I have a 24″ (609.6mm) diameter piece of unknown glass that weighs 72 lbs (32.6 kg) on the nose and is 2″ (50.8 mm) thick exactly. The ROC is 93″ (2362.2) so FL is 46.5″ (1181.1) so focal ratio is F/1.94.  From Mel Bartels’ sagitta calculator I get 19.75mm sagitta. 

Been a long time since university, I think volume =  πr^2h =14826666.20 mm^3 or 0.0148 m^3.

On calculator.net I found a spherical cap calculator (https://www.calculat…calculator.html) which gives a formula of Volume = 1/3  π x h^2 x (3R – h) which gives 2886185.8069929 millimeters^3 or 0.00289 m^3 of material removed to make a F1.94 curve.

This my density comes out to D = M/V = 32.6 / (0.0148-0.00289) =  2737 kg/m^3. I look online and see that glass is 2500 kg/m^3 and pyrex is 2230 kg/m^3 so this would tell me my disk is somewhat more dense than glass or pyrex. 

I posted on Cloudynights to see if anyone knows that this material is – either I have something thats not glass or pyrex or my calculations are out of whack! Before I send it to Andre Taillon at BVCTek.ca for re-annealing I need to know whether he can actually generate enough heat to reanneal the glass!

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