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Tom Mazanec
2015-Sep-25, 02:57 PM
The PGC was first published as an extragalactic database in 1989 by Paturel et al. It contained coordinates and cross-identifications for 73,197 galaxies, with data taken from various sources for between twenty and sixty-seven thousand entries (more for data easily obtained, fewer for data requiring more effort). Due to errors in the references used to compile the catalog there are numerous duplications and non-galaxian entries, but overall it is a reliable reference, covering far more objects than earlier catalogs. Over the years additional data were added to the catalog, and in 2003 a completely new version of the catalog was published (essentially as an online database at HYPERLEDA), which was stated as "restricted to confirmed galaxies, i.e. about one million galaxies, brighter than ~18 B-mag", but the online database actually contained more than a million and a half entries, and as in the case of the original many are duplicate entries or nonexistent or misidentified non-galaxian objects. Over 50 catalogs were used as references for the 2003 version of the PGC, and between the numerous differing ways of listing objects and the inevitable errors in those references, PGC entry numbers actually run into the 4-millions plus (though searches of the database for high numeric entries generally fail, save as the result of a search for a non-PGC designation).
Despite the aforementioned problems, the PGC is the most reliable and consistent database of extragalactic objects, and where a PGC number (or numbers) can be unambiguously assigned to a NGC or IC object, I prefer using the PGC designation to any other method of identifying the object.

http://cseligman.com/text/atlas/ngc00.htm#ngcic

Say what? The best catalog of galaxies has numerous duplications and mistaken "galaxies"?
I just have an old B.S. in Astronomy (Case Western Reserve 1980) but I feel fairly sure that, if I did an assignment with such characteristics, I would:
Definitely get an F for the assignment
Probably flunk the course and
Quite possibly wash out of the program altogether.

How can professionals screw up so royally?

Shaula
2015-Sep-25, 03:51 PM
How many assignments did you do that involved merging a million and a half records with multiple schema? And how long did it take your tutor to mark?!

Tom Mazanec
2015-Sep-26, 10:38 AM
I am looking at his pages of the New General Catalog, and there seems to be a goof or two on each page. The pages are only fifty entries long each. So about 3% of the entries are in error, at a conservative estimate. Surely it would not take long for my tutor to pick this up.
Or were standards looser in 1888?

Shaula
2015-Sep-26, 11:28 AM
My point was more that you are not comparing like with like. An error rate that is acceptable for a catalogue is not acceptable for a piece of coursework.

Essentially it becomes an optimisation problem - it is not the time taken to find a mistake but the time taken to resolve one (and check the resolution is correct - quite possibly involving making an observation) that is important. Below a certain error rate it is simply more efficient to spend your limited astronomer-man-hours doing astronomy than correcting catalogues.

Tom Mazanec
2015-Sep-26, 12:28 PM
Thank you.
But if cataloging is something that has to be balanced with other demands of time, why are there so many catalogs? Why not just stop with one?
https://en.wikipedia.org/wiki/Category:Astronomical_catalogues_of_galaxies

StupendousMan
2015-Sep-26, 01:09 PM
Thank you.
But if cataloging is something that has to be balanced with other demands of time, why are there so many catalogs? Why not just stop with one?
https://en.wikipedia.org/wiki/Category:Astronomical_catalogues_of_galaxies

Serious answer: there are many astronomers. Each one has slightly different interests, which lead each individual to pay special attention to slightly different aspects of the celestial realm. Three astronomers all study galaxies -- but one uses an optical telescope, one a radio telescope, and one an X-ray telescope. The optical astronomers notices 500 galaxies, and measures their properties. The radio astronomer notices 500 galaxies, and measures their properties -- but only 100 of them also appear in the optical. The X-ray astronomer notices 500 galaxies, and measures their properties --- but only 50 of them also appear in the optical, and only 200 in the radio.

So, the optical astronomer makes a list of the objects he can see and measure. The radio astronomer makes another list. The X-ray astronomer makes another list. Three lists. Fifty years later, a scientist interested in the formation of galaxies realizes that she can use properties measured at different wavelengths to explain aspects of her theories ... so she creates a single list of galaxies with any measurements -- that's a fourth list, combining some members of the three original lists. And so on.

Not-so-serious answer, but also not completely flippant:


But if posting is something that has to be balanced with other demands of time, why are there so many posts? Why not just stop with one?

Because some people are curious, and enjoy posting. Or making lists.