To plot USNO-A2.0 data, select the "Stars | Select Catalog" menu.

Before selecting "Plot USNO-A2.0" for the first time, you should click
on the "Options" button that is just below it.

In the "Options" dialog, the most important setting to note is the
"Drive Letter for USNO-A2.0 Data".

If you are reading ALL of MegaStar's data from a hard drive, make sure to
check "No" for the question "Is the same CD-ROM drive used for both MegaStar
and A2.0 data?".

The Field Size Threshhold is the maximum field size (measured in the
"vertical" dimension) for which A2.0 data will be plotted.

In the magnitude filter section, if "Use Min/Max" is selected then the
extremes of either the red or blue magnitudes will be used as the
reference for filtering.

The reference to displaying "unknown" magnitudes refers to magnitudes
in the database that are greater than 25.0, which in general means
that the magnitude is not known.

The reference to displaying "uncertain" magnitudes refers to the "Q"
value (see the notes below) being set to 1.  However, it appears that
most stars have this value set to 1, except for the far negative
declinations.  So in general, you should leave the option to display
"uncertain" magnitudes turned ON.

Notes:

Here is an excerpt from the USNO-A2.0 documentation, which explains the
meaning of "Unknown" and "Uncertain" magnitudes:

The third 32-bit integer has been packed according to the following
format:     SQFFFBBBRRR   (decimal), where:

     S = "-" if this entry is correlated with an ACT star.

     Q = 1 if internal PMM flags indicate that the magnitude(s) might be
         in error, or is 0 if things looked OK.

     FFF = field on which this object was detected.

     BBB = 10 times the blue magnitude.  The range 0 through 250
contains reasonable magnitudes.  500 is reserved for a PMM flux
estimator that was exactly zero, and 501 through 750 are reserved for
PMM flux estimators that were negative.

     RRR = 10 times the red magnitude.  As above.