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Don't shoot the messenger. I was jsut reporting what the sextant claim
said.

George wrote:
"Imagine a thin wedge prism, being deliberately interposed into the
optics,
in just one side of the split viewline, just before the light enters
the
eyepiece lens. "

Instead of imagining it, I tried it. I set up a 24 mm diameter
objective of focal length about 100mm and a -40mm f/l lens as an
eyepiece on an optical bench and focussed it on a collimator(i.e.
object at infinity). With my eye in a normal viewing position I
gradually introduced a prism with faces at 45 degrees from the left.
Nothing appeared to happen until the prism almost covered the whole of
the eyelens, when the target darkened and then disappeared. By moving
my eye to view the left half of the field, not normally viewed, the
darkening and disappearance occurred earlier.

I leave it to the physicists to give a clear explanation, which I
imagine is based on the fact that with a Galilean telescope the eye
cannot be placed at the exit pupil, to receive all the rays from the
objective, and only a small field can be seen from one position.

And Rick wrote:
"Even if the filter is, in effect, fail-proof, the instrument itself
will certainly heat up (after all, the Sun's busy emitting in the IR,
too) and that poses a number problems.  Should the filters either burn
through or the control inadvertently be flipped to "clear", the
observer's eye is at risk."

This may be so with astronomers' telescope with their large objectives
and high magnifications and I certainly do not advocate deliberately
looking at the sun through a telescope, but most of us at various
times must have received an accidental blast of sun light over the top
of the horizon mirror when taking a sun sight(some Heath and Tamaya
sextants have blank shades to prevent this). I certainly have on a
number of occasions and my optometrist has never said that I have
retinal scars. The danger is there, but we blink or look away too
rapidly for damage to occur.

As to heating effects within the optical system of a telescope I would
expect most of the infra red to be absorbed by the glass of the
mirrors and lenses and in any case, the light is not brought to a
focus within the lenses or in the case of the Observator at the
filters. If this were not so, I don't think Mr Wild would provide two
eyepiece shades with his theodolites, nor sextant makers with their
sextants, as there might be a risk that they would heat up and
shatter. The telescope may well heat up but not, I suspect, very much.

Bill Morris
Pukenui
New Zealand

On Aug 15, 9:06�am, "Richard B. Emerson" 
wrote:
> Forget all that.� You're quite right in raising the the issues of allowing 
"raw" sunlight into the scope and questioning the light gathering for star 
shots.� While dimmer stars might still be visible, the horizon may pose a 
problem in that reduced light gathering will mean the horizon can become 
unusable sooner than with a brighter image.� But this is minor compared to 
the following...
> As to allowing unfiltered sunlight into the scope, in astronomical circles 
this generally a major no-no for solar observing.� While Herschel wedges can 
be used as light attenuators, if they fail, the observer will get a blast of 
concentrated sunlight (resulting in anything from corneal burns to retinal 
burns to permanent blindness).� Even if the filter is, in effect, fail-proof, 
the instrument itself will certainly heat up (after all, the Sun's busy 
emitting in the IR, too) and that poses a number problems.� Should the 
filters either burn through or the control inadvertently be flipped to 
"clear", the observer's eye is at risk.� Given this issue alone, concern over 
the filters' surfaces being parallel or not is an exercise in "rearranging 
deck chairs on the Titanic".� (For the record, I also question the reasoning 
that supports "the filters don't have to have parallel surfaces")
> Rick Emerson
> S/V One With The Wind
> All in all, this sextant strikes me as an interesting, but ultimately 
futile, attempt at a better sextant mouse trap.�
> George Huxtable wrote:A further thought about this "Observator" sextant has 
struck me. Bill Morris wrote, in [6123]- Members can read the original patent 
document 
athttp://v3.espacenet.com/origdoc?DB=EPODOC&IDX=EP0082556&F=0&QPN=EP0082556. 
Its claim to originality are that the filters are contained safely within the 
viewing means and that they can be made of cheap material like photographic 
film, as they do not have to have flat parallel faces, lying as they do 
behind the objective lens of a Galilean telescope. ============= Which 
brought this comment from me in [6129]- It's true that the two filters do 
indeed "lie behind the objective", but so far behind it that they are closely 
in front of the Galilean eyepiece. There, they sit side by side, the horizon 
shade to the left and the reflected-light shade to the right, controlled by 
separate adjusting knobs. ============= But is Bill's comment correct, that 
therefore "they do not have to have flat parallel faces"? Imagine a thin 
wedge prism, being deliberately interposed into the optics, in just one side 
of the split viewline, just before the light enters the eyepiece lens. 
Wouldn't that displace the apparent direction of one of those images, by the 
deflection-angle of the prism, as seen through the eyepiece, and not the 
other? If that's correct, then the requirement for optical quality in the 
shade, in its new position, is no less than it is in a traditional sextant 
design. And if so, the suggestion that the accuracy of the instrument would 
not be degraded by the use of photographic film, instead of optically flat 
glass, is at least questionable. George. contact George Huxtable 
atgeorge@huxtable.u-net.comor at +44 1865 820222 (from UK, 01865 820222) or 
at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
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