new nebulae found by Edouard
Stephan at Marseille in the 1870's and 1880's, collected
and published in 1916 by Esmiol under Stephan's direction,
- two long series of observations at Cincinnati Observatory
by Jermain Porter, which appeared in the Publications of
- those done at Strasbourg (then Strassburg) from the late
1860's to the early 1900's (by Kobold, Winnecke, and
Wirtz), again published by the Observatory,
- those published by Herbert Howe of Chamberlin Observatory
in Denver in a series in the Monthly Notices around the
turn of the century.
There are several other shorter lists which are especially
valuable as they, like Bigourdan, provide the details of the
observations. (Modern cataloguing would be far easier if
modern astrophysicists were as careful with their
observations!) Thus, there is no question at all of which
object these visual observers saw. It would, of course, have
been nice if the entire NGC had been as carefully done.
Unfortunately, only about half of its objects were
well-observed by the time it was published.
- Meanwhile, James Keeler at Lick was photographing the larger
and brighter northern nebulae with the 36-inch reflector
there. This work led to corrections being published in several
papers and Lick Observatory publications between about 1900
and 1920. Edward Barnard was also working at Lick and Yerkes
at about the same time, doing visual as well as photographic
surveys of nebulae. The same sort of work was going on at
Harvard under Pickering's direction, but the goal here was
a photographic survey of the entire sky. Plates, primarily
taken in Chile, were being examined by several Harvard
astronomers (primarily Stewart and Frost) not only for new
nebulae, but also for old ones. Wolf, working at Heidelberg,
was another photographic pioneer who contributed many lists of
positions of new and old nebulae. Reinmuth and Schwassmann
continued the Heidelberg work which culminated in Reinmuth's
publication in 1926 of "Die Herschel Nebel", a complete
photographic reexamination of almost all of the northern
nebulae and clusters found by William and John Herschel, as
well as many other neighboring objects. Madwar and Knox-Shaw
at Helwan Observatory in Egypt also published several lists of
new nebulae discovered both visually and photographically,
correcting old problems along the way.
- Starting at about the time of Dreyer's last published papers
on the nebulae, Father Johann Georg Hagen at the Vatican
Observatory began a systematic visual reexamination of all of
the NGC objects north of about -30 degrees. This work
culminated in the publication of several long catalogues in
the 1920's, most by Hagen ("A Preparatory Catalogue for a
Durchmusterung of Nebulae", comprising several zone
catalogues). The publication of the series culminated with the
"Vatican General Catalogue" itself, prepared by Friedrich
Becker under Hagen's direction. Unfortunately, no new
positions were determined for this long work, but lists of
errata in the NGC were given; and the descriptions, with their
frequent records of nearby stars, are valuable in identifying
the objects which Hagen observed.
- In 1940, a list of corrections to the NGC appeared under
Dorothy Carlson's name. This, her only paper, collected most
of the previously published corrections, and merged them with
the long list built up by Hubble during his pioneering work
with the 60- and 100-inch reflectors on Mt. Wilson.
- Gerard and Antoinette de Vaucouleurs, during the work on
their three Reference Catalogues, made notes about NGC and IC
misidentifications concerning the galaxies included in the
Catalogues. Harold Corwin, working on the 2nd and 3rd
Catalogues with them, also contributed many corrections,
primarily in the southern sky with SGC and ESGC (where, as far
as possible, historical sources were consulted in problem
cases), though also in the north to some extent.
Unfortunately, many problems remain in the Third Reference
Catalogue (RC3) as time did not allow the digging required to
sort out all the errors introduced by the modern catalogues
(numbers 7, 8, and 9 just following).
- CGCG (Catalogue of Galaxies and Clusters of Galaxies), UGC (Uppsala General Catalogue of galaxies),
and MCG (Morphological Catalogue of Galaxies) were all inspired by the availability of
the Palomar Observatory Sky Survey (POSS, POSS1, or simply
PSS). CGCG and MCG attempted a complete census of all northern
galaxies to the 15th blue magnitude, while UGC listed all
galaxies larger than one arcminute or brighter than 14.5.
None of these surveys were primarily concerned with
identifications, though Nilson in UGC at least attempted to
correct some of the errors found in or introduced by CGCG and
MCG. Since the compilers of these catalogues had other goals
in mind, the identifications were usually made quickly with no
recourse to the original literature. Nilson, however, does
give a valuable short history of cataloguing of non-stellar
objects, galaxies in particular, in an appendix to the UGC.
This precis is worth reading as an introduction to the topic.
