From: "Dwight K. Elvey" 
To: atm{at}shore.net
Reply-To: "Dwight K. Elvey" 


>From: "Mark Suchting" 
>
>At 07:26 AM 22/12/2002 -0600, Joe Mayenschein wrote:
>>Ok, personally, myself I have never had a "Problem" with
it, because I
have
>>never used one.
>>
>>BUT,,, when holding many many sidewalk sky shows, having one person
>>standing below the line of sight is clearly obvious. and they are many
feet
>>away.
>
>Joe
>
>Pardon me for jumping on the well -worn hobby horse but :
>
>Therein lies the difference between theory and real experience with a true
>binocular.
>
>I think body heat in front of aperture is a great variable depending on
>which way the wind is blowing, how much clothing you have on, is it winter
>or summer, which way is the scope pointing , and what altitude etc.  There
>is no way to even imagine hard and fast rules about the effect, let alone
>write off a particular style of  instrument because of a percieved
problem.
>
---snip---

Hi
 I've actually run some experiments with cardboard
baffles to block the thermal plume. My test setup was a 13" bino.
Looking at a defocused bright source, the thermal plume could be seen
across about 30% of the aperture width in each telescope( defocusing is the
easiest way to identify the plume effect). On a cold winter night, one can
clearly see the difference, looking at Saturn with 120X. There is a
distinct difference in contrast. During summer usage, the distinction isn't
quite as clear.
 All that being said, we still haven't made any permanent
blocking device and still enjoy the bino telescope with plumes. My guess is
that you'll find that the thermal plumes will be the least of your problems
in making a bino scope.
Dwight

--- BBBS/NT v4.00 MP