Previous page  Page 59   Next page    Table of Contents 

used,  and the limiting magnitude used.

   Downloads of each of these datasets accumulates.  If you want
to clear the data,  right-click on an object,  then on "More Info".
You'll see an option there to clear all the stars from that catalog.

   All of the Internet-downloaded catalogs are implemented as
user-added datasets.  You can go into Extras... User-Added Datasets,
click on (for example) "2MASS data (downloaded from VizieR)",  and
see the controls over that dataset.

   Of course,  you may well wonder what each of these catalogs are.
Each has strengths and weaknesses in terms of accuracy,  number of
stars contained,  and the sort of data given for each star.

   The USNO A1.0 and A2.0 catalogs are the eldest of this group,
dating back to 1997.  A2.0 is an update to A1.0,  with recalibrated
(improved) magnitudes and positions,  but they are frequently lumped
together as "Ax.0".  As was mentioned earlier,  this dataset was
distributed on CD-ROMs;  how to use those CDs with Guide is the subject
of the next subchapter.

   The Ax.0 catalogs list about a half billion objects.  Objects   
were found by scanning in wide-field photographic plates,  some of
them dating back to 1950.  Usually,  two plates,  one red-sensitive
and one blue-sensitive,  were used.  This means the catalog can give
an indication of the object's color (not always a very accurate one,
though) and that many spurious objects could be safely ignored:  if
an object was found on only one plate,  it was assumed to be an
error,  or a flaw in the plate,  or a passing asteroid or satellite.

   In this catalog,  and in B1.0 and GSC-2.3,  the magnitude data is
of mediocre accuracy.  USNO was very interested in finding positions
to within a fraction of an arcsecond,  and succeeded in this goal;
but good magnitude measurement wasn't a priority.  The documentation
for A1.0 states bluntly that "the photometry is as bad as it can be
while still being able to claim that the numbers mean something."

   The USNO B1.0 catalog was the successor to A2.0.  It contains     
about a billion objects,  with slightly better accuracy in both
positions and magnitudes.  In this case,  up to five plates were
scanned for each part of the sky.

   GSC-2.3 is the successor to the GSC-1.1 and GSC-ACT catalogs.
It resembles B1.0,  but was assembled by the Space Telescope
Science Institute (STScI),  in some cases from different sources,
and using different methods.  So it may catch some objects missed
by B1.0,  while missing a few that B1.0 includes.

   The UCAC-2 catalog is the second version of what will probably
become the standard catalog of choice for stars down to about mag

 Previous page  Page 59   Next page    Table of Contents