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    Re: "Vernier acuity" of horizon IC tests
    From: Frank Reed
    Date: 2009 Jul 6, 20:37 -0700

    Douglas, you wrote:
    "I note the practical results of horizon index error measurement here by Mr. 
    Rudzinski, and of the results in the paper by Gordon; and the results posted 
    by others here,  amount to around 0.4 minute of arc to one minute of arc.
    The cone diameter is about 0.4 minute of arc.
    'Normal' visual acuity (depending on the target type) is between 0.4 and one minute of arc.
    Curious coincincidence, do you not think?"
    
    But a coincidence nevertheless. The visual acuity of the human eye is NOT the 
    limiting factor in LOP sights such as in that article.
    
    Let's consider a simple experiment. You want to determine the resolution of a 
    telescope and compare it with the normal resolution of the human eye. So you 
    get out a standard eyechart. Let's make it one of those "Snellen E" charts 
    where you look at a row of capital E's in various orientations. Now, wearing 
    appropriate eyeglasses, let's suppose I can read the 20/20 line at a distance 
    of 20 feet and maybe a little farther. So that makes my vision 20/20 or a 
    little better and implies a resolution of 1.0 minuutes of arc or a little 
    better. NOW, we grab a 7x telescope from a sextant case. I assume that this 
    is a decent telescope with good quality optics. I step back to a distance of 
    140 feet with the 7x telescope in my hand. Looking at the chart with my eyes 
    (plus eyeglasses), that 20/20 line is now unreadable --the E's are no more 
    than dots. But when I hold the telescope up to my eye, I can read it just at 
    the limit of my resolution, exactly as I could read it when I was twenty feet 
    away without a telescope. The elements of those E's are reduced in size by a 
    factor of seven because I am seven times further away. But the telescope 
    magnifies them by a factor of seven and my eye is presented with an image 
    that is identical to what I could see at 20 feet without the telescope. The 
    telescope allows me to resolve features that are approximately 0.14 minute of 
    arc in angular size.
    
    -FER
    
    
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