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CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.
Fluorite in Sub Zero Weather
Crystalline Flourite, and to a lesser extent ED glass, is sensitive to thermal shock, but only under certain circumstances. You could take a 6" disc of Fluorite (or ED) and drop it into boiling water, and it would be quite happy. Take it out of boiling water, and drop it into cold water, and it will shatter into a million pieces.
A warm blank of Fluorite heated evenly throughout will crack instantly if a small amount of water is dropped on it. It does not have to be very warm. The amount of heat that water can absorb is quite large, and the sudden contraction of the material surface will cause the surface to erupt and crack. Once a crack has developed, it is only a matter of time before the glass splits into two pieces. This is also true of ED glasses, although they require more cooling shock. It is doubtful that simply exposing warm fluorite to cold air would result in any damage - in fact there is a 10" Fluorite refractor on top of Kitt Peak that has survived several winters without damage, but I think the Fluorite is in the back, protected by a cover element. What can cause the Fluorite to split is to take a warm element into a bitterly cold night and expose the surface to a high freezing wind. I would be very cautious to not let that happen. I have experienced a 4" fluorite blank splitting in half under just such conditions. Fortunately, most fluorite lenses are somewhat protected deep within their cells, and even the dewcap offers a lot of protection against sudden temperature shocks.
As far as using canned air to blow off dust - I would be very careful. I once accidentally caused the surface of an ED element to erupt and crack open when the canned air accidentally spit out a stream of frozen material. The ED was rescued by immediately grinding 3 mm off the surface before the crack had a chance to propagate.
ED glass can withstand high temperatures, but crystalline Fluorite will delaminate above a certain point. I found that out the hard way too. I don't know where that point is, but suspect it begins to happen above 250 degrees F. If the material partially delaminates, it will be very susceptible to moisture attack, and can cloud up eventually. Besides, it destroys the figure. I would not leave a Flourite refractor on the back seat of a black leather interior car on a hot sunny day. Perhaps once or twice is fine, but I would not do it repeatedly. In fact, I would not subject any scope to that treatment.
Finally, a word about coatings. No!, Fluoride coatings are not the same as Fluorite optics. Magnesium Fluoride (Mag Fluoride) has nothing to do with color correction, and is in any event not used to coat Calcium Fluoride or Fluorite optical materials. The type of coatings normally applied to Fluorite are oxide multilayers, which can be applied without using extremely high temperatures, and they are tough and scratch resistent. They won't fall off if you blow compressed air at them.
I hope this clears up some misconceptions about these materials.
- Jon Isaacs, Christopher Erickson, Suavi and 1 other like this