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George wrote:

> What about the dark glass that's used at the side that faces the Sun?
> Presumably, that has to be dark enough to make it comfortable to view the
> Sun image. Agreed, the brightness of that image is reduced somewhat by the
> dimming of reflections in the unsilvered glass, which means that a less-dark
> outer glass may be acceptable, compared with the shade required for a
> sextant. But my concern is this; that the horizon, also, has to be viewed
> through that same dark glass, and unless the horizon is particularly clear
> and bright, it won't be seen sharply enough for a measurement. Is that a
> handicap, in practice?  Is there, or could there be, an additional shade
> glass on an arm, to put it just in the way of the light from the Sun,
> without darkening the horizon view?

Excellent observations.  It was dark the other eve, so could not get a
transmission value on the the unit as a photographic spot light meter would
not be sensitive enough to give anything near usable values under low-light
conditions.  The other obstacle is measuring just the neutral density (ND)
portion of the device (yet to be properly named ;-) without disassembling
it.

Hopefully I can get my little paws on the unit again, and get a rough idea
of the ND or f-stop value.  Until then, one approximation might come from
the transmission value of eclipse-viewing glasses sold in astronomy
magazines.

The only (semi) useful information I can impart at this time is that the
shade glass is colored through, as opposed to surface coated (the difference
being the number of possible reflections created by/within the shade glass;
and that photographic neutral-density filters operate on a log scale, where
0.3 is approx. equal to 1 f-stop loss, 0.6 equal to 2 f-stops loss (1/2 the
transmission), 0.9 equal to 3 f-stops (1/4 the transmission) etc.

Totally off the top of my head, if I recall John used 7% transmission
glasses in his jury-rigged unit, which (just an educated guess) is too
little protection to view the sun with an eyeball.

Yet, as George pointed out, one must be able to make out the horizon as
well. Which, without further investigation, causes me to wonder about the
safety of the device if using the brightest Sun image.

Just thoughts/springboards for discussion--more investigation needed.

Bill

   

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