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    Re: How does the AstraIIIb split mirror work?
    From: Joel Jacobs
    Date: 2004 Apr 23, 18:16 -0400

    This is one of the most active threads I've seen, and there are too many
    participant for me to keep track by name for which I apologize.
    That said, a number of different people presented the proposition that light
    was reflected from the mirrors back surface as well as its front in some
    form of combination. Someone presented some trig to explain the refraction
    of rays from two parallel planes. One person developed a lengthy argument
    which I believe hinged on the proposition that there were two separate
    planes, but I think he said they reflected a single image. If I'm wrong on
    that I apologies again. A figure of 5 mm between surfaces was mentioned.
    FYI, I think most sextant mirrors are 4 mm in thickness if that makes any
    With that highly simplified summary, let me throw a new wrinkle into this
    discussion. No one has asked Jim T, if the split image horizon mirror is
    front or rear silvered. I submit that would make a big difference and if is
    front silvered obviate all the argument about two objects, two sources of
    rays, parallelism, etc. I trust it would make also make a big difference if
    the index mirror was a mismatch and was silvered opposite to the horizon
    It's been my experience that most of Celestaire's mirrors are front
    silvered. But let's check.
    A question for Jim T., is one or both of your mirrors front or back
    Joel Jacobs
    ----- Original Message -----
    From: "Ken Muldrew" 
    Sent: Friday, April 23, 2004 11:36 AM
    Subject: Re: How does the AstraIIIb split mirror work?
    > On 23 Apr 2004 at 8:46, Joel Jacobs wrote:
    > > Ken, and Jim T,
    > >
    > > You may be right, but I have a problem with your explanation because
    > > of the double reflecting principal of a sextant's mirror system.
    > >
    > > We know that when we look through a sextant's telescope, unless it is
    > > set a zero elevation, the clear portion is viewed directly ahead, and
    > > the reflected image we see is actually one half the number of degrees
    > > of arc sighted. That means the eye is seeing an image in the telescope
    > > that is above the viewer's line of sight. If this is correct, then how
    > > can the result of seeing the image in both panes, be caused by
    > > differences in focal length? IMO, the image is that which is reflected
    > > off the index mirror to both sides of the horizon mirror and is from a
    > > single source so that the focal length is equal.
    > The important thing that changes with focal length (remember that the
    > focal length refers to the objective lens of the scope, the actual
    > focus is set at infinity) is the depth of field. The field of view
    > (FOV) can also change, but the Astra sight tube, for example, so
    > severely restricts FOV that this isn't really an issue. You can
    > easily verify the fact that you can (apparently) see right through
    > solid objects that are in front of the focal depth by putting your
    > finger in front of your scope and looking at something far away. You
    > don't see a total obstruction in the middle of the field, instead you
    > see a dimming but everything in there. The reason you can see
    > everything is because the lens in larger than your finger (if you
    > have a small objective and large fingers, you'll have to use
    > something smaller, maybe a pencil) and it gather off-parallel rays of
    > light and redirects them into your eye. This effect fully explains
    > why you can see the straight-ahead view through the horizon mirror
    > and why you can see the reflected view through the horizon glass.
    > Ken Muldrew.

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