Re: Dew Formation Questions & Fluorite Degradation
- From: Martin Brown <|||newspam|||@nezumi.demon.co.uk>
- Date: Tue, 03 Jan 2006 09:26:22 +0000
Mango wrote:
Hi,
What are the circumstances when dews can form outside and inside the lens?? I plan to buy a used Tak FC-100 at good price and will use it in the mountain for clearer view of the sky. But I heard the fluorite at the back of the doublet apo lens is not coated and water can degrade it (and even melt it). How true is this?
Who told you that? Although Fluorite (calcium fluoride) is very slightly soluble in water it is only of the order of 20ppm w/w. That is almost insoluble to a very good working approximation. But you would not want to leave it immersed in bulk water for a long period of time. To put this into perspective one form of crystalline silica (cristabolite) is about an order of magnitude more water soluble. The other forms are insoluble.
I don't want a complicated heater to use with it. Has anyone encoutered such dew soaked lens in cold nights in the mountain?? How many own any older Tak FC here and have you ever encountered dew forming in the lens??
You will get dew on the skyward facing surfaces including the front surface of the lens if you have an inadequate dew shield. You might get some condensation inside the tube in very humid conditions, but I would expect most of it to form on the very coldest parts - namely the upper surface of the metal tube rather than on the back of the lens.
Some active heating for dew prevention may be needed in conditions of heavy dew otherwise the passive dew shield becomes excessively long.
Regards, Martin Brown .
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