The NGC and ICs "Corrected"

     These files contain the explanations behind my decisions for assigning 
the NGC and IC numbers in cases where 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 usually 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 position is at odds with the description, the description is too 
     brief to be helpful, it 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 the several!) "different"
     nebulae.

  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.

  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.

     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 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 (3,665 as of mid-September 2004).  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 all these cases, I've not only gone back to the original observation 
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 4-5 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 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 NASA managers have done 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.

     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.  

     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.  I've also 
abbreviated 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:  Many 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 on 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 ADS.  Or you can write to me 
about the problem.  I'll tackle it as soon as I can.

     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 kicked myself for not having 
had on my own.  Our conclusions as to what object is really what 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 our conclusions 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 go away.  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
November 2004