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    Re: Is the focus on the mirror or on the star
    From: Brad Morris
    Date: 2017 May 24, 19:30 -0400


    On May 24, 2017 12:43 PM, "Frank Reed" <NoReply_FrankReed@fer3.com> wrote:


    There is a phenomenon that can influence the apparent focus. Your eye itself has an optical focussing capability. When you are working close to your mirror assembly, your eyes may be adjusting to a nearby focus. When you then look through your sextant, your eyes will not necessarily relax to infinite focus. This may create the impression that you have to focus differently depending how close you are to the mirror. Try this: just before you look into your mirror, look at some distant hills. Allow your eyes to relax to infinite focus. Then look through the sextant scope. The image in the mirror should remain at apparent infinite distance.


    If the AH is contained within the FOV of the sextant, then the observer may be tricked into changing the focus such that the AH is in focus. 

    Infinity focus for the human eye is generally beyond 20-25 feet.  Distant hills, as Frank recommends, will certainly do!

    Brad 

       
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