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that are being shown on the screen.  But if galaxies (for example) are
currently turned off,  you're obviously unable to click on one in
order to turn galaxies on.  In such cases,  you need to use the Data
Shown dialog.  It's also convenient if you want to change the display
of many types of object at once.
                                                     
   In the Data Shown dialog,  you are given a list of fifteen classes of
objects controlled by the Data Shown menu/dialog:

Messier
NGC + IC
Galaxies
Nebulae
Planets
Variables
NSV
Open clusters
Asteroids      
Gal Clusters
Comets
Globulars
Planetaries
Dark Nebulae
Satellites

   Each object class is followed by buttons to turn that class "on",
"off",  "auto",  or "fixed".  When a particular class of objects is
turned On,  all objects of that class will be shown, regardless of
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

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