- RNGC (Revised New General Catalogue) appeared in the same year as UGC, 1973. This attempt at
bringing modern positions and data to the old catalogue failed
largely because of the intense time pressure to finish the
project in just three summers. As a consequence, RNGC not only
did not correct most of the problems not already fixed in
earlier correction lists, but it actually introduced many
problems by ignoring the historical record, and by relying
almost exclusively on the NGC positions for inspection of the
PSS. NGC numbers were often arbitrarily assigned to nearby
objects without regard to the descriptions or to the known
problems in the original observations. Many errors corrected
by Dreyer himself were perpetuated in the RNGC, and several
plate defects on the PSS now have RNGC numbers!
- The ESO (European Southern Observatory) survey (1982) of the "quick" blue plates taken with
the 1-m Schmidt telescope at ESO in La Silla, had as its goals
the inclusion all of the NGC and IC objects south of
declination -17.5 degrees, as well as all galaxies larger than
1 arcmin, all peculiar galaxies regardless of size, and all
other known or newly discovered non-stellar objects in the
south. So, as with CGCG, MCG, and UGC, the compilers had other
objectives in mind. Thus, they rarely took the time to dig
into the historical record to sort out the NGC/IC
identification problems they found. They did, however, make an
attempt to find all these objects. Those not found by them are
listed for reference at their NGC/IC positions.
- NGC 2000.0 was also produced under time pressure -- it
appeared in 1988, just 100 years after the original NGC. It,
too, ignored the original observations in favor of the modern
corrections, so it too has retained many of the problems of
the original. It was also assembled by the popular print media
rather than the scientific astronomical community, and hence
is missing the normal scientific rigor associated with such
efforts. However, at least some note was taken of the work
done by Archinal, Corwin, Gottlieb, Thomson, Erdmann, and
others, so NGC 2000.0 has been somewhat more successful in
mapping the historical sky than RNGC. It also includes entries
for all the IC objects, though many of these had yet to be
confirmed, especially in the north.
- PGC (Principle Galaxy Catalogue) and NED (NASA's Extragalactic Database) are primarily compilations of modern data. With
few exceptions, the astronomers involved have not concerned
themselves with identification errors in the NGC. Corwin has
contributed some of the historically based corrections to both
projects, but -- in the case of PGC -- Paturel has not adopted
most of them, preferring his own interpretations. Corwin has
also made it a policy to divorce his hobby (for that is what
this work is for him) from his NED work, again because of time
- In 1997, Wolfgang Steinicke in Freiburg, Germany, published a
complete list of NGC and IC objects in the northern sky with
positions based on the objects as they appear in RealSky, a
compressed version of DSS. After RealSky South appeared in
1998, Steinicke presented a revised version of his complete
catalogue containing all the NGC and IC objects. Though not
completely "historically aware", this is still probably the
most complete revision of the three catalogues to appear to
date. Especially valuable are the accurate positions in
Steinicke's list -- they point exactly to the objects to which
he assigned the NGC/IC numbers (it's worth noting, however,
that the positions Steinicke measured with the software
included with RealSky are, on average, 2-3 arcseconds too far
south. Positions for about a thousand galaxies in the north
polar cap measured by Corwin using the same software confirmed
the systematic offset).
- Aside from the present project, perhaps the best NGC debugging
work going on at present is appearing in the Webb Society's
Deep Sky Observer (previously called their Quarterly Journal),
and to a lesser extent, in other publications aimed at amateurs
devoted to exploring the deep sky. There are also several
Internet forums devoted to deep sky astronomy, and
identification questions regularly show up there. Particularly
active in this regard are Steve Gottlieb of Albany, California,
and Malcolm Thomson of Oceanside, California. Gottlieb's
approach is especially valuable, combining as it does historical
rigor with contemporary visual observations made with an 18-inch
reflector. Gottlieb's telescope is large enough that he can see
all but the very faintest of the NGC objects, yet is small
enough that his views of the nebulae are very similar to those
that most of the 19th century discoverers must have had. Other
modern amateurs have also contributed NGC corrections from
time to time. The longest single such list of corrections,
devoted to the "nonexistent" clusters in the RNGC, has been
recently assembled by Brent Archinal of USNO (U.S. Naval Observatory) in Washington,
and has been published as the Webb Society's first monograph.
