Notes and Explanations: The NGC and ICs "Corrected"


Introduction

The files ngcnotes.all and icnotes.all explain my decisions for assigning the NGC and IC numbers to particular objects, as well as more general comments, sometimes even touching on modern astrophysics (though that, of course, is incidental to my main goal of identifying the NGC/IC objects). An additional file notngc.notes has explanations for non-stellar objects known or suspected before the NGC was published but not included in it. All of these Notes generally address cases in which there is some problem with the object's position or with its original description. These 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 relatively common in the offsets 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 mentioned in the description.
  3. The object was assigned more than one NGC/IC number. These are the all-to-common results of poor positions. More than one observer finds the same object and each gives it a position just different enough from the others 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 somewhere in the area of the NGC position. If the object does not fall into one of the categories above, I'm pretty well reduced to simply guessing where the number goes. I try to make "educated" guesses based on what I know about the observer's telescope, observing conditions and constraints, and the other objects found by him or her.
  6. Finally, there is simply no trace of the object on the sky anywhere near its catalogued position and matching its description. Perhaps examination of the original observing logs may be helpful -- assuming, of course, that the logs still exist. Roughly 100 NGC objects fall into this category and the previous one, and approximately 200 IC objects are similarly lost.

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 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 accurate as that for the star. Errors of several arcminutes are the common result, particularly for objects compared to stars with positions known only from the BD. Reference stars with high proper motions are relatively common (the observers chose bright stars as reference stars when they could; brighter stars tend to have larger proper motions simply because they are -- statistically, at least -- nearer). These, too, will affect the calculated position for the object(s).

A common 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, as I noted above, 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 note from the others, and also to simplify a computerized count of the total number of objects that I've examined (5,159 as of 21 Jun 2017). I have begun using a row of five dashes ("-----") within a single note to separate my original note from material added more recently. Some of these "stories" are clearly more complete than others, so regard all as part of a work in progress: I update them as needed.

In 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 modern sky surveys. For the past fifteen or so years, I've used the Digitized Sky Survey (DSS) as presented by SkyView, a service of the HEASARC group at the NASA Goddard Space Flight Center. This was originally a digitization, by NASA's Space Telescope Science Institute, of the red Palomar Observatory Sky Survey (POSS1) plates, the blue-green Southern Sky Survey plates, and special short-exposure plates of rich Milky Way and Magellanic Cloud fields. Later photographic sky surveys are now also available, as are several digital surveys, and many at non-optical wavelengths. 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. Unfortunately, I no longer have access to the prints and films, so am trying to make do with the on-line digital surveys. This is particularly difficult for very large objects, and for missing objects that I suspect are afflicted with "digit errors".

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 thus 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, NGC 7697, 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? Similarly, the 16th magnitude objects at Marth's position for NGC 7830 would seem to be even beyond the visual limit of the 48-inch reflector that he was using. Steve Gottlieb has been working hard on this aspect of the NGC/IC cleanup, and Malcolm Thomson made many similar observations when he was observing in Santa Barbara. Similarly, Courtney Seligman has some interesting stories to tell, so you should certainly check all of these lists of observations should you find a particularly puzzling case. Many other observers have looked at large numbers of NGC or IC objects, too (Jeffery Corder is one whom I know is collecting his observations for publication), so it will be worth checking for others than just those I've cited here.

Finally, it's worth keeping in mind that all but one 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 seen a previously unknown nebula or cluster at the eyepiece -- 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 much 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 frequently 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 -- but I always use "arcseconds" and "arcminutes" when those are the intended units. If there is any possibility of confusion, I spell out "seconds of time" or "minutes of time." As time goes on, I've found that more often than not I simply expand the abbreviations into the full English words. Sometimes, though, this is does not lead to an accurate reflection of the original observations, so abbreviations do still crop up.

On the other hand, I've also usually shortened the names of Sir William Herschel and Sir John Hershel to "WH" and "JH" respectively; their names pop up quite often. 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) at Birr Castle. Where Dreyer has given the name of the original observer at Birr, 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 some 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. I've noticed lately that Wolfgang has apparently sorted out who observed what at Birr Castle; see his files on his web site for more information.

