Previous page  Page 31   Next page    Table of Contents 

fact,  the three .TLEs provided with Guide will already be somewhat
out of date by the time you get this CD.  You might be able to get,
say, an approximate time at which ISS will pass overhead.  But getting
current data is a good idea.

   You can get new .TLE files from several sites on the Internet.  The
site:

http://users2.ev1.net/~~mmccants/tles/index.html

   is recommended,  since it has up-to-date elements for many different
classes of satellites,  and many of the element sets have the magnitude
data Guide uses in filtering out dim objects.  NASA and other government
agencies also maintain element sets.

   Once you have a .TLE file,  click on this option and select that file.
Guide will then simply switch to use of this file for all satellite data.

   The Projections submenu offers a long list of chart projections:   
stereographic (the default),  orthographic,  gnomonic,  Mercator,
and more.  Each serves a different purpose.

   Stereographic projection preserves the shapes of objects quite well,
no matter how large an area is being shown. At Level 1 (180 degree field
of view),  the other projections will show very distorted constellations
near the edge of the chart.  Stereographic projection results in
exaggerated sizes of objects near the edge, but their shapes are okay.
Therefore stereographic projection is used for "all sky" charts in the
center of many astronomy magazines.

   Orthographic projection is best suited for terrestrial maps,  not
celestial charts. In terrestrial maps, one gets an "Earth-from-space"
sort of view.  You may want to use it when displaying
eclipse/occultation paths on the Earth.

   Gnomonic projection involves horrible distortion at large fields
of view.  However,  it shows great-circle routes as straight lines.
It is therefore useful for meteor observers;  draw the paths of meteors
on this chart,  and they will appear as straight lines emerging from
a single point (the radiant).

   Equidistant projection is true in distances and azimuths from the center
of the chart.  It is mostly used for terrestrial charts; short-wave radio
amateurs like its use.

   The Mercator,  Peters,  Miller,  and "simple" projections are almost
never used for sky charts.  They are all cylindrical projections,  in
which lines of latitude are projected as parallel horizontal lines and
lines of longitude are projected as parallel vertical lines.  (For a
sky chart,  read "lines of declination and RA",  or in alt/az coordinates,
"lines of altitude and azimuth.")  The price for this is that the north

 Previous page  Page 31   Next page    Table of Contents