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The Accurate Globular Cluster List is built initially from Brian Skiff's globular cluster list, first published in the Webb Society Quarterly Journal (WSQJ num. 99, Jan 1995) and and subsequently available on the Lowell Observatory FTP server and updated periodically. New positions were measured for many of the globulars in the list, and all globular positions have been hand-checked against the DSS, CCD images made for the purpose, or both.
The list is meant to be accurate in two respects: in identifications, and in positions.
It is worth noting that Skiff's list represents substantial work, probably entailing hundreds of hours. The original compilation involved a complete literature search of over 100 papers involving photometric studies of globulars, and selecting and formatting the data. This list has taken a good deal of this work "on faith," as it were, without pretending to improve or duplicate that work. The genesis of this project was twofold - the testing of observation scheduling software, and a desire to make the list more accessible by incorporating it into SkyMap Pro. The former allowed some value to be added to the latter. It was subsequently realized that this was an opportunity to develop a "reference standard" for accurate star charting software databases, so additional fields and conventions were developed before outputting the results.
Designations are taken wholesale from the source list and used as unique primary keys in this list, except in the case of Eridanus (rekeyed Eridanus 1) and Pyxis (rekeyed Pyxis 1), for database management reasons (these are also the names of constellations).
New astrometry was done for 65 globulars in the list. The remainder of the positions in the source list were retained.
In cases where the declination was accessible from Rockland Observatory (latitude +31.5), astrometry of the globular was performed using a 130cm refractor and CCD camera. Reductions were performed relative to Tycho-2. In the case of very faint or small globulars (e.g., Terzan 2), the astrometry was performed using a 30cm SCT and reductions were relative to USNO A2.0. The goal in all cases was to centroid on the globular itself, rather than to estimate the center of the globular by eyeball.
In cases where the declination is permanently below Rockland Observatory's southern horizon, astrometry was performed using DSS images treated with Gaussian blurring, relative to USNO A2.0.
The new positions given are the mean positions of several reductions, in an effort to reduce the effects of measurement error. Each new position was then hand-checked against the DSS for error.
The improvement or change in the positions is in most cases modest - a few seconds of arc. In a few cases, the corrections are a substantial fraction of the globular's inner core, as visible on the DSS.
Skiff's list also incorporates several other observables, including the integrated V magnitude of the cluster, the diameter at the 22nd and 25th magnitude isophotes, the V magnitude of the brightest stars in the cluster, the colors of those stars, and the V magnitude of the horizontal branch (if you can see stars of this magnitude, the cluster will be substantially resolved). These quantities are taken over into this list.
The confidence that the database maintainer has in the data is graded on a letter system similar to that used in schools in the United States. In short, A is the highest grade, while F is the worst (omitting E).
In general, we assign an "A" grade where we expect "three nines" accuracy - meaning the answer is correct to the adopted precision 99.9% of the time, or in 999 out of 1000 database entries. Typically this means the quantity comes from a source of known reliability. "A" grades are also assigned to any quantity that has been hand-checked for correctness.
"B" grades are not hand checked but come from sources thought to be reliable, such as NED, Simbad, well-regarded journal articles, etc.
"C" grades are assigned to data which is taken from a questionable source but which has been hand-checked and looks reasonable (but which can not be quantitatively confirmed), or which has been generated especially for the database and is not astrometric in nature (I know my limitations).
"D" grades are assigned to data which is taken from sources known to be lacking in significant ways and which is not checked. For example, large integrated observables databases or catalogs compiled from inhomogeneous data and lacking normalization are assigned "D" grades.
"F" grades are assigned to data which is missing, or which is known to be in error.
The format here is a fixed record length text file with PC (not Unix) line terminations, or a SkyMap Pro native database file. It is available by request as a table in an MDA relational database. The text record format is:
Designation 1 22 Designation Confidence 24 34 RA h 26 36 Dec d 39 49 Position Confidence 51 51 Diameter (22nd mag isophote) min 53 56 Diameter (22nd) Confidence 60 60 Diameter (25th mag isophote) min 62 64 Diameter (25th) Confidence 69 69 V magnitude 71 74 V Magnitude Confidence 78 78 V (tip) 80 83 V (tip) Confidence 87 87 B-V (tip) 89 91 B-V (tip) Confidence 96 96 V Horizontal Branch 98 101 V Horizontal Branch Confidence 105 105 Position Source 107 131 22nd mag dia source 132 156 25th mag dia source 157 181 V source 182 206 V (tip) source 207 231 B-V (tip) source 232 256 V Horizontal Branch source 257 281
1.0 (27 Mar 2002) - Initial publication.