Later cataloguers suggesting corrections to the NGC are also mentioned frequently, usually without abbreviation: Reinmuth (Veroff. Sternw. Heidelberg 9, 1, 1926), Carlson (ApJ 91, 350, 1940), NGC 2000 (ed. R. W. Sinnott, Sky Pub. Corp., 1988), and Archinal ("The 'Non-Existent' Star Clusters of the RNGC", Webb Soc., 1993) are among them. I also use "AH" for Archinal and Hynes ("Star Clusters", Willmann-Bell, Inc., 2003), and "RNGC" for Sulentic and Tifft's "Revised New General Catalogue" (Univ. AZ Press, 1973).

I almost always refer to star catalogues by their common abbreviations:
BD Bonner Durchmusterung
GSC Guide Star Catalog
SAO Smithsonian Astrophysical Observatory Star Catalog
FK4 Fourth Fundamental Catalogue
AGK3 Astronomische Gesellschaft Katalog
PPM Positions and Proper Motions Star Catalogue
Tycho-2 Tycho-2 Catalogue of the 2.5 Million Brightest Stars
AC 2000.2 AC 2000.2 Catalogue
UCAC Fourth USNO CCD Astrograph Catalog
CMC Carlsberg Meridian Catalog
2MASS Two-Micron All-Sky Survey
Gaia ESA's Gaia Astrometric Mission
and so on. Similarly, the classic galaxy catalogues of the mid- to late 20th century
SA Shapley-Ames "Survey of the External Galaxies ..."
RSA Revised Shapley-Ames Catalog
RC1 Reference Catalogue of Bright Galaxies
RC2 Second Reference Catalogue
RC3 Third Reference Catalogue
CGCG Catalogue of Galaxies and of Clusters of Galaxies
MCG Morphological Catalogue of Galaxies
UGC Uppsala Galaxy Catalogue
ESO ESO-Uppsala Survey
SGC Southern Galaxy Catalogue
SEGC South-Equatorial Galaxy Catalogue
are usually abbreviated. I've done the same with the names of astronomical journals and magazines: "AJ" for the Astronomical Journal itself, "AN" for Astronomische Nachrichten, "A&A" for Astronomy and Astrophysics, "ApJ" for Astrophysical Journal, "ApJS" for the Astrophysical Journal Supplement Series, "S&T" for Sky and Telescope, "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. References to position sources are in the sources file; these are nothing but abbreviations as they had to fit into four columns in the position files. So, you will find there explanations and references for alphabet soup like "SDSS", "2MSP", "DC", "SPC", "UCAC", and on and on.

At this point in earlier versions of this explanation file, I had "A miscellaneous note" pointing out that a few IC objects did not yet have explanations either for corrections to their positions or descriptions. (I should have added the NGC to that comment as well as the IC.) I believe that I have now provided notes on all the NGC and IC objects needing them. Again, I will be checking these to be sure that I have covered everything. If you find an object without a necessary note, please let me know about it. I'd also encourage you to 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.

I finished the list of NGC explanations for missing objects in December 2006, (or so I thought; I ran across NGC 7830 in May 2016) though am still adding to it as I note unusual circumstances that require some comment. I'm working on a final sweep through the NGC checking those objects that I have not yet looked at personally. There must certainly 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. Another recent "discovery" is that, just as Dreyer said in the notes to the second IC, IC 1281 is identical to IC 1279. And I've found that NGC 952 is a rediscovery of NGC 940 with an incorrectly-identified comparison star.

I have, in fact, done much of this work in response to questions and similar work by other NGC/IC Project team members, or by other correspondents. In particular, Brian Skiff, Steve Gottlieb, Malcolm Thomson, Wolfgang Steinicke, the late Bob Erdmann, Sue French, Chris Watson, Steve Waldee, Courtney Seligman, and Francois Ochsenbein have had a strong influence on which objects I've looked at. All the NGC/IC Project 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 NGC/IC Positions Introduction file.)

I'd also urge you in a much more general way to look at the puzzle solutions 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 unambiguous, or close to it. Once I start comparing my work to that of others, however, this particular strength of our 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 (hcorwin1153@sbcglobal.net); I'll have another look at the puzzle to see if there is anything I've missed previously -- there often is.


Latest update, this file: 21 June 2017