Re: Re[2]: ATM 10in. Mirror--Star Test ?'s

[Richard Schwartz <richas@chelsea.ios.com>]

-- [ From: Richard Schwartz * EMC.Ver #2.5.02 ] --

>>>To do the star test on a street light, how far away does it have to be to
assume parallel light? Would it behave much different if the street light 
was only two miles away?

This all depends on the size of the street light.  Let us assume that an
angular size of .5" is as good as a point source.   And let us assume that
the street light size is one foot.   then 1 foot / sin(.5") =

 57 * 60 * 60 / .5  = 410400 feet = 77 miles.

Of course, I am not an expert on this, so I invite flaming.

However, there is another way to use a street light.  If it has a shiny
curved surface that images the sun, THAT is a damn good point source.  If it
is too close, you will end up with an under corrected mirror, but it will be
a lot better than a sphere.  I have published the formula on this; see the
archives.