Mirror Damage?

[Naemeth]

Hi all, just wondering if you can crack the glass in either refractors or reflectors by taking them inside quickly rather than letting them slowly warm up. From about -14 minimum (maybe less depending on the winter) to 19C. Could it do any harm?

[ScottS]

Wouldn't have thought so.

[NGC 1502]

Agree with post #2.    Thermal shock is a possibility with very extreme temp changes perhaps, but I'd have thought that -14 to +19 was not in that league.

But with the example in post #1, what would happen is instant and severe dewing, including internal dewing as well.

Best to cap the scope, or if possible, with a small scope, put it in a case before bringing indoors, and let it acclimatise more gradually, maybe in a cooler room than lounge temperature.

Next morning, open up the scope to dry out thoroughly.

Regards, Ed.

[Stu_2011]

Hi

I think fluorite containing glass and more specifically calcium fluorite crystal CAN crack with sudden thermal shocks but it has to be quite extreme.

I cracked a CaF window a few years ago by allowing it to heat up to quickly, this was in a lab rather than a scope.

Just be sensible, if it's -15C don't bring the scope straight into a 25C room, try and put it somewhere in between for a bit

Thanks

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[Paul M]

Although I'm not going to try it, I'm sure my Pyrex mirrors would survive intact if I put them in the fridge for an hour then put them in a bowl of boiling water.

Probably wouldn't be much good for the coatings but the glass is made for it!

[Naemeth]

Thanks all, that has alleviated my fears somewhat .

[Paul M]

Thanks all, that has alleviated my fears somewhat .

I wouldn't worry about extreme cold just now.

There is a growing consensus that this winter,or at least the first half is going to be dominated by mild Atlantic weather.

Might be worth getting the optics waterproofed then

[pete_l]

No physical damage. The glass used is low-expansion and it's the rapid expansion of quickly heated glass that causes the cracks.

However, if you bring cold <anything> into a  warm environment, you'll get a lot of condensation on it. While that won't hurt the glass unduly, it can get metallic surfaces damp and prone to rust and electronics can suffer, too.

[Tinker1947]

A garage or shed would be the best place to park it up over night........

[Stu_2011]

No physical damage. The glass used is low-expansion and it's the rapid expansion of quickly heated glass that causes the cracks.

However, if you bring cold into a warm environment, you'll get a lot of condensation on it. While that won't hurt the glass unduly, it can get metallic surfaces damp and prone to rust and electronics can suffer, too.

The glass in mirrors is low expansion but CaF2 crystal and fluoride based glasses have quite high thermal expansion, values of 18-20 x10-6 for CaF2 I seem to recall which is more than some metals. These fluoride glasses are quite fragile in reality with low failure stresses and strains as well.....I could be wrong.

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