https://en.wikipedia.org/wiki/Rotating_furnace https://en.wikipedia.org/wiki/Rotating_furnace discusses that. I think most mirrors cast this way still would ne

https://en.wikipedia.org/wiki/Rotating_furnace https://en.wikipedia.org/wiki/Rotating_furnace discusses that. I think most mirrors cast this way still would need treatment. For example, the mirrors of the Giant Magellan Telescope (https://en.wikipedia.org/wiki/Giant_Magellan_Telescope#Mirrors https://en.wikipedia.org/wiki/Giant_Magellan_Telescope#Mirro...) are being cast using this method to get a rough (for modern astronomy. For example https://www.techbriefs.com/component/content/article/tb/supplements/pit/briefs/40525 https://www.techbriefs.com/component/content/article/tb/supp...: “after the casting, the surface “roughness” is about 2.5 millimeters, or a tenth of an inch, on average. “The polishing and constant measuring are what turn this amazing piece of glass into a mirror,” he said. “By the time we finish polishing, it will be accurate to better than 25 nanometers”) shape for them. That way, there’s a lot less glass to pour, the remaining glass cools down faster, and less glass has to be removed afterwards. (For the GMT, an additional complication is that six of the seven mirrors will be off-axis. I don’t know whether they spin them off-axis, or accept that more grinding will be necessary to give them their final shape) Instead of glass that solidifies, you can also use a container with mercury or another reflective liquid. https://en.wikipedia.org/wiki/Liquid-mirror_telescope https://en.wikipedia.org/wiki/Liquid-mirror_telescope: “Liquid-mirror telescopes are telescopes with mirrors made with a reflective liquid. The most common liquid used is mercury, but other liquids will work as well (for example, low-melting alloys of gallium). The liquid and its container are rotated at a constant speed around a vertical axis, which causes the surface of the liquid to assume a paraboloidal shape. This parabolic reflector can serve as the primary mirror of a reflecting telescope. The rotating liquid assumes the same surface shape regardless of the container's shape; to reduce the amount of liquid metal needed, and thus weight, a rotating mercury mirror uses a container that is as close to the necessary parabolic shape as possible. Liquid mirrors can be a low-cost alternative to conventional large telescopes. Compared to a solid glass mirror that must be cast, ground, and polished, a rotating liquid-metal mirror is much less expensive to manufacture.” Disadvantage is that you can only point the mirror straight up. Also, there’s evaporation of the mercury.