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## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: How does the AstraIIIb split mirror work?
From: Ken Muldrew
Date: 2004 Apr 25, 00:18 -0600

```George Huxtable wrote:

> Then, from outside the house, close to the glass, I observed the view
> of the Sun as observed by reflection in that window from its two glass
> surfaces combined. Even though it's only a small fraction of the
> sunlight that is reflected in that way (most of the light passing
> straight through), it's still bright enough to dazzle, and maybe
> damage, eyes. To look at the Sun that way, you need VERY dark glasses.
> I used a shade from an old sextant.
>
> Question: what would you expect? Should I see two images of the Sun,
> of roughly equal brightness, displaced by some fraction of the
> thickness of the glass, or only one?
>
> Answer: I saw a single image of the Sun. Even though about half the
> light came from the back surface, and half from the front, the two
> images of the Sun coalesced, and were quite undistinguishable.

I wonder if the sun is simply too bright for this experiment to demonstrate
the double image. If your window glass was a meter thick, but plane
parallel (and made of optical glass), and you were a few centimeters
from the front surface, I can't understand how the rear surface reflection
would make it's way back to where your eye was.

If you hold a piece of glass in front of your eyes in a room with normal
light, you see double reflections of objects in the room; one from the front
surface and one from the rear. Do you agree with this? Are you saying
that this double image is only due to the proximity of the objects, and that
as they get further away, the two images will move together so that
objects at infinity will give only one image? I just don't see how this can
happen.

> But just try it and see if you agree with me.

The sun's not up right now but I took a piece of 1/8" float glass outside
and looked at the reflection of a streetlight with something like a 30?
angle of incidence. There is a second image, a little dimmer than the
primary image, but clearly visible. Also Jupiter was reflecting with a
double image. Binoculars made this particularly clear. I checked this
three times because your argument convinced me that it shouldn't be the
case, but it is.

Ken Muldrew.

```
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