Thomson has been working for many years on the NGC and IC, and
has recently posted identifications and discussions of all the
IC objects on the Web site. These are based on his examinations
of POSS1 and DSS, as well as on the original observations as
published in the historical literature. Additionally, Bob
Erdmann, of Chandler, Arizona, has been building a database of
non-stellar objects (The Arizona Database) since 1987 from the
existing professional catalogs. Bob has corrected a number of
the NGC/IC objects based upon inconsistent data in the
professional catalogues, and then digging into the historical
record as well as confirming his work visually with his 16-inch
- "Unpublished" work. When this project began, Corwin had a
thirty-year backlog of unpublished NGC and IC corrections in
his files. He collected all this through his work with RC1 soon
after its publication in 1964, his co-authorship of RC2, RC3,
SGC, ESGC, and his continuing work on the forthcoming OGC. In
doing this work, he has spent countless hours wandering around
in all the major catalogues of nebulae published to date. In
addition, he has started a project to clean out the IC, and has
finished the first 580 or so entries, as well as a couple of
thousand others through the rest of the catalogue. He paid
special attention to the NGC in the southern sky, since one of
the goals of SGC and ESGC was to include all NGC (but not all
IC) objects. Corwin also has cleaned up the NGC and IC
identifications in the Hodge/Wright Atlas of the Large
Magellanic Cloud. Thus, the southern NGC has been almost
completely "debugged." Only a few objects in the Milky Way and
the Small Cloud remain to be checked. Corwin's work in the
north has been systematic only to the extent of his filling in
blanks in the position lists. At the present time, he has
finished that particular aspect of the work through NGC 6560.
Many inconsistencies and questions earlier in the lists remain
to be looked at.
It should be clear from the discussion above that preparing a
modern, fully-corrected edition of the NGC is not a trivial task.
But much of the work has already been done -- it simply needs to be
pulled together into a relatively compact form. What follows is an
outline of the contents of such a catalogue and the steps needed to
- NGC numbers, and a number in the discovery list, or a
reference to the discovery list. The idea behind the whole
task is to associate the NGC numbers as closely as possible
with the objects in the sky to which they originally referred.
To do this, it is obvious that the original sources detailing
the discoveries of the NGC and IC objects must be consulted in
all cases. This has been discussed at length above, but we
cannot emphasize the point enough: the NGC itself does not
usually contain enough information to adequately resolve the
questionable cases. The original sources often have far more
detailed notes on the appearance of the object and the
surrounding field. Furthermore, the original sources usually
provide considerable insight into the observing techniques
employed in determining the positions, descriptions, and
indeed, about the very visibility of the objects.
Other sources which provide valuable information include those
which followed the publication of the NGC by a few years. Many
of these have still useful notes on NGC identifications. Some
are mentioned above. Particularly useful examples include
Dreyer's notes on the NGC printed at the end of each of the
IC's, Bigourdan's observations, Hagen's notes on dubious
objects, and so on.
Modern catalogues provide a much clearer view of what is
actually on the sky, so they can also help in sorting out the
identifications in some cases. But reliance on actual
observations, whether at the eyepiece, or on the Sky Surveys,
is going to be the major method of relating the old
observations to the sky, rather than relying on the secondary
view of the sky provided by the catalogues.
- Number in a modern catalogue. These will be necessary as only
basic data (numbers 3 to 5, following) will be included in our
list. Many observers will want to know more about the object
than we will be able to tell them (e.g. line strengths for
planetaries, proper motions of stars in open clusters,
temperatures and masses of HII regions, line widths for
galaxies, etc etc etc). So, references to the modern
literature -- the modern catalogues at the very least -- will
- Positions must be determined as accurately as possible for the
objects actually on the sky. Ideally, this means performing
astrometric observations in the fields of all the NGC objects.
In practice, perhaps 80 to 90% of the NGC objects already have
accurate positions known. In this context, "accurate" means
those positions that fall within the image of the object as it
appears on the sky. For the majority of the NGC/IC objects,
this translates to an accuracy of about 5-10 arcseconds. This
implies listing the positions to a precision of a tenth of an
arcminute (as in the ESO catalogue), though in practice, we've
strived for one arcsecond or better to avoid roundoff errors
in calculations for precession, coordinate transformation, etc.
For the unmeasured objects, the Digitized Sky Survey and the
Guide Star Catalogue are proving to be valuable aids. The DSS
and GSC astrometric solutions are based on the FK5 system which
systematically agrees with the ICRS (used in the Hipparcos
and Tycho catalogues) to within fractions of an arcsecond.
For any given object, a position can be pulled out of DSS or
GSC that is accurate to about an arcsecond, though plate-to-
plate variations of up to around 2-3 arcseconds exist. These
kinds of accuracies at last make it possible for anyone with
access to a computer to determine positions for celestial
objects that will allow their unambiguous identifications
simply by specifying their celestial coordinates.
- Object types will be expanded from the usual non-stellar
classifications (i.e. galaxy, globular cluster, open cluster,
planetary nebula, reflection nebula, HII region, supernovae
remnant) to cover all the sorts of objects in the NGC. Many
are clearly single or multiple stars, or parts of other
objects. The convention adopted by RNGC in calling these
"nonexistent" is just not correct, and we intend to include
these stars and objects with the proper type as they appear
on the sky.
- Modern data on the objects will primarily be limited to those
useful for identification and classification of the object.
