From: dal3@cbnewsg.cb.att.com (dale.e.parson)
Subject: Re: condensation inside a SCT
Organization: AT&T
Date: Tue, 1 Sep 1992 15:12:29 GMT

In article <1992Aug31.204833.21748@linus.mitre.org> chaloux@mandolin.mitre.org (Dave Chaloux) writes:
>All this talk about hot air, cold air, and sealing the air in a SCT
>resurfaces a question I have wondered about from time to time. Air when
>it cools contracts and when it gets hot expands. The amount of expansion or
>contraction is not insignificant over the range of temperatures typically
>encountered. A BTE calculation for -10 C to 30 C would be about 15%. This
>leads to significant over or under pressure compared to the Atmosphere
>depending on when it was sealed. I remember demos of gas cans being sealed,
>put in icecold water and getting squished. I have very bad images in my
>head of corrector plates imploding or exploding on my SCT. Is this a real
>worry? ...

The most reassuring response I got back from the original post was from a person
who had a C8 8 yrs. & only had this problem once (therefore not a design
problem), and who uses 'dew zapper' heating elements with no adverse effects.
I concluded that it was a short term problem caused by miserably high
humidity levels last week, coupled with a cavalier attitude on my part about,
1) storing the scope downstairs, which is more damp than upstairs at my house,
and 2) letting the scope sit out in the dew after I've switched to meteor
watching or dozing in the lounge chair with my face pointed at the sky
(two descriptions of a single activity).

My conservative solution of letting the scope, with its light absorbing
black body, sit in direct sunlight behind a picture window with the back
off & shaded from dust by a plastic 'umbrella', with the back pointed up,
seems to have done the trick. I was out half the night last night
and had no trouble, although it was about the most dew-free night that I can
remember this summer. I conjecture 2 mechanisms at work in this fix;
remember that I'm not a physicist, I just play one at night:

1) Expansion of air inside the tube through heating pushes some of said
air, along with its portion of moisture, out the back. So long, some of the
H 0.
 2

2) With the room air being both cooler and less humid the the air inside the
scope (it was a clear morning), there is disequalibrium. Gasses move to
balance the disequilibrium, taking more humidity out.

I was leery of blowing hot air in both because I can imagine all the dust that
would go in with it (then I'd have a REAL problem), and because the process
can only be hastened so much. The hole at the back is little, and exchanging
wet air for dry will take time. Expanding the wet air in the scope is all well
and good, but not much use if most of it cools & stays inside the scope.
Given the slow nature of the air exchange, the slow bake in sunlight seemed
more appropriate, also more esthetic.

The liquid nirogen had the 'mad scientist' feel, and I only do that at work.

I keep a tight cover on the back when not in use, but given the fact of
removing it for use, airtight closure doesn't seem realistic. Also I
only want to seal it after it's dried internally--you can expect some humidity
to have gone inside after a long, dewy observing session--recommending
that it dry out IN A DRY ROOM (how could it dry out in a wet room), and that
it be STORED IN SELFSAME DRY ROOM. If the room is the source of dryness,
there is no need for airtight closure.

I appreciate the suggestions I received, including the ones I admit I'm
afraid to use. May you have clear skies with minimal light pollution.


Dale Parson, Bell Labs, dale@mhcnet.att.com

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|    "These words are too solid, they don't move fast enough           |
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|     Suzanne Vega                                                     |
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