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magnitude.  When Off, none will be shown.  When set to Auto or Fixed,
all objects brighter than the magnitude limit for that class will be
shown.

   The difference between 'fixed' and 'auto' is a simple one.  In
'auto' mode,  Guide will automatically adjust the limiting magnitude
as one zooms in or out,  in an attempt to maintain a constant density
of objects. (This is always done for stars,  for example.)  But in
'fixed' mode,  you are essentially telling Guide that you want a class
of object to be shown to a particular limiting magnitude,  and that
the zoom level should be ignored.

   If an object class is turned On or Off,  the magnitude limit is
irrelevant,  and is therefore grayed out.

   Next to these four radio buttons,  there is (usually) a labelling
button and a button to select the color for that class of object.
                                                 
   For "Gal Clusters", the panel shows an extra three-way switch between
Zwicky On,  Abell On,  and Abell + Zwicky On.  The Abell catalog of
clusters of galaxies covers the entire sky, and contains over 5,000
objects. The Zwicky catalog only covers the northern half of the sky,
but it has over 9,000 objects. Some objects are found in both catalogs.
Depending on where you are in the sky,  you may wish to see the Abell
clusters,  the Zwicky clusters,  or both. All three are possible, and a
check mark is placed in the menu beside the current selection.

   The Asteroids submenu also shows some extra options controlling
the labelling of asteroids:  asteroids labelled by number;
asteroids labelled by number if available, provisional designation
otherwise;  asteroids labelled by name, provisional designation
otherwise. By default,  asteroids are labelled with their number.
(Unnumbered asteroids are therefore also unlabelled asteroids,  and are
simply shown as cross-marks.)
                                           
   For the Planets there is a panel with two switches: "Full precision"  
and "Label by Name".  The first toggles between normal precision (the  
default) and full precision for positions.  Planet positions are
calculated using either the VSOP ("Variations Seculaires des Orbites
Planetaires") theory or the PS1996 theory,  both compiled at the Bureau
des Longitudes in Paris.  In each theory planetary positions are
calculated as the sum of a long series of trigonometric terms.  When
full precision is used,  all terms in the optimal theory are used, and
planetary positions are precise to roughly .01 arcsecond. Computing
them can be slow on older computers.  (If you have a more modern
computer,  you will probably not even notice the slowdown.)

   "Normal" precision  provides a precision of about an arcsecond over
the period 0 AD to 4000 AD. It is based on VSOP, but omits many of the
smaller terms.  It is the truncated version used in Jean Meeus'
_Astronomical Algorithms_ (Willmann-Bell, 1991).  For normal use

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