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[TowerTalk] Acuity

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Subject: [TowerTalk] Acuity
From: (Michael Keane, K1MK)
Date: Thu, 18 Mar 1999 13:21:58 -0500
The example given confuses detection with the ability to 
discriminate size (acuity). 

The eye's nominal 1 minute of arc resolution which is being 
referenced implicitly just says you're not going to be able to 
distinguish between 3/16" EHS and 1/4" EHS at 50', NOT that you can't
tell there's a guy wire up there. We can all still see that the top 
set of guys on a 100' tower are still there when we stand under 
it, right?

Your eye has no problem detecting objects that have angular sizes
that are significantly smaller than it's resolution limit. As an 
example, go out tonight after it gets dark and take a look up at 
the sky. Any stars that you happen to see will have angular 
diameters of milliseconds of arc, 4 to 5 orders of magnitude 
smaller than 1 minute of arc.

Detectability of an unresolved source will depend on the 
contrast with its background (within the resolution element). 
The eye can detect roughly a 1% changes in brightness.  For a 
moderatly reflective and linear object like a guy wire seen against 
a more or less equally bright background you might expect to
have to increase the separation by a factor of 10x-100x (order of 
magnitude) beyond the point where the guy becomes unresolved 50' 
in order to make the guy undetectable. 
                              Mike, K1MK

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