"Shiperton Henethe" <shiphen@yahoo.com> writes:

>What about a vacuum, then - suppose the focus
>mechanism would be tricky.

A vacuum would certainly prevent condensation.  The problem is that (a)
everything now needs to be strong enough to withstand the pressure
difference, and (b) you need vacuum-capable seals everywhere.  A
rotating or sliding seal around the focus shaft would be particularly
tricky, though it might be possible to do the focus externally.  (Make
the eyepieces a separate sealed system from the objective/prism, and
focus by changing the distance between the two sealed units).

The nice thing about filling binoculars with *any* dry gas to
atmospheric pressure is that you don't need the extra strength in the
housing, and you don't need perfect seals.  Nitrogen is just the
cheapest dry gas, since 75% of the air is nitrogen.  On the other hand,
atmospheric pressure isn't constant even if you stay at sea level, and
even less so if you climb mountains.  To avoid pressure differences from
distorting binocular frames, even a "sealed" binocular has to be able to
breathe to equalize pressure, and over time this brings humidity from
outside air into the binoculars.  I don't know how the manufacturers
deal with this.

>But thinking about it if you did start with
>Helium what would happen? If helium could get
>out but nothing could get back in, then a
>partial vacuum would be set up, this dimishing
>the flow of helium, but in either cas not
>increasing the weight of the device...

For this to work, you'd again need seals that could withstand a pressure
difference without leaking at all - for years.  And a housing that could
handle the pressure difference too.

	Dave