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George,

I didn't mean my list (vignetting, depth of field, and practical
optical engineering) to be exhaustive.  Yes, the whole-horizon effect
could depend other factors, such as the entendue (the exit-pupil area
times the solid angle).  But I was simply saying I can't understand
the Galilean scope's behavior from elementary ray tracing, which is
the limit of my geometrical optical knowledge.

I have compared all combinations of a 3x15 terrestrial scope, and
6x30, 7x18, and 8x42 prism scopes (all with internal focal planes) and
2.5x24, 3x25, 3.5x40, and 4x40 Galilean scopes.  All of the four focal-
plane scopes show the same whole-horizon effect with trad-horizon
mirrors.  While all of the four Galilean scopes, having no focal
plane, show the half horizon.  Given the range of the various powers
and objective sizes, my tentative thought is that these different
views are not due to the magnification.

BTW, I much prefer the split horizon mirror with the prism scope,
giving me the wide FOV and the whole-horizon view.  It's true that the
6x30 scope is a bit too high powered, but in my experience, coupled
with its both its wider FOV and the whole-horizon view, the 6x power
is usually tolerable, and the combination quite desirable.

John


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