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On 13 Feb 2005 at 23:49, Fred Hebard wrote:

> First, perhaps an expert would know how to adjust the collimation on a
> modern prism sextant scope.  But I see no screws for doing that on mine,
> and disassembled it and examined it carefully.

Are you talking about an internal alignment, so that the eyepiece, prism,
and objective lenses are all parallel, or just aligning the scope to the
sextant frame? If the latter, one could build a ring-mount system to put
the scope in similar to what is used on astronomical telescope finder-
scopes (though it could be pretty tight on a sextant). These are easy to
manufacture out of ABS plumbing pipe.

I wonder how important alignment is? I built my own scope using an exhaust
pipe fitting and a piece of metal that I bent with a hammer and vice until
it matched the 90? of my tri-square. A piece of aluminum channel was
bolted on to it with a groove cut out to mount on the sextant. It seems to
work fine for lunars (better than a sight tube, anyway, though perhaps a
professional scope is much better). I worried about aligning the roof
prism to the lenses more than anything else (but in the end I just wedged
it in with some shims and hoped for the best).

Also, elsewhere in the thread it was mentioned that alignment wires were
helpful. Although wires can't be added to a Galilean scope, a prismatic
scope is different. Depending on the eyepiece design, it may be possible
(and even easy) to string a pair of magnet-winding wires at the focal
plane of the eyepiece. If you can remove the eyepiece, it's a simple
matter to find the focal point (if it's beyond the last lens).

I am neither a machinist, nor an optical expert; the manipulations that
I've described here are within reach of anyone. But it may be wise to
experiment with exhaust pipe before attacking a scope worth hundreds of
dollars.

Ken Muldrew.

   

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