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17d:  ASTEROID OPTIONS      

   The Asteroid Options dialog provides some extra controls over
how asteroids are shown.  You can access it through the Extras...
Asteroid Options menu item,  or by right-clicking on an asteroid,
then on Display,  then on Options.  The options listed are:

Asteroid Labels:
  ( ) Label by number 
  ( ) Label by number/prov
  ( ) Label by name/prov

[ ] Use MPCORB 
Add MPC Comets/Asteroids
Edit Comet Data
 Trail length (in days): 
 [ ] Line of variation (in days): 

   The labelling options allow you to have asteroids labelled only
by number;  by number or provisional designation (such as "1997 XF11",
the sort of designation used for asteroids that haven't been numbered
yet);  or by name or provisional designation.
                                             
   The "Use MPCORB" option is grayed out by default.  It allows you to
switch between Guide's built-in asteroid data,  or data from the MPCORB
dataset.  The MPCORB file is available at:

ftp://cfa-ftp.harvard.edu/pub/MPCORB

   The file is available in two forms:  MPCORB.DAT and
MPCORBCR.DAT.  One is intended for DOS/Windows,  the other for
Unix,  but Guide will use either.  If one of these files is in the
Guide directory,  the "Use MPCORB" option will no longer be grayed
out.  Toggling it on will cause Guide to use the elements in MPCORB,
while ignoring the asteroid data built into the Guide disk.

   Switching to MPCORB has advantages and disadvantages,  and it's
important to understand both.  The data on the Guide disk includes
a lot of pre-computed information regarding where each asteroid will
be over a given time and how bright it will get;  this allows Guide
to display asteroids almost immediately (as long as your "location"
is on Earth).  Elements are provided covering a range of several
decades,  so perturbations are modelled with good accuracy over that
entire range.

   But,  of course,  the "built-in" asteroid data can't be rewritten.
You're stuck with the data as it appears on the CD-ROM,  and newly
discovered asteroids (and orbits updated based on new data) will not
be included.  This is where MPCORB has a great advantage;  the MPC
updates it almost daily.

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