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George Huxtable wrote:

> Here's the point. Parallel rays of light, incident on the objective
> glass of a telescope, are brought to a focus at a single point, no
> matter which part of the objective they enter. And when seen through
> the eyepiece, they all appear to be coming from the same direction,
> from a particular point in the sky.

This would be true if the reflected rays from the front surface and back
surface were parallel. They're not. The reflection from the front surface is
at an angle that is determined by the incident angle. The light that
passes into the glass is refracted. The light that continues on and passes
through the back surface is refracted once again (cancelling the initial
refraction) and it continues on parallel to the incident light. That's why we
see a single image when looking through a piece of glass where the
image extends beyond the edge of the glass. But the light that is
reflected off the back surface has a different angle of incidence than the
original ray. So the angles of reflection from the front surface and the
back surface are different. When this light that is reflected from the back
surface passes back through the front surface, it is is refracted once
again by passing into the air. But unlike the case for light passing all the
way through the glass, this refraction does not cancel the initial refraction
and the light emerges out of parallel with the front surface reflection.

That is why there is a difference between front-surface mirrors and back-
surfaced mirrors (in general, not specifically relating to sextants). Back-
surfaced mirrors are fine when the brightness of the back reflection
swamps out the dim front-surface reflection. We use them normally
because it's much easier to protect the silvered surface behind the glass.
In applications where a bright object reflecting from the front surface
could be mistaken for a dim object reflecting off the silver back surface,
such as in astronomy, one must use a front-surface mirror. If George's
argument was correct, there would be no difference between a front-
surface mirror and a back-surface mirror, since the reflections from the
front surface would always be coincident with the back surface
reflections for a back surfaced mirror.

> eyepiece. The two will combine imperceptibly. The important proviso is
> that there's no ANGULAR displacement,

This is correct but I believe that there is an angular displacement.

Ken Muldrew.

   

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