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    Re: How does the AstraIIIb split mirror work?
    From: Ken Muldrew
    Date: 2004 Apr 25, 16:59 -0600

    Trevor J. Kenchington wrote:
    > Ken,
    > I suggest you check the geometry again:
    > Let the angle of incidence of a ray of light onto the front of a piece
    > of parallel-sided glass be A. The reflected ray will then leave the
    > glass also with angle A but on the opposite side of a line drawn
    > normal to the plane of the glass.
    > The refracted ray will pass into the glass with an angle from the
    > normal of B, such that sineA/sineB is equal to the refractive index of
    > the glass. That ray will then be (in part) reflected from the back
    > side of the glass with angles of incidence of B (the glass being
    > parallel-sided).
    > On returning to the front side of the glass, the light will still have
    > an angle of incidence of B and will be refracted again such that
    > sineA/sineB is equal to the refractive index. Thus, it will leave the
    > glass at an angle A from the normal, just as the ray reflected off the
    > front of the glass was.
    Whoops! It looks like I was reversing my second refraction. The light
    rays coming out of both sides should be parallel, and as George says,
    they should focus to a point when put through a lens. I just can't
    understand why I was seeing double images of Jupiter last night.
    I understand my confusion with front-surface mirrors in telescopes now.
    The objective lens, or primary mirror, has put the initially parallel rays of
    light off-parallel, so in this case, it is like the double image that one gets
    from glass for nearby objects. For objects at infinity, there should be no
    double image from a piece of glass. I'm convinced of that, but I have to
    find where the extra reflection was coming from last night. I am terribly
    confused over this.
    Ken Muldrew.

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