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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, 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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