The NGC and ICs "Corrected" These files -- ngcnotes.[0-7] and icnotes.[0-5] -- contain the explanations behind my decisions for assigning the NGC and IC numbers to particular objects. These are usually cases in which there is some problem with the object or its position. These cases fall into several pretty well-defined categories: 1) The original position alone is not sufficient for an unambiguous identification of an entry in the catalogues, i.e. "missing" or "nonexistent" objects, objects in a crowded field, and so forth. A common subset of this is the digit or sign error -- a position is off by a round number, e.g. 10, 20, or 30 arcminutes, 1, 5, or 10 minutes of time, one degree, one hour, and so forth. Sign errors are often in the offset from a comparison star: the object is claimed to be east of the star rather than the actual west, north rather than south, and so on. 2) The description does not match the object at the given position, is too brief to be helpful, is ambiguous, or is at odds with surrounding stars or nearby nebulae. 3) The object was assigned more than one NGC/IC number. These are the all- to-common result of poor positions. More than one observer finds the same object and each gives it a position just different enough from the other that Dreyer has to include both (or several!) "different" nebulae. Or the same observer discovers the same object more than once, etc. 4) The object is actually a star or a multiple star -- "an asterism". This, too, is common and I suspect arises primarily from poor seeing or insufficient resolution. 5) There is just not enough information to securely assign the NGC number to an existing object. In these cases, I'm pretty well reduced to simply guessing where the number goes. The percentage of these is small, but it is still distressingly large (just over 1%). I am using the historical literature that went into the construction of the NGC and ICs to help solve the problems. Dreyer often omitted information from the NGC/IC "Summary Description" vital to the identification of the nebula or cluster in question. Also, many stars used as position references for micrometric or transit observations had their own positions only poorly known in the 19th century. Thus, while an offset from the star may well have been measured with sub-arcsecond accuracy, the resulting absolute position for the nebula will be only as well known as that for the star. Errors of several arcminutes are the not-uncommon result, particularly for objects compared to stars with positions known only from the BD. The usual layout of an explanation is this: The NGC/IC number is followed by a short statement of the resolution of the problem. Then, I normally present the original observations, stressing those that lead to the solution. For those cases which are easily solved, this is generally enough, so I try to not carry on with irrelevant details. Sometimes, however, the evidence is conflicting -- or simply confusing -- so that all I can do is to give opinions, or more or less educated guesses as to what the original observer really saw. Once in a while, the evidence just is not sufficient to solve the problem. Hopefully, others with access to more data (e.g. unpublished observatory records) or more insight will be able to clear up some of the remaining mysteries. The discussion for each object is followed by a row of five equal signs ("====="). These serve to separate each discussion from the others, and also to simplify a computerized count of the total number of objects that I've examined (4,140 as of mid-January 2007). Some of these "stories" are clearly more complete than others, and all are part of a work in progress: I update them as needed. In almost all of these cases, I've not only gone back to the original observations presented in the late 18th, 19th, or early 20th century literature, but have also looked at the field around the object on the Palomar or Southern Sky Surveys. For the past 6-8 years, I've also used the Digitized Sky Survey (DSS) as presented by SkyView, a service of the HEASARC group at the NASA Goddard Space Flight Center. This is a digitization, by NASA's Space Telescope Science Institute, of the red POSS1 plates and the blue-green Southern Sky Survey plates. The DSS offers convenient views for small fields centered on a given position, while the published POSS1 prints and southern survey films give a wide-field view not easily available on a computer monitor. The two views of the sky are complementary, and each is useful in ways that the other cannot be. (I note, with much astonishment and some sadness, a decision by IPAC's non-scientific management to move the Sky Survey prints and films into "permanent storage" in the summer of 2004. This was done without consulting those of us who used them, and remains one of the sillier things that I've seen NASA managers do over the years. Fortunately, SkyView remains accessible as does STScI's own image server, and wide-field views are fairly easy to fashion onto a monitor in a reasonable amount of time.) As I noted above, Dreyer included in NGC/IC only a "summary description" of the object, and occasionally had to be quite creative in boiling down a long observation into the few characters that would fit in the description column. A lot of useful and interesting information about many objects never made it into the NGC/IC. In general, however, the data that are there are correctly copied or interpreted from the original. Dreyer was a good and conscientious cataloguer, so the problems in the NGC almost always reflect problems in the original observations, not in his compilation efforts (there are a few exceptions, of course; see e.g. NGC 5344 and IC 48). The one aspect of the work that I've generally not been able to do is to look at the objects through an eyepiece on a telescope similar in size to that used by the discoverer. Most of the time, this isn't necessary -- but once in a while, visual confirmation would be good to have. An example is NGC 2491 -- could Swift have really seen the galaxy nearest his position with his 16-inch refractor? Steve Gottlieb has been working hard at this aspect of the NGC/IC cleanup, and Malcolm Thomson (before his retirement about a decade ago) made many similar observations, so you should certainly check their lists of observations should you find a particularly puzzling case. It's worth keeping in mind that almost all of the NGC objects, and over 40% of the IC objects, were found visually. Every observer recording a "nova" clearly thought that he or she