Why bother?

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.

1. 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.

2. 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 volumes.

   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 the Observatory,
   • 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.

3. 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.

4. 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.

5. 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.

6. 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).

7. 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.

8. 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!

9. 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.

10. 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.

11. 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 pressure.

12. 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).

13. 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 reflector.

14. "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.

Desiderata

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 realize it.

1. 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.

2. 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 be needed.

3. 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.

4. 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.

5. 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.

6. 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.

7. 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.

8. 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.

Conclusion

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.

Appendix:

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.

References