Help with selecting mirror

[GarryLazereyes]

Hello, I'm in the process of upgrading my 10" dob to a 16" but having some trouble deciding on the mirror. 

Im concidering two mirror, either pyrex or soda lime glass, the maker says the quality and coating will be the same on both mirrors 1/8, 93-96% and that the only difference is that pyrex handles temperature swings better. 

The price is roughly 450£ extra for pyrex. The telescope will be located in my roll-off roof observatory so it will not need much of a cooldown time (if any) before using, the temperature in the observatory is only one or two degrees higher than outside temp. 

Im observing from Sweden and when talking about temperature swings im not certain how to think about that. A typical night it might be -8C and over a couple of hours drop down to -10-12C, is that enough of a swing to create problems for soda lime glass? 

Besides being more resistent to temp changes is the durability/lifetime of these two types about the same? 

 

Thanks for any input

[Ships and Stars]

I think the only advantage is Pyrex = less expansion = less time to reach optimal viewing IF going from warm to cold. If the telescope is already stored outside and you are looking at temp swings between 2-4C over a few hours I don't think it will make any difference.

I've heard the biggest improvement comes from a barrier fan which blows across the face of the primary mirror (not the back) where thermal turbulence is greatest. I'd just try it first without, personally speaking, and maybe later on try using a 'barrier fan' (my terminology).

The other thing I understand is that Pyrex is harder to work and to figure than 'normal' glass, so takes more time to complete, hence part of the reason for the higher price. 

I would be happy with soda lime glass in the situation you describe above, the £450 could go for other equipment or stay in your pocket!

I also do not think there is any increase in durability with pyrex over soda lime glass, but others may say differently.

Will be interesting to see what others say, but I've been reading on this a fair bit lately. 

Happy observing in Sweden!