had found a previously unknown nebula or cluster -- there is almost always something on the sky that led them to that conclusion. Administrivia First, an aside: I generally use "English" units (feet, inches, etc) rather than metric. This, appearances to the contrary, is not a parochial stance. I am simply copying what Dreyer did more than a hundred years ago before there was any international consensus on standardized units. [When I refer to modern telescopes currently known by their metric sizes (e.g. the twin 10-meter Keck reflectors), I'll comfortably use those metric sizes.] I also stick with the ancient conventions of degrees, arcminutes, and arcseconds for angular measure, and -- for right ascension -- hours, minutes, and seconds of time. I'm not about to try to overthrow several thousand years of historical inertia for the simple convenience of presenting positions in the same units that computers find amenable. Similarly, I use magnitudes rather than flux or flux density for measures of brightness. This is obviously not modern astrophysics that we're involved with here. I've usually adopted the abbreviations used in the original works or in the NGC/IC, but have sometimes expanded them for clarity. In particular, I've used "sts" rather than "st" to mean "stars", and I also use "deg" for "degrees" or the degree symbol, "arcmin" for "arcminutes" and so forth. I use "seconds" to mean "seconds of time" and "minutes" for "minutes of time" when there is little chance of confusion with arcseconds or arcminutes. Otherwise, I spell it out: "seconds of time" or "minutes of time." As time goes on, I've found that I more often than not simply expand the abbreviations into the full English words. This makes the notes clearer, and certainly reduces the possibility of confusion. I've also usually shortened the names of Sir William Herschel and Sir John Hershel to "WH" and "JH" respectively. You'll also often see "d'A" for Heinrich d'Arrest and "CH" for Caroline Herschel. "LdR" is a collective abbreviation for the third and fourth earls of Rosse and their observers (including Dreyer himself). Where Dreyer has given the name of the original observer at Birr Castle, I try to use that. His list of observers and the dates they worked at Birr (in the introduction to his 1880 collection of the observations) is useful, but many of the observations were of course made by the lords Rosse themselves. So, I'm reluctant to say that Dreyer or Ball or Copeland or ... made any given observation based simply on the date. Later cataloguers suggesting corrections to the NGC are also mentioned frequently, usually without abbreviation: Reinmuth (1926), Carlson (1940), RNGC (1972), NGC 2000 (1988), and Archinal (1993) among them, though I do use "AH" for Archinal and Hynes (2003). I almost always refer to star catalogues by their common abbreviations: BD, GSC, SAO, FK4, AGK3, PPM, Tycho-2, AC 2000.2, and so on. Similarly, the classic galaxy catalogues (RC1, RC2, RC3, CGCG, MCG, UGC, ESO, SGC, and so on) are usually abbreviated. I've done the same with the names of astronomical journals: "AJ" for the Astronomical Journal itself, "AN" for Astronomische Nachrichten, "ApJ" for Astrophysical Journal, "PA" for Popular Astronomy, "PASP" for Publications of the Astronomical Society of the Pacific, "MNRAS" -- or just "MN" -- for Monthly Notices of the Royal Astronomical Society, "MemRAS" for their Memoirs, "CR" for Comptes rendus (the second word is not capitalized in French), and so forth. I'll add others to this paragraph as I need and remember them. A miscellaneous note: A few IC objects with corrected positions or names entered in my files of accurate positions do not yet have the explanations for the corrections spelled out. In general, these are discrepancies discovered by other observers over the years and not yet checked by me. These cases have been about evenly split between being correct as shown, or not correct for various reasons. I'm working these off, and have made the end of 2007 a goal for finishing them. Patience is a virtue -- though if your curiosity gets the better of you, have a go at it yourself. Much of the historical literature is now available online at the NGC/IC Project Web site or at ADS. Or you can write to me about the problem. I'll tackle it as soon as I can. (I finished the list of missing NGC explanations in December 2006. Still to be done is a final sweep through the NGC checking those objects that I have not yet looked at personally. There may be a few mistaken identifications remaining. NGC 5441 is an example of one that I found recently, by accident, while checking data for NGC 5440 for NED.) I have, in fact, done much of this work in response to questions and similar work by other team members, or by other correspondents. In particular, Brian Skiff, Steve Gottlieb, Malcolm Thomson, Wolfgang Steinicke, Bob Erdmann, Chris Watson, and Francois Ochsenbein continue to have a strong influence on which objects I've looked at. All the team members have also been valuable sources of ideas that I've often chided myself for not having had on my own. Our conclusions as to which NGC or IC number belongs to which object sometimes differ, too, so I urge you to look at their own discussions before accepting any particularly puzzling case on just my say-so. (A more complete list of acknowledgements is given in the "nipos.doc" file.) I'd also urge you in a much more general way to look at the puzzle solutions of the other team members, and indeed of other observers and literature sleuths -- there have been many. Since I have not yet had time to compare my conclusions in any systematic way with those by other people, we are still reaching these conclusions more or less independently. So, if we agree on a particular case, that is the best possible outcome -- it means that the historical record is clear enough to yeild an answer that is virtually unambiguous. Once I start comparing my work to that of others, however, this particular strength of the group's work will disappear. It will, however, be replaced by other advantages -- particularly that of goading me on to collect more data, to consider other hypotheses, or to simply take more time on an unusually puzzling case; when there may not have been enough of any of these available to me earlier. If you still aren't happy, drop me a note (hgcjr@ipac.caltech.edu); I'll have another look at the puzzle to see if there is anything I've missed previously. There often is -- I'm learning new puzzle-solving tricks all the time. Harold Corwin January 2007