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    Re: How does the AstraIIIb split mirror work?
    From: Joel Jacobs
    Date: 2004 Apr 22, 19:09 -0400

    Jim, et al,
    I was suffering from brain fade when I wrote my original explanation in
    answer to Jim's question. Let me try again leaving the basic premise intact.
    It doesn't make any difference when you look through a split horizon mirror
    whether or not the sextant is held steady or is being rotated. You will
    always see a whole reflected image in the horizon mirror. One half of which
    is on each side of the mirror, and appears as a single image when viewed as
    a whole.
    I submit that the reason for this is that the index mirror captures the
    whole image and reflects it on to the horizon mirror. The clear portion of
    the horizon mirror does have reflective properties and reflects this image
    back to the viewer.
    I hope this does a better job then my first attempt, and I don't get another
    flash of feeling stupid.
    Joel Jacobs
    ----- Original Message -----
    From: "Joel Jacobs" 
    To: "Navigation Mailing List" 
    Sent: Thursday, April 22, 2004 5:56 PM
    Subject: Re: Re: How does the AstraIIIb split mirror work?
    > Jim,
    > If I understand your question correctly, and we're talking about the split
    > mirror, not the whole horizon mirror, here's my two cents.
    > First, do you get an image on both sides when you hold the sextant steady?
    > don't think you do. Unfortunately, I'm not in a place where I can test
    > conclusion.
    > My supposition is, even though the split image horizon mirror is only
    > silvered on half its surface, the clear portion of the glass does have
    > reflective properties of its own.
    > When this is combined with the rotation of the wrist, what I think may
    > happen is the rays reflected from the index mirror on to the horizon
    > allow the object to subtend an arc through both halves. If your seeking an
    > answer based on physics, I haven't got one.
    > In respect to comments about the mirror settling down, that may be due to
    > uneven spring clip tension where some are tightly made with a lot of
    > kickback and one or more less so. But shouldn't that cancel out
    > since everything behind the mirror is rigid?
    > Interesting question.
    > Joel Jacobs
    > ----- Original Message -----
    > From: "Jim Thompson" 
    > To: 
    > Sent: Thursday, April 22, 2004 5:05 PM
    > Subject: Re: How does the AstraIIIb split mirror work?
    > > Yes Doug, I aligned the split mirror and took out all the errors when I
    > > installed it.  I plan to do some star-star checks according to Frank
    > Reed's
    > > suggestions when I get the CN exam done next week.  Still curious about
    > why
    > > the Body seems to appear across the whole field of view though.  "Smoke
    > and
    > > mirrors" -- but the physics eludes me.
    > >
    > > Jim Thompson
    > > jim2@jimthompson.net
    > > www.jimthompson.net
    > > Outgoing mail scanned by Norton Antivirus
    > > -----------------------------------------
    > >
    > > > -----Original Message-----
    > > > From: Navigation Mailing List
    > > > [mailto:NAVIGATION-L@LISTSERV.WEBKAHUNA.COM]On Behalf Of Royer, Doug
    > > > Have all
    > > > the adjustments been made according to the paperwork you get when you
    > > > purchace a new mirror etc.?That will insure all is aligned
    > > > important.

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