Re: Optics - Environmental Sealing
- From: redbelly <redbelly98@xxxxxxxxx>
- Date: Sat, 01 Sep 2007 14:59:50 -0000
On Aug 29, 4:43 pm, "John M. Jarvis" <john.jarvis @ yahoo.co.uk>
wrote:
I'm fabricating an all-weather telescopic device and would ask the newsgroup
about methods to seal the optics in the device from moisture ingress.
In particular, the telescope is a reflecting-type with a 150 mm primary
mirror. I plan to utilize similarly sized front window affixed to the
housing to seal out moisture.
My concern, is that with outdoor temperatures here that vary between 40C
and -40C from Summer to Winter, I won't be able to keep a good seal between
the front window and housing material that will last over time due to
differing coefficients of expansion between the housing and the window. I
anticipate that the window will be glass (TEC ~ 3-4e-6/K) while the housing
will be plastic (still TBD but possibly a polythalimide with TCE ~
50-100e-6/K). This would result in a 0.5 to 1 mm differential expansion
over the full temperature range and could compromise a poorly thought out
weather seal. With a compromised seal, I would be concerned about
water/dew/rain ingress due to the housing "breathing" as a result of normal
diurnal temperature variations.
How does one normally tackle this problem? With adhesives? With o-rings?
Here is a related question... Presuming that there will eventually be
humidity equilibrium established between the atmosphere inside the device
with the environment, how does one prevent dew forming on the optics as the
temperature drops at the end of the day?
Are there any books or articles that deal with this topic? Something like
Phil Hobbs' "Building Electro-Optical Systems..." but with more of a
opto-mechanical bent.
Lexel sealant works (they claim) down to -34 C. How absolute is your
-40C number? You might consider keeping the device warm with some
kind of low-wattage heater during the winter, to be safe.
FYI, Lexel is flexible when it cures, though stiffer than traditional
silicone sealant. I would use it around the outer edge of the seal,
to minimize outgasing effects inside the chamber where your optics
are.
Here is the manufacturer's product info:
http://www.sashcosealants.com/Content/Files/lexel_brochure.pdf
It's available at Ace Hardware stores for $6-$7, in either a
toothpaste-style squeeze tube or a tube that fits into a caulking gun:
http://www.acehardware.com/search/index.jsp?kwCatId=&kw=lexel&origkw=Lexel&sr=1
Possibly available at Home Depot and Lowe's too, though Ace is where I
have bought it. I've used it myself around the home, though not with
optical devices.
Regards,
Mark
.
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