These include diameters (for both major and minor axes),
photographic (blue) and visual magnitudes (or visual
magnitudes and colors), and surface brightnesses when
appropriate. Some additional data, such as morphological types
and distances or radial velocities for galaxies, magnitudes of
brightest stars for clusters, magnitudes for central stars of
planetaries, and so forth could also be provided. But the
cross-references to the other more specialized catalogues will
fulfill most of the demand for these less used data elements.
- As a secondary goal, tied to the cross-referencing, checks
will also be done against as many modern catalogs as possible,
taking into account new data and improved catalogs since 1973.
At least an automatic position and magnitude check should be
done against CGCG, UGC, ESO, SGC, etc for galaxies, Lynga's
cluster catalogue, Djorgovski's recent compilation of
fundamental data for globulars, and Hynes's and/or the
Strasbourg planetary catalogs. Other checks could be done with
The Arizona Database, Lynds's bright and dark nebula, IRAS
point sources, and so on. The main work here will be focused
on resolving identification problems in the modern catalogues.
- A separate textual notes section could be included on object
identifications where there have been problems in the past. As
mentioned above, many such notes have been published recently
by some of the principals in this outline, and Corwin and
Thomson have both independently built up collections of
hundreds of such notes, all now available in other pages on
this Web site.
- Images of each object will provide positive identification for
each object. We are currently taking most of the images from
the DSS. Though this is available on CD-ROM, it's availability
on the World Wide Web means that we can make cutouts of an
appropriate size for each object. Erdmann has so far clipped
DSS images for the first 1200 NGC objects. Other sources may
provide some images of objects of special interest (some of
the spectacular diffuse nebulae and star clusters, M31, M42,
peculiar galaxies, and so on), but we won't depend on these.
In summary, we intend to provide, so far as possible, a)
historically correct identifications for the objects in NGC and
the two IC's; b) accurate positions for the included objects; c)
cross-references not only to the historical sources, but also to
modern catalogues where further data may be found; d) enough data
so that the objects may be unequivocally identified on the sky by
modern observers using a variety of telescopes and instruments; e)
images of all objects to insure positive identifications; and f)
notes to fully describe the reasons for the errors in each case
where the NGC is incorrect in some respect affecting the
identification of the object.
We do not expect to finish this work overnight. It may be that it
will take many years to complete. So be it. Previous efforts have
foundered on the shoals of impatience. We do not intend to repeat
the mistakes of our predecessors, nor can we be pressured by the
thought of commercial gain -- the "market" for our work is small in
comparison to the effort we shall expend to successfully complete
it. In some sense, we are adopting as models, we hope with
enlightened hindsight, the 19th century amateur astronomers perhaps
best exemplified by the third and fourth Earls of Rosse. These men
expected no reward for their efforts beyond the satisfaction of a
job well-done. Our primary goals are those senses of accomplishment
and joy that the Earls must have felt as they aimed the largest
telescope in the world toward the vaguely charted depths of the
clear night sky.
Examples of NGC/IC corrections found by reference to the original publications.
NGC 2529, 30, and 31. Herschel did indeed discover N2530, and this
is the name that, as Steve Gottlieb suggests, should be used for
the galaxy. The other two objects were found by Bigourdan very
close to N2530. Though he examined the field four times, he saw his
two new objects only once. On that one night, he estimated
positions with respect to N2530: N2529 is 1 arcmin distant at
position angle 220 degrees, and N2531 is 1 arcmin distant at PA 150
degrees. There is nothing in either position on the PSS. Bigourdan
also measured a thirteenth magnitude star the same distance away
from N2530 on two nights; it is just where he saw it in PA = 15
degrees. On the second night, Bigourdan claimed to see stellar
objects at the very limit of visibility where he placed N2529 and
N2531 earlier, but he did not attempt to measure them. It's certain
that the two do not exist, so probably are those faint illusions
that we all see now and then when we get tired or try too hard to
push the limits of our optics.
IC 336, IC 341, IC 353, IC 354, and IC 360. These eF diffuse
nebulae were identified by reference to Barnard's drawing in A.N.
3253, and to his drawing and photograph in M.N. 57, 12, 1897.
Barnard's sketch is more or less confirmed by PSS plates O/E441 and
O/E31. Curiously, Dreyer did not include all of the patches of
nebulosity shown by Barnard, and those that he did list in the IC
are not necessarily the brightest. With the possible exceptions of
I353 and I354, these nebulae are probably not associated with the
Pleiades, but appear more likely to be Galactic reflecting nebulae
(see e.g. Sandage, A.J. 81, 954, 1976). Note also that the IC
position of I360 is 5m in error.
[Obvious references here. I'll get them in eventually. hgcjr 19 jun 96]
Alpher, R. A. and Herman, R. 1990, in "Modern Cosmology in Retrospect", ed. B.
Bertotti, R. Balbinot, S. Bergia, and A. Messina, Cambridge Univ. Press,
Cambridge, pp. 129 -- 157.