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. 

After much discussion with various parties including Andre Taillon of BVCTek in Montreal, who has an oven large enough to reanneal my blank for me, I’m leaning on leaving the blank in the box til I can build my own kiln to do the reannealing. Since my bucket list includes larger mirrors than 24″ if just doesn’t make a lot of sense to drop $1000 for Andre to do the work versus spending about the same on a kiln and doing it myself. At the same time I’d like to progress to a 24″ project so I can get a 24″ Black Vitreous Ceramic blank from BVCTek.ca for about the same money ($911+$383 shipping) as a 1.1″ thin meniscus slumped to F/3.3, or the same in lime glass for $660+shipping. Since a 1.1″ mirror is going to cool fairly fast I would tend to go with a lime glass over BVC just for the cost difference.

So my 24″ blank will stay in it’s crate for a while, while I ramp up to attempting the 24″ F/3.3 meniscus mirror by doing the following projects, some of which I’ll sell to fund subsequent projects:

  • 6″ F/3.3 rich field astrograph (underway)
  • 6″ F/20 dall-kirkham planetkiller
  • 10″ F/3.3 rich field astrograph x 2
  • 12.5″ F/8 Corrected Dall Kirkham x 2 (will involve splitting a 12.5″ blank in two but I’ve seen this in a youtube video I can’t find now LOL)
  • 18″ F/3.3 BVC meniscus goto dob

I have glass for all but the last one. I figure this work queue will carry me through to retirement in 2031 and beyond

By Gord

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