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the sky is visible,  at a given time,  with that telescope/eyepiece.

   In entering the aperture size,  you can also enter 15' for a 15
arcminute circle,  or 45.3" for a 45.3 arcsecond circle.

   Also:  when any of these objects appears on-screen,  you can adjust
its display by right-clicking on it,  then clicking "Display".  The
only time it is truly necessary to go through the "Ticks,  Grids,
Etc." menu option is when the object type you want isn't on screen
(perhaps because it is turned off.)

7g:  Background dialog                         

   In the Background dialog in the Display menu you may modify the usual
"white stars on a black background" appearance of the screen. There are
five different types of background: "normal colors",  "chart mode",
"red mode",  "flashlight mode",  and "realistic".

   "Normal Colors" is the default;  the background is usually black.
Printouts always have black stars on white (even with color printers).
                                                           
   "Chart Mode" switches to a white background.  This was first added to
support color printing.  If you have a color printer, you can switch to                  
Chart Mode,  set colors to match what you want to see printed, and then
do a printout.  It would be very clumsy to set up colors if you didn't
have a way to turn the screen background to white. It has turned out to
have other uses, however, and some people prefer it to "normal" mode.
                                                            
   The "Red Mode" option converts everything in the chart,  and many
controls outside the chart,  to shades of red and black.  This can be
significantly less damaging to dark-adapted vision,  and is a common
choice among people running Guide in the field.  If you run Guide in
the field without using this option,  your eyes will never fully adapt
to darkness, and you won't see as much in the sky as you ought to.

   By default,  "red mode" is indeed red.  But you'll see a button
provided with that default shade of red,  and can click on it to select
a brighter or dimmer shade of red.  Or you can choose a different color;
there is a school of thought that holds that a dim shade of green is
really the best color for preserving night vision.

   In "Flashlight mode", the background is red and markings are    
black.  The result is a bright screen that won't damage night vision.
It is sometimes useful in finding dropped eyepieces and such:  you
can switch to flashlight mode, find the missing object,  and switch
back to red mode.  Of course,  most people will use a real flashlight
with a red filter for that purpose.

   "Realistic mode" shows a bright blue sky in the daytime and a dark  
black background at night.  Between sunset and true night (when the sun
is more than 18 degrees below the horizon),  shades of blue denote the
progress of twilight.  This gives you some clue to the visibility of an

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